Hi sen2two:
You have some options here:
For green sand casting:
The first, and probably most obvious is to use the printer to make a loose pattern, commonly a split pattern if your part has geometry that doesn't lend itself well to just laying it on a mold board to ram up the drag.
You need to respect the constraints of a loose pattern; things like shrinkage, drafts and core prints as needed, and you need to finish the pattern smoothly enough to both meet your cosmetic needs and to make it possible to pull the pattern from the sand.
You need to print or insert pins to align the two halves, and you need to decide how robust you want to make the pattern, depending on how many parts you hope to get off it.
If you need to make respectable numbers of parts, you can print a bunch of patterns and mount them on a plywood board to make a matchplate.
You put the sprue, the runners, the gates the risers or riser pads all on the board, and you make the matchplate so the foundry you choose (if it's not going to be you doing the casting) so the foundry can run it on their more automated line.
For investment casting you have a couple of choices as well.
1) you can make the male master, take off a silicone or vulcanized rubber mold and shoot waxes from that which are then invested and cast.
2) you can learn what you need to use for materials, for investment , and for burnout protocol to invest the 3D print directly; making it the burnout pattern.
3) You can print a female mold directly, use it to mold your patterns in foam, or in wax and invest and cast those.
Whatever you choose; the big benefit of using the 3D printer is to eliminate the effort to make the pattern.
When you make a direct pattern, say in styrene or polycarbonate that can be invested and cast you can be much more cavalier about getting the details right, so long as you can invest it completely and you've sprued vented and risered it properly.
As soon as you have an intermediate process somewhere in the chain, whether it be pulling a pattern from a green sand mold, pulling a pattern from a silicone mold and shooting a wax, or whether you're making the mold directly from the 3D printer, you have to get a lot more anal about the details to get it to work; and if you intend a foundry to cast them for you, you must get
everything correct or the foundry will reject your job because you're not worth the hassle.
So in your particular case; for green sand molding, I'd make good quality loose patterns.
Get out the Bondo and the sandpaper when they're printed and make the draft faces super smooth.
If you don't know already, get some books or Google your way to a good knowledge about what a green sand foundry pattern has to look like.
Learn about cores, core boxes, and core prints.
Learn about spruing gating risering and venting.
Learn about split planes and follow boards.
Once you know all that put your 3D printer to work; it's a great tool for stuff like this.
If you hope to investment cast your parts; you have lots more to figure out:
#1 is whether you can do a direct burnout of the 3D print or not.
Many plastics create a lot of toxic shit and a lot of ash when they're burned and are not suitable as burnout patterns.
The four gold standards for burnout materials are wax (best), acrylics (only some kinds), styrenes, and polycarbonates.
Those are the ones I know of; there may well be lots of others.
If your printer can print one of those materials you've got it made; if not, you have to make intermediate molds, preferably in silicone or vulcanized rubber, and shoot waxes into them with a jeweler's wax injector.
Making the molds properly is a whole other topic, and it's a fairly big one.
Oh yeah; not to be deliberately contrary, Bryan, but for low tolerance stuff like casting patterns, any one of the cheap printers on the market these days can be pressed into service very successfully.
The reasons I say this are as follows:
1) You have to finish the patterns anyway, so the better resolution of a better printer isn't worth much for this application.
Even the very best printer isn't good enough for draft faces.
2) If you're hobbying, you don't need to care how long it takes to print the pattern; if you're industrial you'll have proper patterns made anyway.
3) The tinkering is part of the fun; and fiddling with a low end printer gives a lot of guys a lot of satisfaction.
Cheers
Marcus
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