How to Shrink or Enlarge a Casting – Part 3 of 3

This is a third and final part of a three-part series on how to enlarge (also called pointing up) and alternatively, how to shrink the size of castings without the use of laser scanning, CAD/CAM and computer operated CNC machinery. The procedures for both are much simpler than one would imagine and avoids the cost and the complexity of high-tech equipment. The first two parts of this series described the steps on enlarging a casting by enlarging the mold and as an alternate, enlarging the casting. This article will describe a technique on how to shrink a silicone mold to create a smaller casting.

The methods described in the previous articles of enlarging a casting required very little to no measuring out of the enlarging liquids. It is just a matter of keeping the mold submerged in enough liquid, either a solvent, when using silicone, or water for polyurethane, and to keep it covered undisturbed until the enlargement process was completed. A casting reduction also dependents upon the addition of a liquid – specifically a solvent. However, in the reduction procedure the solvent is actually measured out and then combined with silicone catalyst and base before mixing of all the components.

For the reduction method, a fast setting two-part RTV silicone is selected to create the mold. These are either tin or platinum catalyzed one-to-one mix ratio silicones. Part A, the base and Part B, the catalyst are first combined. Then an appropriate amount of a solvent is added and mixed together. The silicone and solvent mix is then de-gassed to remove air bubbles just as in the methods for enlarging process described in the earlier articles in this series.

Since the mold will have an excessive amount of solvent above the normal levels of silicone rubbers, the original (your model or maquette) must be solvent resistant or at least have its surface coated with an application that will seal the original from the effects of the solvent. An example of solvent affected materials would be wax or oil-based clays which would be dissolved by harsh solvents. Care must be taken to thoroughly coat and seal these materials. As an alternative, to solvent-sensitive materials, you may first want to make a regular mold and then create a new casting using a solvent-impervious casting resin.

Once the solvent and the silicone components are mixed together, the solvents will alter cure times. The more solvent added, the slower the curing. But once the mold has cured so that the surface is no longer tacky, the original piece needs to be rapidly de-molded from it. Care must be taken in the de-molding process as the solvent- silicone mold will lack the usual strength of a regular silicone mold and will be more prone to tearing.

As was the case with the enlarging procedures, the solvent will begin to immediately evaporate. As it does, the mold will begin to shrink. The shrinkage amount is contingent upon the amount of solvent mixed with the silicone rubber. A rule of thumb is that a mold made with one part silicone and one part toluene solvent ratio will shrink to about 85% of the original if left for three days undisturbed. If the mold is left to for a seven-day period it will shrink a bit further to about 80% of the original.

These size differences are not as great as the differences that can be achieved in the enlargement processes described in parts one and two. But further reduction can be achieved by continuing to repeat the shrinking solvent-silicone process until the mold meets the target size requirement.

That completes our introduction into the manual processes used to increase or shrink a silicone mold. We hope we have provided you with useful information that you can add to your mold making skills. We want to end by revealing that the generic solvents that we have described here all are known as VOC solvents. That is they all contain VOCs, (volatile organic compounds). VOCs are emitted as gases from certain solids or liquids. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. Concentrations of many VOCs are consistently higher indoors (up to ten times higher) than outdoors. Government studies indicated that while people are using products containing organic chemicals, they can expose themselves and others to very high pollutant levels, and elevated concentrations can persist in the air long after the activity is completed.

Several mold-making and casting material suppliers are now offering silicone solvents that are VOC-free. Though we have described the old-school method that experienced mold makers use to shrink and enlarge molds using traditional solvents, for your health and safety we urge you search out and use VOC-exempt solvents. Finally, all solvents are flammable, and therefore marked as hazardous to ship. Keep them away from open flames, heating devices or equipment that could spark. Close the solvent container immediately after use.

Happy mold making!

Post time: 03-26-2017