Pouring the mould

Now, we mix up our silicone. For the mould itself, we're using a low-viscosity silicone that mixes at a 10-1 ratio. It's much firmer than Gel-10 and has much more resilience. It can even handle up to 500 degrees Farenheit temperatures (in case we feel like making some items out of metal). If you want a mould to last, this is just the kind of material to use.

Once the silicone is mixed, it's a good idea to put your mix into a vacuum chamber. If you don't have access to one, it's okay. But a vacuum chamber helps release any air bubbles in the silicone you just mixed. In the following image, you can see how the silicone froths up. Once a majority of the bubbles have popped, you can release the vacuum and take your silicone out. Be careful though. If your mixing containers are too full, the froth can boil over:

While you're waiting for the bubbles to pop, you can spray some mould-release spray into the mould box with the part inside. Be very liberal with the spray. Make sure everything is glistening. This release spray is an oily material that keeps the silicone from adhering to the box and original part. Don't wait too long though. Some silicones cure very quickly (Gel-10, for instance, completely solidifies in under 10 minutes).

Now, you can pour your silicone into the mould. Tilt the box around to different angles, and even tap on it a bit to shake any air bubbles loose. We need the silicone to get to every area of our part. Pour in enough to completely cover your part, with enough over the top to keep it from tearing when removing parts. You can see the poured mould in the following image. Notice that we added a couple of rubber bands to keep the sides from splitting during the upcoming steps:

Also notice that we left plenty of room at the top of the mould. Why? Because we're going back to the vacuum chamber to make sure we have no air bubbles around the edges of our part in the silicone. In the following image, you can see how far up the silicone froths. We don't want to make a huge mess:

Granted, it's a bit difficult to see past the residue on the clear lid. However, if you look close, you can see that the silicone froth has come up all the way to the top of the mould box. After most of the bubbles have popped, we transfer the mould to the pressure chamber (as shown in the following image):

Again, these chambers are not required. However, it does make for better moulds. You may wonder, "Why vacuum air out, and then put it back in?" Well, we're not injecting air back into the mould. By increasing air pressure around the mould, we're forcing any remaining air bubbles within the silicone to shrink to their smallest size possible. We're also forcing silicone into all the nooks and crannies of the part. As this particular silicone sets in 30 minutes, we wait at least that long before removing the mould from the pressure tank. We increase the pressure to about 40 PSI, and let it sit.

Once the silicone is set (we test by checking the leftovers in our mixing containers), we remove it from the pressure tank, and begin the demolding and curing process.