I've only used Smooth-On's Omoo once, when I was attempting to cast something in transparent resin. The transparent stuff must be cast in a tin-cure silicone mold.
Otherwise I use one of Smooth-On's platinum-cure silicones to make my molds. It costs only a few dollars more for Eco-Flex or Smooth-Sil 920, the tear strength is far greater, the molds will last a long long time (it's important to leave the last pour in the mold for storage). They are detail-sensitive to the degree of registering a fingerprint.
I've never had a shrinkage problem, and in fact I've never heard of any silicone shrinking at the mold stage though, as I say, my experience with Omoo is limited. Shrinking is more often associated with the material used for casting.
Otherwise I use one of Smooth-On's platinum-cure silicones to make my molds. It costs only a few dollars more for Eco-Flex or Smooth-Sil 920, the tear strength is far greater, the molds will last a long long time (it's important to leave the last pour in the mold for storage). They are detail-sensitive to the degree of registering a fingerprint.
I've never had a shrinkage problem, and in fact I've never heard of any silicone shrinking at the mold stage though, as I say, my experience with Omoo is limited. Shrinking is more often associated with the material used for casting.