Mold Making Part 2 - Mold Side 1
Prepare you original; - If you are reproducing a part or existing object you have, clean it well. The item cannot be porous or it will get stuck in the mold. You may have to use a sealer on the surface.
Choose your parting line carefully. You need to decide where your parting line will be. That is where the two halves of the mold will separate. Look for undercuts in your part that will prevent you mold from coming apart. Knowing that you may have a seam visible on your finished part will also help you decide where it should be. Try to line it up with a straight or flat side of you part if you have one. Also make note of the original part's seam, if there is one. Try to match your seam to the original one, if you can.
Build your molding box - I use my son's LEGOS to make the molding box. It's quick and cheap. You can adjust the size to fit you project. Now if you are molding something of size, say bigger than a softball, you may want to build a wood or metal mold box. For smaller hobbyist type of molding, the LEGOS are sufficient.
Add a layer of Sulfur Free Modeling Clay about 1/2 inch thick.
Carefully press your original into the clay. I then use the rounded end of a pen to make registration "holes". These holes will help keep the mold together and align the top and bottom half of the molds. Also notice the "cone" on the top of my part. That will form the funnel opening where the resin will be poured into. You can form this out of clay. In this case I used a piece of plastic and topped it off with a little clay. This opening needs to be on the top of the mold (where you will be pouring the resin in). Try to put it on a spot on your part that will not be noticeable.
Measuring your Liquid Silicone Rubber. The silicone rubber I use has a ratio of 10:1, rubber to catalyst (by weight). Use a postal scale to get an accurate measurement. Put the empty container (plastic cup) on the scale first and zero it out. Then add the silicone rubber.
Degassing your liquid rubber - This is the fun part because you get to use your Vacuum Pump. You'll want a food vacuum 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 its 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. Give the rubber a minute to settle.
Start by pouring the rubber into the mold at a low point. Let it creep up over the part. The less air you trap during the pouring phase, the less chance of a bubble ending up in the wrong place in your mold.