Virtual Engineering for Rapid Product Development

PETRU BERCE, RĂZVAN PĂCURAR, NICOLAE BÂLC

Department of Manufacturing Engineering

Technical University of Cluj-Napoca

B-dul Muncii no.103-105, 400641, Cluj-Napoca

ROMANIA

Abstract: - The paper presents a complete case study focused on studying the accuracy of a part, starting from

the CAD model, to the finite part made by injection molding. The researches has been developed by National

Center of Rapid Prototyping from Technical University of Cluj-Napoca (TUC-N) and Plastor SA company

from Oradea, focused on the manufacturing of the active elements for injection molding tools by selective

laser sintering technology, using a metallic dedicated powder, Laserform St-100. There were revealed the

peculiarities and complexity of this technology, which involve CAD aspects and different issues regarding the

part manufacturing accuracy, correlated with the shrinkage phenomena that appears on the tools in post

processing stage in the oven. A theoretical model of the Selective Laser Sintering (SLS) contractions has been

developed. This research has been the first one in Romania, trying to implement and to test the use of rapid

tooling for injection molding. Experimental obtained results are also presented in the paper.

Key-Words: - Rapid Prototyping, Rapid Tooling, Rapid Product Development, Selective Laser Sintering,

Injection Molding, Finite Element Analysis

1 Introduction Automation of control code generation for the

desired requirements is an emerging research aspect

of Rapid Prototyping. Diane et al [1] classify RP

process parameters into nuisance, constant and

control parameters. Nuisance parameters include age

of the laser, beam position accuracy, humidity and

temperature, which are not controlled in the

experimental analysis, but may have some effect on

a part. Constant parameters normally include beam

diameter, laser focus, material properties, etc. The

control parameters will affect the output of the

process and are controllable in a run. These include

layer thickness, hatch space, scan pattern, part

orientation, shrinkage of the material, etc. Diane et

al [1] concluded that layer thickness, hatch space,

part orientation and depth of cure are the most vital

among the control parameters. Zhou and Hersovici

[2] established that layer thickness and the position

on the built plane are control factors of accuracy.

They employed Taguchi method to find the

functional relationships between different

combinations of control factors and part quality for

standard surface features. However, extrapolation of

these results to complex RP part surfaces is very

difficult. Thomson and Crawford [3] chose build-

time, surface finish and part strength for

manufacturing requirements and developed

numerical methods to quantify the requirements

with respect to the part orientation for the Selective

Laser Sintering process. A genetic algorithm has

been developed by Woodzaik et al [4] to

automatically place multiple parts in a workspace to

reduce build-time, and thereby, increase efficiency.

The parts are enclosed in rectangular boxes and are

rotated 90˚ about the z-axis to aid part packing,

without considering the surface accuracy and the

support structure requirements. Ablani and Bagchi

[5] developed a software system to find preferred

orientations. It rotates a part in increments about the

designer-supplied axes and slices the part to evaluate

the errors due to the stair-step effect. Due to the

complexity involved, most research work has been

focused on the optimization of a single required

parameter.

The research started within Technical University of

Cluj-Napoca and Plastor SA company from Oradea

[6] has proved the complexity of manufacturing

process by using SLS technology, to produce the

active elements for injection molding. There are

many important aspects that need to be taken into

account when speaking about accuracy of the SLS

parts. CAD compensation factors should be

estimated and used, in order to get a good

correlation between the accuracy of plastic

prototype and the injected plastic part, using molds

manufactured by SLS technology. This research is

the first one in Romania trying to estimate and

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correlate CAD model accuracy with the shrinkages

that occurs either, in the manufacturing process of

the plastic prototype parts on Sinterstation 2000

equipment, or especially during metal parts post-

process of the active elements, in the oven.

2 Virtual Engineering for Rapid

Product Development The part selected for the case study undertaken

within the research presented in this paper, is a lid

component of a grass cutting machine (presented in

Fig. 1), made within the Plastor SA, in co-operation

with the Brill company from Germany. The CAD

model of the lid, the punch and die were designed at

Plastor SA using the Pro-Engineer software. Some

shrinkage estimations could be made by using the

finite elements methods. (Fig. 2).

