It should come as no surprise that I immediately deviated from my stated test plan and started using TestrBot to learn about the thing I was most curious about: The effect of solvent treatments on the mechanical strength 3D printed ABS plastic parts.
Acetone is a powerful solvent and when it is applied to ABS plastic the result is similar to what happens when you pour water onto a sugar cube. When the solvent is applied sparingly it will dissolve only the outer surface resulting in a smooth & shiny finish. Vapor polishing is nothing new, people have been doing this for years.
The general assumption held by most people up to this point is that exposing an ABS plastic part to acetone would increase its strength because the smoothed surface would have fewer print lines and micro-cracks (which cause stress concentrations).
But if you’ve been following my work then you know that I’m not one to rely on anecdotal experience, so I setup a little experiment.
And then my little experiment turned into a big one.
I also wasn’t content to use the established methods of applying acetone to the printed parts. The current vapor polishing techniques and their drawbacks include:
- Hot treat via boiling solvent in a crock pot or similar chamber. This presents both fume & fire hazards and doing it is a very hands-on process.
- Cold treat via slow release of solvent from paper towels in an enclosed container. This method is excessively time consuming and you can’t observe parts during the process.
- Spray canned solvent aerosol onto part which produces an inconsistent surface finish and must be done outside.
- Dipping parts directly into liquid solvents which creates an unpredictable/inconsistent finish and will likely over expose smaller portions of the part.
What I wanted is a device with the convenience of a kitchen microwave. Something safe that I can use indoors that will produce a repeatable result without a bunch of hassle. What I ended up building is an Ultrasonic Misting 3D Print Vapor Polisher, and I’ve created an instructable on how you can build one too!
This is the device I used to prepare the specimens prior to testing, as seen in this video:
I concluded that treating abs specimens with acetone vapor caused them to lose strength, despite having given the specimens a full 24 hours to dry out. Curious folks can check out the pdf file for detailed test results here: Acetone Vapor Test Results
This result was surprising. My somewhat speculative explanation for the resulting loss of strength is that the exposure to the solvent caused a permanent chemical change to the surface of the parts resulting in a softening effect which negated the positive effects of the reduced stress concentrations.
Looking at the inside of the parts and it is clear that the treatment did not penetrate very deep. It is interesting to note that the mass of the specimens was measured before and after treatment and right before they were tested; Specimens increased in mass by about 0.2 grams right after the treatment but they went back to their normal weight within 24 hours.
To be fair, this was not an exhaustive study. I tested a handful of specimens in a single orientation. But then again, my standard deviations were low so my experimental results are clearly repeatable.
All things considered, having an ultrasonic vapor treatment chamber could still be beneficial to anyone wanting to improve the aesthetic value of their 3D printed parts.
EDIT UPDATE: 7-4-16
I recently released an updated cleaned up version of this device, and to go with it I performed a dozen more tests on vapor polished parts.
I 3D printed ABS specimens in two different orientations, treated half of them with acetone, and broke them on the Testrbot to see how the orientation and acetone affected the printed parts.
The results were very interesting! (complete report here)
The new testing shows that acetone treatment has two measurable opposing effects on ABS FDM 3D printed parts:
1) A chemical weakening of the material structure
2) A mechanical strengthening of layer bonds via the reduction of surface stress concentrations. (This effect was anticipated but up until now it was never tested for.)
This testing has shown that effect #1 outweighs effect #2 to decrease the part strength by 9% in all stress conditions other than Z-axis loads, in which effect #2 outweighs effect #1 to increase the part strength by 31%.
Regarding effect #1, these new results agree with previous testing done in effect but not in magnitude. My hypothesis is that the additional drying time that I gave the new specimens helped remove all traces of acetone which may have contributed to additional softening of the specimens in the previous testing.
The overall effect of Acetone vapor polishing on ABS effectively makes parts somewhat more isotropic. That is, they react more uniformly to applied loads from various directions. In this case, polishing sacrifices strength in their strong axis to increase strength in their weak axis.