Characterization of continuous fibre reinforced polymers

Elien Van Houwenhove

produced with low-cost 3D printing technologyCharacterization of continuous fibre reinforced polymers

Academic year 2017-2018Faculty of Engineering and ArchitectureChair: Prof. dr. Paul KiekensDepartment Of Materials, Textiles And Chemical Engineering

Master of Science in Sustainable Materials Engineering Master's dissertation submitted in order to obtain the academic degree of

Counsellors: Lode Daelemans, Sander RijckaertSupervisors: Prof. dr. ir. Karen De Clerck, Prof. Ludwig Cardon


produced with low-cost 3D printing technologyCharacterization of continuous fibre reinforced polymers

Academic year 2017-2018Faculty of Engineering and ArchitectureChair: Prof. dr. Paul KiekensDepartment Of Materials, Textiles And Chemical Engineering

Master of Science in Sustainable Materials Engineering Master's dissertation submitted in order to obtain the academic degree of

Counsellors: Lode Daelemans, Sander RijckaertSupervisors: Prof. dr. ir. Karen De Clerck, Prof. Ludwig Cardon

ACKNOWLEDGMENTS i

ACKNOWLEDGMENTS

This thesis project is performed with the support of many people who surrounded me during this interesting

part of my master’s studies. Herewith I would like to thank those people.

I would like to acknowledge Lode Daelemans for being an excellent mentor during this thesis project and

Sander Rijckaert for the great guidance during the project. It was very pleasant and educational to work

together on this topic. I learned a lot during the project and I even found a challenging new hobby.

I would like to express my gratitude to Prof. Karen De Clerck and Prof. Ludwig Cardon for providing me the

opportunity to study this new topic at the department and guiding the project in the right direction. Thank you

for educating me about the fascinating world of polymers throughout my master studies.

Furthermore I would like to thank my fellow students Olivier Verschatse, Laura Truyens, Elisa Van Verre,

Sebastiaan Devrieze, Jozefien De Praetere and Tim De Mits for the very nice experience during our master

studies and for the good support and atmosphere during our thesis project time.

Finally, thank you mom, dad and sister to keep encouraging and supporting me during this thesis and during

my whole academic studies. Thank you Gillis Cornelis to encourage and inspire me.


June 2018

COPYRIGHT NOTICE ii

COPYRIGHT NOTICE

The author gives permission to make this master dissertation available for consultation and to copy parts of

this master dissertation for personal use. In the case of any other use, the copyright terms have to be

respected, in particular with regard to the obligation to state expressly the source when quoting results from

this master dissertation.


June 2018

ABSTRACT iii

ABSTRACT

Characterization of continuous fibre reinforced polymers produced with

low-cost 3D printing technology


Supervisors: Prof. dr. Karen De Clerck, Prof. dr. Ludwig Cardon

Counsellors: dr. Lode Daelemans, Sander Rijckaert

Master’s dissertation submitted in order to obtain the academic degree of Master of Science In Sustainable

Materials Engineering

Department Of Materials, Textiles and Chemical Engineering, Chair: Prof. dr. Paul Kiekens

Faculty of Engineering and Architecture

Academic year 2017-2018

Purpose The need for high performance complex shaped materials with a short design-to-product cycle is

answered by the development of continuous reinforced additive manufacturing. This research focusses on the

development of fused deposition modelling technology for continuous aramid filaments reinfo rced PETG and

mechanical characterization of printed parts.

Approach The project is divided in three phases. First a preliminary mechanical and microscopic investigation is

performed on commercially available ‘state-of-the-art’ material in order to situate the project. The second

phase comprises an explorative study wherein a 3D printing technology for continuous reinforced polymers is

developed. During the third part of the research 3D printed materials are characterized. A tensile test and

three-point bending test provide a mechanical performance analysis. Subsequently the microstructure of the

highly anisotropic layered material is researched through dynamic mechanical analysis and a double cantilever

beam test. As such, the influence of fibre content, altering reinforcing materials and post-processing of the

composite material is analysed.

Results Continuous reinforced specimens up to 56 wt% fibre content are successfully printed. From a tensile

test and a three-point bending test it is concluded that increasing fibre content from 0 wt% to 23 wt%

increases the tensile modulus of elasticity from 1.3 GPa to 10 GPa, the tensile strength from 27 MPa to 288, the

flexural modulus from 0.9 GPa to 6.0 GPa and the flexural strength from 41 MPa to 147 MPa respectively.

Dynamic mechanical analysis leads to insights about the microstructure of the material. The damping capacity

is a potential measure for the matrix/reinforcements interface bonding quality and the amount of

imperfections inside the material. Added to that a novel test for this category of reinforced polymers is

performed: a double cantilever beam (DCB) test in load Mode I can characterize the interfacial fracture

toughness.

Keywords continuous reinforced FDM, fibre content, impregnation, para aramid, PETG

Characterization of continuous fibre reinforced

polymers produced with

low-cost 3D printing technology

Author: Elien Van Houwenhove

Supervisors: Prof. dr. ir. Karen De Clerck, Prof. dr. Ludwig Cardon

Counsellors: dr. ir. Lode Daelemans, ir. Sander Rijckaert

Abstract Continuous fibre reinforced polymers are produced

with a low-cost FDM 3D printer. A 56 wt% aramid reinforced

PETG can be produced. Results for mechanical and dynamic

experiments provide insights for the influence of fibre content on

microstructural characteristics such as impregnation and

matrix-reinforcements interface bonding. The mechanical

performance is increased significantly compared to conventional

FDM products.

Keywords continuous reinforced FDM, fibre content,

impregnation, para aramid, PETG

I. INTRODUCTION

Automotive, medical and aeronautical industries require

high performance materials of complex shapes[1,2].

Additionally, the uniqueness of parts and urgency for fast

deliveries demand for efficient and high quality production

processes. Reinforced additive manufacturing (AM) is a

technology that has the potential to fulfill this need when

mechanical performance of resulting parts can be optimized.

Discontinuous and continuous reinforcements can be

introduced in a fused deposition modelling (FDM) process[3].

The FDM technique produces 3D objects layer-by-layer in the

Z-direction (Figure 1). For each layer the heated extruding

nozzle follows a specific pattern in the X-Y plane while a

thermoplastic polymer is fed in the shape of a filament[4].

Figure 1: Schematic of the FDM technique[5].

Reported studies on continuous reinforced FDM show

significant increases in mechanical strength compared to

E. Van Houwenhove is a master thesis student at the Department of

Materials, Textiles and Chemical Engineering, Faculty of Engineering and

Architecture, Ghent University (UGent), Ghent, Belgium. E-mail:

[email protected]

conventional FDM. The main challenge is to keep improving

performance through increasing fibre content and optimizing

printer settings. Furthermore a consistent set of test methods

designed for these ‘new’ materials is necessary to ensure the

quality and safety of the products [4]. During this project, a

continuous reinforced FDM material is produced and

characterized. The inherent microstructural properties and

mechanisms of a 3D printed material (e.g. filament

impregnation and porosity) are linked to experimental results.

II. SITUATION OF THE PROJECT

A. Literature review

An established commercially available technology for

continuous reinforced FDM was developed by Markforged

(Markforged Inc., headquartered in Cambridge, US). Studies

on Markforged materials report increased performance for

FDM however indicate that the technology does not allow

much design flexibility[6]. Furthermore a couple of in-house

developed techniques are reported in which materials and

settings are easy to adapt but require more knowledge of

electromechanical devices and programming[7–10]. Three

main options are reported to insert the reinforcing material

into the matrix material: pre-nozzle, in-nozzle and post-nozzle

impregnation[5]. On top of that, pre-heating of the reinforcing

material is possible to enhance impregnation with matrix

material[10]. Reinforcing materials are often continuous

carbon fibre, aramid filaments and fibreglass. Commonly used

matrix materials are poly(lactic acid) (PLA), acrylonitrile

butadiene styrene (ABS), polyamide 6 (nylon 6) and glycol

modified poly(ethylene terephthalate) (PETG). The latter is of

interest because of its beneficial mechanical properties (better

than PLA and ABS)[9–12]. Reported by Stephashkin et al. is

continuous 3D printing with poly(etheretherketone) (PEEK),

which is a promising result for further mechanical properties

improvement[13]. Academic studies focus on altering various

parameters of the printing process e.g. printing speed and

extruder temperature[14]. An agreement is found that an

increasing fibre content increases the mechanical properties of

the materials[15].

B. Research objectives

The current research outlines a technology to incorporate

continuous reinforced filaments into a thermoplastic matrix

material through FDM. Initially the technology to produce

such materials is optimized starting from a low-cost

FDM 3D printer. The resulting composite material exists of a

PETG matrix and continuous aramid filament reinforcements.

The second phase includes the characterization of the

reinforced 3D printed specimens. It is aimed to increase the

mechanical performance of the material by increasing the

fibre content and altering the reinforcing filament bundles.

Impregnation quality and imperfections within the

microstructure of the materials are studied and linked to the

mechanical performance.

The specimens are analysed for their mechanical

performance by means of a tensile test and a three-point

bending test. Subsequently dynamic mechanical analysis

(DMA) in flexural and tensile mode was performed. A double

cantilever beam test method is elaborated to characterize

interlaminar fracture toughness, which is not yet reported in

literature for 3D printed composites.

III. ANALYSIS OF EXISTING CONTINUOUS FIBRE FDM

PRODUCED POLYMER

In order to inspect the commercially available technology

for FDM of composite materials, Markforged products are

mechanically tested and visually inspected. Markforged

provides a total package technology of hardware (3D printer

and feed materials) and specialized software; It is noted that

this limits the overall flexibility of product design[6]. Three

kinds of specimens were tested: non-reinforced (‘nylon 6’),

reinforced with larger outer shell (‘FG/nylon 6’) and

reinforced with small outer shell (‘max FG/nylon 6’). The

stress-strain diagram in Figure 3 shows the results of the

tensile tests. A significant increase in strength and modulus

for both bending and tensile tests were recorded: a tensile

modulus and strength increase respectively from 0.35 to 9.4

GPa and from 24 to 228 MPa. A flexural modulus and

strength increase of 0.41 to 7.0 GPa and from 14 to 144 MPa.

The Markforged 3D printed specimens are built up with an

‘outer shell’ of pure nylon. The measured properties are thus

and underestimation for the ‘inner reinforced part’. This is

especially true under flexural loading where the main stresses

develop at the outsides of the specimens, thus in the

unreinforced part for the Markforged specimens. Microscopic

analysis (Figure 2) showed that voids are located inside the

specimen, due to a bad impregnation of filaments by matrix

material during printing. To conclude, the commercially

available 3D printed materials show a significantly lower

performance than conventional fiberglass epoxy composites

(tensile modulus and strength of circa 45 GPa and 1000 MPa

respectively for 60 vol%[16]). Nonetheless the technology

unlocks the opportunity to fast and efficiently produce highly

complex parts of significantly higher quality than

conventional FDM.

Figure 2: Microscopic analysis of the cross section of a Markforged

fibre glass reinforced nylon specimen. (LH): Layer height.

Figure 3: Stress-strain diagram for Markforged specimens with

different fibre contents (specimen label indicated for each

corresponding graph). The curve for non-reinforced nylon 6 goes on

until a strain of 1.4 (140 %).

IV. IN-HOUSE DEVELOPMENT OF LOW-COST FDM

TECHNOLOGY FOR CONTINUOUS REINFORCED POLYMERS

A. Hardware development

First of all, the introduction of reinforcements is performed

through an in-nozzle impregnation technique. The setup is

schematically shown in Figure 4. Multiple lines and multiple

layers are 3D printed alongside and on top of each other,

(Figure 5(c)). Microscopic analysis concludes that the

filament bundle is not homogeneously distributed (Figure

5(b)) and that the actual impregnation happens in two phases.

Inside the nozzle a first entanglement of matrix material

around reinforcing material is pursued. When the next layer is

printed, it reheats the previous one and pressurizes the

filament bundles better inside the matrix material. In Figure

5(d), microscopic analysis of a cross section show pulled out

filaments (indicated with yellow arrows) as the material was

cut after submerging it in liquid nitrogen. This indicates that a

load transfer from matrix to reinforcing filaments took place

causing the filaments to be primarily loaded.

Figure 4: 3D schematic of the setup of a continuous reinforced

composite 3D printer prototype, (a): matrix material filament feed,

(b): cold-end of Bowden setup: matrix filament extrusion control

gear, (c): heating element, (d): fan, (e): nozzle tip, (f): schematic air

flow, (g): reinforcing filament bundle feed, (h): reinforcing material

insertion location.

Figure 5: (a): Ideal homogeneous distribution of aramid filaments in

one 3D printed line. (b): Real situation of one 3D printed aramid

reinforced PETG line. (c): 3D schematic of cross section of a 3D

printed specimen. (d): Corresponding SEM image. In both images

the axes are represented as they are to be coded for the 3D printer.

B. Software development

The printer settings (such as nozzle temperature, printing

speed and matrix material extrusion amount) and printing path

along the X-, Y- and Z-direction are set in a G-coded file. It is

verified that the fibre content is a controllable parameter. At

first an aramid filament tow of 22.2 tex is introduced and by

varying the amount of extruded matrix material, the fibre

content is altered. A second manner to set the fibre content is

possible by changing the count of the reinforcing filament.

The ‘outcome fibre content’ was measured by weighing the

resulting specimens. Introducing a 22.2 tex aramid bundle a

fibre content of 23.3 wt% is reached. With a 158 tex aramid

filament bundle a maximum fibre content of 56 wt% is

reached. From the graphs it is concluded that the set and

outcome values highly agree (Figure 6).

Figure 6: Graphs of set versus outcome fibre content. (a): The fibre

content in this graph was set by altering the amount of matrix

extrusion during printing. (b): Reinforcement material is altered to

increase fibre content.

V. MECHANICAL PERFORMANCE ANALYSIS

A. Materials and methods

1) Materials

As mentioned before, the used materials for the in-house

developed technology are aramid filament bundles and glycol

modified poly(ethylene terephthalate) (PETG) as reinforcing

material and matrix material respectively. PETG is an

amorphous copolymer of poly(ethylene terephthalate) that has

beneficial characteristics for 3D printing as it shows little

crystallization after several heating cycles.

2) Methods

A tensile test and three-point bending test are performed

respectively guided by ASTM D3039 standard and ASTM

D790 standard on Instron 3369 universal tensile machine. The

load modes are represented in Figure 7(c) and (d) respectively

for the tensile and three-point bending test. Dynamic

mechanical analysis (DMA) tests are run on a TA Instruments

DMA Q800 device. The test is performed isothermal at room

temperature in flexural and tensile mode loading using

respectively single cantilever and tension clamps (Figure 7(a)

respectively (b)). The double cantilever beam (DCB) test in

crack opening mode (Mode I) is performed to examine the

interlaminar fracture toughness GI. ASTM D5528 standard for

unidirectional fibre-polymer matrix composites is mainly

followed. The test setup is schematically shown in Figure

7(e). Scanning Electron Microscopy (SEM) was performed

using Jeol Quanta 200 F FE-SEM.

Figure 7: (a): Single cantilever beam setup. (b): Tensile mode setup

on the DMA device. (c): Load mode during tensile test. (d): Three-

point bending test load mode. (e): Double cantilever beam with load

Mode I. (δ): Displacement between the attachment locations of the

hinges. (F): Load orientation pointed with arrow for each figure.

B. Tensile properties and flexural properties

It is observed that the tensile strength and modulus

increases with increasing fibre content (Figure 8). An increase

in tensile elastic modulus from 1.3 GPa for a non-reinforced

specimen to 10.3 GPa for a 23.3 wt% reinforced specimen.

For the same specimen the tensile strength increased from 27

MPa to 288 MPa. For the flexural test the same conclusion

can be made: an increase in fibre content increases the

performance. A flexural elastic modulus increased from

0.9 GPa for a non-reinforced specimen to 6.0 GPa for a

23 wt% reinforced specimen. For the same specimen there

was an increase in flexural strength from 41 MPa to 147 MPa.

It is observed that the flexural strength did not increase as

much as the tensile strength. This is explained in Figure 9.

The loading of the aramid filament bundle is substantially

different in both tests. During a tensile test, the load is directly

transferred to the filaments when in flexural load mode, the

filaments at the bottom part of the specimen are loaded in

tensile mode along their axis, the filaments in the top part will

be loaded in compression mode and might tend to buckle out.

The filaments will only pursue their full reinforcing potential

if the impregnation of the reinforcing filaments is good and

ideally a good bonding exists between both materials.

Comparing the obtained values to conventional aramid

reinforced epoxy composites, with a tensile modulus and

strength of circa 70 GPa and 1300 MPa

Figure 8: Stress-strain diagram for specimens with different fibre

contents (indicated for each corresponding graph).

Figure 9: Side view schematic of the load mode on a specimen

during a three-point bending test: the aramid filaments in the bottom

part are loaded in tensile mode, while the filaments in the upper part

are loaded in compression mode.

C. Storage modulus and damping capacity

Figure 10 presents the storage modulus of a DMA tensile

and flexural experiment on specimens with increasing fibre

content. For both experiments an increasing fibre content

increases the stiffness of the material, which is explained by

the increased presence of stiffer material. This increase is

more outspoken for tensile load mode. In flexural mode,

matrix-filament interface bonding (Figure 12(a)) and

impregnation quality affect the stiffness of the material as it is

important for load transfer. When those are of less quality, the

filaments can buckle out from the load axis and will hardly

contribute to the stiffness of the material.

Figure 10: Resulting graph from the DMA experiment: storage

moduli versus fibre content. (a): Experiment in tensile load mode.

(b): Experiment in flexural load mode.

Consequently, the damping capacity (tan δ) of the

composite material is analysed by a DMA experiment.

‘Damping’ is the act of dissipating energy. It is an intrinsic

property of a material. In a composite material, impregnation

quality (e.g. impurities and porosities cause a bad quality) and

matrix-reinforcement interfaces will additionally affect this

value. Figure 11 shows results for the tan δ measurements of

specimens with different reinforcing filament bundles. It is

concluded that an increasing tex-value significantly increases

the damping capacity of the material. A microstructural

mechanism is schematically presented in Figure 12(a) where

bonding quality is indicated and in Figure 12(b) where

increased imperfections (air inclusion between the filaments)

for increasing tex-value are indicated. It is concluded that the

introduction of a ‘thicker’ filament bundle causes more

imperfections thus exhibits more energy losses.

Figure 11: Flexural damping capacity values for specimens with

different reinforcing aramid filament bundles. The mass percentage

of each specimen is indicated vertically.

Figure 12: (a): Schematic of the cross sections of a reinforced PETG

3D printed line. The frame indicates good and bad interfacial

bonding between matrix and filament. (b): cross section of 3D

printed material with different reinforcing filament bundles. Possible

air inclusions caused by bad impregnation are indicated.

D. Interlaminar fracture toughness

For this experiment, specimens were 3D printed with a non-

adhering foil between two layers in order to create a

preliminary delaminated surface. As such, the specimens have

two ‘legs’. While pulling open the legs, the load and

displacement are recorded and represented in Figure 13. From

the load and displacement values at delamination initiation

(F* and δ* respectively), the interlaminar fracture toughness

is calculated. The obtained values during the experiment vary

from 500 to 1900 J/m² which is assigned to side effects of the

matrix material or slight variation of test equipment

(misaligned hinges). The values fall within a reasonable

range. In literature an interlaminar fracture toughness of circa

500 J/m² and circa 3000 L/m² respectively for conventional

epoxy composites and for pure PETG [17,18]. A delaminated

surface is analysed using electron microscopy (Figure 14).

The patterns of filaments from the bottom delamination

surface are visible as well as one remaining filament that had

enough bonding with the matrix material (encircled with

dotted yellow line).

It is emphasised that future work is required but the test

method is a promising characterization technique for

continuous reinforced 3D printed polymer materials.

Figure 13: Load-displacement graph with F* and δ* indicated with a

cross.

Figure 14: Electron microscopy image of upper part of delaminated

specimen. Encircled with dotted yellow line: one remaining filament.

Inset: schematic of a DCB mode I tested specimen with indicated

with ‘a’ the upper surface whereof image is taken.

VI. CONCLUSIONS

During this thesis project, up to 56 wt% unidirectional

aramid filaments reinforced PETG is successfully 3D printed

with a low-cost FDM technique. The developed 3D printer

has a high adaptability of hardware (3D printer components,

reinforcing material, and matrix material) and software

(printer settings, fibre content and orientation). Increased fibre

content from 0 wt% to 23.3 wt% shows an increased tensile

modulus and strength from 1.3 GPa to 10 GPa and from 27

MPa to 288 MPa respectively, an increased flexural modulus

and strength 0.9 GPa to 6 GPa and from 41 MPa to 147 MPa

respectively. Microstructural imperfections such as poor

impregnation quality and/or interfacial bonding cause poor

load transfer and energy losses within the composite material

and need to be improved. Indicative for further research are

the following suggestions: pre-treatment (sizing) of

reinforcing materials and post-processing procedure of the

specimens to enhance impregnation quality.

2

Situating the obtained materials within the existing

technology found in literature and the commercially available

Markforged technology, it is concluded that the obtained

materials show significant performance values for 3D printed

composites. Compared to conventional aramid reinforced

epoxy composites, the 3D printed specimens perform much

lower. Nonetheless, continuous reinforced 3D printing

provides the opportunity to produce complex design yet high

performance parts efficiently and at lower cost for limited

production quantities.

