Make It or Break It, Now in Color: The effects of colorants on the strength of 3d-printed PLA polymer objects

Make It or Break It
Now in Color:

The effects of colorants on the strength of 3d-printed PLA polymer objects

Acknowledgments

Dr. Dan Fernandez, professor of Physics and Environmental Science at California State University of the Monterey Bay, for the initial idea of studying 3d printed objects, materials, and fill patterns.

My father for the use of the Solidoodle 2 Pro, printing the test objects, for helping build the testing instruments, and for his assistance in conducting the experiments.

My mother for help with proofing and editing.

Fitness Evolution Health Club, Monterey, for allowing me to conduct my compression tests on their equipment.

Dr. Paul Strivers for steering me to read about polymers.

Mr. James Arao for providing tips on an engineer’s perspective of this study.

BFS Landscape Architects for the use of their plotter in creating my presentation board.

Abstract

Motivated by findings in a previous science project about the strength of different 3D printer filaments, this study explores the effect of colorant on the strength of polylactic acid (PLA), a biodegradable filament made from sugar cane, corn or yuca root. In the earlier research, it was found that objects printed with PLA and with the honeycomb fill pattern demonstrated greater durability in stress tests. In the process of researching, it was noticed that objects printed with yellow PLA filament broke more easily than those printed with natural color filament, which gave the impression that printed objects of different colors may react differently in strength tests. For that reason, the earlier study tested white printed objects only. What would happen if objects of different colors were put through strength tests?

Introduction

3D printing remains a growing technology for home and business use. The use of color in the manufactured prints adds variety and allows consumers to be more creative with their print ideas. Color is added to filament prior to extrusion. The granulated PLA (C3H4O2)n resin is blended with an organic pigment, then melted and pushed through a single screw extruder (Smith, 2015). Organic pigments such as azine dyes are widely used today in the making of 3D printer filament.There is not a lot of information available to users to explain why a filament of one color would be weaker than a filament of another color. Frequent checking on Makerbot and Soliforums (3D printing companies with online forums for problem-solving) brought no answers on the subject. Considering that pigment is regarded as an additive and an additive can make a difference in the mechanical properties of plastic, it may be worthwhile to study the behaviors of different color filaments under stress to note differences. Consumers may be interested to know which color filaments are stronger than others.

Although there are no clear explanations available as to why colored filaments may have different strengths, there have been studies of the behavior of plastics in general with colorant and a study that specifically tested the behavior of colored PLA (Evans 2014). As the trend toward organic dyes took over in the last decades, a study done by Kanu et al (2001) looked at the differences between the organic and inorganic pigments. They tested plastics colored with inorganic and organic pigments to determine differences in strength tests. They wanted to understand the effects additives like colorant had on the strength and durability of plastics. Along with the discovery that there was little difference in the behavior of plastic colored with either organic or inorganic colorants, they also found an increase in mechanical properties of plastics colored with organic pigments. In response to the move from inorganic to organic dyes, Christie (2010) asserted that organic pigments could create a tear point in materials and limit performance in some applications such as tensile tests. ”Certain combinations of pigments and dyes can result in a phenomenon called phototendering, in which products lose strength and flexibility from exposure to sunlight” (Christie, 2010 p. 3). Evans (2014) tested colored PLA printed from RepRap printers which are comparable to commercial printers. She also suggested that color was a variable among others that affected the mechanical properties of 3D printed plastic. Testing the elastic modulus in tensile tests with different color beams, she found the natural color beam to be the most rigid, while black performed better in strength tests. She commented that the “tuning” of the printer might have been a variable.

What effect does colorant have on the structure of objects printed with PLA in the Solidoodle2 Pro? For this study, I asked Dominic at Solidoodle Support which colors were the most popular among consumers buying 3D printer filament. Using a Solidodle2 Pro, a 3D printer, PLA filament was tested in the popular colors using a similar method of strength testing as used in a previous research where the strengths of PLA and ABS were compared, with the focus now being the effect that colorant has on the behavior of the filament under different stresses.

PLA was chosen because in the previous research project, it outperformed ABS in strength tests. The honeycomb pattern was used because it proved the most durable compared to other fill patterns. The colors tested were blue, white, black, yellow, and natural. The strength of the colored plastic were tested with the following forces:

Flexure– measuring the flexibility of the object
Shear– finding the breaking point of the object
Torque/torsion– measuring the behavior of the object when twisted
Tension– observing what happens when the object is pulled in opposite directions
Compression– examining how plastic test objects behave under a lot of pressure

Question: Does color affect filament strength? What effect does colorant have on the structure of plastic? Of colors used for PLA (C3H4O2)n 3D printer filament which color is strongest?

