The process of forming a plastic part by shooting hot, molten plastic resin into the cavity of a mold may seem like a scientific exercise. However, many industry veterans will tell you it’s more of an art. In fact, the injection molding process and tooling design practices have evolved over the years through trial and error, with much of the success dependent on the technician’s intuition, creativity and years of experience. It involved constantly adjusting temperatures, pressures, fill speeds, etc.— until the desired part was achieved.
Plastic injection molding grew in an age when there wasn’t much science connected to it, despite the fact that injection molding is one of the most complex manufacturing processes on earth. Think about it: molders start by injecting a hot, molten fluid, often consisting of blends and numerous additives, into a cold mold where it is simultaneously flowing and hardening. The complex interaction of the plastic, mold design, part design, and process all combine to control the mechanical properties, shrinkage, residual stresses, and warpage of the final molded part.
That may be changing, industry observers say. The trend today is toward a more scientific approach. Materials Sciences departments at major universities are now offering degrees in rheology, which is the study of the flow of matter either in a liquid or semi-solid state such as plastics when heated to high temperatures. Computer simulation software has been developed to try to predict the behavior of plastic when it is injected into molds.
The computer simulations have debunked many of the most cherished myths associated with the industry. For example, for years molders believed that the runners, which are the hydraulic channels through which the hot plastic runs into the molds, could never be the source of molding flaws if they were “naturally balanced.” Scientific research showed naturally-balanced runners could indeed by the source of problems in the molding process.
It seems the future of plastic injection molding lies in the university or company lab, and not on the shop floor. You might say the industry is breaking out of the mold of out-dated production methods.