I don't know anything about this stuff, but from YouTube I'd think other options would include:
- machine a form out of wax or delrin (or clay), pour a silicone mold, and then pour liquid plastic into the mold, either a not-yet-polymerized resin such as a thin epoxy, or a molten thermoplastic;
- similarly, but use plaster instead of silicone and burn out the form in a kiln before pouring (not applicable for clay);
- 3-D print the case, and if you don't like the layer lines, smooth them out with either a coat of paint or by smoothing an ABS print with acetone vapor;
- 3-D print a form for molding.
Even for injection molding, you might start with an aluminum mold, which won't last as long as hardened steel but will cost a lot less.
But I don't know anything about this stuff so maybe it's not applicable to this case?
Unless there are products I'm not aware of, mostly thermo plastics require high pressure injection that isn't feasible for a hobbyist setup.
> a not-yet-polymerized resin
There are lots of products like this (a bewildering variety, really), but in general they aren't particularly convenient to use, and you'll have a lot of failures. Factors that can mess you up include temperature, humidity, air bubbles (so many air bubbles!), mixing ratios, over mixing, under mixing, contaminates on the mold surfaces or your mixing containers, pour holes too small, not enough exit holes, entry/exit holes in the wrong places, molds held together too loosely leading to leaks, too tightly leading to distortions, etc.
Except for the possible exception of relatively simple shapes and small items (like dice), just about everything you can imagine going wrong will, and you'll end up with a lot of waste before you get something you can use, and disposing of the failed attempts responsibly is another challenge.
There is a caveat though: if you're trying to make a soft part (whether in a hard or soft mold), the materials are generally a lot more forgiving of errors, but that's tangential to the challenge of making a case.
Thanks! I think I've mostly watched resin casting demonstrations by very experienced people, so I may have unreasonably positive expectations of what is possible, but I think it's fairly common for art students to get some successful resin casts in a single-semester sculpture course? Which I think implies a cost of first success well under US$10000? As I said, I haven't tried it myself. Michal Zalewski makes it sound pretty achievable.
As far as thermoplastic molding goes, aside from the vacuum forming that someone else suggested, there seem to be a lot of experimental options. There is a YouTuber who has built a couple of DIY injection molding machines (http://www.youtube.com/user/AndysMachines), and Star Simpson did successfully mold recycled HDPE plasticized with vegetable oil in an open-top mold (https://www.instructables.com/HomemadePlastic/), but the results are not super smooth. My recollection from melting plastic cups in the oven as a kid (probably polypropylene) is that you end up with a puddle with a pretty flat and smooth surface except for whatever lettering or imagery was on the outside of the cups, so I feel like you might be able to get a smooth top out of an open-top mold, but I don't know which plastic that was, and melt degradation might be an issue. https://youtu.be/KA1UELu33FI shows some apparently successful open-top molding with powder made from recycled polypropylene (skip to the last minute of the video) but doesn't test it for strength. https://www.youtube.com/watch?v=2OYbyzgErvo recycled polypropylene into a two-piece water-cooled mold improvised out of aluminum baking tins, but seems to have suffered major melt degradation possibly due to overheating it in a can on an open fire. https://youtu.be/Ae8_OWju8RM recycled (probably heavily filled) HDPE into a billet in an aluminum can in a thermostat-controlled electric oven at 180 degrees and had a lot of porosity due to high melt viscosity, then turned the billet down on a lathe; the outside of the billet seems to have suffered enough melt degradation to make it machine much more cleanly than the center, which produces the kind of long stringy chips you'd expect from HDPE. https://youtu.be/blSvUqfuiuo makes an HDPE sort of gingerbread man in an open-top silicone oven mold, the kind you'd use to make gingerbread-man-shaped muffins, suffering melt degradation severe enough in parts to turn the HDPE brown, warping, and significant porosity due to high melt viscosity. https://youtu.be/-igxhoGEQFU shows really nice marbled results melting HDPE in a panini press with a silicone mold release, then pressing it into a billet in a particleboard mold with a bench vise, but still suffering some problems with porosity. By turning it on a wood lathe they are immune to problems of warping etc.
