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Hot Runners 12 MoldMaking Technology —— APRIL 2017 This classification of polymers helps to identify the material you are working with, so the appropriate hot runner system is used. By Mohammad Rafi When molding high-temperature resins, direct gating with a hot runner system can save molders time and money. Optimize High-Performance Polymer Molding H ot runner systems were introduced in the 1960s to mold commodity polymers such as polypropylene (PP), polyethylene (PE) and polystyrene (PS). Since that time, technology innovations have improved the capabilities of hot runner systems. The industry has evolved from hot runners direct-gating parts molded from commodity polymers to hot runner systems producing parts from engineering polymers such as polycarbonate (PC), polyethylene terephthalate (PET) and polyoxymethylene (POM), also known as acetal, polyacetal and polyformaldehyde. Engineering resins are high-strength plastic materials that have superior mechanical and thermal properties over commodity plastics. They are often resistant to high temperatures, wear and corrosion. For engineering or high-temperature polymer applications, care must be taken to ensure that the hot runner manifold is thermally stable with no dead spots or areas where the mate- rial can sit and degrade, and without hot or cold spots. These resins are temperature and residence-time sensitive, which means that the material will degrade if exposed to heat for an extended period of time. For this reason, the total runner volume in the hot runner system should not exceed a few shots. The injection molding machine should be sized so that the total shot weight of the parts is about 70 percent of the machine shot capacity. Therefore, the molten plastic will not remain in the bar- rel over numerous cycles. Due to a lack of understanding of this criteria, cold-runner tech- nology has been primarily used to mold parts from high-tem- perature polymers (see Figure 1). Producing parts with a cold run- ner could be unnecessary as engi- neering resins are expensive, and in many cases the cold runners cannot be reground and recycled. Any cold runner applications that FIGURE 1 Figures courtesy of Husky Injection Molding Systems. Classification of Plastics