Sunday, October 19, 2014

Acetone Vapor Baths for Smoothing 3D Printed Objects

Recently for my job, we've been developing some 3-D stimuli for children's psychology experiments. We found a 3-D printing service for rent at the university, and it works great, but the texture on the finished pieces is a bit distracting. In our case in particular, weird object textures might actually affect experimental results. We want to make the texture smoother, but without spending hours with sandpaper. So we decided to try out an acetone vapor bath solution.

This is what the initial texture looks like from my university 3-D printing service. It holds decent detail, but the lines of plastic laid down are still visible, and that texture is going to show through even if painted.


We can address this by exposing the piece to acetone vapor, which melts / reacts with the ABS plastic the 3-D printer uses, leaving a glassy finish. You can see the basic setup below. A large mason jar has a couple of tablespoons of acetone in the bottom of it. It is then also sitting in a hot water bath, which inceases the vapor pressure and speeds the reaction. I microwaved my water until it was boiling. The piece was then suspended from the lid inside the jar to bathe in the acetone vapors without touching the liquid directly:


It is very important that an airtight seal be achieved, one that is strong enough to withstand pressure inside the jar. In the above images, I was trying to use some paint to seal the gap where the screw goes through, but that didn't really work. Later I tried a similar setup with clay as a sealant, and it worked perfectly. This following setup also worked, using super glue, although I don't know how it will stand up long term to the acetone (I switched to clay after just two runs):


The yarn here is wool. If it were acrylic yarn, there's a pretty good chance it would have melted in the acetone fumes and dropped my piece into the drink. Paper clip chains would have worked, too. Double check that everything in your jar is acetone-proof, or test it with a junk piece (which you should probably do anyway), before you rely on it for something you care about.

When the seal is air tight and the water is hot enough, you should see acetone condensation on the side of the jar. That's an indication things are going quickly enough and that the seal is good. If the seal is leaking, you should be able to actually hear hissing if you listen closely, and won't see much condensation.


Here you can see it melting the surface. In the first image, notice that the bottom portion is smoothed out, but texture remains on top. In the second image, there is still some texture on the very inside top, but even that is looking glassy, and all the edges are done:



When the piece looks sufficiently smooth to you (up to an hour for me, and maybe a change of re-heated water bath), take the jar out of the bath and place it in a sink. Now, fill up a glass of water, and have it ready (or the faucet), THEN remove the lid just enough to be able to douse the piece in water.

Don't let it touch the sides of the jar yet, until it is doused. The plastic with acetone still on it is very sticky, like warm caramel, and it will stick to the walls and leave ugly marks on your piece. After dousing with water, it's much more resilient. Enough so to pull out and hang somewhere to dry.

The piece will also attract dust while it is "curing." If you have a spare jar available, you might want to reserve one for drying purposes, to cut down on dust that permanently sticks to the surface. I hung my pieces from the corner of the drop ceiling in my office, and they avoided dust pretty well.


Below is the finished piece. For about another day or so, it will be soft and easily damaged. Not so soft that you will leave fingerprints, but enough for a nail to easily scratch it. The dotted line I drew here was from minimal pressure with a bic pen. If you can, it's best to just leave it hanging somewhere and not touch it at all for a full day.

Notice that the corners definitely get gooey and rounded, so if you need precision, this is not for you. It's an aesthetic thing, like if you're 3D printing a piece of an artwork or similar. (Note: the side corners near the slots in the photo here were actually damaged by the piece being slightly too big for the jar, not by the process).



Here's another piece that shows how easily blemished fresh plastic is out of the bath - I was hanging it by the eye socket there, and just the weight of the piece itself pressing against a metal screw left that gouge mark. So be careful how you hang your piece!


If you prefer, and if you have a big enough glass container, you can also lay the piece into a platform in the bottom instead of hanging it. A piece of wood with nails works well - balance the piece on the tips of 3  nails, and the marks left behind will be minimal. Keep two things in mind, though:
  1. The distance from the piece to the acetone will affect how quickly it melts the top versus bottom relative to one another, because fumes are thicker near the liquid and the water bath.
  2. Try not to leave any large surface flat on top. Tilt it slightly instead. This prevents acetone pooling from condensation and leaving blisters on your piece. You can also do this when hanging the piece, by hanging it slightly off center if possible.

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