July 24, 2018
Metal infused filaments are a really unique subset of materials that you can print with. From brass to stainless steel, they offer a lot of creative possibilities. Now, while they aren’t nearly as strong as an object cast in pure metal, they make for some great decorative or ornamental 3D prints. However, like most 3D printing materials, there is a level of post-processing to bring out those qualities. With most materials, you need to do it by hand with some elbow grease, but with metallic filaments, you can leave a lot of that work to a rock tumbler. However, keep in mind that this method is the “I don’t have a deadline” method.
Brassfill, Bronzefill, Copperfill, Steelfill, Magnetic Iron PLA, or Stainless Steel PLA are all metal-infused filaments, and as such rock tumbling will work really well on them. If you’ve never printed with a metal-infused filament, be sure to read our other article, How to Succeed When 3D printing Metal PLA
While most filaments are pretty sturdy after you print them and need quite a bit of force to break apart, metal-infused filaments are quite a bit more brittle. A 3D Model like Phil would do really well in a rock tumbler, but something intricate like a tree with branches would very quickly have them shear off. Even removing the tree from the 3D printer’s bed could snap them off if you aren’t careful. The main thing is avoiding models that have really prominent protrusions.
Simply enough, the cheapest rock tumblers are just motors that spin containers that you set on them; more advanced and expensive tumblers use vibratory motion to tumble the parts. You can find really cheap ones at Harbor Freight that we’ve used for years and have had to replace only the belts once when they snapped (which was a $3 replacement).
Once you have the tumbler, you’ll need two things for tumbling and some miscellaneous tools:
Media is used to fill in the dead space of the container that is tumbling so your rocks or 3D print aren’t rattling around inside. Media is small, specially shaped ceramic pellets designed to try and get into the various nooks of rocks, and there are different sizes of media too (small, medium, and large). I prefer using a mix of small and medium to fill in the voids of the container, and so they don’t have a lot of weight to thrash into the 3D print. You’ll want to have at least enough to fill each container (if you get a two container tumbler) just to make sure you have everything secure.
Grit is the actual material doing the polishing and smoothing of your 3D prints. You can find refill packs pretty cheap online, since grit is a disposable material. Grit works just the same as it does with sandpaper, except without the paper backing. You’ll progressively dump out, clean, and add a higher grit each time you move from one step of grit to the next.
First, make sure your tumbler container is completely clean. Even if it’s brand new, you’ll want to clean it out. Use a toothbrush and a very small amount of soap to clean it out. If it’s rested too long without being cleaned after a tumble, the easiest way to do clean out all the gunk is to fill it with water, drop in just a little dish soap, fill it about halfway with media and let it tumble for a while. Using this method I turned gray media back into its normal, clean, white color.
Once your 3D print is complete, clean off any strings, zits, or any other artifacts you want to remove. The tumbling process will help with these, but not nearly as much as a sharp knife and a delicate hand will.
Once your part is cleaned up, rest it in the container, then gently pour in the media around it. You want to make sure the container is filled up to the ⅔ level, with ¾ being ideal. With the media and your 3D print in the container, add water to just below the top of the media.
The first pass of tumbling uses the “Step 1” grit, which is somewhere between 60 and 90 grit. Most rock tumbling instructions recommend two level tablespoons per pound of rock, but that isn’t quite 1:1 with a 3D print, since a print with its infill is less dense than a rock. I recommend for most prints you’ll use four level tablespoons in the cup. With the grit added, close up the container, making sure it has a good seal, and turn on the tumbler for seven days. Yup, seven days. At the end of the seven days, check on the print and if it isn’t smooth enough, leave it in there for longer and check on it daily or every other day to see if the layer lines are gone. Once you are happy, set up your screen over the bucket and pour out the container. The gritty mud that is made will clog up your plumbing if you dump it directly down the drain. Make sure that every last grain of grit is off the 3D print and the container, otherwise if it remains into the next steps, you will have to start all over again to clear out the scratches. Use a toothbrush, a little dish soap, and some elbow grease to clean out the mud and the container will be as good as new.
The second, third, and fourth (and final) pass are all the same as step one, just stepping up the grit and making sure that all the previous grit is cleaned out.
While your 3D print may come out smooth after this entire polishing process, it may not be shiny. To make it shine, if that is the intended look, repeat the same process as before EXCEPT use ½ tablespoon of chopped “original” ivory soap (as recommended by rocktumbler.com) per pound (I use 1 tablespoon usually). This time, you don’t need to leave it for seven days; a couple hours is enough time.
Once everything is tumbled, polished, and burnished, display your piece prominently; you’ve put a lot of work into it. From here, you can always patina it or weather it using a variety of different chemicals to oxidize and accelerate the process, like turning a copperfill print green like the Statue of Liberty.
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