ACKNOWLEDGEMENTS

The author would like to acknowledge the promotors Prof.

dr. ir. De Clerck and Prof. Cardon (University of Ghent) and

acknowledge the supervision of dr. ir. Lode Daelemans and ir.

Sander Rijckaert and thank them for the educational and

pleasant cooperation.

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TABLE OF CONTENT x

TABLE OF CONTENT ACKNOWLEDGMENTS ........................................................................................................................................................................................................... i

COPYRIGHT NOTICE .............................................................................................................................................................................................................. ii

ABSTRACT ................................................................................................................................................................................................................................ iii

EXTENDED ABSTRACT......................................................................................................................................................................................................... iv

TABLE OF CONTENT .............................................................................................................................................................................................................. x

LIST OF FIGURES ................................................................................................................................................................................................................. xii

LIST OF TABLES ................................................................................................................................................................................................................... xvi

LIST OF ABBREVIATIONS AND ACRONYMS .......................................................................................................................................................... xvii

1. INTRODUCTION AND LITERATURE REVIEW ................................................................................................................................................. 1

1.1. INTRODUCTION ............................................................................................................................................................................................... 1

1.2. LITERATURE REVIEW ................................................................................................................................................................................... 5

1.2.1. PRINTER TECHNOLOGY FOR CONTINUOUS REINFORCED FDM (HARDWARE AND SOFTWARE)) ................... 5

1.2.2. RESULTING MATERIALS AND CHARACTERIZATION ................................................................................................................9

1.2.3. PRE-TREATMENTS AND POST-PROCESSING ........................................................................................................................... 14

1.2.4. CHALLENGES ........................................................................................................................................................................................... 16

2. RESEARCH OBJECTIVES ....................................................................................................................................................................................... 18

3. MATERIALS AND METHODS ............................................................................................................................................................................... 19

3.1. MATERIALS ..................................................................................................................................................................................................... 19

3.1.1. MATERIALS FOR IN-HOUSE DEVELOPED TECHNOLOGY.................................................................................................... 19

3.1.2. MATERIALS FROM COMMERCIALLY AVAILABLE TECHNOLOGY ..................................................................................... 22

3.2. METHODS ....................................................................................................................................................................................................... 23

3.2.1. SPECIMEN DESIGN AND DIMENSIONS ...................................................................................................................................... 23

3.2.2. MICROSCPIC ANALYSIS ................................................................................................................................................................ 24

3.2.3. THERMAL ANALYSIS...................................................................................................................................................................... 24

3.2.4. MECHANICAL PERFORMANCE ANALYSIS ........................................................................................................................... 25

4. PHASE 1: ANALYSIS OF MATERIALS PRINTED WITH COMMERCIALLY AVAILABLE TECHNOLOGY .................................. 29

4.1. SPECIMEN DESIGN .................................................................................................................................................................................... 29

4.2. MECHANICAL PERFORMANCE ANALYSIS ......................................................................................................................................... 31

4.3. MICROSCOPIC ANALYSIS ......................................................................................................................................................................... 35

4.4. CONCLUSION FOR PHASE 1 ................................................................................................................................................................... 36

5. PHASE 2: DEVELOPMENT OF A LOW-COST CONTINUOUS REINFORCED COMPOSITE 3D PRINTER ............................. 37

TABLE OF CONTENT xi

5.1. MODIFICATIONS OF EXISTING TECHNOLOGY (HARDWARE) ................................................................................................ 37

5.2. PRINTING PARAMETERS OPTIMIZATION (SOFTWARE) ........................................................................................................... 45

5.3. FIBRE CONTENT SETTINGS AND VALIDATION .............................................................................................................................. 47

5.4. MICROSCOPIC ANALYSIS OF RESULTING MATERIAL................................................................................................................. 49

6. PHASE 3: MECHANICAL PERFORMANCE ANALYSIS OF 3D PRINTED MATERIAL ................................................................... 52

6.1. ORGANIZATION OF THE EXPERIMENTS .......................................................................................................................................... 52

6.2. TENSILE MODULUS PREDICTION ........................................................................................................................................................ 53

6.3. TENSILE PROPERTIES .............................................................................................................................................................................. 54

6.4. FLEXURAL PROPERTIES .......................................................................................................................................................................... 55

6.5. STORAGE MODULUS AND DAMPING CAPACITY .......................................................................................................................... 57

6.5.1. INFLUENCE OF FIBRE CONTENT ................................................................................................................................................... 58

6.5.2. INFLUENCE OF REINFORCING MATERIAL ......................................................................................................................... 60

6.5.3. INFLUENCE OF SPECIMEN DIMENSIONS ............................................................................................................................. 61

6.5.4. INFLUENCE OF POST-PROCESSING....................................................................................................................................... 62

6.5.5. INTERPRETATION OF DMA COMPARED WITH TENSILE AND FLEXURAL RESULTS ........................................ 63

6.6. INTERLAYER TOUGHNESS ...................................................................................................................................................................... 63

6.6.1. EXPERIMENT DEVELOPMENT ........................................................................................................................................................ 63

6.6.2. RESULTS.............................................................................................................................................................................................. 65

6.6.3. DELAMINATED SURFACE ........................................................................................................................................................... 66

6.7. SITUATION OF DEVELOPED TECHNOLOGY ................................................................................................................................... 68

7. CONCLUSION............................................................................................................................................................................................................ 70

BIBLIOGRAPHY .................................................................................................................................................................................................................... 72

APPENDIX A: G-CODE ....................................................................................................................................................................................................... 76

APPENDIX B: TABLES ........................................................................................................................................................................................................ 78

LIST OF FIGURES xii

LIST OF FIGURES

Figure 1: Schematic representation of three commonly used additive manufacturing techniques[8,9]. (a): FDM setup, (b):

SLA setup, (c): SLS setup ................................................................................................................................................................................................................................. 3

Figure 2: Summarizing graph of mechanical performance of the materials produced by existing polymer additive

manufacturing techniques[16]. Included are the non-reinforced materials produced with commercially available

techniques (SLS, SLA and FDM), short carbon fibres and carbon nanotubes reinforced polymers materials created

with FDM technology (reinforced FDM) [17–19] and the results from the research of Matsuzaki et al.: carbon fibre

reinforced thermoplastic polymers (CFRTP).................................................................................................................................................................................... 4

Figure 3: This figure is taken from the report of Baumann et al. [25]. (a) Pre-nozzle insertion is possible either as

pre-impregnated filament or separately guided along each other. (b) In-nozzle impregnation. (c) Two-nozzles

printer head: reinforced filament 3D printing is separate from 3D printing of the matrix material. ..................................................... 6

Figure 4: (a): Carbon filaments deposited on the bottom part of the 3D printed specimen. (b): Carbon fibres put under

tension while top part is printed [26]. (c): Impregnation of the carbon fibres via a heated hypodermic needle[25]. ............... 6

Figure 5: (1): An in-nozzle impregnation where the reinforcing filament is inserted perpendicularly to the print bed

surface. [22,23,28]. (2): Carbon fibre is fed along with the PLA filament [29]. (3): The same principle as in (2) [30].

(4) Fibre encapsulated AM (FEAM) technology [27]. (5): In-nozzle system to 3D print continuous carbon fibre

reinforced thermoplastic polymer [14]. .............................................................................................................................................................................................. 8

Figure 6: Microscopic image from cross section of a carbon fibre reinforced PLA 3D printed composite [29]. ................................................. 10

Figure 7: 0° and 90° load direction schematically represented. The dark blue lines represent continuous reinforcing

filaments and the light blue area represents the matrix material. ............................................................................................................................... 11

Figure 8: Continuous aramid reinforced 3D printed PLA structures with complex infill patterns in order to create

lightweight materials with high mechanical performance[28]. ......................................................................................................................................12

Figure 9: Described by Yang et al. are the ‘three scale interfaces’. This figure shows with red arrows on schematics where

each interface is located in the printed material[22]. ............................................................................................................................................................ 13

Figure 10: (a): Scanning electron microscopy (SEM) image of a uniaxial tensile loaded carbon filament reinforced 3D

printed PLA specimen without pre-treatment. Voids between matrix and reinforcing material are visible.

Encircled with a yellow dotted line: ductile ‘bridges’ phenomenon appearing from the ductile failure of the PLA.

(b): SEM image of the same experiment with pre-treatment. Encircled with a blue dotted line: A pulled out

filament on which a large amount of load had to be put, confirming a good micro scale interfacial strength[29]. ..................15

Figure 11: Results from flexural loading test on originally printed and recycled carbon fibre reinforced PLA. The original

specimen contains 25 vol% of reinforcing filaments, the remaufactured specimen contains 8.9 vol%[32]. ................................... 16

Figure 12: (a): Originally 3D printed carbon fibre reinforced PLA. (b)-(c): Recycled and remanufactured carbon fibre

reinforced 3D printed PLA showing a better impregnation of the matrix material around the continuous carbon

fibres[32]................................................................................................................................................................................................................................................................ 16

Figure 13: (a): SEM image from a cross section of a carbon fibre reinforced PEEK printed specimen [16]. The air inclusions

between the continuous filaments are pointed with a red arrow. One well impregnated filament is pointed with

a white arrow. (b): Cross section of a fibreglass/nylon composite produced with Markforged technology[35].

17 vol% of porosity inside the material is estimated. ............................................................................................................................................................... 17

Figure 14: Microscopic image of the cross section of two materials printed by Markforged technology. (a) Cross section of

lower fibre content shows considerable amount of voids, (b) cross section of specimen with higher fibre content

does not show as much voids[41]. ......................................................................................................................................................................................................... 17

Figure 15: Schematic of the project methodology. ............................................................................................................................................................................................ 18

Figure 16: Chemical structure of p-phenylene terephthalamides (PPTA), more commonly known as para-aramid under the

brand names Kevlar® (DuPont) and Twaron®/Technora® (Tejin). [44] ....................................................................................................................... 19

Figure 17: First and second heating cycle in a DSC measurement of several shapes of PETG: (a): PETG pellets, (b): PETG feed

filament before 3D printing, (c): 3D printed PETG. Red dotted line: glass transition range. For all three shapes the

LIST OF FIGURES xiii

glass transition lies within the range of 70-80 °C. Green dotted line: the recommended printing temperature

range. .........................................................................................................................................................................................................................................................................21

Figure 18: Chemical structure of poly(ethylene terephthalate glycol) (PETG). ..............................................................................................................................21

Figure 19: FTIR scan of the nylon 6 matrix material used in Markforged Mark One Composite 3D printer. ............................................................. 22

Figure 20: (a): Schematic of the print path of the nozzle for one layer of a long beam specimen (b): schematic of a printed

long beam specimen; (L) the line spacing is the distance between two laid down lines; (l): the length of a printed

specimen. ...............................................................................................................................................................................................................................................................23

Figure 21: Schematic of the printed long beam specimen and the ‘cut-out’ represents the inside view; (L) the line spacing is

the distance between two laid down lines; (LH) layer height, programmed Z-distance over which the nozzle

moves up before printing the next layer; (l),(w),(t) the length, width and thickness of a printed specimen. ............................... 24

Figure 22: Plate-shaped specimen. Length, width and thickness are indicated with (l), (w) and (t) respectively. ............................................. 24

Figure 23: (a): tensile test setup on Instron 3369 with indicated gauge length (Lg) of 70 mm, (b) schematic of the load

direction. ................................................................................................................................................................................................................................................................. 25

Figure 24: Schematic stress-strain curve of a tensile test or three-point bending test. (E): Moduli of elasticity, (σB): stress at

breakage, (σmax,): maximum stress. Three possible responses are sketched: (1) linear response, (2) yielding occurs,

(3) ‘bilinear’ response [49] and [50]. ................................................................................................................................................................................................ 26

Figure 25: (a): Three-point bending test setup on Instron 3369 before test. (b): The same setup during test. (c): Schematic

of three-point bending test setup with indicated on the figure: the blue dots represent the cross section of the

cylinders with diameter of 5 mm, (F): direction of the load, (L): support span. ................................................................................................... 27

Figure 26: (1) and (2): schematic for the setup of the DCB Mode I test setup before testing. (F): Applied load that is

transferred vertically by hinges with respect to the non-delaminated part of the specimen in both figures, (a):

delamination length from the pre-crack tip to the load point where the hinges are applied to transfer the load,

(δ): displacement recorded by the device, thus at the height of where the hinges are glued. ................................................................ 28

Figure 27: Schematic load-displacement graph for calculations of the interlaminar fracture toughness GI [51]. ............................................... 28

Figure 28: Picture from Markforged Mark One composite FDM 3D printer, (b): schematic of the reinforcing filament

incorporation into nylon 6 matrix material and how the 3D printing process is performed[21]. (c): Picture of the

double-nozzle printhead[35]. ................................................................................................................................................................................................................. 29

Figure 29: Screenshot of the Markforged software interface: schematic of the infill pattern design for ‘FG/nylon’. ....................................... 30

Figure 30: Picture of the resulting Markforged 3D printed GF/nylon composite (‘FG/nylon’) pointed with (1) and non-

reinforced nylon (‘nylon’) specimen pointed with (2). .......................................................................................................................................................... 30

Figure 31: Schematic cross section of the first design of a Markforged printed specimen (‘FG/nylon 6’). The horizontal lines

in this schematic represent the layered structure. (A1): cross section area of reinforced part, (A2): cross section

area of non-reinforced ‘outer shell’, (A1) + (A2) represents the total cross area of the specimen. .......................................................... 31

Figure 32: Stress-strain diagram for Markforged specimens with different fibre contents (specimen label indicated for each

corresponding graph). The curve for non-reinforced nylon 6 goes on until a strain of 1.4 (140 %). .......................................................32

Figure 33: (a)-(b): Results from three-point bending test, (c)-(d): results from tensile test. Both tests are performed as was

described in paragraph 3.2. For both reinforced designs, it is calculated how much higher the performance value

is compared to the non-reinforced specimen. ............................................................................................................................................................................ 33

Figure 34: Schematic of the Markforged specimens loaded in flexural mode (load F orientation pointed with arrows). The

lines in this schematic represent the reinforcing continuous fibreglass. (a): FG/nylon 6, (b): max FG/nylon 6. ........................... 33

Figure 35: Graphs showing the storage modulus and tan δ values of the dynamic mechanical analysis in bending (a)-(b)

and tensile (c)-(d) mode. ........................................................................................................................................................................................................................... 34

Figure 36: Microscopic analysis of the cross section of a Markforged fibreglass reinforced nylon specimen. The reinforced

part is in the middle (with reinforced nylon lines of 0.1 mm layer height (LH)) surrounded by ‘walls’ of pure

nylon. Pointed with the red circle are voids between the non-reinforced wall and the reinforced middle section.

The orange circle shows a void between the reinforced lines. Encircled in green is approximately one laid down

line with impregnated filaments over the whole cross section. .....................................................................................................................................35

Figure 37: (1)-(2): SEM image from the cross section of a ‘FG/nylon 6’ specimen. (a): fibreglass, (b): nylon 6 matrix. .................................... 36

Figure 38: General setup of Malyan M200 FDM 3D printer. Indicated in the picture: (a) filament feed, (b) gear regulating the

extrusion rate, (d) fan. ................................................................................................................................................................................................................................. 38

LIST OF FIGURES xiv

Figure 39: The insertion location of the reinforcing filament on all images is pointed with (h), direction of the reinforcing

filament bundle with (g) and matrix material filament with (a). (1): The reinforcing continuous filament bundle

is inserted along the PETG filament at the remote extruder end of the Bowden setup. (2): Insertion happens

before the heating element. (3): Insertion happens between fins of the heating element. ...................................................................... 39

Figure 40: 3D schematic of the setup of a continuous reinforced composite 3D printer prototype, (a): matrix material

filament feed, (b): cold-end of Bowden setup: matrix filament extrusion control gear, (c): heating element, (d):

fan, (e): nozzle tip, (f): schematic air flow, (g): reinforcing filament bundle feed, (h): reinforcing material

insertion location. ........................................................................................................................................................................................................................................... 39

Figure 41: 3D schematic of one extruded strand existing of reinforcing filaments and matrix material. The red line indicates

how the extruded filament was cut. That surface is then investigated using SEM (Figure 42) ............................................................... 40

Figure 42: (a)-(c): SEM images of the cross section of one extruded strand. The red arrows in (a) and (c) indicate voids (air

inclusions) between reinforcing filaments and between reinforcing filaments and matrix material due to bad

micro-scale interface bonding (between reinforcing filaments and matrix material as described by Yang et

al.[22]). A better bonding is shown in (b) with a green arrow.......................................................................................................................................... 41

Figure 43: Final setup of the tuned continuous reinforced composite 3D printer. Indicated on the picture: (a) PETG filament

feed, (g) aramid filament bundle feed, (j) BuildTak printer bed. ................................................................................................................................... 42

Figure 44: (a): Cross section of a 3D printed specimen. The white box show the area whereon images (b) and (c) zoom in.

(b) and (c): Microscopic image of the specimen cross section. Pointed with red arrows are reinforcing material

filament bundles. ............................................................................................................................................................................................................................................. 42

Figure 45: (a): Microscopic image of top side of a 3D printed beam-shaped object that exists of multiple lines next to each

other and multiple layers on top of each other. (b): Bottom side of that specimen. The bottom side shows clearly

non-impregnated filaments that lay up the specimen. On the bottom side a full cover with nylon is visible. (c):

Cross section of a 3D printed specimen. The imbedded filament bundle of 22.2 tex has an elliptic cross section

(with marked dimensions). Pointed with white arrows are presumable pores between filament bundles and

between filaments and matrix material. ........................................................................................................................................................................................ 44

Figure 46: (a): 3D schematic of a laid down strand where the para aramid filament bundle is pulled to the upper surface of

the strand (main friction point is indicated with a green arrow), (b) Long distance microscope image of the

nozzle during 3D printing. ......................................................................................................................................................................................................................... 45

Figure 47: 3D schematic of ideally impregnated aramid filament bundle in one laid down strand (a) and a realistic filament

bundle in the PETG matrix (b). ............................................................................................................................................................................................................... 45

Figure 48: (a): Schematic of the print path of the nozzle for one layer of a long beam specimen. (b): Schematic of the

printed long beam specimen. (L): The line spacing is the distance between two laid down lines. (l): Length of a

printed specimen. ............................................................................................................................................................................................................................................ 46


the distance between two laid down lines; (LH) layer height, programmed Z-distance over which the nozzle

moves up before printing the next layer; (l),(w),(t) the length, width and thickness of a printed specimen. ................................47

Figure 50: (a) Set versus outcome fibre content for different extrusion. (b): Set versus outcome fibre content for different

aramid filament bundles............................................................................................................................................................................................................................ 49

Figure 51: (1): 3D schematic and (2): corresponding SEM image of cross section of an ‘S_5’ specimen. The cut was obtained

by using liquid nitrogen. In both mages the axis are represented as they are to be coded for the 3D printer. ........................... 50

Figure 52: (1): 3D schematic of a broken specimen and orientation of the SEM imaging of the specimen, (2): SEM image of

liquid nitrogen cooled, cut and folded specimen. (a), (b) and (c): Different printed layers, (d): ductile failure of

matrix material, (e) upper filament bundles. ................................................................................................................................................................................51

Figure 53: Stress-strain diagram for specimens with different fibre contents (indicated for each corresponding graph). ......................... 54

Figure 54: (a): Graph with tensile modulus results for in-house 3D printed specimens with fibre content as varying

parameter. (b): Graph with tensile strength values. The data-points in light blue in graph (a) represent the

estimated moduli values resulting from the stiffness prediction method............................................................................................................. 54

Figure 55: (a): Flexural strength performance of specimen with increasing fibre content, (b): flexural modulus of elasticity

of specimens with increasing fibre content. ................................................................................................................................................................................ 56

LIST OF FIGURES xv

Figure 56: Side view schematic of the load mode on a specimen during a three-point bending test: the aramid filaments in

the bottom part are loaded in tensile mode, while the filaments in the upper part are loaded in compression

mode. ........................................................................................................................................................................................................................................................................ 57

Figure 57: (a): Single cantilever beam setup and (b): Tensile mode setup on the DMA device. ........................................................................................ 58

Figure 58: (a): Results for storage modulus (b): Results for tan δ for increasing fibre content of DMA in tensile mode. ............................. 59

Figure 59: Schematic of micro-scale interfaces within a specimen. Good versus bad interfacial bonding are indicated in the

figure with green and respectively red arrows. ......................................................................................................................................................................... 59

Figure 60: (a) and (b): respectively results for storage modulus and tan δ for increasing fibre content of DMA in bending

mode (single cantilever beam setup). .............................................................................................................................................................................................. 60

Figure 61: (a): Flexural storage modulus of beam-shaped specimens with different reinforcing aramids. (b) Flexural

damping capacity values for those specimens. The mass percentage of each specimen is indicated vertically. ......................... 61

Figure 62: Schematic of the cross section of 3D printed material with different reinforcing filament bundles. Possible air

inclusions caused by bad impregnation are indicated. .......................................................................................................................................................... 61

Figure 63: (a): Graph with flexural storage modulus for three different aramid/PETG composite specimens of beam shaped

