Re: 3d mold making

But the entertainment value of the two of you together can be too much, sometimes, and I have to make myself go back to work. :-)

I seem to have missed the beginning of this thread, but I gather that someone wants to slip-cast letters. Assuming these are letters on a tile or the like, you may find it easier to just press mold them. Make a plaster mold of the tile by sticking it down in a box, face up, and pouring plaster over it. (Use a release compound like soap unless the original is smooth plastic or similar.) Remove the master, let the mold dry thoroughly, and then press a slab of clay into it. You might want to dust the mold with a little talc first. Let it sit for a while (5-30 minutes depending on how wet the slab was) and then the piece will pop out. (Same effect as slip-casting: The clay shrinks as water is drwan into the plaster.)

HOWEVER, if you want letters with a lot of relief, or even free-standing, this may be more of a problem no matter which way you do it, simply due to shrinkage of the clay. Most letters have some enclosed portion, such as A, B, D and such, and when the clay starts to shrink in the mold, it will tend to crack around that "island" for the enclosed part. Even C and E will have this problem.

If this is what you want to do, use a lot of draft angle on the master to help squirt the part out as it shrinks. Then (assuming you press-mold these), invert the mold and suspend it a fraction of an inch over a flat surface (or whatever) to catch the part as shrinkage squirts it out of the mold.

Good luck!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

After carefully avoiding this thread, I'll chime in that this definitely sounds like a press-mold job, and that's how such things are made in commercial quantities.

Before somebody demands bona fides, no, I don't know ceramics. I do know gypsum tooling. And U.S. Gypsum has some information on the grades of plasters to use for press molding, slip casting, and related work. You may have to call to get your hands on it. There's one guy in the entire company, based somewhere in the upper Midwest, who maintains the old tooling data for USG. I have copies of most of it, having done a lot of research on it in the mid-'80s (I wrote a series of articles on gypsum sweep-molds). If you need some USG data, there's a good chance I have it.

BTW, you can't do reliable slip-casting with just any old grade of Plaster of Paris. It's pretty touchy. It has to have just the right absorbance or it will either stick, or it will let too much slip run to the bottom of the mold. And that's also a function of the percentage of water mixed with the slip-casting plaster. You can learn it by fiddling around, but you will waste enormous amounts of time that way. USG can save a lot of time for you, if you can reach the right people there.

Ed Huntress

definitely

So you don't recommend, as Michael did, slip casting clay in Silicone or Latex molds? Like he says his wife does?

I don't think that Michael foresees any problems .... even after all the posts ... after all, he has a (claimed) wife in the basement and the daughter "paints" ceramics too ..

It started on alt.machines.cnc and was cross-posted in midstream, Bob. I don't think there was any indication that the letters were to be undercut. Actually, they're to be 3/4"-square tiles, 1/8"+ thick, with recessed letters on top. The original poster wants to make sets of all 26 letters.

As you suggested, it's a press-mold job. But the molds don't have to be plaster. He has an offer by someone else to machine them out of another material.

I think somebody got confused about ceramic casting processes, and used the wrong terms.

Ed Huntress

The problem with any sort of machined mold will likely be that wet clay is pretty sticky, and you will be using pressure to drive it into the mold details. If the mold doesn't absorb any moisture, the piece may not release. Some production press-molds use porous molds that release the piece with air pressure, so in theory you could machine a non-porous mold and then drill a bunch of teeny holes in it. But plaster is a lot easier and cheaper! You can make your originals in plasticine modelling clay (the kind that never dries out) and just pour plaster over them to make the molds. You don't even need a parting compound. Or, have the machinst make models and cast the molds from them.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

A good point, and the reason that USG's main tooling application for its materials is press-molds. Considering that many press-mold jobs are long runs and that a metal mold certainly is a lot less labor-intensive than making many copies of press molds, it seems that the industry has settled on plaster as the best mold material for that work. The really big power mixers are used mostly for making press-molds, for example.

Ed Huntress

"That guy went off on a press mold rant too, again, "

Thanks,

Bob, thanks for coming through with lots of info. I'm the original troublemaker, and what began as a thought of trying slip casting in a non-pourous material is now evolving into wanting to find out more about press molding. Do you know of any good books on the subject? I would like to read a little about it to see if it's the best choice for me. Machining a pattern won't be a problem, and while one fellow has offered to cast and fire them for me for free, I don't know how economical it will be in the long run shipping them from Georgia to Washington. That, and when it comes to how it's glazed, I don't know how hard I can push the free button. I'm willing to bet he'd say no big deal, but favors shouldn't be abused. The machinist group has a few folks who have come forth with a lot of good information, but it's nice to see an outsider with some experience pop in once in a while. I hope the pissing contest didn't make you think we're a bunch of idiots. Thanks again.

Far and a way the best post I've read in a while. Good job Charlie.

Thanks, Bill. Every now and then......................