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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