(light green) and plate shaped (dark green) dimensions. (b): tan δ-values for the same set of specimens.................................. 62

Figure 64: Graph with flexural storage modulus for three different aramid/PETG composite specimens. Post-processed

specimens are represented in orange. (b): tan δ-values for the same set of specimens. ............................................................................ 63

Figure 65: Schematic of DCB specimen, 3D printed with non-adhesive foil in between the second and third layer. (a)

Distance between crack initiation location and load transfer point (hinge adhesion location), (h) hinges, (n): non-

adhesive foil. ....................................................................................................................................................................................................................................................... 64

Figure 66: Front view of the 3D printer during printing of a DCB specimen. (b) Top view. .................................................................................................. 64

Figure 67: Setup of the double cantilever beam Mode I test loaded in the DMA Q800 device. (a): before the test, (b): two

‘legs’ are pulled apart in Mode I configuration during the experiment. ................................................................................................................... 65

Figure 68: Load-displacement graph obtained for specimen ‘DCB 1’ with F* and δ* indicated with a cross. The inset graph is

a zoomed in part of the same graph. ................................................................................................................................................................................................ 65


a zoomed in part of the same graph. ................................................................................................................................................................................................ 66

Figure 70: (1): Microscopic analysis if the DCB Mode I successfully tested specimen (DCB 1). (2): Detail from the bottom

surface where the crack initiation location was and where the delamination failure started (red dotted line) (a):

Initial surface before the experiment, (b): delaminated surface. .................................................................................................................................. 67

Figure 71: (1): SEM image of upper part of a delaminated specimen. The figure inset shows a schematic of a DCB Mode I

tested specimen with indicated (a): upper surface, (b): bottom surface. The patterns of previously laid in

filaments are visible as well as one remaining filament (encircled with yellow dotted line). (2): SEM image of

bottom part of delaminated specimen. The filaments are visible and small amount of matrix material is present

(encircled with yellow dotted line)..................................................................................................................................................................................................... 68

Figure 72: Fragment of G-code for 3D printing of an S_4 specimen ..................................................................................................................................................... 77

LIST OF TABLES xvi

LIST OF TABLES

Table 1: Listed AM techniques for polymers[1,3,7]. ..........................................................................................................................................................................................2

Table 2: Properties of reinforcing materials for in-house developed technology ................................................................................................................ 20

Table 3: Datasheet for materials used along the research ..................................................................................................................................................................... 22

Table 4: Markforged specimen design details ................................................................................................................................................................................................ 30

Table 5: Printer settings for aimed fibre content ......................................................................................................................................................................................... 49

Table 6: Stiffness prediction calculated for the long beam specimens..........................................................................................................................................53

Table 7: Results of tensile test. ..................................................................................................................................................................................................................................55

Table 8: Results for three-point bending test. ................................................................................................................................................................................................. 57

Table 9: Varying parameters overview for DMA test. ................................................................................................................................................................................. 58

Table 10: Results of successful DCB tests. ............................................................................................................................................................................................................ 66

Table 11: Selection of G-code commandos important for the project[58]. ................................................................................................................................... 76

Table 12: Reinforced FDM 3D printing technologies overview. .............................................................................................................................................................. 78

Table 13: Overview of existing research on continuous reinforced FDM 3D printing. ............................................................................................................ 79

Table 14: Specimen details for performed experiments. ........................................................................................................................................................................... 80

LIST OF ABBREVIATIONS AND ACRONYMS xvii

LIST OF ABBREVIATIONS AND ACRONYMS

Abbreviation or acronym

Full name

AM Additive Manufacturing

SLS Selective Laser Sintering

PCL Poly(caprolactone)

UV Ultraviolet

PA Polyamide

MIT Massachusetts Institute of Technology

FDM Fused Deposition Modelling

SLA Stereolithography

ABS Acrylonitrile butadiene styrene

PLA Ply(lactic acid)

CAD Computer Aided Design

STL Surface Tessellation Language

FEAM Fibre Encapsulated Additive Manufacturing

PEEK Poly(etheretherketone)

VAS Volume Average Stiffness

FEA Finite Element Analysis

ISO International Organization for Standardization

ASTM American Society for Testing Materials

SEM Scanning Electron Microscopy

PPTA p-Phenylene terephthalamides, commonly known as (para-)aramids

PET Poly(ethylene terephthalate)

PETG Glycol modified poly(ethylene terephthalate) or poly(ethylene terephthalate glycol)

FTIR Fourier Transform Infrared

DSC Differential Scanning Calorimetry

DCB Double Cantilever Beam

DMA Dynamic Mechanical Analysis

wt% Weight percent

vol% Volume percentage

INTRODUCTION AND LITERATURE REVIEW 1

1. INTRODUCTION AND LITERATURE REVIEW This thesis project report comprises an explorative topic which has not been studied extensively in academic

research. In order to receive a better view on the existing technology and the importance of further research,

this first part of the report consists of introducing chapters. A general introduction describes the additive

manufacturing (AM) process and its applications, followed by a literature study of continuous reinforced AM.

Finally, challenges for the technology are described and the research objectives are indicated.

1.1. INTRODUCTION

The interest for AM of polymers is encouraged by a constant ambition to be more energy efficient, by the

increasing complexity of small parts and by the speed and cost of production. The high degree of geometrical

freedom and the fast design-to-product cycle are inherent characteristics of the 3D printing technology [1,2].

For decades, AM is widely used for prototyping. Since recently it is becoming more and more important as a

stand-alone production technique for functional parts. Initially most AM techniques resulted in materials with

poor properties compared to other production technologies. The increasing interest and importance of AM

requires an increase in the performance level of the parts [3]. The aerospace and automotive industry start to

implement additive manufacturing into their production process scope. The commercial motor is driving

innovation and pushing the limits of the technology and the materials for AM [1,2,4].

AM is also referred to as 3D printing, rapid prototyping or solid free forming. A number of techniques to build

up a material layer-by-layer are listed in this paragraph and summarized in Table 1. Selective laser sintering

(SLS) is an AM technique where a powder reservoir is providing a layer of powder on a vertically traveling ‘print

bed’, the building platform of the AM device (Figure 1 (c)). The energy of a laser beam causes the material to

sinter together along a prescribed pattern. The remaining powder is removed. Depending on the laser power,

molecular diffusion is possible which will result in stronger bonding between layers. The particle size, scan

speed and scan spacing are influencing parameters for the resolution of the produced part. A drawback for the

technique is the limitation of materials that can be chosen. Molecular diffusion is best possible for polymers

such as poly(caprolactone) (PCL) and polyamides (PA) [3]. Stereolithography (SLA) produces a 3D object by

advancing a UV laser-beam over a layer of photocurable resin along a specific path. Layer-by-layer the object is

built, with each layer formed along a prescribed pattern[1]. In 1993 a powder-liquid 3D printing technology

was invented by the Massachusetts Institute of Technology (MIT). For this technique, an inkjet head drops a

physical binder liquid according to prescribed quantities and locations. A predetermined 2D-path is followed

layer-by-layer. The properties of the binder fluid are determining for the resolution and mechanical

performance of the part [5]. During 3D plotting a viscous polymer is extruded along a prescribed path

alongside the sacrificial support material. Subsequently that polymer is cured by heat or UV-radiation.

During a fused deposition modelling (FDM) production process, a thermoplastic filament is brought into a

liquid state and is extruded from the nozzle (Figure 1 (a)). FDM requires a relatively stable polymer with a

melting temperature below 300 degrees. Thermoplastics such as poly lactic acid (PLA), polyamide (PA),

polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) are often used for low cost 3D printing

technologies [1,6].


Table 1: Listed AM techniques for polymers[1,3,7].

Technique Feed material Feed material form

Principle Advantages/ disadvantages

Selective laser sintering (SLS)

PCL and PA Powder Energy supply by the laser, molecular diffusion results in bonding between adjacent particles

Less material flexibility(-) Rough surface finishing(-) good mechanical strength(+)

Powder-liquid 3D printing

Any polymer in liquid state

Powder and liquid binder

‘Drop on demand’ along prescribed path layer-by-layer

More material flexibility(+) Low-cost(+)

Fused deposition modelling (FDM)

Thermoplastics Filament Reheating in the nozzle, pressure build up and extrusion: lay down along a prescribed path, layer-by-layer

Low-cost(+) High printing speed(+) Visible layer structure(-) Rough surface finishing(-)

Stereolithography (SLA)

Photo-curable resin polymers such as epoxy or acrylate

Liquid Laser induced crosslinking along a prescribed path layer-by-layer

High resolution(+) Degradation due to photosensitivity(-)

During a general procedure of an additive manufacturing process a 3D model is generated via computer aided

design (CAD) software to subsequently produce a surface tessellation language (STL)-file wherein the object is

meshed. A slicer software then decides how the object is built up line per line. For an FDM process the 2D-route

of the nozzle in the X-Y plane is prescribed in a G-coded file for each layer in the Z direction. The nozzle

temperature and the print bed temperature are as well set by G-code. G-code can be written by either a slicer

program or by the user in any text software [1]. In order to gain an idea of a G-coded program, see Appendix A

of this report.


Figure 1: Schematic representation of three commonly used additive manufacturing techniques[8,9]. (a): FDM setup, (b): SLA

setup, (c): SLS setup

Technologies to 3D print reinforced polymer composite materials are widely developed and investigated. In

general the group of reinforced polymers is divided between continuous and discontinuous reinforced

polymers. Discontinuous reinforced materials contain small particles or short staple fibres with the aim to

enhance properties. Continuous reinforced polymers contain long filaments, mostly to enhance mechanical

performance[3]. Until today, discontinuous reinforced 3D printing is mostly investigated because of its low cost

and ease of adaptation of existing AM technology[10,11]. SLS, SLA ad FDM technologies allow to introduce

reinforcing particles. Adding short staple material can improve a material in different characteristics. The study

of Kalsoom et al. describes the inclusion of diamond particles in an acrylate resin in order to improve in the

heat transfer rate[3,12]. By adding iron and copper particles to an FDM printing process, the value for thermal

expansion is greatly reduced which results in a better print quality [3,13]. Predominantly in the field of

reinforced additive manufacturing, research focusses on adding reinforcements to improve mechanical

performance of the 3D printed material. In the summarizing graph in Figure 2 it is observed that continuous

reinforced FDM 3D printed polymers outperform the conventional additively manufactured polymers and the

discontinuous reinforced FDM polymers in the outlined mechanical properties [14]. It must be mentioned that a

conventional continuous carbon fibre reinforced epoxy composite (with fibre volume percentage of 60 vol%)

shows a longitudinal tensile modulus and strength of respectively 145 GPa and 1240 MPa, which is far higher

than materials shown in the graph[15]. In the literature review of this chapter, continuous reinforced FDM

technology is intensely investigated.


Figure 2: Summarizing graph of mechanical performance of the materials produced by existing polymer additive

manufacturing techniques[16]. Included are the non-reinforced materials produced with commercially available techniques

(SLS, SLA and FDM), short carbon fibres and carbon nanotubes reinforced polymers materials created with FDM technology

(reinforced FDM) [17–19] and the results from the research of Matsuzaki et al.: carbon fibre reinforced thermoplastic

polymers (CFRTP).

3D printing of objects is already spread in a wide variation of industries where the technology provides smart

and efficient solutions. Examples of which are the logistic management of spare parts and helping tools for

surgeons to optimize their performance skills [2]. The National Academy of Sciences describes the importance

of 3D printing technology in aerospace industry[4]. One commercially available 3D printer is presented by

Markforged (Markforged Inc., founded in 2013 by Gregory Mark, headquartered in Cambridge, United States),

claiming to print parts that are stronger and stiffer than ABS and as easy producible than conventional

non-reinforced polymer fused deposition modelling. For their newest model they claim to outperform 6061

aluminium in strength while being 40% lighter. At the same time speed of production is increased and cost is

reduced [20].


1.2. LITERATURE REVIEW

Continuous reinforced fused deposition modelling is a recent production method whereof relatively little

studies exists up until today. Matsuzaki et al. reported their technology in 2016 [14]. An increasing amount of

studies are launched and provide improvements in the various aspects of the production process. Those

aspects are mainly hardware (3D printer and materials) and software (printer settings) developments. The

technology is each time validated by testing of the produced material in various ways, of which an overview is

provided [21–23].

In this literature review the currently available academic studies are outlined. First, both in-house developed

technologies as well as commercially available printers are outlined. Secondly, materials can be printed with

various settings which include the chosen reinforcements and matrix materials, fibre content of the material,

printer settings such as nozzle temperature and printing speed. Finally the materials are characterized and

tested using microscopy and several mechanical test methods for reinforced plastics. However, it can be noted

from this literature review that no standards specifically for fused deposition modelled reinforced polymers

have been established yet. Finally the challenges for the technique and some potential solutions found in

literature are summarized.

1.2.1. PRINTER TECHNOLOGY FOR CONTINUOUS REINFORCED FDM (HARDWARE AND SOFTWARE))

1.2.1.1. HARDWARE

As AM of composites is a hot topic yet challenging to implement, every improvement for a processing

technique of such materials is of great interest to the industry [11]. In general two different fields of academic

research into continuous reinforced 3D printed materials exist. Either the tested specimens are manufactured

using an in-house developed or modified 3D printer or either the researchers use a commercially available 3D

printer. The line of Markforged ‘Mark’ printers (Markforged Inc., founded in 2013 by Gregory Mark,

headquartered in Cambridge, United States) are currently the only commercial FDM-based 3D printer on the

market that allows printing of a continuous fibre reinforced polymer. Currently a Markforged Mark Two can be

purchased online for 13 499 dollars. [24]

Most researches using a self-designed 3D printer report the difficulty of inserting the filament into the matrix

material. Until now in literature, three different ways of filament inclusion in the matrix material is found

(Figure 3)[25]. First of all there is pre-impregnation of the filament. This technology is used in Markforged 3D

printers. Tian et al. found that the pre-impregnation method is very effective and beneficial for the mechanical

performance. Secondly in-nozzle insertion and thirdly two-nozzles insertion is performed. Post-nozzle

insertion techniques are described by Baumann et al. and Yao et al. [25,26]. Both researches printed the

bottom part of a sample, then introduced the reinforcing filaments and subsequently printed the top part on

top of the filaments. In total four different techniques are proposed: direct overprinting the laid in filaments

without (by Baumann et al.) and with (by Yao et al.) pretension. Yao et al. first coated the carbon tow with an

epoxy resin adhesive and subsequently applied them at 2 N constant tension while the top layer of the

specimen is printed on top. Two additional techniques are proposed by Baumann et al.: injecting the filaments

with a heated needle that melts the matrix material or implementing them on the matrix in a small solvent

film. That solvent will solve a small bit of matrix material and include the filaments. Subsequently the solvent


will evaporate and the filaments are more surrounded by matrix material on which the top matrix layers are

printed.

Figure 3: This figure is taken from the report of Baumann et al. [25]. (a) Pre-nozzle insertion is possible either as

pre-impregnated filament or separately guided along each other. (b) In-nozzle impregnation. (c) Two-nozzles printer head:

reinforced filament 3D printing is separate from 3D printing of the matrix material.

Figure 4 (a): Carbon filaments deposited on the bottom part of the 3D printed specimen. (b): Carbon fibres put under

tension while top part is printed [26]. (c): Impregnation of the carbon fibres via a heated hypodermic needle[25].


Yang et al. developed an FDM 3D printing system where the carbon filaments are pre-heated in order to

enhance the in-nozzle impregnation (Figure 5(1))[22]. Matsuzaki et al. describe a pre-nozzle heating of the

reinforcing filaments with the aim to optimize impregnation of reinforcing filaments with matrix material

(Figure 5 (5)). Cox et al. developed a technique that is merely to be used for 3D printing electric circuits. It is

called ‘fibre encapsulated additive manufacturing’ (FEAM). Copper wire can as such be introduced into the 3D

printing process which opens up opportunities to produce complex circuits in a fast way (Figure 5 (4))[27]. Hou

et al. used a material extrusion head controlled in space by a robotic arm. Their in-house designed 3D printer

produced the continuous fibre reinforced samples by feeding both material filaments simultaneously to the

extrusion head which impregnates the melting matrix material into the reinforcing filament Their aim was to

create continuous reinforced composite lightweight structures and the reached 11.5 vol% of fibre content[28].

Stephashkin et al. successfully 3D printed poly(etheretherketone), a material generally known under the

acronym ‘PEEK’. This material is of great interest because of its beneficial chemical, thermal and mechanical

behaviour. The possibility to 3D print this material is thus a great next step towards flexible and high quality

production of parts for the aerospace, automobile and medical industry. The challenge for this material is its

high processing temperature. Stephashkin et al. used the FDM technology to produce continuous carbon fibre

reinforced PEEK [16]. Different printing setups and a comparison of different methods and materials used for

continuous reinforced polymer 3D printing and are shown in Figure 5 and Table 12 respectively. In Figure 5 for

each technology both a schematic design (a) and a picture (b) are shown.


Figure 5 (1): An in-nozzle impregnation where the reinforcing filament is inserted perpendicularly to the print bed surface.

[22,23,28]. (2): Carbon fibre is fed along with the PLA filament [29]. (3): The same principle as in (2) [30]. (4) Fibre

encapsulated AM (FEAM) technology [27]. (5): In-nozzle system to 3D print continuous carbon fibre reinforced thermoplastic

polymer [14].


A pre-nozzle incorporation system is developed by Markforged company. They developed a commercially

available technology that allows private individuals to produce continuous reinforced parts at home. They

present the technology as a total package to 3D print reinforced nylon. Fibreglass, carbon fibre and high

modulus para-aramid filaments are available as reinforcing material in the shape of pre-impregnated

filaments (only available by Markforged web shop and selected distributors). The printer has a double nozzle

printhead that extrudes non-reinforced matrix material (nylon 6) from one nozzle and pre-impregnated

reinforcing fibres from the other, which is schematically represented in Figure 28 (b)-(c).

1.2.1.2. SOFTWARE

The wide variety of parameters important in the FDM process creates a lot of optimizing opportunities. Process

parameters such as extrusion speed, printing speed, nozzle temperature, infill design, layer height and band

width influence the quality and thus the mechanical performance of the material. An important varying

parameter in reinforced 3D printing is the fibre content, which is often characterized in literature with units of

volume percentage of fibre in the matrix [1,31]

1.2.1.3. MATERIALS

Up until today a limited amount of researches succeeded to 3D print continuous reinforced materials. The

different materials used are depicted in Table 12. For matrix materials it is often opted to use materials that

are already widespread in conventional FDM technology such as acrylonitrile butadiene styrene (ABS)

([22,23,25,28,32]) and poly(lactic acid) (PLA) ([14,26,29,30]), because of its easy operation. Stephashkin et al.

succeeded to print reinforced poly(etheretherketone) (PEEK) which required a more advanced printer setup

(see earlier discussed in this paragraph) [33]. The continuous reinforcing material is either carbon fibre

([14,22,23,26,28,32,33]), fibreglass ([25]) and aramid filaments ([30]). Nylon 6 is used as a matrix material in

the commercially available Markforged technology, which is researched using the three reinforcing materials

previously mentioned [21,34–36].

Other conventional polymers for FDM technology are poly(ethylene terephthalate glycol) (PETG) which is an

almost totally amorphous co-polymer of poly(ethylene terephthalate) (PET). PETG is claimed by commercial

reports and private individuals to be stronger than ABS and PLA and show less shrinkage [37]. Nylon is a

stronger material and more flexible than PETG and thus more fit for construction elements produced to

withstand high forces[38]. On the contrary the processability of nylon 6 is less straightforward because of its

hygroscopic character and higher processing temperature[37].

1.2.2. RESULTING MATERIALS AND CHARACTERIZATION

1.2.2.1. FIBRE CONTENT

Several studies demonstrate the influence of fibre content on the mechanical properties of a 3D printed

composite. An overview is provided in Table 12 in Appendix B of this report. In general for academic research,

the FDM technology is set to print as much reinforcement as possible inside the composite with the aim to

increase the performance of printed parts. In order to vary fibre content mainly the following parameters are

altered: the count of the reinforcing material, the printing distances (layer height and line spacing) and the

extrusion rate of matrix material[23,25]. As a result, increasingly higher volume percentages are reported. In

most researches the fibre content was calculated by amount of material that is inserted into the composite


during the 3D printing process. Tian et al. report the success to 3D print PLA containing 27 volume percentage

( vol%) continuous carbon fibre [23]. The fibre content value was obtained by means of simple calculus: the

total volume of one laid down line is obtained by multiplying the nozzle distance (length of one line), the layer

height and the line spacing. The actual fibre volume inserted was determined by multiplying the fibre count by

the nozzle distance and dividing it by the density of the fibrous material (carbon). The ratio of fibre volume and

total volume yields the fibre content in volume percentage. Li et al. obtained a 34 vol% of carbon fibre into PLA

matrix [29]. In their research they estimated the fibre content by normalizing a pixel area from the microscopic

image shown in Figure 6 by real dimensions. A ‘unit volume’ is indicated with dotted lines and subsequently

the area of encircled regions within that unit area is determined. That area is divided by the total area in the

unit volume resulting in the estimated fibre content in volume percentage [29].

