Statistical Analysis of the Tensile Properties and Effect of Fill Density in Various Polymers

Processed through 3D Printing

Luke Buckner, Oak Ridge High School

Hahn Choo, Peijun Hou, Yuan Li & Zane Palmer

Abstract –

The polymers written about in this paper are PLA (Polylactic acid), ABS

(Acrylonitrile butadiene styrene), and Bio-ABS. They are processed using the FDM

technique on a Lulzbot Mini following the ASTM (American Society for Testing and

Materials) standards. After the polymer specimens were printed they were tested by being

pulled until fracture on an Instron APEX 60UD. The data obtained by doing this test was

then used to calculate the numbers used to create an Engineering Stress-Strain curve for

each polymer specimen. These curves were than used to determine the Modulus of

elasticity and the ultimate strength of each polymer. Then the data of the varying fill

densities was reordered in order to see if there was a seemingly linear relationship for

ultimate strength and elastic modulus with fill density.

I. INTRODUCTION {all caps}

– The importance of the project was to enable me to obtain an understanding of the concepts

of material testing. To gain knowledge of the terms in the field and why the data found is so

important to the field. To learn how to test materials for their mechanical (tensile) properties

while following the ASTM (American Society of Testing and Materials) guidelines for the

given material. To utilize my prior knowledge of CADing in Solidworks to the design the

specimen to the ASTM specifications.

II. LITERATURE REVIEW

– Additive Manufacturing is a new processing method developed for creating a 3-

Dimensional object by adding layer upon layer of material whether it is a polymer, metal,

concrete or even human tissue. The main techniques used in Additive Manufacturing are,

SLA (Stereolithography), FDM (Fused deposition modeling), MJM (Multi-jet modeling),

3DP (Three dimensional printing), and SLS (Selective laser sintering). Additive

Manufacturing has begun to be utilized in many fields including aerospace, automotive, and

even bio-medical. Back in April 2011 Boeing revealed the first flight of the Boeing Phantom

Ray, a stealth un-manned combat air vehicle, which was developed in Skunkworks using

rapid prototyping and iterative design utilizing additive manufacturing both SLS and FDM

(for models). In the automotive industry, AM (Additive Manufacturing) has opened up doors

to newer lighter and stronger car designs while also decreasing the cost.

The ASTM Standards D 638 – 03 “Standard Testing Method for Tensile Properties of

Plastics”, explains the proper way to testing various polymers for the tensile properties both

reinforced and unreinforced plastics. It describes the dimensions of each test sample

depending on the type of polymer being tested as well as how to set up the test in order to

obtain the most accurate results. It also explains the process of watering down the data to

obtain the elastic modulus and the ultimate strength and maximum elongation.

III. METHODOLOGY

-In order to test the polymers for the tensile properties, the test specimen was first designed in

Solidworks following the ASTM standards for tensile testing polymers using the Type 1

specimen dimensions. The CAD file was then converted into STL and sliced to create G-code so

that the printer could print the mechanical testing specimens.

The specimens were scaled down 75% to print decent sized specimens at a decent rate. The

first specimens printed by each polymer (PLA, ABS, and Bio-ABS) had a 25% fill density with a

1 millimeter top and bottom layer as well as a 1 millimeter shell encasing the specimen. After

fifteen of each specimen at 25% fill were printed I proceeded to print ABS specimens at varying

fill densities (20%, 40%, 60%, 80%, and 100%) printing five of each fill type.

After all the printing was finished I went about testing each polymer on the Instron

APEX 60UD with a strain rate of 5.6 * 10-4 with the temperature conditions ambient. All samples

were pulled to fracture and each test varied from two to three minutes. The data recorded by the

Instron was then saved as an excel spreadsheet so that the calculations for each specimen could

be done with relative ease. The strain was calculated by dividing e displacement at any given

time by the gauge length of that specimen. The stress was derived by the load at any given time

by that specimen’s cross - sectional area. Those values where than used in a program called

Origin Pro as the x and y coordinates for the Engineering Stress-Strain curve for each test

specimen. I then derived the elastic modulus of each material by finding the linear fit for the

linear set of data points and took the slope of that line as the Elastic modulus of that material.

IV. RESULTS

– The top figure is a representation of the data made in Origin 8 software. This compares the

Stress Strain Curves of the three polymers tested in this project. The bottom figure shows the

elastic modulus of the three polymers side by side.

- This top bar graph shows compares the ultimate strength and maximum elongation of the

three different polymers the graph below shows the ABS stress strain curves at varying

densities

PLA ABS Bio-ABS0

5

10

15

20

25

30

35

31.483776

17.953034 18.658038

2.4 4.41.8

Ultimate Strength & Maximum Elon-gation of Various PolymersUltimate

Strength (Mpa)

Max Elongation (mm)

- The top graph has the three data points of the average ultimate strength at each fill

density and it shows that there seems to show that ultimate strength is linearly

proportional to fill density. The bottom graphs has three data points of the average elastic

modulus at each fill density and it shows that there seems to be a linear relationship

between elastic modulus and fill density.

-The rest of the graphs show the stress strain curves of each specimen in each polymer category

V. DISCUSSION

– The testing of the three polymers and using 3D printing as the way to process the material was

a success. The testing of the 3D printed specimens brought to light the tensile properties of PLA,

ABS, and Bio-ABS. I found that of the three polymers PLA exhibits the highest strength under

the given processing and conditions. And that ABS and Bio-ABS have almost identical strengths

and properties. This is promising for Bio-ABS since is looking to be the future more

environmentally friendly polymer to replace ABS. Since ABS is petroleum based it is not

biodegradable in landfills and bad for the environmentally conscience during its production.

While this specific Bio-ABS is 95% composed of recycled materials and can biodegrade in a

landfill. Also the effect of fill density clearly shows that elastic modulus and ultimate strength

increased linearly with fill density.

VI. CONCLUSION

-I could continue to test more test specimens to see if there is any greater variation, and then do a

more in depth statistical analysis into the polymers mechanical properties. I could also throw my

results up against the Solidworks simulation and see how it compares up against my hard data. I

would also find it interesting to look into the fill patterns effect on the strength of the polymer

and the overall elongation of the part and maybe look into bio-inspired fill patterns.

ACKNOWLEDGEMENTS

– I would like to especially to thank Dr. Choo for all of his guidance and help, and then his

amazing students Peijun Hou and Yuan Li. I would also thank Zane Palmer and Chris Wetteland

for teaching me and allowing me to use their lab space and printers to conduct my research and

tests. I would also want to thank Erin Wills and Chen-fei for accepting me into this program

allowing me to enjoy a great learning environment and place to learn and understand how to

conduct proper research.

REFERENCES

– http://dimensionpolymers.com/store/

- ASTM Standards D 638 – 03- http://additivemanufacturing.com/