Plaster for Moldmaking...Has Anyone Tried This?

Treadhead · Post by **Treadhead** » Thu Jun 15, 2006 10:55 am

I'm dithering with the possibility of using plaster instead of RTV rubber for making a rather large mold (13in by 8in). I've had success with RTV, but the cost of the stuff gives me pause.

Has anyone had experience with using plaster? Aside from having to use mold release, are there any extra precautions with using it?

Any info would be greatly appreciated

DennisH · Post by **DennisH** » Thu Jun 15, 2006 11:28 am

Plaster is fragile and you can not have any undercuts on your pattern or it will never come out without breaking either the part, the mold or both.

What are you making a mold of? You could possibly use the plaster to make a mother mold to save on the amount of RTV used.

Dennis

Post by **TER-OR** » Thu Jun 15, 2006 10:11 pm

Yeah, backing rubber with plaster does work - I've seen molds made that way.
But you won't have any flexibility - big things need rigidity anyway.

REL · Post by **REL** » Thu Jun 15, 2006 10:56 pm

Use a thickening agent to make the silicone brushable, start with a super thin coat then use thicker coats to get it about 1/2" thick. Then cover it with a plaster jacket reinforced with burlap strips, making sure to key the silicone before covering the rubber with plaster.

That saves on rubber and negates the bubble issue.

http://sovereignreplicas.com/SM-9.jpg

Treadhead · Post by **Treadhead** » Fri Jun 16, 2006 12:08 pm

REL wrote:Use a thickening agent to make the silicone brushable, start with a super thin coat then use thicker coats to get it about 1/2" thick. Then cover it with a plaster jacket reinforced with burlap strips, making sure to key the silicone before covering the rubber with plaster.

That saves on rubber and negates the bubble issue.

http://sovereignreplicas.com/SM-9.jpg

Thanks Rel -- and others for their help.

Can you explain the keying part?

I'm going to put serveral coats of RTV on the pattern then cover that with plaster (it is all going to be in a rectangular mold box)

Chacal · Post by **Chacal** » Fri Jun 16, 2006 2:07 pm

Silicone doesn't like to stick to things, so it won't stick to the plaster. Keying is done the same way you would key a two-part mold, making raised bumps or indentations on the area of the Silicone that the plaster will cover later, so that you can place the silicone "blanket" (it will be thinner than a conventional mold, of course) on the right place inside the plaster mold, so the negative shape of what you want to duplicate remains exact.

Spacephrawg · Post by **Spacephrawg** » Sun Jun 25, 2006 11:26 pm

a plaster mold is for master models that are made of clay or otherwise soft things and above all, are for one copy only. They are called "waste molds" because you have to break the mold to get at the cast, hence "when they made so and so, they broke the mold."

I have some experience pulling waste molds from clay masters. Thats the only way it will work.

Use something flexible for a mold. always.

fernieo · Post by **fernieo** » Wed Jun 28, 2006 6:41 pm

Here's some info:
http://paleo.cc/casting/silsum.htm
and another:
http://www.themonsterlab.com/vance-matrixmold.html
and this:
http://www.duke.edu/~mtb3/castingmanual/chapter_2.html
and yet another:
http://www.conceptart.org/forums/showth ... light=mold
also google-"Silicone jacket mold: for a lot more info.

Dakota_in_the_sky · Post by **Dakota_in_the_sky** » Sat Jul 01, 2006 2:09 am

I have used plaster to make molds of of clay patterns. I was able to make more than one because I was careful with the clay. It just needed some minor repair afterwards.

Here's a link:

http://public.fotki.com/robertburns/my_ ... e_process/

Keep in mind clay shrinks. The master needs to be removed while the plaster is still warm. When it cures it shrinks and will crack if the master is still inside.

Cleanup is a hassel until I bought a rubber bowl from Micro Mark. It works very well.

http://www.ares-server.com/Ares/Ares.as ... t&ID=83246

I love making molds with plaster. It's fun, easy, and inexpensive.