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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 expand (also called pointing up) and alternatively, how to reduce the size of the casting without the use of laser scanning, CAD / CAM and computer operated CNC machines. Both systems are much simpler than one might imagine and avoid the cost and complexity of high-end equipment. The first part of this article described the steps to increase the composition by increasing the mold. The second part provides another way to cast expansion by expanding the mold, rather than the mold as described before, in the first part in the expansion process.
This second method of rubber expansion uses a specially formulated flexible polyurethane rubber that when immersed in water expands evenly over time to create expansion. Polyurethane and water are generally not miscible. For example, in areas of high humidity, curing times are adversely affected. If this composition comes into contact with moisture during curing, the joint surfaces, will have foam that leaves an incomplete cast or mold. However, after the polyurethane is cured it is not waterproof.
But with this particular expansion design, something different is happening. Polyurethane actually attracts water and passes through it. When water comes in contact with its surface, the water molecules are attracted to the charge on the atoms of the polymer molecules. The polymer molecules actually pull water away from the surrounding water molecules and absorb the water inside. As water is absorbed, the casting expands to accommodate the incoming water. Water is held so tightly in the casting that it feels dry to the touch. Even cutting or crushing the polyurethane casting will not easily release the liquid water. The water molecules migrate around each polymer molecule, evenly stretching the polymer matrix in all directions. Through this process, this unique polyurethane design produces a uniform expansion of the molded shapes by submerging them in normal tap water.
To make an expanded casting, expanded polyurethane rubber is mixed together in a one-to-one mix ratio. For best results it should be degassed as described above. After the degassing is poured into the mold it is treated with mold release. Silicone is the best candidate for molding materials, as polyurethane adheres quickly to polyurethane, so polyurethane mold should be avoided. Polyurethane rubber cures overnight. Cured rubber should not be delayed.
Be careful when molding so as not to tear the cured rubber. Fill the container with tap water and submerge the cured casting in the water. Make sure it is completely covered with water. As any casting area left without water will not expand properly. Cover the container and leave it undisturbed for three weeks to allow the water to absorb.
After three weeks the absorption should be increased to about 160%. Remove from container and dry. The mold should be made as soon as possible to expand as once the expansion of the silicone molding process, the evaporation will decrease the casting. Usually silicone molding is recommended for expansion so when the finished molding is done now it will be 160% the size of the original and the right size. The expansion process can be repeated many times, limited only by the size of the container.
This concludes Part-2 of this How to reduce or increase Casa. Part-3 will explain how to reduce the frame.
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