Evaluating Shrinkage in Mold Compounds
Shrinkage from the jewelry master to the final piece of finished jewelry depends on many variables. Shrinkage of the mold material is one variable we can isolate. We do this by vulcanizing a block of the mold material in a mold frame, then measuring the block and comparing it to the orginal dimensions of the mold frame.
Bill Mull at Zero-D Products advises:
“Measure the inside dimensions of your mold frame at room temperature. Use a vernier or dial caliper and take care to get the readings correct.
“Starting with everything at room temperature, very carefully measure the length and width of the vulcanized block. Avoid squeezing the rubber with the caliper. Record your dimensions to three decimal places and then calculate the shrink in both directions. The long dimension should show more shrink than the short dimension. Average the two and you will have a fair comparison.”
This gives us a method to compare the original length plus width of the inside of the mold frame to the vulcanized dimensions of the rubber block.
Michael Knight at Castaldo® uses a different method to measure shrinkage:
“We do it optically. No contact with the rubber. Take a mold frame, fill the bottom half with rubber, and then imbed a short section of a metal millimeter ruler in the rubber. Fill the rest of the mold frame with rubber, vulcanize and let it cool for a day to allow shrinkage to stabilize. Then compare the ruler with the impression in the rubber using a magnifying glass.
“The same rubber compound molded around the same metal master model can produce highly variable final casting shrinkage rates depending on the mold maker’s and caster’s skill, knowledge, precision and attention to detail. Perfection is not automatic.”
In fact, most mold compounds shrink. The injected wax shrinks. The investment shrinks. The cast metal shrinks. The surface of the cast piece is usually abraded and burnished in the polishing process so there is shinkage there. But awareness of the amount of shrinkage in a mold compound is where it all starts.
Knowing the basic shrinkage properties of your mold compound guides the model making process to success.. Compare the various shrinkage rates of molding compounds using the online mold compound comparison chart.