Colour Matters

Colour Matters

Have you ever wondered about the strength of 3D printed parts and how they behave under load? A while back I did some research to look at this focusing on the deformation behavior of FDM 3D printed parts compared to equivalent injection molded parts. It was during the DOE (design of experiment) portion of my work that I came across some very interesting findings relating to how material colour affected its strength. I learned that different colours of the same material had different strength properties. That’s right, forget about PLA vs. ABS or Nylon vs. Flex, colour matters far more than you may have thought.

When you think about how 3D printed parts are made it makes sense that they will behave differently under load than injection molded parts. 3D printed parts are made through a layered approach and usually have sparse internal structures, unlike their solid injection molded counterparts. In order to compare them I used a conventional testing method called a uniaxial tensile test where a mechanical device stretches a sample until it fractures.

In order to assure the accuracy of my results it was important to perform a design of experiment exercise to understand the factors affecting the tensile strength of the components. Part of this was to do a sensitivity analysis on several factors of the printing method, including: the printing settings (infill density and infill raster), orientation, as well as feedstock properties (such as filament vender, and filament colour). The idea is to understand which factors the tensile strength is sensitive to so that I can ensure the tests remain constant between samples. I remember during these tests I only included filament colour to be complete, but I did not think it would have any affect on the mechanical properties… I was wrong.

Below you can see an engineering stress vs. strain graph for the deformation of several 3D printed uniaxial tensile samples.

One way to think about this plot is that it is showing the load the samples exerted on the testing machine, (y-axis) as it was stretched a given amount (x-axis). The end of the data after it flattens out represents the point where the sample fractured as the load dropped to zero.

From the plotted data we can see that the Red and Black PLA had similar tensile strength, about 25% higher on average than the yellow PLA. Equally as staggering was the difference in ductility between the yellow PLA and the others. The yellow PLA had a strain to fracture on average over 60% higher than the others. Although there were not enough tests performed to be statistically significant, even this rough data is quite compelling.

What this shows us is that filament of the same kind but with different colours can exhibit different mechanical properties. The red PLA I tested was the most rigid but was not very tough or ductile. This would make it an ideal candidate for when I print part for my rep-rap as I need them to be very rigid. On the other hand when I print a camera mount to attach a digital camera to my snowboard I would choose to use the yellow PLA because it can be deformed quite significantly without fracturing. Pairing these two materials together however (with SEEM perhaps) you could imagine a composite structure with a rigid internal skeleton in a tough external matrix that gives you the best of both worlds.

This only scratches the surface of what the future of 3D printing has to hold for not only multi-colour but also multi-property prints. It was really surprising to have discovered how sensitive some mechanical properties can be to material colour proving that colour matters. Hopefully within the next few years material vendors will be able to provide specification sheets for their materials that include mechanical properties that we can take into account during the design phase.