December 13, 2021
When it comes to mold making, you have a lot of options for what to make the mold out of. In many cases Mold Making Silicone is the best option for making a mold, but there’s so many different kinds it’s hard to decide which one is the best one for your project. Fear not, we’re here to help! In this post we are going to be focusing on the types of silicone carried by AeroMarine Products, but there are several other types of silicone that we don’t carry that are outside of the scope of this post.
To begin, a little background on what silicone is and what it does: Silicone is a polymer derived from silica, the same material you’ll find on the beach. A little chemical wizardry is done to transform it from a hard, clear, crystal to a more opaque and liquid resin and there you have it: liquid silicone just waiting to for a catalyst to turn it into a firm rubber. This brings us to the next part: catalyst.
When polymers are in a liquid state, the molecules form flexible chains and can slide past each other like a plate of oiled up spaghetti noodles. To become a solid, polymers need something to trigger that change. In epoxy this change is brought on by heat but for silicone it requires a second chemical, called the catalyst, to be added. Once the catalyst is added it causes the oiled-up spaghetti noodles to form something more like dry spaghetti noodles that line up in orderly rows taking it from a liquid to a solid rubber.
Silicone is special because you can do different things with the same liquid silicone by changing the catalyst. For example, with the base white AM128 silicone resin you can use our purple catalyst to create a pourable silicone that resists sticking to things, or you can use our pink catalyst to make a thicker silicone that has more adhesive properties which you can apply to a variety of surfaces to make a thinner glove mold (but it will still peel off when it’s cured).
Within the realm of silicone catalysts, they come in two major forms: Tin-Condensation cure, and Platinum cure. Like the name says on the tin, each type uses a different metal in its chemistry to catalyze the silicone into becoming rubber. In addition to making one significantly more expensive than the other, this also makes it so that even if the silicone has the same pourable properties, the other properties may end up different.
Tin-condensation cure catalyst is significantly cheaper than platinum cure catalyst, but also tends to be less temperamental about what it touches. Silicone in general is somewhat sensitive to what the substrate is made from, or what is on it. Some things like sulfur-containing clay or sulfur-vulcanized rubber will prevent all kinds of silicone from curing. This can be mitigated however by coating your piece with a coat of sealant, like spray acrylic, primer, or AeroMarine Products 300/21 epoxy.
Tin-condensation cure silicone also tends to suffer from shrinking a little more while curing than platinum cure silicone. Apart from water, most materials get smaller when they go from being a liquid to a solid. Silicone is not immune to this and will shrink in all dimensions as it cures. Tin-condensation cure silicone generally is worse for this than Platinum cure silicone, however this is not a universal rule. AeroMarine Products AM128 silicone for example, will shrink by less than 1% of its volume as it cures which makes the math for figuring out your final dimensions a lot easier.
Next is Library Life, or how well a cured silicone mold will store. After silicone cures and all of the little monomers link up into polymer chains, there’s still going to be various forms of oil and other materials in the silicone that helps it maintain its soft, rubbery nature. These other components are somewhat volatile so they’ll slowly, over a long period of time, evaporate out of the silicone eventually leaving you with a dry crumbly mold that isn’t really good for anything anymore. Again here, cheaper Tin-condensation cure silicone loses out a bit to Platinum cure silicone. In general cured tin-condensation cure silicone will not store as well for as long as cured platinum cured silicone, but again AM128 silicone is the exception and enjoys a library life almost, but not quite, as long as most platinum cure silicones.
Another consideration for silicone is using it with food. After a silicone mold has been made, the volatile oils and other components will slowly leach out as we discussed previously. This means that not only will they come out over time while you’re storing it, but also if you try to use food with it. Platinum silicone is ideal for use with food because the rate of leaching is so low that it won’t affect the food. Even AM128 is not immune to this, however AeroMarine Products does carry an FDA-approved Food Grade Silicone that is great for use with molding chocolates, candies, and other food items.
So what silicone do you need for your project? If you’re making a mold from non-sulfur clay, for casting in things that won’t be eaten, and you don’t need to be able to store it for years at a time, then a tin-condensation cure silicone like AM128 or AM125 will be for you. While we prefer AM128 for being a little more economical and everything we talked about above, AM125 is very popular with new mold makers for its ease of use. The decision between AM128 and AM125 really comes down to your preference for mixing; AM128 uses a thin, liquid catalyst while AM125 uses equal volumes of liquid silicone.
For food projects you’ll need food grade platinum silicone. For this we carry both FDA approved Food Grade Silicone, and food grade Silicone Putty. The silicone putty sets and cures pretty quickly, but is fantastic for making new molds on the fly. The FDA approved Food Grade Silicone takes more time to cure, but works well for making a mold that allows you to create a mold that lets you do multiple items at once.
Still have questions or maybe a project that doesn’t fit neatly into one of the categories outlined above? Give us a call at 877-342-8860 or email us at [email protected] and we will be happy to discuss your project with you.