by CosmoWenman, published
Cosmonaut figure as Lucifer, the Lightbearer.
See my video for an overview of casting metal figure directly into printed ABS mold: http://youtu.be/ZvKtsrt9kTo
I'll try to write these notes out in a tidier format soon, but I'm totally high on molten bismuth fumes at the moment, so it's stream-of-consciousness for now:
[UPDATE: I have to point out that all the low temp "cerro--" alloys have lead and cadmium, and are toxic. You need to use proper ventilation, and keep it away from food preparation areas. In fact, I'm going to be looking into "Field's metal", which has neither lead or cadmium, and I'll report back how well that works in ABS. The objects you make won't be suitable for food preparation or food containers or utensils of any kind, and probably not for kids' toys either. FWIW, I've seen several gunsmithing how-to videos that don't mention toxicity of the Cerro-lines at all, and they aren't using ventilators either, but they should. For all I know it gives off gamma rays too. Here are some Material Safety Data Sheets I found: http://www.bendalloy.co.uk/Cerrosafe.pdf
And some usage instructions, with no mention of ventilation or toxicity in normal use:
Use at your own risk.
If you want to try these molds with friendlier materials, you might try casting simple plaster of Paris in the "Lamassu" (winged bull creature) mold. That one is a thin, break-away mold: no solvents required. See the photo I've added.]
Casting bismuth alloys directly into printed ABS molds:
I've uploaded the .stl of the mold shown in the video, as well as my gcode and sg3 files. With these the finished figure is about 2.25 inches tall.
I have printed this figure a bit larger with much better results - the bigger it is, the better the detail-to-defect ratio. It just consumes a lot of metal - and I ruined the bigger casting trying to grind the plastic off.
The mold envelope of the "Mold" stl is much more bulky than necessary, and wastes plastic and print-time, and dissolve-time. I just haven't had time to make a more close fitting one. I've included the figure's "positive" stl too, if you want to redesign the mold shell (or just print the figure in plastic).
This process is related to lost wax casting, but the "manufacture" of the original object happens onscreen, and the wax burnout step is skipped completely. In theory, any print-problematic overhangs should translate into air pocket problems, but at the small scale I've been working at, I haven't had any problems; the metal is thin and heavy enough it's displaced all the air pockets no problem - just a little tapping here and there. If you work at a larger scale, or have air void problems, you may have to start printing in several pieces, or using vents and sprues, olde school style. But just design them into the object!
I've been playing with bismuth alloys off and on for a couple weeks now, but I haven't been making controlled tests or keeping careful notes, so I don't have a completely reliable workflow figured out. Some of my castings-in plastic turn out well, some don't. For example, the low-relief Lamassu (winged bull figure) also shown in the video turned out much sharper than my Lucifer figure. Why? Good question.
There are a couple different bismuth alloys out there under a variety of trade names, including "Cerrosafe" (which seems to be the most popular), and "Cerrobend". Gunsmiths use them for taking casts of rifle chambers and barrels. It has a very low melting point â€“ around 158F, and it flows very thin. Its melting point is actually lower than ABS' so... do the math.
[UPDATE: I have to recommend trying "Field's Metal", even though I haven't tried it myself, not the "Cerro---" products. "Field's Metal" has neither lead or cadmium in it, and should be safer to work with. Please do your own research into the best alloy for your purposes.]
The manufacturer of the "Cerro" line seems to have been bought out, and I'm not sure which alloys are which from the new supplier. I've just been buying "bismuth casting alloy, 158 Degree Melting Temperature" from McMaster-Carr ( http://www.mcmaster.com )
It seems to take pretty good impressions of fine details. From what I've read, the metal contracts a bit over the first half an hour after it's solidified, and I haven't figured out yet if that's good or bad for my purposes, or how I should try to time my work in plastic.