Fig. 1 Lid component of a grass-cutting machine

Fig. 2 The part prototype and the punch

2.1. Finite Element Analysis of Tools During

Post-Processing in the Oven The CosmosWorks module of the SolidWorks

software was used in order to perform the finite

element analysis. The 3D model was transferred to

the SolidWorks software CAD system and a plate

was added, which is necessary for the bronze

infiltration process, in the oven. The model beeing a

symmetrical one, only half of it was analysed, in

order to reduce the amount of the required

calculations. The results of the computations are

also valid for the second half. Theoretical

estimations of the thermal shrinkage has been made

using the FEA method, by taking into account the

following material properties: Young’s Modulus,

Poisson’s Ratio, Shear Modulus, Mass Density,

Coefficient of Expansion, Thermal Conductivity and

Specific Heat. These material characteristics of the

Laserform St-100 powder were taken from 3D

systems website (the manufacturer of the SLS

System) and have been introduced into the

CosmosWorks software [7]. Figure 3 illustrates such

a dialog box and presents the values of those

parameters.

Fig. 3 Material’s characteristics, used in FEA

The constraints have been established, in order to

lock the rigid body motions of the 3D model. The

translation movements were locked along the x, y

and z axes: One movement constraint on x axis,

placed on the infiltration plate, without influencing

the deformation possibility for the punch; Y – the

base is considered as being fixed; Z – the

movements within the symmetry plane were

blocked, as the model is a symmetrical one.

Figure 4 illustrates the constraints imposed to the

analysed part, along the x, y and z axes. The model

has been meshed using tetrahedral elements. A total

number of 9978 elements with 16315 nodes have

been generated.

Fig. 4 The x, y and z constraints of the model

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Finally, some estimation regarding the shrinkages of

the model after post-processing in the oven were

made (as illustrated in Figure 5), first of all in the

warm-up cycle and second of all in the cool-down

stage. The maximum estimated shrinkages in the

warm-up cycle were: 1.912 mm, on X-axis

direction, 0.4974 mm, on Y-axis direction, and

0.5696 mm on Z-axis direction. Following the same

steps of analysis for the cool-down stage, changing

only the thermal expansion coefficient 51055.1 −

⋅=α K-1

, which correspond to a classical

tool steel alloyed material, the maximum estimated

shrinkages on the 3 axis were, -1.880 mm on X-axis,

-0.4879 mm on Y-axis and -0.5557 mm on Z-axis.

As we observed the maximum estimated shrinkages

are on the X-direction in both cases.

The conclusion is that the shrinkages are quite high,

around 0.3 mm, mainly because of the post-

processing temperature in the oven, which is around

1070 degrees.

Compensation factors must be used to modify the

dimensions of the CAD parts, before these tools are

produced on the Selective Laser Sintering (SLS)

machine.

Fig. 5 Maximum deformations along the x axis

2.2. The Flow Simulation, Using the

Moldflow Software The Moldflow software was used to simulate the

molten plastic flowing to fill up the cavity. This

simulation provides also some information regarding

other aspects, such as: mold’s cooling down, stresses

and displacements of the final part. The part’s mesh

contains a network of tetrahedra, where the

calculations could be made for every node of the

network.

The part’s material is ABS Terluran 867 M, having

275°C as maximum melting temperature and 60°C,

as the recommended mold’s temperature. Other

purpose of the Moldflow simulation is to find the

optimal injection parameters. After establishing the

injection point and the temperatures for the mold

and molten plastic, a 3 coordinates diagram is

generated, as illustrated in Figure 6, where the small

red sphere represents the optimal injection

conditions.

Fig. 6 The optimal injection conditions diagram

These optimal injection parameters could be

displayed also like a table. The important parameters

for the analyzed part are: 70°C, as the optimal mold

temperature, 245°C, as the optimal molten plastic

temperature and 0.58 seconds, as the optimal

injection time. Other useful information obtained

from the Moldflow simulation software include:

cooling time before the part is pushed out from the

mold, in different points of the part, percentage of

the solidified material, during the filling up stage, in

different points, injection pressure variation or

estimated 3D contractions of the part, as illustrated

in Figure 7.

The estimated volumetric shrinkage were around 1,2

%, which correspond to approximate 0.1 mm in

ABS plastic parts case.

Fig. 7 Injected plastic part – estimated volumetric

shrinkage MoldFlow FEA analysis

Compensation factors must be taken into account in

order to modify the dimensions of the CAD parts,

before these plastic parts are produced on the

Selective Laser Sintering (SLS) machine. A

correlation between manufacturing of the prototype

part and the active elements tools scale factors on

the same selective laser sintering equipment is also

required.

