How to Shrink or Enlarge a Casting – Part 1 of 3

Introduction

This is a three-part article 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.

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How to Shrink or Enlarge a Casting – Part 2 of 3

This is the second part of a three-part article on how to enlarge (also called pointing up) and alternatively, how to shrink the size of a casting 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 part of this article described the steps on enlarging a casting by enlarging the mold. Part two provides an alternate method of casting enlargement by actually enlarging the casting, rather than the mold as previously described, in the first part in the enlargement process.

This second method of casting enlargement employs the use of a specially formulated flexible polyurethane rubber that when immersed in water expands proportionally over time to create the enlargement. Polyurethane formulations and water usually don’t mix. For example, in high humidity environments, cure times are adversely effected. If these formulations come in contact with moisture during curing, the surfaces that are in contact, will foam leaving an imperfect casting or mold. However, once cured polyurethane is then impervious to water.

But with this particular enlargement 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 formulation produces a proportional expansion of molded shapes by soaking in ordinary tap water.

To create the enlarged casting, the expanding polyurethane rubber is mixed together at a one-to-one mix ratio. For best results it should be de-gassed as described above. After degassing it is poured into a mold that has been treated with a mold release. Silicone is the best candidate for the mold material, as polyurethane sticks fast to polyurethane, so a polyurethane mold should be avoided. The polyurethane rubber cures overnight. Cured rubber should not be tacky to the touch.

Be careful in de-molding so as not to tear the cured rubber. Fill a container with tap water and submerge the cured casting in the water. Make certain that it is completely covered with water. As any part of the casting that is left out of the water will not expand properly. Cover the container and leave it undisturbed for three weeks to allow the water to be absorbed.

After three weeks the casting should enlarge to approximately 160%. Remove it from the container and pat dry. A mold must quickly be made of the enlargement as just as with enlargement of the silicone mold process, evaporation will shrink the casting. Usually a silicone mold is recommended of the enlargement so when a finished casting is created it will be now be 160% the size of the original and in proper proportion. The enlargement process can be repeated many times, limited only to the size of container.

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