Rubber molds made with silicone can be quite versatile and, when made thick enough, don’t require the support of a rigid support shell, or mother mold. Working with silicone doesn’t require a degree in chemistry, but it sure wouldn’t hurt! What I mean is, silicone is an interesting creature, and I think it is worthwhile for me to go into some of its quirks and idiosyncrasies so that you won’t literally get yourself into a mess that is difficult to clean up. I will go into more detail about silicones in Chapter 6, but here are some basics.
Do you remember the silicone teaser in Chapter 3? Two basic types of silicone are used for mold making and for makeup effects: condensation cure and addition cure. The ways that these silicones cure are fundamentally different and are not intermix- able. Condensation-cure silicones require moisture in the air for the reaction that’s needed for them to cure. These silicones are also called RTV silicones. RTV stands for room-temperature vulcanization, which means that RTV silicones don’t need the application of heat in order for them to cure.
Condensation-cure silicones used for makeup effects mix in two parts, both liquid; one part is a rubber base, the other a catalyst (activator) that triggers the reaction. There are also one-part condensation-cure silicones that are used mainly as adhesives, caulking, or sealants and are usually thick and paste-like. Of the two types of silicone, condensation cure and addition cure, condensation-cure silicone is the most tolerant of outside influence. These silicones use tin as part of the curing process and are also known as tin-cure silicones as well as RTV and condensation-cure silicones. Of the two main types,- condensation – cure silicone is most widely used in mold making due to its resistance to cure inhibition (not setting up because of contamination by an outside agent).
The second type of silicone, addition cure, cures by a self-contained chemical reaction. Addition-cure silicones will cure in a vacuum, and there is virtually no shrinkage, though the shrinkage with condensation silicones is also nominal. However, being able to cure in a vacuum is one of addition-cure silicone’s benefits. I guess that’s true if you work in outer space. Addition-cure silicones are also mixed in two parts, and when they’re mixed, air bubbles get trapped in the thick liquid silicone, which is bad. The silicone must be degassed, which requires a vacuum chamber. Right now you’re probably thinking, "Oh, man! Where the heck am I gonna get one of those?!" Not to worry. If you feel compelled to buy one, Harbor Freight sells a 2-gallon pressure/vacuum pot for less than $100. They’re really not that hard to find. If you know what you’re doing, and you will because you’re reading this book, you don’t have to have a vacuum chamber to get rid of the air bubbles, but it could help.
A couple of rules to remember here:
1. Mix the two parts of your silicone gently, thereby minimizing the number of air bubbles you create when you stir.
2. Many silicones take several hours to cure— some as long as 24 hours—so, the air bubbles can usually find their way to the surface of a mold in plenty of time to escape before the silicone kicks. On the other hand, some addition-cure silicones will go off in minutes. Why the huge difference? Well, addition-cure silicones are also known as platinum-cure silicones, and the more platinum they contain, the faster the reaction (and the more it costs).
Addition-cure silicones also come in two varieties: RTV and HTC (high – temperature cure). The HTC platinum silicones need heat to cure, and an oven or other heat source is recommended. Platinum RTVs will also cure faster with the application of heat, but it isn’t a requirement.
Ordinarily, silicone will stick to silicone, but virtually nothing else will, which is why silicone is very, very difficult to paint. That topic is covered in Chapter 6.
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Tin silicone will stick to tin silicone, and platinum silicone will stick to platinum silicone, with no outside help. Tin silicone will also stick to platinum silicone, but platinum silicone will not stick to tin silicone; it won’t even set up. Yuk; very messy.
The biggest thing to know and remember about addition-cure silicones is that they are extremely sensitive to outside contaminants. They will not cure against or in the presence of condensation-cure silicones. They’re also finicky about anything that’s been in contact with sulfur, latex, foamed latex, ammonia, tin (platinum silicone will not set up on cured tin silicone, but tin silicone will set up on cured platinum silicone), and a variety of other substances. Just be very aware that addition-cure silicones are very sensitive to cure inhibition.
Knowing how sensitive platinum silicones are to cure inhibition is very important because of the materials you choose for your prosthetic appliances. If you make foam latex prosthetics in a mold and later decide you’d rather make a silicone version, it can’t be platinum silicone, because the foam latex will prevent it from curing. Likewise, you can’t cast platinum silicone prosthetics in a mold with tin silicone parts, because it won’t cure either. Ah, chemistry! Gotta love it. I’ll save more about silicone for the next chapter.