Casting in silicone?

March 28, 2006 9:47 AM

Casting (molding?) a solid object in silicone or similar...

I am interested in casting solid objects out of silicone/silicone rubber, latex, foam rubber or a similar soft, flexible, pliable material. I need to do this very inexpensively, and I don't have access to any professional equipment. I have been putting a lot of thought into it, and I can't think of how the silicone or similar material will cure in the center of the mold, where it is an inch or more thick.

I conducted an experiment to see if I could use regular GE silicone rubber sealant that you can get at Home Depot in caulk tubes. I put it into a 2in diameter by 2in long section of PVC pipe and left it since Friday night to see if it would cure through. It shows no signs of ever curing on the bottom, and surely in the center, where no air can get it.

Can anyone recommend an alternative (inexpensive) material with similar physical properties, or an alternative method to get the material to cure through?

posted by bradn to media & arts (6 comments total)

two part silicone cures like epoxy.
posted by hortense at 10:09 AM on March 28, 2006

two part silicone cures like epoxy.

Thanks! Looks like Smooth-Sil 900 might be just what I need. Any ideas where to find it locally?
posted by bradn at 11:06 AM on March 28, 2006

You don't specify what the usage of your mold is, so let me offer a few alternatives that I've used:

Moulage is an alginate which is used for life casting. Melted in a double-boiler, it stays gooey at a little warmer than body temperature. It pickes up termendous detail, but needs some care for air bubbles and voids. It should go in a single layer, about 1/2" thick and then coated with plaster gauze to keep it firm. Moulage is ideal for life casting, but the molds tear easily. Moulage can be torn up and re-melted. Moulage molds will accept plaster and other materials that won't melt the moulage.

Plaster works great for materials that either have no undercuts or the casting medium is flexible. Plaster captures a lot of detail and makes molds that can be reused. It takes some practice to get used to mixing it in the right proportions and to get out air bubbles. I've made one and two-piece molds in plaster. The object to be cast needs to be liberally coated in a release compound. I've used spray-on vegetable oil since it works moderately well and isn't toxic.

Liquid latex (actually, you want the stuff that is the consistency of pudding) is brushed on in many thin layers. It's a little tedious to work with because it has to be put on thinly. It can be cured with a hair drier to make the process go faster. It stretches well and is therefore friendly to undercuts. I've usually used it only as a casting medium and not as a molding compound. It will not stand high temperatures.

As noted, silicone casting (RTV) is best done with a two part compound and lots of mold release. Watch the safety warnings on the mold release. Last I checked, most varieties were pretty nasty.
posted by plinth at 11:14 AM on March 28, 2006

Smooth-On sells a wide variety of two-part silicone, urethane and polysulfide compounds for mold-making applications.
posted by RichardP at 11:28 AM on March 28, 2006

Thank you, I think Moulage might be a great solution also.

Just to clarify, I am searching for a material to make the final positive form out of, not the negative mold.
posted by bradn at 11:35 AM on March 28, 2006

Moulage is not a final form material - it's too weak. You might look at Plastisol (from Nasco), which needs a mold that can stand 355F or one of the RTV silicones.
posted by plinth at 12:21 PM on March 28, 2006