Why not?! Background: I wanted to create my own bullet mold because I'm building a 350 Legend made primarily for subsonic rounds (1:10 twist, 10" barrel, will have a suppressor on it.) Basically, an American 9x39. I want a big heavy bullet that I can design myself. The problem is, if I have a custom mold made, it's expensive and takes a considerable amount of time. Worst yet, if I don't get the design right - I've paid a lot of money for a mold I'll never use again.
I've been working on a process (and yes, it's a process) to make it easy for those of us who have ventured into 3d printing to actually create a usable mold to cast into. I did a ton of reading/watching about casting lead into various materials (there are some high temp silicone molding out there, but they aren't up to lead temps and degrade fast if you do) but settled on Plaster of Paris (PoP.) The idea is that you 3d print a master mold from which you cast PoP molds that can be used to test your design. It's rapid prototyping for boolits. Once you've got a definitive working design down - you can send it off to a proper mold maker (or, potentially use a small CNC router to cut your own from aluminum.)
My initial design/process was to create a master face mold that had indexing bumps/dips built-in but I found that the bumps didn't want to release from the master very well so I ditched that idea in favor of making a 3d printed "carrier" that had indexing bumps in that are symmetric (so you only need one design file for it.) This also prevents the mold from breaking when you use a small clamp to keep them together; you're clamping the carriers, not the mold itself. Print your master mold with the same corresponding indexing bumps and they'll line up perfectly every time. Pour your PoP mixture down in the cavity and use something to vibrate the air bubbles up to the top. (I use an old Dremel engraving tool with the tip removed; loud, but extremely effective.) I also went through a few different items for mold release (silicone spray, liquid car wax) before I settled on Murphy's Oil Soap (big bottle of it costs about $4 at Walmart.) It's cheap and works perfect every time now; just dip the tip of a cheap toothbrush in it and coat just the master face mold with it. Once the PoP has mostly dried so I can demold the face, I stick it in my dehydrator I use for brass for several hours on a low temp to completely dry it out (but not so high that it melts the 3d printed plastic carrier.)
Keep in mind that I have a Fused Deposition Modeling (FDM, aka filament) printer that I made my masters and carriers from; a Stereolithography Apparatus (SLA, aka "resin") printer would create a much higher resolution master (and thus, a much smoother PoP mold.) I ordered an SLA printer and it should be here soon but I wanted to test the process before I got one. So far, it's looking extremely promising. I'll be trying that here in the next week or so.
I'm sure there will be plenty of comments on my design - this was literally a design I whipped up in TinkerCAD in less than an hour and I have no idea if it will be worthwhile. It's a rebated boat tail with a short-ish ogive. Nothing scientific, it was just a "this looks cool" design to test the plausibility of being able to rapid prototype boolit molds. How many casts can I get before degradation of the mold? I have no idea. I haven't gotten that far yet but the videos I've seen using PoP seem to show you can get quite a few. I wanted to release my results and see if anyone else is interested in trying this with me. Below is a picture from this afternoon from a mold I made in about 2 days - the length of time is more because of the PoP having to dry (I poured one at a time) than anything but it seems like it takes about 4-6 hours in the dehydrator to dry completely. When I poured the bullet shown, there was no bubbling (like you'd find in a PoP mold that isn't properly dried) and it fell right out without having to "smoke" the mold. The mold is a bit on the fragile side so not something you'd make thousands of bullets with so much (though the carrier does help with rigidity,) but one you can probably knock 20 or so out of to test with before you decide if you like the idea. If you don't? Snap the PoP mold out of the carrier, print a new master and pour two new molds. Pennies to make, only a couple of days (which can be knocked down to an afternoon if you print two masters and pour both at the same time) to finish off. Design in the morning, casting boolits by the evening.
There are still some flaws in mine; there's a tiny bit of flashing that I'm still figuring out how to avoid but it's extremely minimal. I think part of this is consistency of the PoP mix itself, too. The biggest part is making sure you get the master face coated well with mold release so it doesn't stick to the PoP (and as such, pull some off ruining the mold.) Otherwise it retains every detail (even the ones you don't want.) Again, an SLA (resin) printer will solve the resolution issue.
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Originally Posted by RKJ