Hypothesis: Filament with no dye or added color (natural) will out perform the plastics with color in stress tests. Of the colored filaments, yellow will be the weakest and white will be the strongest.

Independent Variable: The colors of the plastic objects

Dependent Variable: The force needed to break the objects

Controlled variable: The colors, shapes, densities of the objects and all the filaments used for printing are PLA-based.

Materials

  • 2in C clamp
  • 1in wide ratcheting tie down
  • 2in wide 1/8in thick Aluminum sheet cut into 12in lengths
  • Four 2in corner brackets
  • Two 2 and 1/2in corner bracer
  • Two 3 and 1/2in corner braces
  • Twenty 3/8in flat head screws
  • Two 5/8in flat head screws
  • One sheet of Plywood 3/4in thick 16×14
  • One set of vice grips
  • One fish scale
  • Solidoodle 2 Pro 3D printer
  • PLA 3D printer filament
  • 4x1in mending plate
  • slotted angle 1.5in x 14ga x 48in

Procedure:

Print ten sets of test objects in three shapes: rectangular (1cm x 1cm x 8cm), rectangle with circles on each end (dumbbell shape with two cylinders 1cm deep and of 1cm radius joined by a 1cm x 1cm x 3cm rectangular prism), and half cylinder (1cm deep 1cm radus). All shapes should have a honeycomb fill pattern, and each set will be printed each of five different colors: blue, black, white, yellow, and natural. The fill density should be 40% with three vertical shells for all tests but shear and compression, for which the density should be 15% with two vertical shells. Objects are printed at 195ºC on the first layer at 190ºC on the rest of the layers, except that this particular white PLA will have to be printed starting at 185ºC and dropping to 180ºC. The bed temperature is 60 degrees.

Test Procedures

Flexure

  1. Mount metal brackets and fix C-clamp to one end of the test object
  2. Insert test object into Metal Brackets
  3. Hook fish scales through space in C-clamp
  4. Pull slowly and consistently on fish scales until object breaks
  5. Record the highest reading from the fish scales

Tension

  1. Loop one end of first nylon strap through knot onto carriage bolt fixed to end of slotted angle.
  2. Loop knot at other end of same nylon strap over bulge at one end of test object.
  3. Loop knot at one end of second nylon strap over bulge at other end of test object.
  4. Loop other end of second nylon strap around fish scales.
  5. Connect fish scales hook to hook at end of ratcheting tie-down.
  6. Hook ratcheting tie-down’s second hook into a hole in the slotted angle.
  7. Tighten the straps by ratcheting the tie-down until the object is pulled apart.
  8. Record the highest reading from the fish scales.

Shear

  1. Slide rail until two sets of corner braces are aligned.
  2. Insert first test object between corner braces, spanning mounted plate and sliding plate.
  3. Attach fish scales hook to bolt at end of sliding plate.
  4. Pull slowly and consistently on fish scale along path perpendicular to test object, away from test object, so that the rail slides, attempting to move half of the object away from the other half and eventually breaks the object.
  5. Record the reading on the fish scales from the moment of the break.

Torsion

  1. Place the test object between metal brackets at edge of board so that round end hangs over the edge.
  2. Attach the vicegrips to one end of the object, firmly pinching but not crushing the flat sides of the overhanging rounded end of the object, perpendicular to the object.
  3. Hook the scale to one end of the vicegrips and pull, causing the rounded end of the object to twist in relation to the rest of the object.
  4. Record the highest reading from the fish scales.

Compression

  1. Set weight selector on leg press at a weight below expected break point.
  2. Apply force, as in exercise, to raise the stack of weights.
  3. Place test object on exposed, unraised weight’s upper surface.
  4. Slowly, lower stack of weights onto test object and note if it is crushed.
  5. Repeat steps #1 through #4, increasing the weight applied, until the object breaks.
  6. Record weight under which object breaks.

Results

I recorded my observations of the stresses applied to each object in the tables below. All values are in Newtons. Altogether 130 tests were conducted over a period of two weeks. According to the tables the yellow PLA showed the highest degree of strength in all of the tests.