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3 Experimental Research As we can observe from the schematic diagram

presented in Figure 8, when speaking about

manufacturing the prototype and the active elements

(punch and die) on the same equipment,

Sinterstation 2000, it is very important to have a

good correlation between manufacturing processes,

in order to have a good result in the final level, when

the plastic parts are manufactured by injection

molding [8].

Fig. 8 The correlation between manufacturing

processes (schematic diagram)

3.1 Part Prototype and Tool Manufacturing

by Selective Laser Sintering (SLS) The virtual models of the parts were transferred to

the rapid prototyping machine, via the *.STL files. It

was possible to manufacture on the same machine,

both the part prototype from plastic (Duraform

polyamide) and the active elements punch and die

from a dedicated metallic powder (Laserform St-

100). All these experiments were carried out at the

Technical University of Cluj-Napoca (TUCN),

within the National Center of Rapid Prototyping,

using the Sinterstation 2000 equipment (Figure 9).

Fig. 9 Sinterstation 2000 machine – Technical

University of Cluj-Napoca

The compensation factors were used within the

manufacturing process, in order to get the same

values, both at the prototype part made on the SLS

machine and at the injected part made by injection

molding, using the active tools made on the SLS

machine as well.

The most important technological parameters used

for manufacturing the part prototype and the tools

on the SLS equipment are presented in Table 1.

Table 1

It is important to mention the fact that if for rapid

prototype made by Duraform Polyamide on the SLS

machine a post-processing stage is not necessary,

while speaking about tools manufacturing, in this

case, after manufacturing on the machine, a post-

process stage is needed in the oven. (Figure 10)

While post processing in the oven, the tools went

through the following steps:

- Melting (burning out) the polymer (at 450°C -

650°C), which was surrounding the metal grains

- Get fully sintered metal parts, while increasing the

temperature to about 700°C

- Infiltrating with bronze, at about 1050°C - 1070°C

- Cooling down the parts (natural / slow cooling).

The applied scale factors for the tools are taking into

account the shrinkages that occur mainly during the

post-processing stage, in the oven.

Fig. 10 SLS post processing in the oven, TUCN

Parameter Plastic

prototype part

(Duraform

Polyamide)

Punch and

die

(Laserform

St-100)

Scale factors X=1.03180,

Y=1.03309,

Z=1.01800

X=1.02054,

Y=1.02144,

Z=1.00950

Fill laser

power

5 W 28W

Slicer fill scan

spacing

0.15 mm 0.08 mm

Powder layer

thickness

0.1 mm 0.08 mm

Manufacturing

temperature

183˚C 98˚C

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3.2 Finishing the Tools There were some dimensional contractions during

the SLS process and post processing. That is why, it

was necessary to do some finishing operations to the

punch and die. These finishing operations were

performed at SC Plastor SA, using the PNC 320

CNC lathe and some hand tools for finishing, in

order to obtaining a perfect close of the active

elements, as presented in Figure 11.

Also, there were necessary some other machining

operations (turning, milling, grinding, drilling) to the

plates, in order to get a correct positioning of the

SLS active elements into plates.

Fig. 11 SLS punch and die positioned into plates

3.3 Injection Molding The tests of the SLS tools behavior were made at SC

Plastor SA, using a Krauss Maffei 90/340 A

injection molding machine. The punch’s plate was

fixed onto the mobile assembly of the injection

molding machine and the die’s plate onto the fixed

assembly of the injection molding machine, as

illustrated in Figure 12.

Fig. 12 Injection molding machine (Plastor SA) and

SLS punch and die (made at TUCN), at work

The injection molding tests were made using both

ABS and polypropylene materials. A couple of the

injection molded parts (using the SLS tools) are

presented in Figure 13.

Fig. 13 Injection molded parts, using the SLS tools

3.4 The SLS Accuracy

Measurements were taken in the Regional Center of

Industrial Metrology from Technical University of

Cluj-Napoca, using a Zeiss-Eclipse CMM

equipment (as illustrated in Figure 14)

Fig. 14 Measurements taken by the Zeiss Eclipse

CMM (TUCN)

By having the possibility of manufacturing the

plastic prototypes on the Sinterstation equipment, it

is very easy to check if there are errors at CAD stage

and correct them before producing the molds. The

tables 2-4 present the results we obtained. It can be

seen that there are differences between the CAD

dimensions and the parts manufactured by SLS.

