Measuring your Silicone Rubber - The silicone rubber I use has a
ration of 10:1, rubber to catalyst. Use the postal scale to get an
accurate measurement. I put the empty container (plastic cup)
on the scale first and zero it out. Then add the silicone
rubber.
Once the scale has settled , add catalyst equal to 10% of the
weight of the rubber. I then mix it thoroughly
using a popsicle stick and transfer it to a larger container before
the degassing step.
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. I
added another row of Legos at this point because I had mixed quite a bit
of rubber and 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.
Carefully flip over the mold and take out one of the Lego
pieces. This will allow you to slowly peal the clay off of the
part you are molding.
You don't want to disturb the part as it is half buried in the bottom
half of the silicone mold. Peal it off leaving the part in place.
