Shrinking a model is a common request from users of mold making and casting materials. The challenge is to shrink a model uniformly and with a high degree of predictability. The procedure should also be repeatable. Overview; A user can shrink a silicone rubber mold 18% - 22% by mixing NOVOCS silicone solvent with the silicone.
It is possible and extremely easy to reduce the size of an original without any special equipment. Comparison of original sized model measuring 3” (7.6 cm), a casting out of a mold reduced in size to yield a casting that is 2 3/8” (6 cm) and a casting out of a mold reduced to yield a casting that is 1.9” (4.8 cm).
A decorative rosette model, that measures 3" (7.6 cm), will be used for this demonstration. A small base is used to mount the model and then secured with a hot glue gun. After that, a plastic cup is cut and used to contain the liquid silicone.
Mold Star™ 16 will be used in this project because of its quick setup and demold times. NOVOCS™ is a silicone solvent that does not contain VOC’s. Mold Star™ Parts A and B are dispensed in equal amounts. 2 parts of NOVOCS™ are dispensed. Proportions are 1A:1B:2 parts NOVOCS™. All components are mixed thoroughly for 3 minutes. Viscosity of the mixture will be very low.
Silicone is then poured over the model and allowed to cure for 2 hours.
The plastic cup is removed from around the mold. The rubber mold is carefully peeled away from the model. Rubber is very weak at this point and has low tear strength.
The mold is allowed to shrink for up to 10 days. Mold is ready for casting and will yield castings that at least 20% in size vs. the original. In the gallery below is a comparison of original sized model measuring 3” (7.6 cm), a casting out of a mold reduced in size to yield a casting that is 2 3/8” (6 cm) and a casting out of a mold reduced to yield a casting that is 1.9” ( 4.8 cm).