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How to Shrink or Enlarge a Casting


Here you will find the secret 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. This knowledge comes in handy for many mold making and casting projects and artistic commissions, saving the time and aggravation of re-sculpting a model to a needed size.

Two Ways to Enlarge a Casting

There are two ways to enlarge a casting. The first is to enlarge the mold before the casting in created. While the second method is to enlarge the casting after the mold has been made. Both methods of enlargement, if properly done, will yield a finished casting in correct proportion and detail as its original. Each method depends on a different material. To enlarge the mold, the material used is known as addition-cured silicone. To enlarge the casting the material used is a specially formulated polyurethane rubber.

Method One - Enlarge the Mold

We begin with instructions on how to enlarge the mold in order to enlarge the casting. The first method to create an enlargement of a casting, is approached the same as for making a duplicate size casting, 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 mold is targeted rather than the casting because the enlargement process involves the solvent saturation of the rubber. Saturating the casting would make it awkward to work with and difficult to re-mold. So it is more efficient to enlarge the mold instead.

Before beginning, follow the usual mold making protocol. You should first de-gas the rubber using a vacuum chamber, lacking that, a pressure pot will do. If you don't have either of those basic mold making tools, make certain you vibrate your mold while it is curing to drive 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 container that can be sealed, 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 of the recommended solvents are flammable. So you do not want to expose to open flame or a heat source such as an electric heater. In addition, the fumes are extremely toxic so that when pouring any of these solvents, you must have good ventilation and you should wear an approved NIOS mask.

Once the mold is fully immersed, the silicone rubber will begin to absorb it. Over time (weeks), you can expect an almost doubling in size could be reached with all its details in proportion to the original model. That is, providing that the mold is completely covered in solvent, if not the mold will distort. Patience is required when resizing the mold, as an example, a mold soaked in toluene takes seven days to expand to approximately 130% of the original size. Larger sizes require more time in the solvent. Molds that have been expanded using solvent are very fragile and must be handled more carefully than usual to prevent tearing or damage.

When you are satisfied with the size, or after three weeks the mold has ceased to expand it should be removed from the solvent bath, and the surface should be thoroughly dried. These are volatile liquids which immediately begin to evaporate when exposed to air. As evaporation continues the mold will begin to shrink. So you must be quick to create your casting. Once the surface is dried, a good casting choice is a two-part polyurethane casting resin. Though silicone doesn't ordinarily need a mold release, we do suggest using a silicone-based release as it will be de-molding much easier.

Now that the new casting has been created, you can repeat the entire process of silicone mold enlargement using solvent immersion to further increase the size of the final casting.

Method Two – Enlarge the Cast

The second method of casting enlargement employs a specially formulated polyurethane rubber that when immersed in water expands proportionally over time to create a perfect enlargement of the casting. All polyurethane rubbers are hydrophobic (doesn’t like water). However, with this polyurethane formulation something different takes place. The polyurethane is actually attracting water to and through it. When water comes in contact with its surface, water molecules are attracted to the atomic charge of the polymer molecules. The polymer molecules actually pull water away from the adjacent molecules of the surrounding water and absorb the water internally. As water is absorbed, the casting stretches to accommodate the incoming water. The water is so tightly held in the casting that it feels dry to the touch. Even cutting or crushing the polyurethane casting will not easily release liquid water. Migrating water molecules uniformly surround each polymer molecule, evenly stretching the polymer matrix in all directions. Through this process this specialty polyurethane rubber produces near perfect expansion of molded shapes by soaking in ordinary tap water.

To create the expanded casting, the polyurethane rubber is thoroughly mixed together at a one-to-one ratio. It then should be de-gassed as described above. After degassing it is poured into a silicone mold that has been treated with a mold release. The urethane rubber cures overnight and is then de-molded. It has a slight rubber-like feel.

The new casting is then placed in a container that can be sealed and completely cover with tap water. The container is sealed and left undisturbed for up to three weeks. After three weeks it has enlarged to approximately 160% and kept it proportions. Once it is removed from the water, a mold must quickly be made of the enlargement, as just as with enlarged silicone mold, evaporation immediately begins which results in a shrinking casting.

Traditionally a silicone mold is made of the enlargement so when a finished casting is de-mold from it, it will be 160% the size of the original and in proportion too. The enlargement process can be repeated many times limited only to the size of the container of water.

Reducing a Casting

The methods described here of enlarging a casting requires very little to no measuring 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, to keep it covered. A casting reduction also dependents upon the addition of a liquid – specifically a solvent. However, in the reduction procedure the solvent is actually combined with silicone catalyst and base before mixing of all the components.

Thus, for this reduction method, a fast setting addition-cured silicone is selected to create the mold. These are platinum catalyzed one-to-one mix ratio silicones. Part A and B are mixed along with the solvent and then de-gassed to remove air bubbles just as in the methods for enlarging described above.

Since the mold will have an excessive amount of solvent above the normal levels of silicone rubbers, the original 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 this would be wax or plastic clays which would be dissolved by harsh solvents. Care must be taken to thoroughly coat and seal these materials. As an alternative you may first need to make a regular mold and casting it in a casting resin which is impervious to solvents.

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 piece needs to be rapidly de-molded. However, the solvent-added silicone mold will lack the strength of a regular mold and will be more prone to tearing, so added care must be used when de-molding.

As was the case with the enlarging procedures, the solvents will begin to evaporate. As they do, 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 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. If the mold is left to for a seven day period it will shrink further to about 80% of the original.

These size differences are not as great as the differences that can be achieved in the enlargement methods described above. But further reduction can be had by continuing to repeat the process until it meets the target size requirement.