Silicone Mold Making Guide
Here's part three of my Silicone Rubber Mold Making Guide. Be sure to start with part 1.
De-gassing your rubber. This is the fun part because you get to use your Vacuum Pump. You'll want a container that can hold about 4 times the amount of rubber you will be mixing up. During the degassing process, the mixture will expand 2-3 times it's original size as the air expands. Gather the kids as they'll want to watch this part.
Here's a close-up of the rubber bubbling. Any air that got into the rubber while mixing will expand. When the bubbles get big enough they will pop. It will then stop expanding and reduce back down to where it started. This may take about 5 minutes. Remove it from the vacuum chamber. There may be a few small bubbles, but don't worry about them at this point.
Pour the rubber into the mold at a low point. Let it creep up over the part. You want the rubber to cover your part by at least 1/2 inch. Add another row of Legos at this point if you have extra silicone. A thicker mold is better.
After the mold is filled, you will see some small bubbles come to the surface. Some pros use a vibration table to force the bubbles to the surface. I happen to have a table sander mounted to the corner of my work bench. I turn this on for 3-4 minutes and it vibrates the table pretty good. Any stubborn bubbles that raise to the surface and don't pop, I poke with the end of an Exacto knife.
Allow the rubber to cure at least 16 hours. I usually allow 20 hours.