Table 2

CAD model Plastic injected part

De = 56.6 mm

Di = 54 mm

de = 15 mm

di = 13 mm

Dem = 56.5 mm

Dim= 53.72 mm

dem = 15.04 mm

dim = 12.93 mm

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Table 3

Table 4

Referring to the plastic prototype and the injected

part using molds manufactured by SLS (Figure 16),

we can conclude that the differences between

prototype and injected parts are around 0.1 mm.

This value corresponds to the shrinkage value that

occur, either while manufacturing on SLS

equipment or because of plastic material shrinkages

during the injection process.

Fig. 16 Injected part, plastic prototype and active

elements (punch and die) manufactured by SLS

When speaking about the dimensions of the active

elements of the moulds as compared to CAD

dimensions, it is important to notice that the

shrinkages are about 0.3 mm. The shrinkage is

higher at the moulds manufactured by SLS, as

compared to plastic prototype shrinkage, mainly

because of the post processing stage needed in this

case, when at 1070 C degrees, the bronze is

infiltrated.

4 Conclusion The research started within Technical University of

Cluj-Napoca and Plastor SA company from Oradea

has proved the complexity of manufacturing process

by using SLS technology, to produce the active

elements for injection molding. There are many

important aspects that need to be taken into account

when speaking about accuracy of the SLS parts.

CAD compensation factors should be estimated and

used, in order to get a good correlation between the

accuracy of plastic prototype, and the injected

plastic part, using molds manufactured by SLS. This

research is the first one in Romania underlying that

compensation factors should be used, in order to get

a good correlation between the plastic prototype, the

active elements (punch and die) and the final part

made by injection moulding into the active elements

made by SLS, in order to fit the dimensions of all

these, with the designed CAD dimensions.

References:

[1] A.S. Diane, K.-R. Chu, D.C. Montgomery,

Optimising Stereolitography throughput, Journal

of Manufacturing Systems 16(4), 1997, 290-303

[2] J.G. Zhou, D. Hersovici, Parameter tuning and

optimization for SLA Rapid Protoyping

manufacturing processes, Proceedings of the

International Conference on Manufacturing

Automation (ICMA’97), Vol.2, 1997,pp.894-902

[3] D.C. Thompson, R.H. Crawford, Computational

quality with orientation, Proceedings of the Solid

Freeform Fabrication Symposium, 1995, pp.362-

368

[4] J.R. Woodzaik, G.M. Fade, C. Kirschman, A

genetic algorithm for optimizing multiple part

placement to reduce build-time, Proceedings of

the 5th International Conference on RP, 1994

[5] M. Ablani, A. Bagchi, Quantification of errors in

Rapid Prototyping processes, and determination

of preferred orientation of parts, Transactions of

the North American Manufacturing Research

Institution of SME, Vol. 23, 1995, pp.319-324

[6] Bâlc N., Berce P., Păcurar R., Active elements

tools made by selective laser sintering,

Proceedings of the 8th ESAFORM Conference on

Material Forming, vol.2, 2005, pp.715-718, Cluj-

Napoca, Romania

[7] Bâlc, N.; Berce, P. & Păcurar, R., Injection

Molding Tools Made by Selective Laser

Sintering, Society of Manufacturing Engineers,

Rapid Prototyping Journal, Vol. 12, no.3/2006,

pp.1-7, SUA. (www.sme.org/rtam ).

[8] Bâlc N., Berce P., Păcurar R., Barisic, B., Using

the CAD compensations factors to improve the

accuracy of the SLS parts, Proceedings of the 8th

International Conference Modern Technologies

in Manufacturing, 2007, pp.43-46, Cluj-Napoca,

Romania

Punch

CAD dimension Manufactured model

on SLS

Interior diameter

Di = 56.6 mm

Exterior diameter

De = 70 mm

Interior diameter

Di = 56.31 mm

Exterior diameter

De = 69.75 mm

CAD model Plastic prototype

part

De = 56.6 mm

Di = 54 mm

de = 15 mm

di = 13 mm

Dem = 56.43 mm

Dim= 53.6 mm

dem = 14.95 mm

dim = 12.85mm

WSEAS International Conference on ENGINEERING MECHANICS, STRUCTURES, ENGINEERING GEOLOGY (EMESEG '08), Heraklion, Crete Island, Greece, July 22-24, 2008

ISBN: 978-960-6766-88-6 200 ISSN 1790-2769