Figure 6: Microscopic image from cross section of a carbon fibre reinforced PLA 3D printed composite [29].

Fibre content is often used as a varying parameter during academic research in an attempt to compare

mechanical performances. The 34 vol% continuous carbon fibre reinforcement in PLA matrix of Li et al.

performed 80 MPa in tensile strength[29]. Melenka et al. tested structures made with a Markforged Mark One

3D printer containing 4 to 10 vol% of reinforcing aramid filaments. Their assessment shows that increasing

continuous reinforcing fibre volume percent, increases the mechanical performance: the Young’s modulus

shifts from almost 2 GPa for 4 vol% to 9 GPa for 10 vol%[34].

1.2.2.2. MODULUS PREDICTION

Besides mostly experimental tests Al Abadi et al. theoretically calculated the influence of fibre content on the

performance of 3D printed composites[36]. In the research two methods to determine a prediction for the

elastic modulus and poisons ratio are used. First, a volume average stiffness method (VAS method) is

established as an analytical method to forecast the modulus value of a material with fibre content as the

varying parameter. It includes a simple rule of mixture with volume percentages as measures how much the

value of each material counts in the ‘mixed’ value of the composite. Besides from that, a finite element

analysis (FEA) model is realized to see the 3D printed composite’s behaviour under various loads and to

validate the VAS method. Subsequently both the VAS and FEA methods for modulus and respectively failure

behaviour prediction are compared with experimental tests. It is found that the estimations are in good

compliance with the experimental test results from their own research. For a specimen with 40 vol%


reinforced aramid filaments in a nylon 6 matrix, a mismatch of only 3.7 % was found[36]. Al Abadi et al.

validated their predictive model with results from Melenka et al., who 3D printed materials of 4 and 10 vol% of

reinforcing aramid filaments in a nylon 6 matrix and obtained an elastic modulus of respectively 1.8 and 9.0

GPa. The lower fibre content showed a significantly higher mismatch between the prediction model and the

experimental values (57 % mismatch for 4 vol% versus 0.1 % mismatch for 10 vol%) [34]. Al Abadi et al. explain

that a high mismatch for lower fibre content is resulting from poor bonding between the reinforcing material

and matrix material. This physical occurrence is not calculated in the model [36].

1.2.2.3. SPECIMEN INFILL DESIGN AND TESTING LOAD DIRECTION

In most researches the reinforcing filament orientation is designed to be uniaxial and with a 100% infill of the

part. When designing parts for a uniaxial tensile test, the filament orientation is laid either in parallel (0°

loading) or perpendicular (90° loading) to the load axis (schematically shown in Figure 7). This was for

example performed by Justo et al. to then make a conclusion on both directions’ mechanical performance of

fibreglass reinforced nylon printed specimen[35]. The research concluded a tensile strength and modulus of

elasticity of respectively 575 MPa and 26 GPa in 0° loading direction and 10 MPa and 1 GPa in 90° loading

direction. A specific research on infill design where filaments are loaded in various directions at once is

performed by Hou et al.[28]. During their research they fabricated 3D printed continuous fibre reinforced

composite structures (14.5 tex Kevlar® aramid as reinforcing material and PLA as matrix material) with special

infill design (Figure 8). The aim was to produce lightweight structures with high mechanical performances.

Complex infill of parts were designed and produced with reinforced FDM technology and subsequently tested

for compression strength. A fibre content of 11.5 vol% was reached resulting in a compression strength of

17 MPa. The main objective was to relate the design and process parameters to the mechanical performance of

the complex shapes. They report reinforced FDM to be a feasible production method to produce complex

infilled lightweight-high performance structures in an efficient and cheap way [28].

Figure 7: 0° and 90° load direction schematically represented. The dark blue lines represent continuous reinforcing

filaments and the light blue area represents the matrix material.


Figure 8: Continuous aramid reinforced 3D printed PLA structures with complex infill patterns in order to create lightweight

materials with high mechanical performance[28].

1.2.2.4. INTERFACE BONDING

Three types of interfaces within a reinforced fused deposition modelled part are defined by Yang et al. The

‘three scale interfaces’ are meso-scale, micro-scale and nano-scale interfaces which are the interfaces

between lines on top of each other, between filaments and matrix material and between the polymeric

material of adjacent lines respectively[22].

First of all, the set layer height, line spacing and printing surrounding conditions have an effect of the air

inclusion in general in the part between all laid down lines. The printing temperature has an effect on the

nano-scale interface, where polymer chains from adjacent lines can be intermingled when given enough

energy. The nozzle temperature is to be set according to the matrix material used. In order to print with

continuous reinforced filaments impregnated in a thermoplastic polymer, the nozzle needs to be heated

enough so the viscosity is low enough to impregnate the reinforcing filaments but not too high so the polymer

does not degrade. The influence of the nozzle temperature is investigated by Tian et al. [39]. It is set as target

parameter to vary the melting viscosity of the PLA matrix material in search for an optimization of the

impregnation of carbon fibres in the matrix material. When the nozzle was heated to 180 °C, the viscosity of

PLA was reported to be too high to incorporate the reinforcing carbon fibre. Once the temperature was

increased above 240 °C, the PLA showed a very low viscosity which was reported to be too low for the

continuous reinforced FDM process. Eventually, a nozzle temperature of 210 °C was suited for the process.


Figure 9: Described by Yang et al. are the ‘three scale interfaces’. This figure shows with red arrows on schematics where

each interface is located in the printed material[22].

During the project of Abott et al. on non-reinforced ABS 3D printing, the influence of printing speed on the

bonding strength between laid down lines is investigated[40]. It was concluded that a changing printing speed

influences the mechanical performance of the specimen and thus implies an influence on the bonding between

layers. It was reported that for a decrease from 50 to 10 mm/min resulted in an increase of 10 to 20 MPa in

tensile strength. A better interlayer bonding (and thus better mechanical performance) is related to the time

and temperature optimization via printing speed adaptation. The duration of how long the polymer is heated

above its glass transition temperature during extrusion of the line causes a right amount of energy increase in

the adjacent material, which mobilizes the polymer chains. Bond formation between adjacent lines occurs

thanks to diffusion an entanglement of the molecules. Additionally, the influence of the layer height is

investigated, stating that a smaller layer height allows more heat to flow to material from the previous layer

and ‘post-process’ (anneal) the material. That reheat of material is proven in this research to be beneficial for

the bonding and so the mechanical properties [40].

1.2.2.5. TEST METHODS

The report of Dizon et al. describes standards by the American Society for Testing Materials (ASTM) and the

International Organization for Standardization (ISO) that are generally used in the academic world for

reinforced as well as non-reinforced additive manufactured materials [1]. The tensile test standards used for


determining those values are ASTM D3039. For the compression test and in plane shear stress ASTM D695

standard and ASTM D3518 standard are used respectively. Tian et al. performed flexural performance analysis

according to ISO 14125 standard while Li et al. conduct tensile test, flexural test and dynamic mechanical

analysis on their produced materials following no standard method but putting own testing parameters for

each test[23,29]. A Sharpy impact test was performed by Tian et al. on continuous carbon fibre reinforced FDM

produced parts. A composite’s impact strength is related to the stiffness and strength and is of utmost

importance to register the composite’s ability to absorb and dissipate energy[39]. Matrix debonding,

delamination and fibre pull out are described as the three working mechanisms during impact. However,

Sharpy impact test are not always representative for the mechanical loads to be expected in composite

applications, where impact typically happens in the out-of-plane direction, which is not considered by a Sharpy

test and could differ substantially [32].

Dizon et al. express the importance to have test standards with regard to the safety and reliability of the 3D

printed parts [1]. The generally used standards for the continuous reinforced FDM technology were originally

established for conventional composite materials. It is questioned whether the FDM printed composites should

fall under this category because they appear to be a different category of materials. The manifest difference

with conventional composites is the high porosity volume which is reported in several studies[21,33,35].

According to Justo et al. porosity is caused by the lack of applied pressure during production[35]. Justo et al

performed a test in different loading directions in order to characterize different loading modes on the

material, though found difficulty to produce similar specimens (required for a valid comparison) for different

materials. Since the carbon fibres were too brittle for the design of a specimen for 90° loading test, they had

to be printed in a different way, which could influence the test results. Disputed by Melenka et al. are the

disputable tensile test results when breaches were visually reported at the corners of the dogbone-shaped

specimens where continuous reinforcements commenced and make a ‘turn’ that induced stress

concentrations[34]. In order to compare and validate existing technologies, the need for a streamlined set of

standard tests for reinforced additive manufactured materials is emphasized.

1.2.3. PRE-TREATMENTS AND POST-PROCESSING

The study by Li et al. emphasises the importance of the interface interactions between the reinforcing material

and matrix material which is attempted to be enhanced by a pre-treatment of the carbon filaments[29]. The

following treatment procedure is performed: a PLA particles/methylene dichloride solution is initially

emulsified and subsequently slowly added to deionized water with added PLA sizing agents. Finally the carbon

fibres are placed in this mixture to become infiltrated. Results of the mechanical experiments of the modified

continuous carbon fibre reinforced 3D printed PLA show an increase in tensile strength (from 80 to 91 MPa)

and flexural strength (from 59 to 156 MPa) because of enhanced micro-scale interactions (between filaments

and matrix as is described by Yang et al. [22]). An improvement was reported in dynamic mechanical analysis

for the pre-processed carbon filaments. This was explained by an enhanced impregnation of matrix material in

between the filaments. The storage modulus increased from 0.72 to 3.25 GPa. Electron microscopy showed a

clear difference in interface morphology of treated and non-treated carbon fibres (Figure 10). For the pre-

treated carbon filaments a different failure surface is visible as it shows adhering matrix material to the

filaments and not much ‘bridging’, meaning a good load transfer took place (Figure 10(b)). The morphological

dissimilarity explained the difference in mechanical performance of both materials. As such, the demand for


investigation of micro scale interaction enhancements was a well predicted solution by Yang et al. to improve

continuous reinforced FDM material quality[29]. Yang et al. use a prelaminar sizing action on the carbon

filament before including the filament bundle in the FDM 3D printing process[22].

Figure 10: (a): Scanning electron microscopy (SEM) image of a uniaxial tensile loaded carbon filament reinforced 3D printed

PLA specimen without pre-treatment. Voids between matrix and reinforcing material are visible. Encircled with a yellow

dotted line: ductile ‘bridges’ phenomenon appearing from the ductile failure of the PLA. (b): SEM image of the same

experiment with pre-treatment. Encircled with a blue dotted line: A pulled out filament on which a large amount of load

had to be put, confirming a good micro scale interfacial strength[29].

Matsuzaki et al. heat up the reinforcing filament before guiding it in the nozzle. They describe that this has a

positive influence on the infiltration of the matrix material between the carbon filaments due to lowering of

the viscosity until the centre of the filament bundle once they are ‘mingled’ in the nozzle of the extruder

head[14]. The pre-heating spiral is visible on the schematic Figure 5 (5.a). Tian et al. optimized printer settings

for continuous carbon fibre reinforced FDM printed parts. They attempted to recycle the parts and reuse it in a

similar 3D printing process. This investigation was motivated by the need for recyclability of present day

composite products. Material recovery of the carbon filaments was 100 % while of PLA 75 % was recovered, the

loss is due to the remaining polymer in the nozzle. Aging of the PLA occurs due to it being subjected to high

temperature cycles. The shortening of the PLA chains reduce the viscosity which is even more beneficial for the

impregnation of the carbon filaments. The loss of molecular weight is partly compensated by adding virgin PLA

to the polymer melt. However the mechanical testing results show an interesting effect of the recycling of the

carbon filaments. Since the recycling process results in an impregnated carbon filament tow, the interactions

between matrix material and reinforcing material become much better and voids between the filaments are

reduced. An improvement in mechanical properties versus fibre content ratio is recorded: the original 3D

printed material had 25 vol% of carbon fibres in the PLA matrix and showed a flexural strength and modulus of

respectively 335 MPa and 30 GPa. The recycled and remanufactured carbon fibres in PLA matrix contain

8.9 vol% and show flexural performance in the same order 263 MPa and 13.3 GPa. The results of the mechanical

test are represented in Figure 11 and the microstructure of the specimen are shown in Figure 12. The

microscopic analysis shows good impregnation of the remanufactured carbon filaments, which correlates with

the mechanical test results. This investigation proves the importance of the interaction between continuous

reinforcing material and the matrix material. [32]


Figure 11: Results from flexural loading test on originally printed and recycled carbon fibre reinforced PLA. The original

specimen contains 25 vol% of reinforcing filaments, the remaufactured specimen contains 8.9 vol%[32].

Figure 12: (a): Originally 3D printed carbon fibre reinforced PLA. (b)-(c): Recycled and remanufactured carbon fibre

reinforced 3D printed PLA showing a better impregnation of the matrix material around the continuous carbon fibres[32].

1.2.4. CHALLENGES

The impregnation of the reinforcing continuous filament bundle is often reported as a key phase of the

reinforced 3D printing process. When impregnation is insufficient, poor mechanical properties are obtained due

to porosity. Porosity between layers was observed using electron microscopy analysis during the investigation

of Stephashkin et al. [16]. They report that the observed cracks and ‘fine scale intralayer defects’ are due to

thermal and mechanical changes for the laid down material. Furthermore there is a high degree of porosity

between the filaments due to bad impregnation of the filaments with matrix material. Porosity in 3D printed

PEEK material was investigated by comparison of density with casted composite material. Results show that

the density of the 3D printed reinforced PEEK is decreased compared to cast reinforced PEEK of the same fibre

content. Microscopy confirmed this finding: porosities inside the printed material are found (Figure 13).


Figure 13: (a): SEM image from a cross section of a carbon fibre reinforced PEEK printed specimen [16]. The air inclusions

between the continuous filaments are pointed with a red arrow. One well impregnated filament is pointed with a white

arrow. (b): Cross section of a fibreglass/nylon composite produced with Markforged technology[35]. 17 vol% of porosity

inside the material is estimated.

Justo et al. quote the presence of porosity in reinforced materials produced with FDM technology (Figure

13(b))[35]. They indicate the porosities through microscopic analysis and calculate thereof an estimated

porosity volume fraction in the material. This estimation resulted in 12 and 17 vol% for respectively carbon and

aramid filament reinforced nylon using Markforged technology to print the parts. Yang et al. talk about weak

micro-scale interfaces between the reinforcing filaments and matrix material in their 3D printed

composites[22]. That weak attraction causes poor tendency for the matrix material to flow around the inner

filaments of the bundle (poor micro-scale interactions, Figure 13(b)). As possible solutions to be examined,

Yang et al. describe filament surface modification, pre-treatment and post-treatment actions. Van Der Klift et

al. noticed voids inside the Markforged printed materials (Figure 14) [41].

Figure 14: Microscopic image of the cross section of two materials printed by Markforged technology. (a) Cross section of

lower fibre content shows considerable amount of voids, (b) cross section of specimen with higher fibre content does not

show as much voids[41].

a b

RESEARCH OBJECTIVES 18

2. RESEARCH OBJECTIVES The need for fast and efficient production techniques of high performance materials drives the research on

continuous reinforced additive manufacturing. Previous studies provide certain check points that can result in

high quality 3D printed materials. A continuous reinforced material with high fibre content is pointed as an

appropriate way to fulfil this need in the industry. It is in general aimed to increase the printable fibre content

in the thermoplastic material, improve the quality of impregnation and reduce the impurities inside the

material (porosity, voids, misalignment of filaments,…).

At first, the research is situated by analysis of 3D printed continuous reinforced polymer from established

commercially available technology. In ‘Phase 1’ the mechanical performance and microstructural

characteristics of the material are studied. Consequently in ‘Phase 2’ it is aimed to 3D print the materials of

interest ‘in-house’ in an efficient and low-cost way. The used materials are continuous aramid filaments as

reinforcements and PETG as matrix material. Aramids show a better abrasion resistance than fibreglass, which

is beneficial in the first stage of 3D printer development[42]. The aim is to produce a technology with easy

adaptation of material use and printer settings. Subsequently three main parameters are altered in the

production process. First, the fibre content is increased. Secondly the reinforcing material is altered: aramid

filament bundles of different linear densities are introduced into the process. Finally the production process is

extended with a post-processing annealing action with the aim to further enhance quality of the printed

materials. In ‘Phase 3’ of the project, the following set of experiments are used to characterize the 3D printed

specimens: a mechanical performance analysis is completed through a tensile test and three point bending

test, followed by DMA to investigate stiffness and damping capacity and a double cantilever beam test to

characterize the interlayer toughness. The material is subsequently visually analysed using microscopy.

Figure 15: Schematic of the project methodology.

MATERIALS AND METHODS 19

3. MATERIALS AND METHODS

3.1. MATERIALS

3.1.1. MATERIALS FOR IN-HOUSE DEVELOPED TECHNOLOGY

Preliminary research showed that fibreglass is not easily printable: a non-treated or non-pre-impregnated

filament bundle breaks easily. As such to start the developments of the 3D printer with accessible yet high

performing materials, a non-treated aramid filament bundle is chosen as reinforcing material. PETG is chosen

as matrix material due to its good processability and mechanical performance of more advanced level than

commonly used ABS or PLA in 3D printing.

Three sorts of fibre bundles were inserted into the polymer matrix. Initially a Twaron® filament bundle (tow) of

40.5 tex provided by Teijin was used (Aramid 2), next to that para-aramid filament bundles (tows) of 22.2 and

158 tex are used (Aramid 1 and3). Since only a limited data sheet is available for each aramid, characterization

tests are performed to fill in the basic properties. A simple weighing and measuring handling is performed to

measure the count of the filament bundles (tows). Subsequently a tensile test of the separate filaments is

performed on a Textechno FAVIMAT textile testing device (Textechno Herbert Stein GmbH & Co. KG Textile

Mess- und Prüftechnik, headquartered in Mönchengladbach, Germany)[43]. Results of the test provide the

count of the filaments and the tenacity and initial modulus. Using the density of the para-aramid, the results

are calculated to tensile strength and tensile modulus of elasticity (Young’s modulus). All information is

summarized in Table 2 and the chemical structure schematic of a poly(para-phenylene terephtalamide) is

provided in Figure 16.

Figure 16: Chemical structure of p-phenylene terephthalamides (PPTA), more commonly known as para-aramid under the

brand names Kevlar® (DuPont) and Twaron®/Technora® (Tejin). [44]


Table 2: Properties of reinforcing materials for in-house developed technology

Property Unit Property value Machine / source

Aramid 1 Aramid 2 Aramid 3

tow linear density dtex 222.00 405.00 1580.00 material specification label

filament linear

density/count

dtex 1.79 1.72 1.72 Textechno FAVIMAT

filament initial modulus cN/dtex 609.58 758.02 698.80 FAVIMAT

filament tensile modulus

of elasticity

GPa 87.78 109.22 100.69 calculated from initial

modulus and density

filament tenacity cN/dtex 18.78 18.51 17.60 FAVIMAT

filament tensile strength GPa 2.71 2.67 2.54 calculated from tenacity

and density

Material density g/cm³ 1.44 1.44 1.44 [38]

PETG is used as matrix material. It is fed to the 3D printer in the shape of a transparent filament with a

diameter of 1.75 mm to feed to the 3D printer. The brand name is ‘Jupiter Serie’, obtained from the company

123-3D.com. The chemical structure is represented in Figure 18. PETG is an amorphous copolymer of PET where

the ethylene glycol is partly replaced by cyclohexane dimethanol to react as a diol during the esterification

polymerization process. The non-linearity of the copolymer causes the almost total amorphous character. As

preliminary research, a differential scanning calorimetry (DSC) analysis was performed. Two heating cycles

within a range from -50 °C to 300 °C are scanned at a ramp of 20 °C/min. Three different forms of PETG with

the same co-monomer ratios (as used during the project) are investigated: (a): PETG pellets, (b): PETG feed

filament for a conventional FDM 3D printer as described above and (c) PETG feed filament that was extruded

by the nozzle of the FDM 3D printer (Figure 17). The main conclusions of the DSC analysis are that PETG is

almost totally amorphous and that it shows no degradation until 300 °C. The glass transition is for each

processed PETG sort located between 70 °C and 80 °C. It is concluded that PETG is beneficial for 3D printing as

it shows little to no crystallization after several heating cycles.


Figure 17: First and second heating cycle in a DSC measurement of several shapes of PETG: (a): PETG pellets, (b): PETG feed

filament before 3D printing, (c): 3D printed PETG. Red dotted line: glass transition range. For all three shapes the glass

transition lies within the range of 70-80 °C. Green dotted line: the recommended printing temperature range.

Advantages of PETG are that it has a strong interlayer adhesion. It is as easy to print as PLA but is as strong as

and even more sustainable than ABS. It does not expel an odour while ABS does [45,46]. The printing

temperature range is prescribed from 220 to 250 °C. The PETG filament is stored in a controlled environment

dry box because of the slightly hygroscopic character of the polymer. The aim is to avoid water adhesion onto

the polymer which could cause air bells during extrusion by the heated nozzle resulting in imperfections in the

printed material. The mechanical properties of bulk PETG material are provided in Table 3.

Figure 18: Chemical structure of poly(ethylene terephthalate glycol) (PETG).


