We discussed how to make the mould bigger, and this method needs little to no measuring of the solvents. It is just a matter of keeping the mould submerged in the solvent to keep it covered,
A casting reduction also depends on the addition of a solvent, however, this time the solvent is added to the rubber and catalyst before mixing of all the components.
Once again, this only works with addition (platinum) cured rubbers.
Use a 1:1 Addition cure rubber. Part A and Part B are mixed together with solvent and de-gassed.
Since the mould will have excessive amount of solvent, the original model must be solvent resistant or at least have a suitable surface coating that will seal it against attack from the solvent. Original models made of wax or plastic clays would be susceptible to solvent attack. Care must be taken to thoroughly coat and seal these models.
An alternative would be to make a mould of the original in the normal way and cast with a casting resin that is impervious to solvent attack.
Once the solvent and silicone components are mixed together, the solvents will alter the cure times of the rubber. The more solvent added the slower the curing time.
Once the mould has cured so that the surface is no longer tacky, the piece must be rapidly demoulded. However the solvent added silicone mould will lack the strength of a standard mould and will be more prone to tearing, so added care must be used when de moulding.
As in the case of the enlarging process the solvents will immediately start to evaporate. As it does the mould will begin to shrink. The shrinking amount is dependent on the amount of solvent added to the silicone at the beginning.
As a rule of thumb a mould made with 1 part silicone and 1 part solvent ratio, will shrink to around 85% of the original if it was left for 3 days. If the mould is left for a 7 day period it will shrink further to about 80% of the original. i.e. the longer it is left the smaller it will get. After 7 days there will be no further reduction.
The size differences are not as great as the differences that can be achieved in the enlargement method. Further reduction can be had by continuing to repeat this process.
