Just as I mentioned in the plastics section, this is a guide to using rubbers for making molds for beginners, by a beginner in beginner terms.
I will only mention 2 types of rubber, as these are the only two I have any experience with. While I'm sure there are many other types of molding rubbers, these two would have to be the most accessable and inexpensive. Let's start with the first one, which is latex rubber.
Latex is used to make all kinds of products, from rubber bands, clothing, yes, condoms, and also molds. But latex is a mongerel. I only say that now because I've since found something better (which will come later). The reason I'm a little down on the stuff is that it is so incredibly time consuming to use it to make up a mold.
Latex is a layer by layer product, which means, that make a mold from an object, say, a puzzle piece, you need to paint a layer of latex rubber and wait for it to dry. As you can imagine, latex, being a rather unny material, it does take a very long time (quite a few days) to sufficiently built up enough layers to complete a strong stable mold.
To best illustrate this, take a look back at what I mentioned latex is usually used for. Clothing, condoms, etc, are thin layer items. A mold is usually a block of rubber with a hole in it, the shape of your object. The other problem with latex is that is does eventually rot or decay.
Take the humble rubber band for example. Notice a rubber bands do eventually go brittle and kind of crusty. Latex molds tend to go this way too.
The final, and my biggest problem with latex, is that it tends to have a chemical reaction with polyester resin. Not a huge reaction, but when a polyester piece is removed from a latex mold, sometimes the wall where resin contacts latex is left sticky, and your mold will need to be scraped free of sticky material.
As for polyurethane plastic, I never tested it with a latex mold so I don't know if any side effects occur between the two substances. However, to its credit, latex is cheap and accessable and can be a lot of fun to work with on temporary projects or just to fool around with and learn the process.
Superior is every way with a price tag to match. Unfortunately, better matterials do cost more but they last so much longer, that in the long run, latex rubber would cost you more in time. As an example, my container of latex cost $30AUD for one litre. The 500g container of silicone and it's hardiner (catalyst) cost me $41AUD.
A mold in latex would take me 7 - 14 days to complete whereas the same mold in silicone would take 24 hours.
Unlike latex, which requires a layer by layer method, silicone rubber can me mixed with it's catalyst and poured over the piece. You would construct a box, perhaps made of lego (see working with plastics), and pour the rubber in, and leave it to set.
You'll see pictured here, some Dow Corning 3110 silicone rubber and its hardiner. If you live in Sydney, Australia, you can contact Daystar polymers. In the USA and other countries, I'm not sure where you would get it but you can start at www.dowcorning.com, then go to a reseller in your country. Besides 3110, there is also 3120 which is a red colour ( just a dye) but can withstand higher temperatures if you decide to fast-cure your plastic piece in an oven. Should be the same price as the 3110.
MOLD RELEASE AGENTS
Doug also reminded me about mold release. You will need this. I find that silicone spray is cheap and last for ages. It's also very effective. You spray this into your rubber mold and allow it to dry. It will still be there, leaving a slippery coating. Don't drown the mold, that would be pointless.
When you pour in your plastic and let it set, even without a release agent, sometimes your piece will simply pop out itself once cured. Other times, it will stick firmly against the walls and you may have to use so much force to get your piece out that the mold may become weak or simply tear open. Use silicone spray or some other slippery material. I wouldn't recommend vasoline as a release mold simply because vasoline, by its very nature, leaves smears along the walls of your mold.
This is no big deal of itself, but because resins are so senstive to texture (which is a good thing) it will duplicate those smear marks into the piece.
Email me if there's something I haven't mentioned that you think I should. Two-part molding techniques coming up later on.