Polycaprolactone deserves a mention because above its glass transition you can mold the thermoplastic with your bare hands, but pouring it into a mold is out of the question because of its high viscosity, and as a result of that low Tg you tend to get a lot of creep, and parts become unusably weak in a warm room.
So I think you've convinced me that thermoplastic casting without injection molding equipment is not a practical option for making low-volume parts. Thank you for sharing your experience!
> I think I've mostly watched resin casting demonstrations by very experienced people, so I may have unreasonably positive expectations of what is possible, but I think it's fairly common for art students to get some successful resin casts in a single-semester sculpture course?
It is much easier to get good results with a bit of in-person guidance, and in a controlled environment. There are also optional materials (eg. mold release) and equipment (pressure pot, vacuum chamber, or vibration table, all of which help with both mold-making as well as casting) that make success easier which are harder to justify for a low volume or one-off project.
> Which I think implies a cost of first success well under US$10000?
Well, sure. But you aren't comparing like with like. If you outsource the production of a prototype to someone else using resin casting, it will also probably be cheaper than $10k for the first one (say, $1-5k, because resin casting more or less requires you to have built one with some other method before you can even make molds) but scaling up is going to be more expensive (dozens or hundreds of $ per unit), in part because silicone molds wear out quickly and have to be replaced after being used as little as 5-10 times.
Conversely, if you already have your own machine shop, you may be able to produce your own metal injection molds for much less than $10k and hand it to someone to do a small run.
The two methods trade off (access to) expensive equipment for expensive materials, and up front cost for per unit cost at scale, but if you really only want one neither is attractive.
You're welcome. I just realized there is a bit of nuance missing from my comment. The unattractiveness of casting diminishes even if you only want one of a thing if it is the case that you will want many things rather than only one thing ever. Many of the videos on the subject seem to be of experienced makers creating many different unique objects. This does of course allow you to amortize the equipment costs across many projects, just as you can for machine tools, and the optional equipment I mentioned (whether purchased or DIY) becomes much more approachable.
Still, it is really hard to beat vacuum forming for approachability in making a one off: The only equipment you don't have laying around can be made with a few chunks of wood. If you have a small oven (even a toaster oven) that only limits the size of your parts. If you have a vacuum cleaner with crappy suction, that just means you can only form thin sheets.
BTW, all these techniques combine. In the case of resin casting, I mentioned that you really need to have made a one-off in order to create the silicone molds. In fact, I have in the past vacuum formed a blank shape (this was before 3D printing was a thing), sanded or machined it to more precise tolerances, added details (dedents, screw posts or holes, flanges, ribs, ridges, textures, etc.) and then used silicone molds and resin casting to make duplicates. Sometimes you actually do need several identical parts in order to just make one object...
Yes! Today I bought 20 identical poorly-injection-molded T-fittings for aquarium air hose, and I was thinking about your comments. It occurred to me that the existence of injection molding strongly favors designs that have several instances of the same part, like four identical wheels or 15 identical spring clips.
In some cases I think there are materials that are a lot easier to shape to make a mold pattern than whatever you're going to make your final part out of. Wood is the traditional choice, except that wax is the even more traditional choice, and clay, styrofoam, foamed waterglass, and pumice also come to mind. I guess in theory polycaprolactone might work, too, and as you say, poly(lactic acid) is a popular modern choice, sometimes painted or sanded to smooth out the print lines. And I've been wondering if it's feasible to strengthen aluminum foil by spray-painting or cathodic mineral deposition in order to make such one-off items, because aluminum foil is very easy to form, usually too easy. Lots of stuff to try!
- machine a form out of wax or delrin (or clay), pour a silicone mold, and then pour liquid plastic into the mold, either a not-yet-polymerized resin such as a thin epoxy, or a molten thermoplastic;
- similarly, but use plaster instead of silicone and burn out the form in a kiln before pouring (not applicable for clay);
- 3-D print the case, and if you don't like the layer lines, smooth them out with either a coat of paint or by smoothing an ABS print with acetone vapor;
- 3-D print a form for molding.
Even for injection molding, you might start with an aluminum mold, which won't last as long as hardened steel but will cost a lot less.
But I don't know anything about this stuff so maybe it's not applicable to this case?