Bismuth is machineable; a dremel cuts through it really easily. It's also drillable, sandable, and, I believe, weldable (not sure on that one though). I don't know how well it polishes, if at all, but it's bright silver when cut or freshly cast. One of my plastic molds (the Lamassu figure) was a tear-away - no acetone - so the casting's been very lightly handled, and it has stayed a very shiny silver so far. I haven't tried any patina acids on it yet.
I've been heating the ingots in a small pot on a stovetop, keeping the temperature as low as possible. If the temperature gets too high, the liquid metal will get clumpy, and make for a bad casting. It doesn't seem to reconstitute well either, so I think you can ruin a whole batch by overheating it. And I've had a batch that didn't clump, but seemed to crystalize weirdly as it cooled, and didn't take any details - I think that batch was too hot too.
I think there's a bit of lead in these alloys (though possibly not in the "Cerrosafe" product, I'm not sure) so wear a ventilator if you're so inclined. But I've been breathing the stuff for weeks and I feel fiej.
[Update: I double checked, and all the low temp alloys from McMaster-Carr have lead and highly toxic cadmium, so a using ventilator and not working in your kitchen would be a good idea. The objects you make won't be suitable for food preparation, probably not for kid's toys either. I'll be trying "Field's metal" next, which contains neither lead or cadmium. Do your own research into the best alloy for your purposes.]
DO wear eye protection. I made the mistake of pre-cooling an ABS mold in icewater, plus I really overheated the metal; when the molten metal hit a few stray water droplets, they flash vaporized and splattered molten bismuth all over me and my kitchen.
Also, too hot, and it will melt through or soften the plastic mold.
I've tried saving plastic and print time by making some of my molds hollow, and they didn't turn out well. The metal flows so thin that it seems to seek out even a microscopic pinhole defect and fills any secondary cells in the mold, which allows the metal to surround the cast object in a metal shroud you have to grind and cut off.
Whether you print solid or hollow, use lots of shells - at least 4 or 5, or else the metal will find its way out of the main cavity and leak out.
As long as you don't overheat it, the bismuth is allegedly "infinitely" reuseable - you can cast something, re-melt it, cast it again, over and over.
I've been beating my brains out trying to figure out how to print MakerBot's water-soluble PVA, with no luck at all - I can't even get it to feed through the print heads reliably. If you've had better luck with it, the concept here is the same, and it's probably a lot easier to dissolve PVA in water than to scrape off acetone-softened ABS. PVA has a lower melting point, so if you run into problems with the metal melting the PVA, look for alloys with higher Indium content. I have an ingot of pure Indium from McMaster-Carr that's marked "Cerrolow - 117" (i.e., melts at a very low 117F), but it was much more expensive than the Bismuth ingots. I haven't used it yet - maybe I will if I ever get PVA figured out and realize I need lower temperature metals.
It takes many hours of soaking in acetone to soften up a thick mold of ABS. Freshen or change the acetone every couple hours - it seems to speed it up a lot. I'm impatient, so I scrape the ABS with my hands as soon as it's soft, to expose fresh ABS to the acetone. The melted ABS gets everywhere, forming a second skin, but it peels off easily in a couple minutes. Afterwards my skin feels luxuriant.
I'll post my molds of the Lamassu figure and the Burgonet helmet as separate derivative objects soon. Here are the originals, from Makerbot's Met museum hackathon:
If you've got an object you want to mold, delete a big flat face, then make shell around the rest and give it a try.
How about a T-1000 style Colbert?
If you want to see lots of clear diagrams showing how traditional lost wax bronze casting works, I recommend "Methods for Modern Sculptors", 1980, by master sculptor Ron Young. It'll help you wrap your head around what will work, and what will be tricky.
If you want to watch all the steps of traditional lost wax casting, see this great video with Katherine Michaels: http://www.youtube.com/watch?v=Hkb_8X-7bv0 You can see which steps are skipped by casting into printed plastic, and which steps are retained in analogous forms. Stick with it past the long intro; it shows all the stages really well, and the conjectures about the animist and alchemical thinking of the first metalworkers and their chthonic cults are pretty cool.
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