Molding Silicon?

So I need to make some waterproof gaskets and I wondered if I could use silicon calk in a mold and then bolt it down to produce an airtight seal.

sort by: active | newest | oldest
There is such a thing as castable silicone. You mix it two liquid components together, and you get something that flows fast enough to be poured into a mold before it completely polymerizes. You really should see this stuff in action, so that's why for the video link:

This company, Tap Plastics(r), will sell this stuff to you, but their prices are kinda higher than what it would cost you to buy a tube of silicone caulk from your local hardware store or big box retailmonger.

One of the problems with the silicone caulk-in-a-tube is that it does not flow very well.  You cannot just pour it into a mold and expect the pressure of its own weight to force it into the corners.

I think for a gasket mold you may be able to use some sort of screed or scraper tool to force the silicone caulk into the mold and make it level with the top of the mold, similar to the way a screed is used to level concrete.

Another problem with the caulk-in-a-tube style silicone lies in the way it cures.  According to legend, small amounts of water are necessary to catalyze the polymerization reaction.  After all, what is it that keeps the silicone caulk from hardening inside the tube before you bring it home from the store?   Supposedly this is due to an extreme lack of water inside the tube.    When the silicone caulk is forced outside its tube, small amounts of water vapor in the surrounding air are sufficient to catalyze the polymerization. 

However, water from the air has trouble penetrating deeply into the curing silicone.  Thus if you have a really big blob of the stuff, only the outside layer will cure, leaving a soft, gooey, uncured center.

One way to fix the gooey-center problem is to mix in with the silicone caulk, a small amount of water, plus emulsifier to help it mix, and the usual practical way to do that is using a smidgen of water-based acrylic paint. As a bonus the paint pigment gives a visual indicator of when you have it completely mixed.

Of course the gooey-center problem should not arise for something as thin as a gasket.

There may also be problems with the silicone sticking to your mold, and you might have to use something to keep it from sticking.  It seem to recall Tap Plastics(r) sold a bunch of different mold-release type sprays and stuff, but something cheap like grease, or wax paper, would probably work as well.

Just noticed this:

I was going to say something about the addition of solvents like, "paint thinner, or "white gas", to make the uncured silicone goop so it would flow better.  That's part of the folklore  too.

kelseymh6 years ago
You mean silicone caulk (spelling is important if you want to search successfully). You certainly can use marine-grade silicone caulk for a watertight seal. However, it won't be a "gasket" in the re-usable sense. When you unbolt, you'll need to clean out the residue and reapply.
Agreed. You can even get 'gasket' caulking - designed for various locations/temperatures.

As for molding silicon...that's a fun story :)
Now here's a slightly off-topic question: what is the difference between "caulk" and "caulking"? Just regional dialect?
Burf kelseymh6 years ago
Caulk refers to the process of sealing a joint and the material used, including resin impregnated fibers. Caulking is a shortening of the term 'caulking compound', generally the sealant you get in a tube. Both words; caulk and caulking, can be used as verbs or nouns in a sentence.
You say Duct, I say Duck...
jj.inc (author)  kelseymh6 years ago
Ok, "silicone caulk" sorry, I was just posting this really quickly so I could get a fast answer but thanks, I have used some before and I had a huge glob sort of blast out which I didn't expect. It dried and became this rubbery chunk of silicone so I guessed I could just mold a rectangular gasket on waxed paper and compress it for a good seal many times, but it looks like I can't correct.
kelseymh jj.inc6 years ago
Yes, it dries rubbery, and it does make for good and flexible watertight seals (I use it all the time for things like bringing coax in through a wall).

I don't think you're going to have good results using standard silicone caulk to mold a gasket; you're better off putting down a thin ring of it on one of the two surfaces, then bolting the two surfaces together. If you're doing this for something like a pipe flange, you should put the caulk close to the outside, so you don't get contaminants into the pipe.

There are moldable gaskets available, or you can cut one out of a thin sheet of rubber.