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Can I use hardware store silicone / rubber for moldmaking?

I need a flexible, reusable, poured mold but it needs to be as cheap as possible. The RTV silicone that I see available specifically for moldmaking is too expensive for me. I know that there are lots of different formulations of silicone and rubber for home improvement and automotive use, and most are inexpensive. Are any of these good for making molds? Which is best?

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emcelhannon7 years ago

A little glycerine lowers the surface tension, and breaks the water droplets down for a more thourough mix.  It should speed the curing time considerably.  I don't know how much, but recipies are out there.  Google it.  You can thin it down with mineral spirits, but experiments say that xylene works best.  Definitly not as good as the industry 2 part mix, but a small project doesn't have to cost 30 bucks, and the caulk will keep for longer.  I've used heat, as in a blowdryer when spreading rubber on a mold.  It thins it and causes the bubbles to temporarily expand, helping them to lift of your project surface.
Try this site.
http://www.myheap.com/book/chapter-07/siliconecaulkmolds.php

This method works completely. I've even written an instructable based on this research.
guyfrom7up7 years ago
I've done it before, the only problem with using the silicone calk is the airbubbles, on the inside of my mold that i used the calk there was a bunch of tiny bumps caused by the airbubbles.
Prfesser7 years ago
Most silicone caulk is formulated to cure by reaction with water, from water vapor in the air.  The inside of a thick layer (more than about a half inch or so) will not cure, or will "soft cure" giving a gummy texture.

There may be some way to make a thick layer of silicone caulk cure.  If anyone knows what that method is, I am sure lots of people would like the information.

A 10 oz tube of silicone caulk is about $5; on eBay the moldmaking silicone is about $60 for a half-gallon, which is about twice the price of the caulk; or $20 for a pint (16 oz).  I messed around with several tubes of caulk before breaking down and buying the real thing.  Which was well worth it.

Mix the silicone caulk with water or, so you can see if it's mixed well, acrylic paint.  It's sticky and doesn't fill fine detail well, but can work.  I think it was about a tablespoon of water per tube of caulk. 
I've had pretty good results using the cheapest store-brand clear silicone caulk they had. Like JimFlo said, it's all about the release agent. I use slightly warmed up petroleum jelly and a soft brush.
Koosie7 years ago
I've noticed that marine silicone is quite strong but still flexible, like normal silicone.
JimFlo7 years ago
Use a release agent and should work fine in a pinch. I have used cooking spray (veg oil). If you are doing more than one small thing, invest in latex mold materials. BTW, it doesnt pour, but you will work it out.