Discussion: Strengths and Limitations of the Study

The strengths of this research were that I learned that the additives in filaments include more than just color. The additive affects the mechanical structure of a 3D printed object. I was able to find that the white filament contains an additive that is another type of plastic, yet it was weaker compared to the prints of other colors. My findings confirmed that color makes a difference in the mechanical properties of the printed objects.

The research is not free from limitations, however. Due to technical issues with the printer, the printing of objects was sometimes put off for weeks. I also found it difficult to control how prints themselves were realized. The prints are not uniform. This is because of the machine itself. Some prints are sloppy and noticeably more brittle. The variables might have been better controlled if I had used a professional testing instrument which would also have provided more detailed and consistently accurate information. In the tension test the ratchet often prevented me from seeing the whole number. I was only able to record the number before the decimal. I might have found clearer results if I had found a way to limit the amount of friction in the shear test. A more mechanical apparatus would have minimized the human element. The environmental variables (time of day of printing, temperature of the room, etc) were not controlled, even though most prints were completed during the afternoon to evening out of direct sunlight. I might have been able to eliminate some potential inconsistencies if I had printed all of the objects at the same time of day. A larger sample size would have made it possible to have more statistical inferences.

Conclusion

This study found that, contrary to my hypothesis, the “natural,” uncolored PLA was not the strongest in any of the stress tests, rather, the yellow selected for the study was consistently the strongest. This does indicate that the colorant makes a difference in the strength of printed objects

I set about this research project wanting to find the effects of colorant on the PLA polymer for 3D printers. This research proved that that color causes differences in the mechanical structure of the plastics; in fact the color sometimes serves to strengthen the plastic. The colorants vary by chemical makeup and are additives which cause changes in the mechanical properties of plastics with which they are blended. I observed that each individual color behaves differently with PLA. One example is the fact that the white filament melted differently than the other colors and was messy and gummy when printing. It turns out that the white filament is colored and reinforced with another plastic which is why it behaves differently in extrusion. Most of the other dyes are azo dyes and there are differences in the way that the colorants “fix” or do “not fix” to the polymer. Another factor that is beyond the consumer’s control is the way that the filament is manufactured. The temperature of the water used for cooling during the process can determine the strength of the filament or the objects printed (Smith 2015). Consumers may be interested to know that it is not just a question of colors. There are other variables that may affect the strength of the object. These questions of manufacturing differences do not come into play in this specific study, as my PLA was all ordered at one time and of the same “Elephant” brand.

Further research is strongly suggested taking other variables into account. Correlation studies between different printers with the same filament may also prove helpful.

Resources

Crawford, R.J., Spence, A.G., Silva, C. (1992). “Effects of pigmentation on the Impact Strength of Rotationally Moulded Polyethelene”. Retrieved, Jan 5, 2015. http://www.centroinc.com/Handler.ashx?item_D019F21F-0FBF–AB0F–E92F0635D60F

Christie, J. (2010) “Tips and Techniques: The Art Science Behind Getting the Color Right” Plastics Technology, Oct. 2010 Issue. Retrieved November 2, 2015. http://www.ptonline.com/articles/tips–and–techniques–the–art–science–behind–getting–the–color–righ

Evans, C. (2014). “PLA Color Comparison: Color vs. Strength”. Retrieved, Jan 5 2015 from http://blog.fictiv.com/posts/PLA–color–comparison–color–vs–strength

Kanu, R.C., Chesebrough, M. Spotts, T.H., (2001) “The Effects of some Organic and Inorganic Pigments on the Tensile Impact Properties of Injection-molded Polypropylene.” International Journal of Modern Engineering, vol 2 No. 1. Retrieved Dec. 28, 2015. http://ijme.us/issues/fall2001/articles/polypropylene.htm

Naitave, M. (2008). “Additives and colorants score dramatic advantages’. Plastics Technology Magazine. March 2008. Retrieved Feb. 16, 2016 www.ponline.com/articles/additives–and–colorants–score–dramatic–advances

Price, H. (2002). “Cellulose textile dyes”. Retrieved Feb. 16, 2016 www.chm.loris.ac.uk/webprojects2002/price/cellulose.htm

Royte, E. (2006). “Corn Plastics to the Rescue” . Smithsonian Magazine. pp. 1-4. Retrieved Sept. 14, 2014 from www.smithsonianmag.com/science_natur/plastic.html

Smith, J. (2015) “Makergeek.com” Taped interview February 11, 2015, Retrieved Dec. 11, 2015. https://www.youtube.com/watch?v=OEkksADFjP8