Silicone Rubber

[Stephen Tashiro]

I saw a website that claimed you could cast silicone rubber objects by mixing clear silicone caulk with plaster of paris. This seemed improbable, but I tired it and it works in a crude way. I got a tube of clear silicone caulk from Walmart and mixed 1 part plaster (by volume) to six parts of the caulk. I stirred it up till it was uniform color and spooned it into a plastic jar lid. It hardened overnight. You are supposed to use the kind of caulk that smells strongly of acetic acid and it must be silicone caulk not urethane or latex. The surface of the cast had the marks of bubbles and smelled of the acetic acid. In a few days the smell went away. This method would be good enough to cast a big washer or gasket that didn't have to be utterly smooth..

It would be nice to find some way to thin the mixture enough to where the bubbles would rise and pop before it hardened.

[Dick Strauss]

maybe vinegar and a vacuum pump?

[Chuck Wintle]

yes you will need a vacuum pump to remove the bubbles and one that can achieve vacuum into the low mTorr.

[Dave Anderson NH]

You can use a small amount of toluene to thin the mixture, but be careful and use it in a well ventilated are. Toluene is a known carcinogen and when used as the carrier for silicone resin primers is always used under a fume hood with the operator wearing a dual organic vapor cartridge respirator and nitrile gloves. A bell jar will work well to remove the air bubbles if you have access to a vacuum pump.

[David G Baker]

Don't know if the stores are still in business but I would go to Tap Plastics when I lived in California and they had all kinds of molding products. My brother would make plastic tail lights for classic cars, pistol grips that looked like ivory, coins and many other things that I don't remember.

[Cliff Rohrabacher]

vacuum into the low mTorr.


Oh look~!! An old timer~!! People look at me funny when I talk about molecular flow regimes of 3 or 4 times 10 to the negative power of 14 Torr.

All the cool kids have gone Metric. I think they have all been duped but don't tell them.

[Stephen Tashiro]

The point of using caulk as silicone rubber is that it's cheap. There are expensive two part "RTV" silicone rubbers. The expensive one's I have tried are fairly thin and if you are careful you don't get many bubbles.

Using a vacuum pump to de-bubble viscous casting liquids is probably good advice for industrial operations. I've never seen a convincing way that the home hobbyist can do it. For one thing, my experiments with vacuum pumps show that making a vacuum chamber is a big problem. They must be very substantial or they implode or deform! Buying a typical laboratory bell jar is one solution, but that's not very satisfying for a do-it-yourselfer.. It's also hard to know what the home hobbyist is supposed to use for a vacuum pump. Again, laboratory grade equipment is one expensive solution. I think for the DIY world, a less viscous casting liquid is the better approach.

I wouldn't know where to buy toluene (is it sold by that name for use in wood finishing or construction?). It sounds too scary to use. I may try turpentine or acrylic paint, which are also mentioned on the website.
.

[Cliff Rohrabacher]

A company named Bel-Art Products, Inc., sells a polycarbonate and propylene Vacuum bell jar pretty cheap. http://www.belart.com/shop/
http://www.belart.com/shop/420300000-space-saver-desiccator-nonvacuum-140mm-p-420300000.html
It's cheap at $40.00



They are lots cheaper than glass. I've used ‘em exclusively for de-gassing epoxies using a twin stage Sergeant Welsh Rotary Vane vacuum pump. It's gets you all the way down to the molecular flow regime


An alternative is a Pressure Cooker. It lacks a window which can be a serious issue but, you can make one from 1/2" plexi and cut the port in the lid then silicone glue the thing in place.


I wouldn't know where to buy toluene (is it sold by that name for use in wood finishing or construction?)Ace Hardware.




It sounds too scary to use.It is just another VOC.
If you can live with lacquer thinner you can live with MEK, Xylene, and Toluene.

[David Keller NC]

"You can use a small amount of toluene to thin the mixture, but be careful and use it in a well ventilated are."

By the way, guys. The sale of toluene is more restricted than it was 15 to 20 years ago, as it is the base material for T-N-T. There are other solvents that have similar properties to toluene that are not restricted because they can't serve as the raw material for an explosive. Xylene is one such substitute and has similar chemical properties to toluene.