Casting with plaster

[Treadhead]

Has anyone tried casting with plaster? I'm thinking of using this medium to create vacuform plugs as plaster will withstand the heat without being damaged in the process.

How much does plaster shrink in comparison to resin? Does it demold easily? Also will it damage a silocone rubber mold?

Thanks in advance for the help.

[Kylwell]

Don't know the shrink ratio offhand, IIRC it's a few %, but I do know it gets hot enough to cause 2nd degree burns (after all, it's related to quicklime).

[Joseph Osborn]

Plaster shrinks a bit more than resin but will not damage silicone molds at all. You could probably cast hundreds of plaster copies in RTV with no breakdown of the mold. It demolds very easily, too. Depending on the mass of your plaster part, it will get quite hot when curing but it shouldn't hurt a good quality RTV. I used to cast a lot of parts in Hydrocal plaster because it was cheap and would hold decent detail.

[eeun]

I've used plaster as a mother mold, bracing an inner latex mold for casting concrete garden sculptures.

Tried it with small stuff and had poor results, but what I was doing was better suited to a flexible mold.

If you're using it to make plugs you might get good results, though. Plaster can be waxed or oiled, and as long as the plaster has fully dried, it'll withstand the brief heat of vacu-forming. It's fairly easy to shape, and dirt cheap enough that if it doesn't work, you aren't out a lot and can always use it to patch the walls.

The prop makers handbook has extensive info on using plaster for both casting and molding.

[Treadhead]

If I need to do a large pour of plaster, would it be better to do in it in smaller pours? Let one pour cure, then add another until the mold is filled? Would this also reduce shrinkage?

[Antenociti]

depends on the plaster - the harder alpha-plasters shrink far less than the cheaper versions, are much harder and can easily be pressure cast to remove bubbles.

pouring layers of plaster is not a good idea though as you can get fracture lines which are very week and can sheer apart easily - especially if there is moisture between the two layers.

even large blocks of plaster dont really get very hot, certainly not compared to resin, though we haven't commercially cast anything over about 12 x 12 x 12 inches in size, so beyond that i dont know what the temperature increase might be.

You can cast "forever" with plaster in silicone moulds, it doesnt harm them at all and the only damage you may ever get is tearing from over use. it demoulds very easily and you dont need a release agent.

For minimal shrinkage try getting the technical info on your countries Dental plasters - these are usually the better alpha-plasters like Crystycal Alpha K, Alpa R, Herculite Stone Powder and so forth - the names tend to change from one country to another though.. I think in the USA Merlins Magic or Excalibur is a favored type???

We stopped doing commercial plaster casting last year, but i still do the odd bit nowadays as we carve masters from wet-plaster a fair amount.

[Chacal]

You can even use plaster to make molds for resin—all you need is an original with no undercuts, and an acrylic-based demolding agent, which can be airbrushed on. If you use good plaster and stir it gently so you don't get bubbles, you get really clean castings, comparable to silicone rubber.

[Dakota_in_the_sky]

I think the plaster I used had a shrink rate of 3%. Another brand was 8%. I can't remember what they were. I recomend using hemp rope to make the plaster stonger.

[modman06]

i use, ultracal 30, its a dental stone. used by dentists, but also for special effects make up. but works great for vac plugs, especially canopy plugs has a great smooth surface..

you can get any where they sell concrete, 50 lb bag for 20 bucks.


Al