1, For casting big mould, Platinum cure type silicone will be better than tin cure type silicone.
Because tin cure type silicone has a shrink after cured (~0.3%), so it is more serious if the mould too big. However, platinum cure silicone is no shrink on cured, it’s size can keep very stable.
2, if you use tin cure silicone to cast a big mold, below are some suggestions.
a), Do not add too much hardener, too much hardener will let the silicone curing too fast, you don’t have enough time to pour.
Our recommended mixing ratio is silicone & hardener 100:2~3 by weight. (not by volume). if it’s still too fast, you can try to reduce to 1%~1.5%.
You can control the pot-life (working time) by reducing or increasing the ratio of hardener. Less hardener longer pot-life; more hardener shorter pot-life.
b), Applying mold release agent on the original model is necessary, recommend release agent: wax, vaseline, soap water etc.
c), Before pouring, vacuum degas silicone rubber is a must, otherwise, the bubbles inside the mould will break the strength of the mold, also, bubbles can affect you by copying fine details from the original model.
d), To get a best strength, your mold require enough thick (generally > 3cm).