Last night I started work on a second RTV Silicone mold of a troll. I intend to cast a couple of units to use in a Throgg list and I'll convert them after casting for variation in the unit.
Here is a photo guide of how I built this mold.
Step one: Plasticine and lego
roll out some plasticine to about 1cm thick, Press the master into the plasticine so it is about half way in. Smooth the plasticine flat next to the master. Square up the edges and build a Lego retaining wall. Marbles pressed into the corners help the mold lock together. strips of sprue allow the resin to flow between the base and the troll. I press a Lego brick into the plasticine around the model to help it lock together.
Once this is done I coat the whole lot in a very fine film of vaseline to act as a mold release agent.
Step two: weights and measures
I covered the mold with a sheet of cling film then filled the mold with flour to get the weight of silicone I would need. Further down the line I found this didn't work well enough as there was too much air in the flour so I hadn't mixed enough silicone. I'll use uncooked rice next time as that worked well for the first mold I made. I needed 175g of Silicone to fill the mold so I measured 17.5g of the catalyst as the silicone I'm using mixes at 10% catalyst by weight. I then made up the weight to 175g with the Silicone.
I carefully mixed the silicone so it was an even colour. any unmixed bits do not set leaving a sticky runny mess.
Step three: Pouring
I drizzled the silicone over the model carefully to try and minimise air bubbles. There is no rush to complete this next stage. The silicone stays workable for at least an hour and a half.
once I'd got this far I poured the rest of the silicone from a reasonable height in a thin stream into the lowest spot of the mold. This stretches the silicone to remove air bubbles. Air bubbles are the enemy in mold making as they will wreck the detail on your master and weaken the mold.
After pouring I vigorously jiggled the mold to rid it of as much air as possible. Professional mold makers use a vacuum chamber to pull out as much air as possible but that equipment is expensive.
Step four: the waiting game
Once you are happy that you have got as much air out as you can. Put the mold somewhere flat and level and go and do something else. It takes at least 12 hours before you can do the next bit.
I'll add the next stage later. I'm just about to go and do it