This is the second part of a three-part article on how to enlarge (also called Pointing up) and alternatively how to reduce 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 might imagine and avoid the cost and complexity of high-tech equipment. The first part of this article describes the steps to enlarge a casting by enlarging the mold. The second part provides an alternative method of casting enlargement by actually enlarging the casting, rather than the mold as described above, in the first part of 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 generally do not mix. For example, in high humidity environments, cure times are adversely affected. If these formulations come into contact with moisture during curing, the surfaces in contact will foam, leaving an imperfect mold or cast. However, once cured, polyurethane is impervious to water.
But with this particular magnification formulation, something different happens. Polyurethane actually draws water into and through it. When water comes into contact with its surface, the water molecules are attracted to the atomic charge of the polymer molecules. The polymer molecules actually draw water from adjacent molecules in the surrounding water and absorb the water internally. As the water is absorbed, the mold stretches to accommodate the incoming water. The water is so tight on the cast that it feels dry to the touch. Even cutting or crushing polyurethane casting will not easily release liquid water. Migrating water molecules uniformly surround each polymer molecule, uniformly stretching the polymer matrix in all directions. Through this process, this special polyurethane formulation produces a proportional expansion of the molded shapes when soaked in running tap water.
To create the enlarged casting, the expanding polyurethane rubber is mixed in a mixing ratio of one to one. For best results, it should be degassed as described above. After degassing, it is poured into a mold that has been treated with a mold release agent. Silicone is the best candidate for the mold material, as polyurethane quickly adheres to polyurethane, so a polyurethane mold should be avoided. Polyurethane rubber cures overnight. The cured rubber should not be sticky to the touch.
Be careful when unmolding so as not to break the cured gum. Fill a container with tap water and immerse the cured cast iron in the water. Make sure it is completely covered in water. Any part of the plaster that is out of the water will not expand properly. Cover the bowl and let it sit for three weeks to allow the water to absorb.
After three weeks, the cast should enlarge to about 160%. Take it out of the container and pat it dry. An enlargement mold should be made quickly, as as with the enlargement of the silicone mold process, evaporation will shrink the casting. Usually a silicone mold is recommended for enlargement, so when a finished casting is created, it will now be 160% the size of the original and in the proper ratio. The enlargement process can be repeated many times, limited only to the size of the container.
This completes Part 2 of How to shrink or enlarge a casting. Part 3 will describe how to shrink the cast.