3.1.2. MATERIALS FROM COMMERCIALLY AVAILABLE TECHNOLOGY

From a Fourier Transform Infrared (FTIR) scan (Figure 19) the identity of the nylon matrix used in the

Markforged technology is confirmed. Nylon 6 is used as matrix material. In the FTIR spectrogram the vibrations

at 1635 and 1537 cm-1 are characteristic for C=O and C-N stretching respectively (orange arrows in Figure 19).

Those are called the amide I and amide II bands. Subsequently a distinction between different polyamides is

possible by analysis of the peaks in the region between 1500 and 900 cm-1. In this spectrogram the peaks at

1461 and 1259 cm-1 indicate the identity of polyamide 6 (green arrows in Figure 19) [47]. Fibreglass is used as

reinforcing material as this was the available technology within the scope of the thesis project. Details of

generally used fibreglass for reinforced polymers (E-glass) and pre-impregnated fibreglass used in the

Markforged technology are provided in Table 3.

Figure 19: FTIR scan of the nylon 6 matrix material used in Markforged Mark One Composite 3D printer.

Table 3: Datasheet for materials used along the research

Property Unity Fibreglass (E-glass) [15]

Pre-impregnated fibreglass (Markforged) [48]

PETG [37] Nylon 6 (dry)[37]

Density g/cm³ 2.58 1.5 1.3 1.1

Tensile modulus GPa 3.5 21 2.2 2.7

Tensile strength MPa 72 590 53 74

Flexural modulus GPa 22 2.1 3.1

Flexural strength MPa 210 77 100


3.2. METHODS

3.2.1. SPECIMEN DESIGN AND DIMENSIONS

The specimens are characterized using mechanical performance tests and visual analysis. The techniques and

equipment that were used is described in this paragraph. For most techniques, existing standards for

composites were used as a guideline. Since continuous fibre reinforced fused deposition modelled materials

are still relatively new in the academic world, no specific standards or many example tests exist yet. In Table

14 in Appendix B, a list of the tested specimens is provided and which tests were performed on each specimen.

Two 3D designs are used: beam-shaped specimens and plate-shaped specimens. Figure 21 and Figure 22 show

a schematic of the design of a beam-shaped specimen and of a plate-shaped specimen respectively. Figure 20

shows the nozzle’s printing path (in the X-Y plane) to produce one layer of the beam-shaped specimen.

Figure 20: (a): Schematic of the print path of the nozzle for one layer of a long beam specimen (b): schematic of a printed

long beam specimen; (L) the line spacing is the distance between two laid down lines; (l): the length of a printed specimen.

L



the distance between two laid down lines; (LH) layer height, programmed Z-distance over which the nozzle moves up

before printing the next layer; (l),(w),(t) the length, width and thickness of a printed specimen.

Figure 22: Plate-shaped specimen. Length, width and thickness are indicated with (l), (w) and (t) respectively.

3.2.2. MICROSCPIC ANALYSIS

3.2.2.1. OPTICAL MICROSCOPY

A cross section of the samples is prepared for examination with an optical microscope (Olympus BX51). The

specimens were cold imbedded in an epoxy matrix (RIM 135 and curing agent in a 100/30 weight ratio, density

of the epoxy is 1.15 g/cm³) to facilitate the grinding and polishing action. The grinding cycle exists of the

following steps in grit of the grinding plate: 240 - 320 – 400 – 800 – 1000 – 1200 – 2000. Each time a new

grinding plate is used, the sample is rotated 90°.

3.2.2.2. SCANNING ELECTRON MICROSCOPE

Scanning Electron Microscopy (SEM) was performed using Jeol Quanta 200 F FE-SEM. The samples were first

gold-coated with the sputter coater Balzers Union SKD 030 in order to avoid charging during the analysis as

the specimens are not conductive.

3.2.3. THERMAL ANALYSIS

3.2.3.1. DIFFERENTIAL SCANNING CALORIMETRY

Differential Scanning Calorimetry (DSC) is performed using Q2000 Tzero DSC from TA instruments is used.

Three PETG specimens with a different processing background are analysed in a differential scanning

calorimeter (DSC). Small parts of a PETG pellet, a PETG filament and extruded PETG from the 3D printer. In a

L


small pan, a piece of material of circa 10 mg is mounted and sealed. A temperature program of 3 cycles is

performed on each piece. The program exists of a first heating cycle (H1), a cooling cycle (C1) and a second

heating cycle (H2). The temperature range is from -80 °C to 300 °C at a rate of 20 °C/min. The test is

performed in a DSC Q2000 device of TA Instruments.

3.2.4. MECHANICAL PERFORMANCE ANALYSIS

3.2.4.1. TENSILE TEST

In-plane tensile properties are investigated guided by the ASTM D3039 standard [49]. The specimens are

tested in the fibre direction (0°) using an Instron 3369 universal tensile machine with a 2 kN load cell and

wedge clamps. The samples are provided of cardboard strips at the end in order to avoid slippage at the

clamps and better transfer of the load. A gauge length of 70 mm was set up. The test is performed with a

displacement control of 2 mm/min. Load and displacement are recorded and from these results the ultimate

tensile strength (‘σmax ‘) is determined as the maximum force encountered in the test divided by the cross

sectional area of the specimen. The Young’s modulus (tensile modulus of elasticity ‘E’) is determined as the

slope of the stress-strain curve. The calculations required to obtain the results are given below (Equation 1 and

2). The instantaneous strain is defined as the infinitesimal displacement between one data point and the

previous one divided by the initial gauge length (70 mm). The instantaneous stress (σi) is defined as the load

(Fi) at that specific instant (data point i) divided by the surface perpendicular to the direction of the implied

load (the cross section of the specimen (A)). From a stress-strain curve the tensile modulus of elasticity is

obtained by calculating the tangent of the first linear part of the curve. In general a region from 0 to 0.2%

strain is chosen. The ultimate tensile strength is equal to the maximum stress value of the curve (Figure 24).

[49]

Figure 23: (a): tensile test setup on Instron 3369 with indicated gauge length (Lg) of 70 mm, (b) schematic of the load

direction.

b


𝜎𝑖 =𝐹𝑖

𝐴

(Equation 1)

𝜎𝑚𝑎𝑥 =𝐹𝑚𝑎𝑥

𝐴

(Equation 2)

휀𝑖 =(𝑥𝑖 − 𝑥𝑖−1)

𝐿𝑔

(Equation 3)

Figure 24: Schematic stress-strain curve of a tensile test or three-point bending test. (E): Moduli of elasticity, (σB): stress at

breakage, (σmax,): maximum stress. Three possible responses are sketched: (1) linear response, (2) yielding occurs, (3)

‘bilinear’ response [49] and [50].

3.2.4.2. THREE-POINT BENDING TEST

The flexural strength and stiffness is determined following the ASTM D790 standard. This is a common

standard for determining flexural stiffness and strength properties of polymer matrix composite materials.

The mechanical equipment is Instron 3369 universal tensile machine, used in compression mode. A specimen is

laid on two cylindrical shaped supports which both have a diameter of 5 mm with the specimen’s bottom layer

(the first layer of the additive manufacturing process) facing down. The distance between the two cylinders’’

radial axis is called the ‘span’ (L) (Figure 25(c)). The specimen is then loaded by one cylindrical shaped ‘loading

nose’ with the same radius as the bottom two. A span-to-thickness ratio of 16:1 is set and is adjusted for each

individual sample. The imposed strain rate is 1 mm/min. The generic stress-strain curve given in Figure 24 is

also valid for the bending test. Though for the performed test it is more practical to obtain the results from a

load-displacement curve. The calculations to obtain flexural strength σf and flexural modulus of elasticity Ef

are given in Equation 4 and 5 respectively. The slope of the load-displacement curve is represented by ‘m’, the

width and thickness respectively by ‘w’ and ‘t’.


𝜎𝑓,𝑚𝑎𝑥 =3𝐹𝑚𝑎𝑥𝐿

2𝑤𝑡²

(Equation 4)

𝐸𝑓 = 𝐿3𝑚

4𝑤𝑡3

(Equation 5)

Figure 25: (a): Three-point bending test setup on Instron 3369 before test. (b): The same setup during test. (c): Schematic of

three-point bending test setup with indicated on the figure: the blue dots represent the cross section of the cylinders with

diameter of 5 mm, (F): direction of the load, (L): support span.

3.2.4.3. DYNAMIC MECHANICAL ANALYSIS (DMA)

Dynamic mechanical analysis tests are run on a TA Instruments DMA Q800 device. The test is performed in

flexural and tensile mode loading using respectively single cantilever and tension clamps. The frequency for

both tensile and cantilever tests is set to 1 Hz, using an amplitude of 2 and 20 µm respectively. The tests are

performed isothermally at room temperature.

During a dynamic mechanical analysis the material is mounted between two clamps. A controlled sinusoidal

displacement is applied and the ‘response’ of the material is accurately recorded as the force signal. A ‘storage

modulus’ expressing the elastic behaviour of the material and a ‘loss modulus’ expressing the energy

dissipating behaviour are obtained from the measurements. The ratio of the loss to the storage modulus is

called the ‘tan δ’, this is the lag between imposed displacement and response force signal. It is a measure for

the damping capacity of the material or ‘how well’ the material dissipates energy under cyclic load[38].

3.2.4.4. DOUBLE CANTILEVER BEAM (DCB) TEST

The double cantilever beam (DCB) test in crack opening mode (Mode I) is performed to examine the

interlaminar fracture toughness GI. ASTM standard D5528 for unidirectional fibre-polymer matrix composites is

mainly followed using Universal Analysis/TA instruments DMA Q800 device with tensile clamps. The following

procedure is followed: a “DMA strain rate”-mode is set where a ramp displacement from 2000 µm/min is

instructed. A preload of 0.05 N is applied. The test is carried out as follows: on the two beams a force is applied

and the stress is concentrated at the tip where the delamination failing takes place. There is a critical amount

of energy required to overcome the interlayer strength. Before that, an elastic section in the curve is observed.

This value indicates the energy required to initiate the delamination propagation.

a b c


𝐺𝐼 = 3𝐹∗𝛿∗

2𝑤𝑎

(Equation 6)

The interlaminar fracture toughness GI is calculated using the above formula ((Equation 6). ‘F*’ and ‘δ*’

respectively represent the load and the displacement data point at the intersection of the load-displacement

curve and the line with a slope of the curve’s elastic part tangent value minus 5% (Figure 27). The distance

from the delamination tip to the hinges attachment location is represented by ‘a’ and the width of the sample

by ‘w’. The method is schematically depicted in Figure 26 (1) and (2).

Figure 26: (1) and (2): schematic for the setup of the DCB Mode I test setup before testing. (F): Applied load that is

transferred vertically by hinges with respect to the non-delaminated part of the specimen in both figures, (a): delamination

length from the pre-crack tip to the load point where the hinges are applied to transfer the load, (δ): displacement recorded

by the device, thus at the height of where the hinges are glued.

Figure 27: Schematic load-displacement graph for calculations of the interlaminar fracture toughness GI [51].

1 2

PHASE 1: ANALYSIS OF MATERIALS PRINTED WITH COMMERCIALLY AVAILABLE TECHNOLOGY 29

4. PHASE 1: ANALYSIS OF MATERIALS PRINTED WITH COMMERCIALLY AVAILABLE TECHNOLOGY With the prospect to situate in-house developed technology between state-of-the-art technologies, this

project commences with the investigation of an established commercially available composite 3D printing

technology. A Markforged Mark One composite 3D printer (Figure 28) is used to print specimens for mechanical

and visual analysis. The 3D printed composites in question are continuous fibreglass as reinforced nylon 6.

Figure 28: Picture from Markforged Mark One composite FDM 3D printer, (b): schematic of the reinforcing filament

incorporation into nylon 6 matrix material and how the 3D printing process is performed[21]. (c): Picture of the double-

nozzle printhead[35].

4.1. SPECIMEN DESIGN

The beam-shaped specimens are 3D designed with the following dimensions: a beam of 120 mm length, 7 mm

width and 2 mm thickness. The design includes reinforcing filaments aligned in a unidirectional way.

Subsequently the software slices the object and a printing path for both reinforced and non-reinforced

material is set. The software automatically generates ‘walls’ which means an outer shell of non-reinforced

nylon 6 is set. This shell will influence the mechanical performance. Two reinforced parts are printed: the first

one having an outer shell of four layers on the top and bottom part of the specimen, the second one only one

layer. The encrypted software does not allow to leave out the ‘walls’ because Markforged wants to ensure

dimensional accuracy, water resistance and successful printing of reinforced layers. This automated setting is

beneficial for an individual but not for specific academic research on the reinforced material. The second

design is thus considered as the maximal amount of reinforcing material inside the specimen. Calculated fibre

contents are provided in Table 4. This is an estimated value and is to be considered as an overestimation

because air voids (of which evidence visible in Figure 36) That topic is further elaborated in the course of this

a b

a

c

a


paragraph. The resulting dimensions and specimen names are assigned. All specimens are 100% infill and are

printed so the filaments have a unidirectional alignment parallel with the longest side of the beam. A

screenshot of the ‘Eiger’ software (Figure 29) shows the design of the specimen.

Table 4: Markforged specimen design details

Specimen name Description Length x width x thickness (mm x mm x mm)

vol% FG

wt% FG

nylon 6 non-reinforced nylon 6 120 x 6.8 x 2.0 0 0

FG/nylon 6 fibreglass reinforced nylon 6

120 x 6.8 x 2.1 12.5 25.1

max FG/nylon 6 maximal fibreglass reinforced nylon 6

120 x 6.7 x 2.0 18.8 35.0

Figure 29: Screenshot of the Markforged software interface: schematic of the infill pattern design for ‘FG/nylon’.

Figure 30: Picture of the resulting Markforged 3D printed GF/nylon composite (‘FG/nylon’) pointed with (1) and non-

reinforced nylon (‘nylon’) specimen pointed with (2).

The fibre content of the Markforged printed fibreglass/nylon 6 composites is estimated. This is not as

straightforward since the composition of the pre-impregnated filament is unknown and not released explicitly

by Markforged company. An estimation of the fibre volume content of a printed part in volume percentage can

be made using available general data of fibreglass and specific data of Markforged materials. First of all in the

datasheet of the Markforged materials it is provided that the density of the fibreglass reinforced nylon 6 part

1

2

l

w


for is 1.5 g/cm³. This is valid for printed materials with a 100 % infill and unidirectional orientation of the

filaments [48]. The density of the nylon 6 matrix filament is 1.1 g/cm³[48]. The general fibreglass (E-glass)

density is used: 2.58 g/cm³ [15]. From a general rule of mixture calculation the fibre content of the reinforced

area in the part is estimated (Equation 8).

𝜌𝑟𝑒𝑖𝑛𝑓𝑜𝑟𝑐𝑒𝑑 𝑝𝑎𝑟𝑡 = 𝜌𝐹𝐺 ∗𝑣𝑜𝑙%𝐹𝐺

100+ 𝜌𝑛𝑦𝑙𝑜𝑛 6 ∗ (1 −

𝑣𝑜𝑙%𝐹𝐺

100)

(Equation 7)

𝑚𝑎𝑠𝑠%𝐹𝐺 = 𝜌𝐹𝐺 ∗ 𝑣𝑜𝑙%𝐹𝐺

𝜌𝐹𝐺 ∗ 𝑣𝑜𝑙%𝐹𝐺 + 𝜌𝑛𝑦𝑙𝑜𝑛 6 ∗ (1 − 𝑣𝑜𝑙%𝐹𝐺)

(Equation 8)

The fibre content for the reinforced part of the Markforged printed material is estimated at 27 vol%. For the

two different reinforced test specimens (‘FG/nylon 6’ and ‘max FG/nylon 6’) the Markforged software

automatically introduces ‘walls’ to the print design. This means four top and bottom layers are printed for

‘FG/nylon 6) (visible in Figure 36) and one top and bottom layer is printed in the second design (‘max FG/nylon

6’) print designs. In Figure 31 specimen ‘FG/nylon 6’ is schematically represented. The real fibre volume is

calculated for cross section area A1. From the dimensions of the interior design schematic the ratio of the real

fibre volume over the total volume (the cross section area (A1)+(A2) for 1 mm in the plane) is taken. A fibre

content of 12.5 vol% is obtained for ‘FG/nylon 6’ and 18.8 vol% for ‘max FG/nylon 6’. From those results and the

density values of fibreglass and nylon 6, the fibre content in mass percentage is 25.1 and 35.0 respectively

(Equation 8). It should be mentioned that the obtained values are valid for an ideal composite material

without voids or imperfections. For the further course of this research, it is important to notice that the density

of fibreglass is almost twice as high as for aramid filaments which means for the same volume percentage a

much higher mass percentage will be obtained for a fibreglass reinforced thermoplastic polymer.

Figure 31: Schematic cross section of the first design of a Markforged printed specimen (‘FG/nylon 6’). The horizontal lines in

this schematic represent the layered structure. (A1): cross section area of reinforced part, (A2): cross section area of non-

reinforced ‘outer shell’, (A1) + (A2) represents the total cross area of the specimen.

4.2. MECHANICAL PERFORMANCE ANALYSIS

In Figure 33 the results for the tensile test and three-point bending test are represented and compared to the

non-reinforced specimens. For all performance values there is an obvious increase for the reinforced material.

Additionally it is observed that a higher fibre content results in a higher performance value for both

A1 A2


experiments. This is shown in a stress-strain diagram in Figure 32; the maximal reinforced specimen shows a

more stiff behaviour. An emphasis is put on the difference in interior design of the two reinforced specimens.

The amount of top and bottom non-reinforced layers were reduced from 0.4 mm (four layers) on top and

bottom side to approximately 0.1 mm (one layer) on each side in ‘FG/nylon’ specimen and respectively ‘max

FG/nylon’ specimen. For the flexural load mode, the filaments on the bottom part are more loaded. As there are

more reinforcements on the outside of the ‘max FG/nylon’ specimen, this results in a higher performance. The

mechanism is schematically indicated in Figure 34. Additionally the influence of interfacial bonding between

reinforcements and matrix material becomes more important in flexural load mode. When a good bonding

exists, the reinforcing filaments will take up the load and thus increase the strength and modulus

performance. It is stressed that the outer ‘shell’ of non-reinforced nylon 6 clearly influences the obtained

values. As such these values are not showing the actual performance of the reinforced inner part of the

specimen. The value for a reinforced 3D printed part without side effects from a ‘shell’ will probably perform

better in both tensile and flexural mode.

Figure 32: Stress-strain diagram for Markforged specimens with different fibre contents (specimen label indicated for each

corresponding graph). The curve for non-reinforced nylon 6 goes on until a strain of 1.4 (140 %).


Figure 33: (a)-(b): Results from three-point bending test, (c)-(d): results from tensile test. Both tests are performed as was

described in paragraph 3.2. For both reinforced designs, it is calculated how much higher the performance value is

compared to the non-reinforced specimen.

Figure 34: Schematic of the Markforged specimens loaded in flexural mode (load F orientation pointed with arrows). The

lines in this schematic represent the reinforcing continuous fibreglass. (a): FG/nylon 6, (b): max FG/nylon 6.

Figure 35 shows the test results of the dynamic mechanical analysis. First of all the increase in storage

modulus for both bending and tensile mode means that the stiffness of the material is increased as more

reinforcing filaments are added. A value increase from 0.45 to 3.2 GPa in flexural storage modulus and 0.63 to

12.04 GPa in tensile storage modulus.


Regarding the results for the damping capacity (tan δ), fibreglass is a very stiff material (tensile modulus of 72

GPa [15]) so as expected, the influence of the fibreglass’ lower damping capacity than nylon 6 reduces the

overall damping capacity of the composite material. When comparing the first reinforced specimen (‘FG/nylon

6’) with the second specimen where a maximum amount of reinforcing filaments is included (‘max FG/nylon

6’), it becomes clear that the damping capacity is increased for a higher fibre content. This is plausible when

looking at the microstructure of the material: more reinforcing filaments included means more micro-scale

interfaces present. Those interfaces have a high tendency to form voids (porosities caused by air inclusion in

between the filament bundles) when impregnation during printing has not happened in an optimal way.

Additionally the interfacial interactions between the materials influence the damping capacity. As explained

before, a good bonding will transfer the load to the reinforcements. When a bad bonding is present, the

different materials can slide over each other and dissipate energy, which increases the damping capacity.

Therefor it can be concluded from the increasing damping capacity values that a higher fibre content results in

a higher amount of voids and (bad) matrix/filament interfaces. The second explanation is at the meso-scale

interface: the layers on top of each other do not have an ideal bonding. Voids and pores between the layers

also dissipate energy and subsequently increase the damping capacity. Since in the maximum reinforced

specimen the amount of reinforced layers is higher, the amount of dissipated energy in between the layers is

increased and thus the damping capacity is increased.

Figure 35: Graphs showing the storage modulus and tan δ values of the dynamic mechanical analysis in bending (a)-(b) and

tensile (c)-(d) mode.


