Introduction
This is a three-part article on how to extend (also called pointing up) and, alternatively, how to reduce 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 might imagine and avoid the cost and complexity of high-tech equipment. This knowledge is useful for many mold making and casting projects and artistic commissions, saving the time and hassle of re-sculpting a model to size.
Two ways to enlarge a cast
There are two ways to enlarge a cast. The first is to enlarge the mold before creating the casting. While the second method is to enlarge the casting after the mold has been made. Both enlargement methods, if done correctly, will produce a finished shell in the correct proportion and detail as your original. Each method depends on a different material. To enlarge the mold, the material used is known as addition curing silicone. To enlarge the casting, the material used is a specially formulated product polyurethane rubber.
Method one: expand the mold
We start with instructions on how to enlarge the mold to enlarge the cast. The first method of creating an enlarged mold is approached in the same way as making a double-sized mold, which is to create a normal silicone mold. In this case, the silicone material of choice is an addition-cure (platinum-catalyzed) silicone, as condensation-cure silicone will not work. Once the mold is created, the next step is to enlarge it. The goal is the mold rather than the casting because the enlarging process involves solvent saturation of the rubber. Saturating the plaster would make it difficult to work and re-mould it. Therefore, it is more efficient to enlarge the mold.
Before you begin, follow the usual mold making protocol. First you need to degas the rubber using a vacuum chamber, if you don’t have one a pressure cooker will do. If you don’t have any of those basic mold-making tools, be sure to vibrate your mold while it’s curing to push air bubbles to the surface. A power tool such as a hand sander placed against the mold is very effective. Once created, place the mold in a sealable container such as a 5 gallon bucket. Then add solvent to the container such as toluene, xylene, or naphtha.
When handling volatile solvents, we recommend extreme caution as all recommended solvents are flammable. So you don’t want to expose it to an open flame or heat source like an electric heater. Also, the vapors are extremely toxic, so when pouring any of these solvents, you must have good ventilation and you must wear a NIOS-approved respirator.
Once the mold is completely submerged, the silicone rubber will begin to absorb it. Over time (weeks), you can expect it to almost double in size with all its details in proportion to the original model. That is, as long as the mold is completely covered in solvent, otherwise the mold will be deformed. Patience is required when changing mold sizes, for example, a toluene-soaked mold takes seven days to expand to about 130% of the original size. Larger sizes require more time in the solvent. Molds that have been solvent-expanded are very fragile and must be handled with more care than usual to avoid breakage or damage.
When you are satisfied with the size, or after three weeks, the mold has stopped expanding, it should be removed from the solvent bath and the surface should be thoroughly dried. These are volatile liquids that immediately begin to evaporate when exposed to air. As evaporation continues, the mold will begin to shrink. So you have to be quick to create your cast. Once the surface is dry, a good choice for casting is a two part polyurethane casting resin. Although silicone doesn’t normally need a release agent, we suggest using a silicone-based release agent as it will release much easier.
Now that the new casting has been created, you can repeat the entire solvent dipping silicone mold enlargement process to further increase the size of the final casting.
That completes the first part of How to shrink or enlarge a cast. The second part will provide details on how to enlarge the mold itself. The third part will provide the details on how to shrink the cast.