by Fomen38 » Tue Jul 08, 2014 12:56 pm
Obviously I'm not Larry or Mike, and I know they know infinitely more about mold making than I do, but I do have experience in making RTV silicon molds. There are several different types of RTV silicons that you can buy, each differing in flexibility and shore hardness when cured. The HS3 is the most flexible. It's perfect for making molds of things that have deep undercuts that would normally require a 2 piece mold. The HS2 is a little harder and a little less flexible. There are many types of silicone that Alumilite offer, and most (or all) will list the cured shore hardness. The higher the number, the harder the cured product. If you Google search "shore hardness scale", it will show you a graph of shore hardness, and comparable textures to give you a gauge of what the finished product will feel like. If you're making a 2 piece injection mold, the harder the better.
I have a cast piece that I inject casting resin into. It's a 2 piece silicon mold that takes a lead core insert. I then use a BBQ marinade syringe to inject the mixed casting resin into the mold. It works like a champ. My advice would be to make the sidewalls of the mold thick (at least 1" on each side). This will minimize contortion. Also, I use a couple of cut to fit wood planks to put on the outside of the mold, then gently squeeze them together with some ratcheting clamps. The wood pieces are the exact size of the bottom of both halves of the silicon mold. Don't over tighten or your piece will get contorted and not look like your original. You also need to make sure you add adequate ventilation ports. Remember, whatever direction is "up" on your mold, if you have a high spot and there isn't a vent hole, you WILL trap air there, and your plastic piece won't look exactly like your original.
As far as detail is concerned, the silicon is FAR superior to the aluminum. They use a CAD program and a CNC machine to carve out the negative in block of aluminum. Depending on the size of the bit they use, you can get decent detail. However, when you pour a silicone mold, it will pick up every detail- even fingerprints and hair. My advice to you to avoid any air bubbles in your silicone is this: When you mix your silicon with the catalyst (10 to 1 ratio) an stir it in, let it sit for about 5-7 minutes in the mixing container so all the air bubbles can rise to the top. If you have a vacuum pump and can degas the silicon, that's even better. But most people don't. So when you've let it sit for a few minutes, pour a paper thin ribbon of silicon over your original. Once it's covered, get a small, very soft bristled paint brush and paint the silicon in a very thin layer over the piece. This will eliminate small air bubbles. Make sure you tug on the bristles of your brush before doing this to remove any lose bristles. Once you paint the thin layer on, proceed in pouring the rest of your silicon into the mold box you've made. Since the layer covering the piece is bubble free, it doesn't matter if the rest of the silicone has bubbles in it. The bubbles will not penetrate that painted layer. Keep in mind, the brush will be toast after this, so don't buy an expensive brush. You will need 2 of them if you're making a 2 piece mold.
You'll be surprised how well your plastics look when using a silicon mold. Just remember, the properties of silicone are different than aluminum. You may have to let the plastic sit a minute or two longer before extracting it from the silicon. Also (to answer your last question) you should get hundreds or perhaps into the thousands of pours into the silicone. It is well suited to take the heat from hot plastisol. If you want to be absolutely safe though, and think heat might be an issue, get the high temp silicon. It will take over 500 degrees F, and it's very stiff when cured. It's perfect for 2 piece molds where you're injecting molten plastisol inside.
Hope that helps.