4.3. MICROSCOPIC ANALYSIS

Microscopic investigation of the printed specimen is performed (Figure 36 and Figure 37). A first important

point to address is the voids visible in Figure 36. The interfaces between which voids occurred would be named

by Yang et al. as ‘meso-scale interfaces’ (the interfaces between adjacent layers)[22]. The air inclusion is

unfavourable for a good load transfer to the reinforcing filaments and will cause a non-ideal behaviour of the

specimen. From the SEM image (Figure 37) it is concluded that Markforged 3D printed fibreglass is considerably

well impregnated. A lot of filaments in one image are captured and still matrix material is observed in

between. This good result is assigned to the pre-impregnation of the filament bundle. When looking at the

specimen design, the fibre content is quite low. As such this leads to the suspect that the filaments are not

well dispersed over the whole cross section of the specimen. On the contrary the visual inspection of the

overall filament distribution in Figure 36 reveals a satisfying microstructure. The layered structure of the

specimen is visible but filaments are distributed over the whole cross section of one line.

Figure 36: Microscopic analysis of the cross section of a Markforged fibreglass reinforced nylon specimen. The reinforced

part is in the middle (with reinforced nylon lines of 0.1 mm layer height (LH)) surrounded by ‘walls’ of pure nylon. Pointed

with the red circle are voids between the non-reinforced wall and the reinforced middle section. The orange circle shows a

void between the reinforced lines. Encircled in green is approximately one laid down line with impregnated filaments over

the whole cross section.


Figure 37: (1)-(2): SEM image from the cross section of a ‘FG/nylon 6’ specimen. (a): fibreglass, (b): nylon 6 matrix.

4.4. CONCLUSION FOR PHASE 1

Markforged technology is considered as a good commercial technology though some disadvantages are to be

mentioned. A lack of design flexibility in the software and the limited choice of materials limit the user in

producing a reinforced part. The setting of fibre content is not straightforward and is limited due to pre-

impregnation of the reinforcing filament bundles. From the mechanical analysis it becomes clear that an

increase in reinforcing filament content results in a higher flexural and tensile modulus and strength. The

damping capacity is increased when more reinforcement is added since more matrix/filament interfaces are

present and more tendency towards void formation exists wherein the energy can dissipate.

Continuous fibreglass reinforced 3D printed nylon 6 by Markforged technology shows a considerably higher

performance than conventional non-reinforced materials printed with FDM technology. A flexural and tensile

modulus of elasticity of 7.0 and 9.4 GPa are obtained respectively. A flexural and tensile strength of 144 and

288 MPa are obtained respectively. A microscopic analysis showed imperfections (air inclusions) inside the

reinforced structure. An estimation of fibre content resulted in approximately 18.8 vol% which translates into

35 wt%. To conclude, the commercially available 3D printed materials show a significantly lower performance

than conventional fiberglass epoxy composites (tensile modulus and strength of circa 45 GPa and 1000 MPa

respectively for 60 vol%[15]). However the technology provides the opportunity to create complex parts more

efficiently.

PHASE 2: DEVELOPMENT OF A LOW-COST CONTINUOUS REINFORCED COMPOSITE 3D PRINTER 37

5. PHASE 2: DEVELOPMENT OF A LOW-COST CONTINUOUS REINFORCED COMPOSITE 3D PRINTER In the second chapter of this project, a low-cost continuous reinforced polymer 3D printer is developed by

modification of a commercially available FDM 3D printer. It is concluded by preliminary research that the FDM

technology is the most feasible option to transform an existing device into a continuous reinforced polymer 3D

printer[3]. A pre-nozzle alignment and in-nozzle impregnation technique is conducted. This is made possible by

several uncomplicated and efficient modifications of the existing low-cost printer. PETG and aramid filaments

show a good processability and were chosen in this explorative study with the view on altering materials to

further optimize the overall technology in a later stage. The stepwise process of adapting the hardware is

outlined in the first section of this chapter. First, the location of joining reinforcing material and matrix

material, subsequently the mixing chamber optimization and finally the print bed modifications. Visual

inspection and preliminary microscopic analysis are used to provide direct feedback about the print quality

(integration and homogeneous distribution of a continuous reinforcement) during modification of the printer

setup. The second part of this development phase focuses on the resulting 3D printed material and

reproducibility. Crucial parameters for printing the materials are the integration configuration of the

continuous reinforcement in the final print, the amount of reinforcing material that can be included and the

impregnation of the reinforcing material into the matrix material.

5.1. MODIFICATIONS OF EXISTING TECHNOLOGY (HARDWARE)

During this project, a low-cost FDM 3D printer (Malyan M200) was used to make continuous reinforced

composite materials. This printer was selected as it is a low-cost 3D printer that is well rated under amateurs

while being conceptually similar to more advanced FDM printers such as the high end Stratasys technologies

[52]. A modification to the printer is applicable to a general FDM printer.

The matrix material is fed into a 3D printing head in the shape of a polymer filament, pointed with (a) in Figure

38. This printing head is extruding whilst moving along a programmed sequence of X-, Y- and Z-directions. The

printer has a ‘Bowden setup’ which means the extrusion of the filament is controlled by a motor (gear (b)) not

attached to the printing head. That gear is called the ‘remote extruder end’ of the Bowen setup[53]. That

printing head consists of a tubular nozzle surrounded by a heating element (c), referred to as the ‘hot end’ of

the Bowden setup and a fan (d). The heating is controlled by a guided air stream towards the heating element

and the nozzle tip (e) using a by-pass. That air flow (f) accelerates solidification of the extruded reinforced

material strand. This is an important detail for the polymer cooling rate, which is mentioned specifically during

the discussion on reinforcing filament incorporation into the matrix material. The previously described parts

are indicated with an arrow on Figure 38-40.


Figure 38: General setup of Malyan M200 FDM 3D printer. Indicated in the picture: (a) filament feed, (b) gear regulating the

extrusion rate, (d) fan.

In order to print with a continuous reinforcement, the reinforcing material needs to enter the nozzle where it is

combined with the liquid thermoplastic (in-nozzle impregnation) It is attempted to be inserted as close to the

nozzle as possible to minimize the friction on the reinforcing filament during printing as this could lead to

unnecessary fibre breakage, filament stalling or uneven fibre distribution.

A first setup brings in the reinforcing filament together with the matrix filament at the Bowden tube inlet near

the cold-end (Figure 39 (1)). The major drawback of this setup is that there is a lot of friction in the Bowden

tube (diameter 2 mm for 1.75 mm of matrix feed filament diameter) due to the close tube/filament fit that is

necessary to prevent buckling of the matrix filament. That friction causes difficulty in the co-extrusion and

results in the formation of tensile forces acting on the reinforcing filament that prevent a stable printing

process. Moreover, the high tensile forces acting on the filament result in fibre breakage as the reinforcing

continuous fibres undergo friction at the edge of the metal 3D printing nozzle. This drastically reduces the

performance of the printed material. In a second setup (Figure 39(2)) the filament bundle is introduced into

the printing process right before the heating element and in a third one it is introduced in between the cooling

fins of the heating element. This is the closest feasible incorporation location possible for this FDM 3D printer.

The tension forces were greatly reduced and co-extrusion now runs more smoothly. This third setup was

therefore used during the further course of the thesis project.


Figure 39: The insertion location of the reinforcing filament on all images is pointed with (h), direction of the reinforcing

filament bundle with (g) and matrix material filament with (a). (1): The reinforcing continuous filament bundle is inserted

along the PETG filament at the remote extruder end of the Bowden setup. (2): Insertion happens before the heating

element. (3): Insertion happens between fins of the heating element.

Figure 40: 3D schematic of the setup of a continuous reinforced composite 3D printer prototype, (a): matrix material

filament feed, (b): cold-end of Bowden setup: matrix filament extrusion control gear, (c): heating element, (d): fan, (e):

nozzle tip, (f): schematic air flow, (g): reinforcing filament bundle feed, (h): reinforcing material insertion location.

The next focus point is to optimize impregnation of the reinforcing filament bundle with matrix material. The

melt chamber of the nozzle serves as an impregnation chamber (Figure 38 (i)). It is enlarged by replacing the

standard nozzle of 12.5 mm length by a longer E3D Volcano nozzle of 21.1 mm length. The nozzle diameter is

0.8 mm, which is larger than typically implemented for non-reinforced FDM technology but significantly

smaller and thus with a higher layer resolution than generally practiced in academic research during. (1.4 mm

diameter nozzle was used by Matsuzaki et al. [14]). Bettini et al. reports that for a flexible aramid continuous

g

1 2 3


fibre reinforcement, a smaller nozzle is feasible for carbon fibres but for fibreglass reinforced 3D printing

there is danger of fibre breakage due to the more brittle character of the continuous fibres[30]. Although

reported by Yang et al. is the use of a 0.8 mm nozzle for carbon fibre reinforcements obtaining 10 wt% fibre

content[22].

Figure 41: 3D schematic of one extruded strand existing of reinforcing filaments and matrix material. The red line indicates

how the extruded filament was cut. That surface is then investigated using SEM (Figure 42)


Figure 42 (a)-(c): SEM images of the cross section of one extruded strand. The red arrows in (a) and (c) indicate voids (air

inclusions) between reinforcing filaments and between reinforcing filaments and matrix material due to bad micro-scale

interface bonding (between reinforcing filaments and matrix material as described by Yang et al.[22]). A better bonding is

shown in (b) with a green arrow.

At first, one strand is extruded vertically from the nozzle without laying down on the print bed as it is

schematically shown in Figure 46. The strand exists of matrix material and reinforcing material and it is

observed that the matrix is surrounding the filament bundle. SEM Images (Figure 42 (a), (b) and (c)) show that

the matrix material is not intensely mingled between the filaments yet. The images indicate a poor distribution

quality.

Secondly the printer head is programmed to lay down the composite material strands on the buildplate. First

of all, the sharp point of the nozzle damaged the extruded filament bundle whereupon the nozzle’s sharp tip

was flattened by a grinding action. Secondly, difficulties of printed material adhesion to the print bed are

observed. The tension build up within the impregnation chamber of the nozzle has to be counteracted by the

adhesion force between the print bed and the laid down strand. The device’s standard aluminium print bed’s

adhesion force is not sufficient to do so. The print bed temperature is set at 50 °C. A higher temperature would

avoid the high sudden temperature change from nozzle to print bed but no stable higher temperature could be

reached by the Malyan M200 3D printer. This parameter is thus kept constant during the thesis project. Hence a

poly-imide layer was attached to the print bed as is shown on Figure 38. The result is still insufficient so a

BuildTak print bed cover is applied (indicated with (j) on Figure 43). No information about the composition of


this cover bed is released by the company (Ideal Jacobs Corporation, headquartered in New Jersey, USA). It is

presumed that a polymer coating is interacting with the hot matrix material. This estimation is confirmed by

visual analysis of the printer bed after several 3D prints on the same coordinates.

Figure 43: Final setup of the tuned continuous reinforced composite 3D printer. Indicated on the picture: (a) PETG filament

feed, (g) aramid filament bundle feed, (j) BuildTak printer bed.

Figure 44: (a): Cross section of a 3D printed specimen. The white box show the area whereon images (b) and (c) zoom in. (b)

and (c): Microscopic image of the specimen cross section. Pointed with red arrows are reinforcing material filament bundles.


The printed material is now investigated using optical microscopy and electron microscopy. The microscopic

images of the cross section of a printed material with fibre content of 2.85 mass percentage (Figure 44) show

that the aramid filament bundle is imbedded in the matrix material with a non-circular cross section. As the

filament is laid down together with the matrix material, the fibres are spread out and even better impregnated

than a not laid down line (an extruded strand of matrix and reinforcing material as shown in Figure 46. I tis

concluded that the flattened nozzle tip and pressure of layers onto each other are beneficial for the

impregnation of the reinforcing filaments into the matrix and result in more contact surface between the two

materials.

However the reinforcements’ impregnation and dispersion quality is an important point of attention, some

issues are observed during microscopic analysis. First, the reinforcing material lays in the upper part of the laid

down strand. This is visible on microscopic Figure 45 (a) and (b) from top and bottom part of a printed

specimen. The bottom part shows almost full matrix material (PETG) and on the top part almost only filaments

are visible. Secondly some filaments on the top part of the printed material failed. The combination of high

tensile forces and sharp nozzle tip result in filament breakage as the reinforcing fibres undergo friction at the

edge of the metal 3D printing nozzle. This failure on filament level drastically reduces the performance of the

resulting printed composite material. Therefore the nozzle tip was made less sharp by simple grinding the top

part and making it flat. An additional advantage besides preventing the breaking of the filaments is that now

the filaments are ‘pulled open’ to let the molten matrix material diffuse between them. Thirdly, visible by

microscopic analysis are air inclusions between filament bundles and between filaments and matrix material

(Figure 45(c)). Compared to the microscopic analysis of Markforged printed material, the pores are of much

lower dimensions. Pores and voids are observed in the Markforged material (Figure 45(c)) while for the own

printed specimen the voids have a diameter of approximately 50 to 100 µm.


Figure 45 (a): Microscopic image of top side of a 3D printed beam-shaped object that exists of multiple lines next to each

other and multiple layers on top of each other. (b): Bottom side of that specimen. The bottom side shows clearly

non-impregnated filaments that lay up the specimen. On the bottom side a full cover with nylon is visible. (c): Cross section

of a 3D printed specimen. The imbedded filament bundle of 22.2 tex has an elliptic cross section (with marked dimensions).

Pointed with white arrows are presumable pores between filament bundles and between filaments and matrix material.

In Figure 47 the concluding model is sketched of how one strand of the modified FDM 3D printed composite is

laid down on the print bed. Figure 47 (a) shows an ideal distribution of reinforcing filaments over the cross

section and (b) shows a schematic of the real impregnation. Herewith it is concluded that an improvement for

this non-distribution phenomenon should be looked for to optimize properties. In order to direct the cross

section in the shape of (a), the layer height and line spacing are already reduced. Further investigation on the

quality (increasing filament distribution by optimization of layer height and line spacing) and reproducibility

(validation of desired composite materials versus outcome composite materials) of the printed specimen is

described in the next paragraph.

407.7 µm

123.9 µm

c


Figure 46: (a): 3D schematic of a laid down strand where the para aramid filament bundle is pulled to the upper surface of

the strand (main friction point is indicated with a green arrow), (b) Long distance microscope image of the nozzle during 3D

printing.

Figure 47: 3D schematic of ideally impregnated aramid filament bundle in one laid down strand (a) and a realistic filament

bundle in the PETG matrix (b).

5.2. PRINTING PARAMETERS OPTIMIZATION (SOFTWARE)

The FDM 3D printer movements in X-, Y- and Z-directions, extrusion rate, printing speed and temperature

settings are all controlled by a G-coded program. A G-coded program with explanation of mainly used

commandos is provided in Appendix A of this report. After calibration of the zero point of the modified nozzle,

an own written G-code navigates the nozzle and extruder to generate an object that consists of strands that

are laid down along a defined path. The program is written initially to create a ‘standard specimen’ that is

reproducible. Therefore the layer height, line spacing, printing temperature and printing speed are optimized

by repeatedly investigating the printed material and altering the G-code. The target for the 3D printed

composite material is to be comparable with a conventional composite material. In 3D printing language this

means a 100 % infill design (solid block infill, unlike typical FDM printed objects which have a raster infill

design to save material) is aimed whereof the reinforcing filaments are completely impregnated in the matrix

material.

It is observed that the printing speed is of great importance for a good impregnation of the para aramid

filament into the PETG matrix. When the nozzle is moving too fast, the reinforcing filaments cannot be

contained optimally by the matrix material since it is not yet cooled down enough and viscosity is as such not

high enough. Subsequently the reinforcing filament will be pulled out of the matrix material. The printing

speed during this thesis was set at 50 mm/min. It could be increased when cooling air flow created by the fan

a b


is improved but this is not included in the scope of the thesis project. The printing temperature additionally

influences the viscosity of the extruded material. This effect is not further elaborated within the scope of this

project. The nozzle temperature is kept constant at 250 °C during the project. Layer height and line spacing

(Figure 48) are important settings to decrease the possibility of air inclusions and to increase the total

distribution of the filament bundle, as is shown in Figure 47 (a). It is to be decided how much overlap or ‘air

width’ between two lines there should be and how high the layers should be distanced on top of each other

(layer height). A model of that is presented in Figure 49 (b), which is based on the observation that the

reinforced material is laid down with an elliptical cross section. To counteract the phenomenon of ‘porosity’,

the lines are laid down with an overlap of 40-60 %. This allows the adjacent layer to slightly heat up again. The

polymer chains are given extra mobility and a bonding interaction with the new laid line is possible now. At the

same time diffusion of the matrix material between the aramid filaments is possible. This is beneficial for the

impregnation of the filaments within the PETG matrix.

Figure 48: (a): Schematic of the print path of the nozzle for one layer of a long beam specimen. (b): Schematic of the printed

long beam specimen. (L): The line spacing is the distance between two laid down lines. (l): Length of a printed specimen.

L



the distance between two laid down lines; (LH) layer height, programmed Z-distance over which the nozzle moves up

before printing the next layer; (l),(w),(t) the length, width and thickness of a printed specimen.

5.3. FIBRE CONTENT SETTINGS AND VALIDATION

When the ‘standard specimen’ is set up, the next step is to increase the fibre content. This is done by pre-

setting a fibre mass percentage-value and then calculating the required amount of matrix material to be

extruded. A first set of specimens is produced with as varying parameter the fibre content. In order to produce

this set the following procedure is followed: first of all, the fibre content is decided. From that ‘aim’ the matrix

filament extrusion rate is calculated and finally the aimed value is validated by weighing the resulting

specimens. Each step is described in this paragraph and finally a graph is provided where ‘aimed fibre content’

is set out versus ‘resulting fibre content’.

The 3D printing path (material build-up procedure) of the reinforced part of the specimen is coded as shown in

Figure 49: six lines are extruded next to each other of which four layers are printed on top of each other.

Aiming at a 100 % infill of matrix material ideally leaving no space for voids, the layer height and line spacing

are set (both indicated in Figure 49 and outlined per sample in Table 14). This nozzle path is defined as the

‘basic design’ for the reinforced part of the specimen. In order to change the fibre content matrix material

extrusion amount per path is adjusted. The first three specimens (S_1 to S_3 in Table 5) consist of a non-

reinforced PETG top and bottom layer, this is inspired by the Markforged printed materials. The Markforged

‘Eiger’ software sets a minimum amount of top and bottom layers without reinforcements is set automatically

by the software to allegedly create products of higher quality.

Eight variations on the ‘basic design’ are printed. This is done by changing the extrusion rate of the matrix

material and retaining the four layers of six lines, which means the length of the impregnated reinforcing

filament bundle is unchanged but the specimen cross section dimensions are altered. The count of the aramid

filament bundle is 22.2 tex and the diameter of the PETG feed filament is 1.75 mm. The value for E is to be

interpreted as the length of feed PETG filament that is extruded per line. This value sets how much volume of

PETG is extruded per line and thus the amount of matrix material that is present per line. This is the method

used to alter fibre content per specimen.

A second set of specimens has as varying parameter the kind of implemented reinforcing material. This time

the fibre content calculation must alter the tex-value. The different reinforcing materials details are provided

in paragraph 3.1.

L

a b


Finally to validate the set fibre content, the following procedure is followed once the specimens are printed.

The path length of the aramid filaments inside the samples is estimated as equal to the movement of the

nozzle path programmed in G-code. The density of para-aramid material is 1.44 g/cm³ and of PETG is

1.23 g/cm³. The specimens are weighed and their mass is used to calculate the mass percentage for each piece

as described in Equation 9-12. A mean value is obtained and represented versus the aimed fibre content value

in Figure 50. It is concluded from the graph that the fibre content is accurately modifiable through this

procedure. This counts for the 22.2 tex aramid filament bundles as well as for the 40.5 tex and for the 158 tex

bundles.

𝑚𝐾𝑒𝑣𝑙𝑎𝑟 = [𝑝𝑎𝑡ℎ 𝑙𝑒𝑛𝑔𝑡ℎ]𝑚𝑚 ∗ [𝑡𝑒𝑥 𝑣𝑎𝑙𝑢𝑒]𝑔

1000 ∗ 103𝑚𝑚 (Equation 9)

𝑚𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 = 𝑚𝑚𝑎𝑡𝑟𝑖𝑥 + 𝑚𝐾𝑒𝑣𝑙𝑎𝑟 (Equation 10)

𝑚%𝐾𝑒𝑣𝑙𝑎𝑟 = 𝑚𝐾𝑒𝑣𝑙𝑎𝑟

𝑚𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 (Equation 11)

𝑣𝑜𝑙%𝐾𝑒𝑣𝑙𝑎𝑟 = 𝑚𝐾𝑒𝑣𝑙𝑎𝑟/𝜌𝐾𝑒𝑣𝑙𝑎𝑟

𝑚𝑚𝑎𝑡𝑟𝑖𝑥/𝜌𝑚𝑎𝑡𝑟𝑖𝑥 + 𝑚𝐾𝑒𝑣𝑙𝑎𝑟/𝜌𝐾𝑒𝑣𝑙𝑎𝑟

(Equation 12)

A special remark is to be made for the representation of the fibre content. In literature, often volume

percentage is used because of its established use in conventional composite research. Though for 3D printed

composites, the void formation is less negligible than for vacuum infused moulded composites. Because air

volume inclusion measurements are time-consuming and challenging to decide accurately, this was not

included in the thesis research. As a mass percentage representation is an accurate and efficient measure to

appoint and assuming the air inclusion in all specimens of the ‘basic design’ is approximately equal, this will

be used predominantly. So far, in previous research the effect of air inclusion is ignored, which actually results

in an overestimation of the fibre volume content of the material.


