Choosing The Best Mold Material
A few months back, we discussed the differences between Liquid Molding and Injection Molding and described when each method is appropriate. As a follow up, we thought it would be important to focus on Liquid Molding and to discuss some of the tooling options as well as the advantages and disadvantages to using each.
Aluminum is the most commonly used metal when it comes to mold making. Extremely tight tolerances are possible with today’s CNC milling machines. Close to mirror finishes are possible straight from the machine. Mold designers will ideally design in draft to the mold for easy part removal; however, zero draft angles can be accommodated.
Production rates for aluminum molds are limited to one part per cavity per day. Releasing and preheating the mold is often necessary and should be factored in when considering turnover time. Once part removal and mold cleaning are factored in, yield rates of 15-20 parts per month can be expected.
Plastic or composite molds are usually made backwards. A pattern is created first using wax, clay, foam, etc. and then, via liquid molding or fiberglass layup, a mold is formed around it. This method is done using thermoset materials; however some thermoplastic materials can be milled like aluminum. A good example of this is High Density Polyethelene(HDPE) which, because of it’s self-releasing properties, can decrease cycle times.
Liquid Molding and fiberglass layup should be considered for larger parts with 2 or more dimensions. Some of the primary benefits are a lower cost and design change flexibility. Surface finishes are typically as good as the original patterns. Even a fingerprint on the on the original pattern will show up on the cast mold.
Much like plastic or composite molds, silicone and urethane rubber molds rely on a pattern as the primary tooling element. A flexible rubber, such as RTV silicone, is poured over the pattern and allowed to cure. The pattern is then removed and a liquid plastic is then poured into the cavity, replicating the part.
Three-Part Silicone Mold
Silicone rubbers, while more expensive than urethane rubbers, do not require release agents on the pattern or finished mold. They are also rated for much higher temperature environments which should be considered when a post cure is necessary.