Table 5: Printer settings for aimed fibre content

Specimen name

(aramid count)

Aimed fibre content

Resulting fibre

content

E (Extrusion amount of

PETG filament)

LH (contiuous reinforced

layer height )

L (line spacing)

Bottom and top layer?

(wt %) (wt%) (mm) (mm) (mm) (yes/no)

S_1 (22.2 tex) 3 2.85 14 0.4 0.7 Yes: 0.5 mm

S_2 4 4.70 10 0.4 0.5 Yes: 0.3 mm

S_3 5 5.25 9 0.3 0.4 Yes: 0.3 mm

S_4 9 9.02 8 0.3 0.4 No

S_5 12 11.85 6 0.2 0.4 No

S_6 15 15.35 4 0.2 0.4 No

S_7 20 22.71 3 0.2 0.3 No

S_8 23 23.33 2.7 0.1 0.2 No

D_1 (40.5tex) 25 23.98 4 0.2 0.4 No

D_2 (158 tex) 55 55.63 4 0.2 0.4 No

Figure 50: (a) Set versus outcome fibre content for different extrusion. (b): Set versus outcome fibre content for different

aramid filament bundles.

5.4. MICROSCOPIC ANALYSIS OF RESULTING MATERIAL

In this paragraph the layered microstructure and the impregnation of the aramid filament bundles in the PETG

matrix material are is investigated with microscopic analysis. An obvious ‘cutting action’ is visible in Figure

51(b), whereby the aramid filaments are pulled out. Because of the liquid nitrogen, most of the matrix failed in

a brittle way except for the areas around the aramid filaments, which show now a ductile behaviour. With this

interpretation, print lines are visible (arrows (b)). Even more ‘pulled out’ are the fibres pointed with (a). There


seems to have been a better impregnation of the matrix material between the reinforcing filaments which

caused the cooling down of a brittle failure between the filaments and a less ‘clean’ failure surface as we see

here compared to the other areas on the image. It is thus concluded that some impregnation of aramid

filaments was realized.

In Figure 52 (2) the layer structure in the Z-direction (pointing out of the page surface) is observed. Orange

arrows (a), (b) and (c) show the different layers: fibres-matrix-fibres-… In the X-direction, not much distinction

can be made between the different lines. The need for better impregnation becomes clear by this evaluation.

Arrow (e) shows again the upper filament bundles that lie ‘open’ (without impregnation by matrix material) on

the top part of the specimens because they were printed in the last layer. Arrow (d) shows how the matrix

material failed in a ductile way in some areas. Those areas are showing a pattern. It appears to be each time

above or beneath a filament bundle of one line. This brittle/ductile failure structure allows to recover the print

path of the specimen and proves that the reinforcing filaments have been loaded and an interactions of the

filaments with the matrix material exists.

Figure 51: (1): 3D schematic and (2): corresponding SEM image of cross section of an ‘S_5’ specimen. The cut was obtained by

using liquid nitrogen. In both mages the axis are represented as they are to be coded for the 3D printer.

1

2


Figure 52: (1): 3D schematic of a broken specimen and orientation of the SEM imaging of the specimen, (2): SEM image of

liquid nitrogen cooled, cut and folded specimen. (a), (b) and (c): Different printed layers, (d): ductile failure of matrix

material, (e) upper filament bundles.

To conclude the explorative chapter of the project, the following key points are highlighted about the

development of an FDM 3D printer for continuous reinforced composite materials.

It is important that the insertion of continuous reinforcing filament is performed as close as possible

to the nozzle to avoid friction forces on the reinforcing filaments before it enters the nozzle.

Extrusion of the right amount of matrix material along with the reinforcing filament of a certain

count is required. A calculation can be performed by forehand. When there is too less extrusion of

matrix material, the hazard of void formation exists. Over-extrusion on the other hand lowers the

dimensional accuracy and will result in unwanted dimensions of the printed part.

The nozzle tip needs to be well designed in order to 3D print the reinforcing filaments in a ‘smooth’

way.

The print bed/3D printed material adhesion should be set on point.

Controlled cooling of the extruded material increases the cool down rate, which is important for the

increase of viscosity of the matrix material. ‘A sticking force’ or ‘tracking force’ as described by Li et al.

towards the aramid filaments can counteract the friction force that was built up in the nozzle[29].

The filaments are impregnated inside the matrix material in two phases: the first impregnation action

happens inside the nozzle and the second phase happens when the next layer is printed on top of it.

That second phase reheats the material from the bottom layer and compresses it by a next line of

material. It was observed that this compression exists and definitely surrounds the aramid filaments

of the bottom layer.

In contrast with previous research, unit of 'mass percentage' is chosen instead of 'volume percentage'.

This is done in the interest of emphasising that additively manufactured composites show different

properties than conventionally produced composites. A fibre reinforced polymer produced with an

FDM 3D printer is susceptible to contain voids which are not negligible. This was already reported by

Justo et al.[35].

PHASE 3: MECHANICAL PERFORMANCE ANALYSIS OF 3D PRINTED MATERIAL 52

6. PHASE 3: MECHANICAL PERFORMANCE ANALYSIS OF 3D PRINTED MATERIAL The third prospect of the project is the investigation of mechanical performance of the continuous reinforced

3D printed polymers. Four different test methods are performed. During a uniaxial tensile test and a three-

point bending test, the basic mechanical characteristics are analysed. A dynamic mechanical analysis (DMA)

measures values for the storage modulus and tan δ. Those values express a measure for the elastic behaviour

and respectively the damping capacity of the material. Finally an innovative test method for continuous

reinforced 3D printed materials was developed whereby a value for the interlaminar toughness can be

determined. This value is already of interest in studies on conventionally produced composites to determine

the hazard for delamination failure.

6.1. ORGANIZATION OF THE EXPERIMENTS

First of all, a comparing analysis is made of the specimen with increasing fibre content. The fibre content is

varied by altering two parameters: altering the amount of matrix material that surrounds the parts and

altering the count of the aramid reinforcing filaments (see paragraph 5.3 on fibre content settings). It is

important to mention that the dimensions of the specimen varies while the amount of aramid filament is not.

A third variation is a post-processing procedure of the specimens. The results are compared to non-post-

processed specimens. The sizes for all specimens are depicted in Table 14. It is to be mentioned that a shift in

dimensions was made to accomplish better experiments.


6.2. TENSILE MODULUS PREDICTION

As a preface for the determination of the modulus of elasticity, an estimation is done based on a rule of

mixture with fibre volume percentage, as is performed in several previous academic studies[17,30,54]. The

value for the Young’s modulus for an ideal composite is estimated using a volume average stiffness method.

‘Ideal’ means that the filaments are homogeneously distributed over the cross section of the composite and

incorporated in parallel inside the PETG matrix (Figure 47). All load is then transferred to the filaments without

interruption by air voids or impurities between the matrix and the reinforcing material.

Through a volume percentage rule of mixture (Equation 13), an estimation of the modulus of elasticity can be

made.

𝐸𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 = 𝑣𝑜𝑙%𝑚𝑎𝑡𝑟𝑖𝑥 ∗ 𝐸𝑚𝑎𝑡𝑟𝑖𝑥 + 𝑣𝑜𝑙%𝑎𝑟𝑎𝑚𝑖𝑑 ∗ 𝐸𝑎𝑟𝑎𝑚𝑖𝑑 (Equation 13]

The values for the predicted elastic moduli for the matrix material (PETG) and reinforcing material (22.2 tex

para-aramid) are outlined in paragraph 3.1. A value from own measurements for the aramids is chosen because

of different history of the filament bundles (e.g. exposure to sunlight which leads to degradation) which could

cause the filaments to fail earlier as expected. In the next paragraph the estimated values are shown in Figure

54 (b). The actual physical experiments are an experimental validation for this prediction. This is a valid action

since the printed materials have the same main characteristics as conventional composites. In literature the

estimations are often compared to experimental values and an overestimation is often reported[34]. Al Abadi

et al. explain that this is because bonding interactions of reinforcements and matrix material are not included

in the formula[36]. Overall, the real 3D printed composites are far from an ideal composite which is why an

over estimation is expected.

Table 6: stiffness prediction calculated for the long beam specimens

Sample name Description wt%

aramid vol%

aramid Rule of mixture for tensile

modulus (GPa)

S_0 non-reinforced PETG 0.00 0.00 2.20

S_1 22.2 tex aramid/PETG 2.85 2.52 4.36

S_2 22.2 tex aramid/PETG 4.70 4.17 5.77

S_3 22.2 tex aramid/PETG 5.25 4.66 6.19

S_4 22.2 tex aramid/PETG 9.02 8.04 9.08

S_5 22.2 tex aramid/PETG 11.85 10.60 11.27

S_6 22.2 tex aramid/PETG 15.35 13.79 14.00

S_7 22.2 tex aramid/PETG 22.71 20.58 19.82

S_8 22.2 tex aramid/PETG 23.33 21.16 20.31


6.3. TENSILE PROPERTIES

A uniaxial tensile test according to previously described method (see paragraph 3.2) on an Instron universal

tensile test device provides information about the strength of the material when loaded along the direction of

the filaments. This axial direction is the optimal load mode for the aligned composite material: all filaments

are loaded directly. The dimensions of the specimens are beam-shaped instead of prescribed dogbone-shaped.

Stress could be concentrated at curved paths of the reinforcing filaments and that will cause the specimen to

fail preliminary to the overall material strength [1]. Melenka et al. reported failure at the starting location of

the fibre reinforcement when investigating Markforged printed materials with a dogbone-shape[34].

Figure 53: Stress-strain diagram for specimens with different fibre contents (indicated for each corresponding graph).

Figure 54: (a): Graph with tensile modulus results for in-house 3D printed specimens with fibre content as varying

parameter. (b): Graph with tensile strength values. The data-points in light blue in graph (a) represent the estimated moduli

values resulting from the stiffness prediction method.


Table 7: Results of tensile test.

Fibre content (wt%) Tensile elastic modulus (GPa) Tensile strength (MPa)

0.00 1.32 27.37

4.70 3.06 55.25

9.02 4.45 110.60

15.35 5.76 158.77

22.71 8.59 265.10

23.33 10.30 288.31

A significant increase in strength is recorded when comparing a non-reinforced specimen to a 23 wt% aramid

reinforced specimen (Figure 54(a)). The strength performance for the highest reinforced specimen is almost

ten times as much as a non-reinforced specimen. As the rule of mixture for tensile modulus predicts and

previous studies report[17,30,54], it is recorded during this experiment that the increase of fibre content

results in the increase in tensile modulus of elasticity (Figure 54(a)).

The obtained Young’s modulus values are only half as much as the predicted values (Figure 54(b)). This

overestimation is assumedly due to porosity between filaments and between filament bundles and matrix

material. The volume percentage that is accounted for in the rule of mixture is not accurate since the air

volume in the specimen is not negligibly small. Additionally the bonding interactions between matrix material

and reinforcing material have an influence that is not calculated in the estimation formula. Additionally, the

filament bundles are not homogeneously distributed over the cross section of the specimen, which they should

be in an ideal composite. Finally, a characteristic for additively manufactured parts is the layered structure

whereof the interlayer and interline interactions are of great influence to the mechanical performance and

create an anisotropic behaviour different than bulk material. The rule of mixture prediction method therefore

highly overestimates the mechanical performance.

6.4. FLEXURAL PROPERTIES

In Figure 55 it is shown that a 23.3 wt% continuous aramid reinforced PETG 3D printed specimen has an

increased flexural strength and modulus compared to non-reinforced material of circa three times and

respectively six times as much. Though under deflecting load, the tendency of increasing performance with

increasing fibre content is less clear. First of all, reinforcing filaments are in this setting not loaded ideally. An

‘ideal loading’ for the fibre reinforced polymer would be along the longitudinal orientation. It is recorded that

the filaments are not of such added value as they are in tensile mode. In Figure 56 the load modes are

schematically represented. The filaments located on the bottom of the specimen will be loaded in tensile

mode, which will occur their longitudinal strength properties to become useful. Whereas the upper lying

filaments are loaded in flexural mode, which will less contribute to the flexural stiffness and strength

performance. When looking on a microscopic level, the bonding between matrix and reinforcing materials can

influence the performance in flexural loading mode. The non-ideal impregnation of the aramid filaments can

result in the following occasion: when a compression load is set, the reinforcing filaments can buckle out of

the loading plane and as such not contribute anymore to the flexural performance of the material. A special

note about the low modulus performance of the ‘S_7’ specimen (22.7 wt%): this value is not yet explainable. It

might be due to a bad processing that caused imperfections inside the material. It is possible that the amount


of extruded matrix material and the layer height and line spacing settings were not adequate. The latter one is

more plausible since the flexural strength does not differ that much from a 23.3 wt% specimen’s performance.

In order to determine the cause more in depth research is required.

For the flexural strength an increasing value with increasing fibre content is observed when leaving the

specimens of 9 and 12 wt% out. Those two might perform less because of less impregnation of the filaments

since their layer height and line spacing settings were the same as the specimens with lower fibre content but

the amount of input matrix material is lower. Subsequently, there was more ‘space’ for air inclusion during the

production process. Again this is an assumption that should be further investigated.

From the overall results it is concluded that increasing fibre content increases the flexural performance. The

improvements are not as high as for tensile loading. The theory behind this phenomenon is the importance of

impregnation of the filaments into the matrix. Under tensile loading, the stress is directly transferred to all

fibres even if they are not well impregnated. For the flexural loaded experiments, that impregnation quality

does make a difference. The better the impregnation, the better the load is transferred and filaments provide

support to the material resisting the forces as they are schematically represented in Figure 56. As such, the

obtained strengths and moduli are much higher for the tensile experiments than for the flexural experiments.

On top of that, these values are far beneath what is usually obtained for traditional continuous fibre reinforced

polymer materials where typical flexural moduli are between 20 – 80 GPa[55].

Figure 55: (a): Flexural strength performance of specimen with increasing fibre content, (b): flexural modulus of elasticity of

specimens with increasing fibre content.


Table 8: Results for three-point bending test. Flexural elastic modulus (GPa) Flexural strength (MPa)

Fibre content (wt%) Mean Standard deviation Mean Standard deviation

0.00 0.89 0.10 40.82 1.96

2.85 1.70 0.03 63.39 1.98

4.70 2.04 0.08 79.43 4.14

5.25 1.70 0.07 63.60 2.74

9.02 3.46 2.31 77.97 0.94

11.85 5.67 1.98 94.94 8.52

15.35 4.65 0.49 152.69 15.59

22.71 2.11 0.73 91.18 36.26

23.33 5.97 1.41 147.30 33.39

Figure 56: Side view schematic of the load mode on a specimen during a three-point bending test: the aramid filaments in

the bottom part are loaded in tensile mode, while the filaments in the upper part are loaded in compression mode.

6.5. STORAGE MODULUS AND DAMPING CAPACITY

A dynamic mechanical analysis is performed on the in-house 3D printed composites. In Figure 57(a) and (b) the

test setup is presented. A DMA measures the storage modulus, which represent the elastic behaviour of a

material and damping capacity, which is an inherent material property for the dissipation of energy.

Two important characteristics of the materials will change its performance: the reinforcing fibre content and

the impregnation quality. Those two main influences are investigated through various sets of specimens. First,

the long beam-shaped specimens are assessed and represent increasing fibre content as varying parameter.

Those specimens are tested in tensile and in flexural mode to see what is the difference in both loading modes.

Secondly the reinforcing material is altered from smaller to larger aramid filament bundles. Since the printed

beam specimens showed slight over-extrusion at the edge, possibly affecting the experiment results, a plate-

shaped geometry is tested. The last set of specimens is post-processed to see whether an annealing procedure

affects the mechanical performance.


Table 9: Varying parameters overview for DMA test.

Varying parameter Range

Fibre content 0 wt% to 23.3 wt%

Reinforcing material 22.2 tex; 40.5 tex; 158 tex

Dimensions Beam-shaped versus plate-shaped

Post-processing (pressurized in a mould for 1h @ 90 °C ) Not post-processed versus post-processed

Figure 57: (a): Single cantilever beam setup and (b): Tensile mode setup on the DMA device.

6.5.1. INFLUENCE OF FIBRE CONTENT

Figure 58 shows the effect of increasing fibre content on the tensile storage modulus and tan delta. During the

experiment the composite material is loaded along the filaments’ axis and as such a direct loading upon the

filaments is possible. Hence the bonding between the reinforcements and matrix material does not influence

the results that much. An increasing fibre content results in an increasing storage modulus and compared to

the reference unreinforced material, the damping capacity is increased when reinforcing filaments are

inserted.


Figure 58: (a): Results for storage modulus (b): Results for tan δ for increasing fibre content of DMA in tensile mode.

For the flexural experiments, more information can be gained about the reinforcing filaments/matrix bonding

interactions and imperfections inside the 3D printed material. The explanation is as follows: when the bonding

between reinforcements and matrix material is good (Figure 59), the properties of the reinforcing filaments

will be utilized well and the storage modulus of the material will increase. On the other hand, when the

bonding and impregnation is not well, these ‘loose’ filaments will hardly affect the stiffness ad they can

realign, buckle and shift inside the specimen. The damping capacity (tan δ) is to be interpreted as energy

dissipating ability and is an intrinsic property of the material. For composite material, added to the intrinsic

properties are the microstructural effects of both materials together: imperfections exist wherein the energy

can flow away. This insight in the field of continuous reinforced 3D printed polymers might provide a new

characterization technique to verify the microstructure of a printed part.

Figure 59: Schematic of micro-scale interfaces within a specimen. Good versus bad interfacial bonding are indicated in the

figure with green and respectively red arrows.

In Figure 60(a) and (b) the results of a flexural dynamic mechanical analysis of specimens with increasing

fibre content are shown. First of all, the addition of filaments into the PETG matrix increased the storage

modulus more than four times. A further increase in fibre content did not result in the expected improvements,

which might be explained by a decreased impregnation quality. For the specimen around 22 wt% (pointed with


red arrow in Figure 58(a) and Figure 60(a)), the lower performance in both tensile and flexural mode might be

explained by extra imperfections due to a bad printing process of this specific specimen.

Regarding the damping capacity, it is observed that the reinforced PETG shows a higher damping capacity than

the non-reinforced printed parts. More filaments inside the material means more matrix/reinforcement

interfaces and more possible void formations. This higher amount of imperfections inside the composite

material results in a higher damping capacity. It is however recorded for the flexural load mode that the

damping capacity of the 22 wt% and 23 wt% specimens show a lower damping capacity than the non-

reinforced specimens (Figure 60(b)). This might be due to the fact that less matrix material is present which

will increase the contribution of the reinforcements’ intrinsic lower damping capacity. Further research is

required to draw solid conclusions.

Figure 60: (a) and (b): respectively results for storage modulus and tan δ for increasing fibre content of DMA in bending

mode (single cantilever beam setup).

6.5.2. INFLUENCE OF REINFORCING MATERIAL

For the next analysis, three different reinforcing aramid filament bundles were introduced during 3D printing

of the specimens. A 22.2 tex, 40.5 tex and 158 tex filament bundle was introduced and respectively 23 wt%, 24

wt% and 56 wt% were obtained. The influence of the different reinforcements is analysed during this DMA test.

The results are given in Figure 61.

Comparing the two specimens with 22.2 tex and 40.5 tex filament bundles and close fibre contents a higher

storage modulus can be observed for the higher tex aramid filament bundle. This is presumably caused by the

increased presence of stiffer material. The recorded damping capacity of those two specimens indicates that

energy is more dissipated within the material that was 3D printed with higher tex aramid filament bundle. A

‘larger’ bundle of filaments is harder to impregnate by the matrix material and will as such have more air

inclusions between the filament bundle (Figure 62). Comparing the storage modulus for 40.5 tex and 158 tex

specimens, the value does not increase significantly despite the increase in fibre content of more than two

times. Earlier calculations for moduli predictions using a volume average rule of mixture, forecasted a larger

increase. This indicates that a lot of losses are present within the material. In order to fully conclude this


argumentation, further in depth research is necessary in which specimens with the same reinforcing fibre

content are printed using different reinforcing bundles.

Figure 61: (a): Flexural storage modulus of beam-shaped specimens with different reinforcing aramids. (b) Flexural

damping capacity values for those specimens. The mass percentage of each specimen is indicated vertically.

Figure 62: Schematic of the cross section of 3D printed material with different reinforcing filament bundles. Possible air

inclusions caused by bad impregnation are indicated.

6.5.3. INFLUENCE OF SPECIMEN DIMENSIONS

For the different reinforcing materials, each time a long beam-shaped specimen and plate-shaped specimen is

tested in flexural mode. Before looking at the results it must be mentioned that fibre contents were not equal

per dimension nor for the different reinforcing materials and consequently a strict comparison is not valid. The

results are set out in a performance versus fibre content graph (Figure 63). An overall conclusion can be made

that with increasing fibre content the material shows an increased storage modulus because of more

reinforcements contributing to the performance and an increased damping capacity due to the higher amount

of imperfections inside the material. The plate specimens exhibit a lower trend for the storage modulus

normalized with fibre content, however the results tend to be more consequently following a straight


correlation. A similar conclusion is drawn for the tan δ-values. This is probably explained by decreased side

effects in the plate-shaped 3D printed specimen.

Figure 63: (a): Graph with flexural storage modulus for three different aramid/PETG composite specimens of beam shaped

(light green) and plate shaped (dark green) dimensions. (b): tan δ-values for the same set of specimens.

6.5.4. INFLUENCE OF POST-PROCESSING

Finally the influence of post processing is analysed. A preliminary DSC-scan of the PETG matrix material is

performed. The glass transition occurs between 70 and 80 °C. Based on this knowledge the following post-

processing program is defined: leave the specimens for one hour isothermal at 90 °C. In theory, the PETG

molecules gain a certain mobility at that temperature: they are given the opportunity to diffuse between the

filament bundles and thus it is expected that the impregnation of the filaments is enhanced. Especially in

flexural mode (single cantilever beam setup on the DMA device) it is expected that the post-processed parts

will have a higher storage modulus value and a lower damping capacity because the impregnation quality

contributes to the load transfer between matrix and reinforcing filaments.

From the results for the storage modulus in flexural mode (Figure 64 (a)), there is no spectacular increase in

mechanical performance visible for the 22.2 tex and 40.5 tex aramid filament bundle reinforced PETG

materials. This could mean that either the filaments were already well impregnated or that the PETG

molecules were not mobile enough to further diffuse between the filaments. For the 158 tex aramid bundle,

there is a slight increase though further research is required to conclude upon the cause. A first step in the

further investigations could be the increase of post-processing temperature.


Figure 64: Graph with flexural storage modulus for three different aramid/PETG composite specimens. Post-processed

specimens are represented in orange. (b): tan δ-values for the same set of specimens.

6.5.5. INTERPRETATION OF DMA COMPARED WITH TENSILE AND FLEXURAL RESULTS

Despite the different origin of the storage modulus (obtained from a dynamical mechanical test) and the

modulus of elasticity (obtained from a non-cyclic tensile or bending test), the values signify the stiffness of the

composite material Qualitatively it is concluded from both analysis techniques (dynamic and non-dynamic)

that an increasing fibre content increases the modulus value in tensile mode as well as in bending mode.

Considering that the obtained values are much lower than the predicted modulus values (paragraph 6.2), it is

concluded that a considerable amount of imperfections are present in the material (e.g. bad impregnation,

porosity, bad interfacial bonding, bad printing process, bad alignment of filaments,…) Nonetheless the

qualitative agreement shows the significance of each test to characterize the specimens.

Concluding from the experiments, the following insights on the microstructure are obtained: impregnation

quality of the reinforcing filaments within the matrix material and interfacial bonding of reinforcement and

matrix material influence the performance of the 3D printed structure. Mechanisms following from the quality

of this microstructure are load transfer (as measured in three-point bending test) and energy dissipation (as

measured in DMA). As both impregnation and bonding are good, the flexural performance will increase as the

reinforcements will achieve an increased contribution to the performance. As for the damping capacity, this

will decrease when impregnation is optimized and bonding is good. Those insights are in the same line with

the study of Li et al., mentioned in the literature review of this thesis report. During their microscopic and

mechanical experiments it was observed that remanufactured reinforcing filaments with much better

impregnation quality correspond to much better mechanical performance relatively to the fibre content[29].

6.6. INTERLAYER TOUGHNESS

6.6.1. EXPERIMENT DEVELOPMENT

In order to obtain a measure for the interlaminar fracture toughness between two printed layers on top of

each other, a double cantilever beam test setup (with Mode I load direction) is developed. For this experiment,

the specimens are produced as follows. Four layers are printed on top of each other. A strip of non-adhering

foil (such as Teflon foil, baking foil or aluminium foil) was placed between the second and third layer in order

a b

a


to create a delamination initiation area. Metal hinges are stuck onto the two beams at a certain distance from

the pre-crack tip to grip the specimen in the tensile machine (Figure 65). Consequently the specimens are

loaded as is shown in Figure 26. During the first attempts to perform the DCB test, the specimens had a

relatively low modulus and high toughness compared to conventional fibre reinforced composite materials.

This resulted in very large displacements of the DCB specimen legs during testing before any crack initiation

occurred. This could be circumvented by using a thicker specimen (printing more layers on both sides of the

crack initiation surface), or by reducing the distance of crack initiation to the hinge attachment. An initial

delamination of 3 mm was therefore used which is a valid distance according to the range provided in the

ASTM standard (paragraph 3.2).

A value for the interlaminar fracture toughness is determined from the load-displacement graph using the

5 %/max-rule from the ASTM standard. The load and displacement values at crack initiation (respectively F* and

δ*) are indicated in Figure 68 and Figure 69 for DCB specimens 1 and 2 respectively.

Figure 65: Schematic of DCB specimen, 3D printed with non-adhesive foil in between the second and third layer. (a) Distance

between crack initiation location and load transfer point (hinge adhesion location), (h) hinges, (n): non-adhesive foil.

Figure 66: Front view of the 3D printer during printing of a DCB specimen. (b) Top view.

a b

n

h


Figure 67: Setup of the double cantilever beam Mode I test loaded in the DMA Q800 device. (a): before the test, (b): two

‘legs’ are pulled apart in Mode I configuration during the experiment.

6.6.2. RESULTS

Initiation of the delamination is pointed in the graph and is to be interpreted as follows: this is the amount of

displacement of the two legs (at the location of the hinges) where the top and bottom layer come loose from

each other. Further opening of the legs results in crack propagation through the specimen. Equation 6 in

paragraph 3.2 provides how the GI –value is calculated. The result will be an overestimation according to the

ASTM standard [51] because no correction was performed for the rotation action at the crack tip.

Figure 68: Load-displacement graph obtained for specimen ‘DCB 1’ with F* and δ* indicated with a cross. The inset graph is a

zoomed in part of the same graph.

a b



a zoomed in part of the same graph.

Table 10: Results of successful DCB tests.

Specimen P* (mm) δ* (mm) a (mm) w (mm) GI (J/m²)

DCB 1 6.6 0.64 3.0 4.3 491

DCB 2 8.2 2.91 3.0 4.2 1865

Comparable experiments on conventional composites can be found in literature. The obtained curves resemble

to the shape of the curves in the study of Daelemans et al. where interlaminar fracture toughness values of

around 500 J/m² are reported[56]. A study of Martinez et al. on the fracture toughness of pure PETG resulted in

values from 2590 to 3800 J/m² [57]. Both conclusions indicate that the results of this experiment are useful.

However the curve in Figure 68 of current report shows a stiffer behaviour than the curve in Figure 69. It is

remarked that this is probably due to the not optimized experiment conditions: not well aligned hinges or a bit

of glue that leaked in between the hinges or the ‘legs’. It is herewith emphasised that even though future work

is required to optimize the conditions, the test method already demonstrated remarkable potential to be used

as a characterization technique for materials of interest.

6.6.3. DELAMINATED SURFACE

After performing a DCB Mode I experiment, the delaminated surface is visually inspected using microscopic

analysis. Figure 70 (1) represents a 3D schematic of a double cantilever beam Mode I delamination between

two layers. The blue material schematically represents the extruded PETG matrix and the yellow strands

represent the inserted high strength aramid filament bundle. Indicated on the figure are the upper and bottom


surface of the delaminated interlayer (Figure 70(a) and (b)). The Mode I load direction is indicated with black

arrows

From electron microscopic analysis (Figure 71 (2)) it is observed that the upper surface from the delaminated

layers contains very clear imprints of the filaments, which means impregnation around the outer filaments of

the reinforcing filament bundle has taken place. The yellow dotted line in Figure 71 (2) indicates a filament

that had enough bonding with the matrix material and stayed attached to the upper surface. That bonding was

already shown in the high GI-values obtained during the experiment. On the other side of the failed surface in

Figure 70 the aramid filaments are clearly visible in the bottom layer. The obtained images show similarities

with delaminated surfaces from conventional composites [56].

Visual analysis confirms the layered structure of the printed material. It can be concluded that initially, the laid

down lines have the inserted aramid filament in the top half of the cross section. Secondly, the PETG material

of a printed line has closely surrounded the filament bundles from the previous layer at least on the outside of

the bundle.

Figure 70: (1): Microscopic analysis if the DCB Mode I successfully tested specimen (DCB 1). (2): Detail from the bottom

surface where the crack initiation location was and where the delamination failure started (red dotted line) (a): Initial

surface before the experiment, (b): delaminated surface.

1 2

a

1

b

1

hinge


Figure 71: (1): SEM image of upper part of a delaminated specimen. The figure inset shows a schematic of a DCB Mode I

tested specimen with indicated (a): upper surface, (b): bottom surface. The patterns of previously laid in filaments are

visible as well as one remaining filament (encircled with yellow dotted line). (2): SEM image of bottom part of delaminated

specimen. The filaments are visible and small amount of matrix material is present (encircled with yellow dotted line).

6.7. SITUATION OF DEVELOPED TECHNOLOGY

To conclude the third phase of the thesis project, an overview of the currently available continuous reinforced

FDM 3D printing technologies is provided in Table 13. From that overview it is emphasized that the studies until

now are not ‘organized’. Along the existing technologies, different reinforcing and different matrix materials

are used which are then processed into composites with varying fibre contents. No set of standardized tests is

followed. For example the determination of fibre content differs from volumetric estimations to visual

determinations. No appropriate specimen dimensions are defined. The mechanical results of current research

are obtained using test methods mostly taken for reinforced plastic materials or own created test methods.

The table aims to situate the accomplishments of this thesis project amongst existing technologies and

studies.

Melenka et al. and Bettini et al. investigated continuous aramid reinforced polymers that were produced with

FDM technology[30,34]. Respectively they report the production of 10 vol% reinforced nylon and 12 vol%

reinforced PLA. Reported in both studies, the material exhibits a tensile modulus of 9 GPa. In current project, a

modulus of 10 GPa is obtained for a specimen of 21 vol% aramid reinforcements. As for the tensile strength, the

reinforced PETG from this project exhibits a performance of 288 MPa whereas the reinforced nylon and

reinforced PLA perform a tensile strength of 84 and 203 MPa respectively. The 18.8 vol% fibreglass reinforced

3D printed specimen by Markforged showed a tensile modulus and strength of respectively 9.4 GPa and

228 MPa. This is lower than the obtained values for in-house printed specimens. It must be mentioned that the

matrix materials are different and subsequently no valid comparison can be made. It can be concluded that

altering the matrix material to PETG and increasing the fibre content to 21 vol% resulted in a slightly increased

tensile modulus and strength for 3D printed composites reported in literature.

The materials produced during this project show a much lower performance than conventional aramid

reinforced epoxy composites (tensile modulus and strength of circa 75 GPa and 1400 MPa respectively[15]). For

parts with simple dimensions, a conventional production process will provide the best performance. However


when a one-of-a-kind part with a complex design is desired, continuous reinforced AM is a beneficial

production technique.

CONCLUSION 70

7. CONCLUSION This explorative thesis project consists of three coherent stages. In the first phase a polymer composite

material produced with state-of-the-art FDM 3D printer technology (by Markforged) is mechanically tested and

visually inspected. Subsequently the development of an in house created FDM 3D printer for continuous

reinforced polymers is explained in phase two. A composite material with para-aramid filament bundles as

reinforcing material and PETG as matrix material is printed with a tuned low-cost FDM 3D printer. An aramid

filament content of up to 56 wt% (53 vol%) can be obtained. With the Markforged technology, a material with

an estimated glass fibre content of up to 35 wt% (18.8 vol%) is produced. In academic literature the

achievement of 3D printed composites of 5 to 34 vol% are reported. Various reinforcing and matrix materials

are used. Repeatedly the challenges of porosities and impregnation are reported. During this study porosities

were observed with microscopy in the Markforged specimen and own printed specimen.

For an increasing fibre content from 0 wt% to 23.3 wt% the tensile modulus and strength increases

respectively from 1.3 GPa to 10 GPa and from 27 MPa to 288 MPa. The flexural modulus and strength is

increased respectively from 0.9 GPa to 6.0 GPa and from 41 MPa to 147 MPa. In literature, the highest tensile

strength and modulus values for 3D printed aramid reinforced polymer are 9.3 GPa and 203 MPa

respectively[30]. The commercially available 3D printed materials performed a tensile modulus and strength

of 9.3 GPa and 228 MPa respectively. Despite the great variance in materials and fibre content, the results of

this thesis project show a significant performance compared to the average continuous reinforced 3D printed

polymer. Conventional composites show a much higher mechanical performance than 3D printed composites.

Nonetheless the continuous reinforced 3D printing technology shows high potential to produce complex

functional parts of significantly higher quality than conventional FDM fast and efficiently.

From the experiments of current study the following insights on the microstructure of the 3D printed

composite are found. Impregnation quality and reinforcement-matrix bonding are more important for load

transfer during flexural loading, which explains why an increasing fibre content shows relatively less

amelioration in performance. In general for all specimens, the predicted stiffness is higher than experimentally

measured, from which is concluded that a significant amount of imperfections are present in the 3D printed

composite material. Through dynamic mechanical analysis the damping capacity of the specimens is measured.

As the impregnation of the reinforcing filaments is increased and the interfacial bonding is better, the damping

capacity is lowered due to less energy losses. Results for various sets of specimen confirm this theory and

further research is required. Finally a DCB test in load Mode I was introduced for continuous reinforced 3D

printed polymers which can characterize the interfacial fracture toughness. Compared to conventional

composites (average GI-value of 500 J/m²), a significantly high toughness was recorded (GI-value of 1400

J/m²). This result supposes that the material shows less hazard for delamination. Further conclusions from this

experiment require further research.

It is concluded that a success of printing a material with a reinforcing fibre content of 56 wt% is a significant

result amongst reported fibre contents. Suggestions for further research on this topic are to implement more

different reinforcing materials and improve matrix/reinforcements interface bonding. This could be done by

pre-processing (sizing action) of the filament bundles. Likewise the post-processing of printed specimens can

CONCLUSION 71

be further optimized. Looking further in depth to results from DMA could provide a better insight in the

microstructure of the materials. The novel DCB test can be further developed and subsequently utilized as

interlayer characterization technique.

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APPENDIX A: G-CODE 76

APPENDIX A: G-CODE The FDM 3D printer settings are controlled by a G-coded file. The most important commandos are listed in the

table below (Table 11). Furthermore, a screenshot of a program for a specimen is provided in Figure 72.

Table 11: Selection of G-code commandos important for the project[58].

Code Action

M140 S## Set the temperature of the print bed to ## °C

M104 S## Set the temperature of the extruder head (with printing nozzle) at ## °C

M190 S## Wait until the set value of ## °C for the print bed is reached

M109 S## Wait until the set value of ## °C for the extruder head is reached

GO Fff Xxx Yyy Zzz Move the extruder head with a speed of ff mm/min to location on the defined X- , Y-, Z-axis of the printer (xx,yy,zz)

G1 Fff Xxx Yyy Zzz Eee Move the extruder head with a speed of ff mm/min to location on the defined X- , Y-, Z-axis of the printer (xx,yy,zz) while extruding ee mm/min of feed filament

APPENDIX A: G-CODE 77

Figure 72: Fragment of G-code for 3D printing of an S_4 specimen

APPENDIX B: TABLES 78

APPENDIX B: TABLES Table 12: Reinforced FDM 3D printing technologies overview.

FDM Printer technology Matrix material

Continuous reinforcing material

Pre-treatment of the reinforcing material?

Maximum obtained fibre content

Reference

Markforged technology: pre-nozzle

impregnation of reinforcing material

Nylon 6 Kevlar®, Fibreglass, Carbon fibre - - [21,34–36]

Pre-nozzle insertion PLA Carbon fibre yes, in a methane

dichorlide solution

34 vol% measured using

micrographs with

calibrated pixel dimensions

Li et al. [29]

Pre-nozzle PEEK Carbon fibre - 5 wt% Stephashkin et al.[33]

Fibre encapsulated additive manufacturing

(FEAM)

ABS Copper wire (not as

reinforcement but to generate

printed circuits)

- - Cox et al. [14]

In-nozzle impregnation ABS Carbon fibre reinforcing material is

pre-nozzle heated

10 wt% Yang et al., Hou et al., Tian et

al.[22,23,28]

In-nozzle insertion of reinforcing filaments PLA Para aramid filaments - 8.6 vol%, 9.5 wt% Bettini et al. [13]

In-nozzle impregnation PLA Carbon fibre reinforcing material is

pre-nozzle heated

6.6 vol% Matsuzaki et al.[14]

Post-nozzle impregnation ABS Carbon fibre, fibreglass lay-up, via heated needle

or with thin solvent film

6 vol% for carbon fibre but

did not test it yet , 0.6 for

FG

Bauman et al.[25]

Post-nozzle impregnation under tension PLA Carbon fibre Curing epoxy agent was

applied to align the

filament bundle

- Yao et al.[26]


Table 13: Overview of existing research on continuous reinforced FDM 3D printing. (CF = carbon fibre, FG = fibreglass, PETG = poly(ethylene terephthalate glycol, PLA = poly(lactic acid), ABS = poly(acrylonitrile butadiene styrene), Markforged between brackets means that Markforged© technology is used for specimen production)

Research Materials Fibre content

Tensile strength

(MPa)

Tensile modulus

(GPa)

Flexural strength

(MPa)

Flexural modulus

(GPa)

Melenka et al.[34] aramid / nylon 6 (Markforged) 10.1 vol% 84.0 9.0 - -

Dickson et al.[21] CF / nylon 6 (Markforged) 10 vol% 198.0 8.5 - -

Van Der Klift et al.[41] CF / nylon 6 (Markforged) 18 vol% 464.0 36.0 - -

Current research FG / nylon 6 (Markforged) 18.8 vol% 228.4 9.3 144.2 7.0

Current research aramid / PETG 23.3 wt% 288.3 10.3 147.3 6.0

Matsuzaki et al.[14] pre-heated CF / PLA 6.6 vol% 185.2 19.5 133.0 5.9

Bettini et al.[30] aramid / PLA 9.5 wt% 203.0 9.3 - -

Li et al.[29] pre-treated CF / PLA 34 vol% 91.0 - 156.0 -

Tian et al.[23] CF / PLA 27 vol% - - 335.0 30.0

Yang et al.[22] CF / ABS 10 wt% 147.0 4.2 127.0 7.7

Bauman et al.[25] laid in FG / ABS 0.6 vol% 48.0 2.1 - -

Stephaskin et al. [33] CF / PEEK 5 wt% - - - -


Table 14: Specimen details for performed experiments.

Additional details: printing speed = 50 mm/min, LH = layer height, L= line spacing asdepicted in Figure 21. All S-samples consist of 4 layers (long beam-shaped specimen), all P-samples consist of 2 layers (plate-shaped specimen) and the DCB specimens consist of 4 layers with a delamination initiation between the second and third layer.

Specimen name

Colour code

Description LH (mm)

H(mm)

Lines per layer

wt% aramid

vol% aramid

l x w x t (mm x mm x mm)

Tensile test

Three-points bending test

DMA single cantilever beam

DMA tensile

DCB Mode I

S_0A blue non-reinforced PETG 0.3 0.4 6 0.00 0.00 110 x 3.9 x 1.9 x x x x

S_0B blue non-reinforced PETG 0.1 0.2 6 0.00 0.00 110 x 3.7 x 1.1 x x x x

S_2 blue 22.2 tex aramid/PETG, 0.4 0.5 6 4.70 4.17 110 x 4.1 x 2.4 x x x x

S_3 blue 22.2 tex aramid/PETG 0.3 0.4 6 5.25 4.66 110 x 3.9 x 2.2 x x x x





S_8 blue/ green

22.2 tex aramid/PETG 0.1 0.2 6 23.33 21.16 110 x 2.1 x 0.89 x x x x

D_1 green 40.5 tex aramid/PETG 0.2 0.4 6 23.98 21.77 110 x 3.2 x 1.2 x x

D_2 green 158 tex aramid/PETG 0.2 0.4 6 55.63 52.51 110 x 3.7 x 1.8 x x

P_0 dark blue

non-reinforced PETG 0.1 0.2 30 0.00 0.00 55 x 7.6 x 0.77

P_1 dark 22.2 tex aramid/PETG 0.1 0.2 30 21.26 19.23 55 x 7.1 x 0.71 x x


blue

P_2 dark blue

40.5 tex aramid/PETG 0.1 0.2 30 33.00 25.00 55 x 7.5 x 0.76 x x

P_3 dark blue

158 tex aramid/PETG 0.2 0.6 10 40.21 30.28 55 x 7.2 x 1.0 x x

P_1 pp orange 22.2 tex aramid/PETG 0.1 0.2 30 21.26 19.23 55 x 7.1 x 0.71 x x

P_2 pp orange 40.5 tex aramid/PETG 0.1 0.2 30 33.00 25.00 55 x 7.5 x 0.76 x x

P_3 pp orange 158 tex aramid/PETG 0.2 0.6 10 40.21 30.28 55 x 7.2 x 1.0 x x

DCB_1 purple 22.2 tex aramid/PETG 0.1 0.2 16 21.26 55 x 4.2 x 1.4 x

DCB_2 purple 22.2 tex aramid/PETG 0.1 0.2 16 21.26 55 x 4.1 x 1.4 x


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Characterization of continuous fibre reinforced polymers