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moldmakingtechnology.com 29 through an induction coil. This, in turn, generates a magnetic field that induces eddy currents on surrounding metal objects (see Figure 1). These electrical cur- rents flow in a circular path and generate Joule heating, which is the generation of heat from a current flowing through a conductor (similar to the element of a lightbulb). The conductor is a very thin layer (often less than 1 mm) of metal, such as the magnetic tool steels that are used as inserts. Induction heating provides more flex- ibility for controlling which areas of the mold are heated and what temperature each of those areas reaches. Specific spots on the mold surface can be target- ed for heating by using highly magnetic inserts in those areas to focus the electromagnetic effects. For parts with varying surface textures, applying these inserts in locations where high gloss is required can result in significantly improved surface quality (see Figure 2). With induction heating, the temperature that a particu- lar area of the mold reaches is directly proportional to the amount of time the current is applied. While the rate of heating decreases over time with water and steam heating methods, there is no practical upper limit to how hot the mold can get with induction heating (see Figure 3). This means the mold can be heated appropriately for high- and low-temperature poly- mers with the added benefit of shorter cycle times and lower energy consumption. The induction heating process can be made more efficient by machining grooves into the channels where the induction coils will rest (see Figure 4). This will focus nearly all of the heating power on the molding zone, nearly doubling efficiency. More heat is generated with less power in the area where it is desired, while the bulk of the mold is heated very little. This results in reduced cooling time and reduced overall mold cycle times. Designing an induction heating system comes with its own set of challenges, however. The layout of the coil needs to accu- rately follow the curvature of the part, and it is often designed in the core around other mold components (for example, hot runners, nozzles, lifters, sliders and cooling channels). In addi- tion to the shape and placement of the induction coil, other variables also need to be optimized to ensure that the induction heating system performs as desired. These variables include mold materials, frequency, current and cooling system design. Process Insight Simulation provides a relatively low-cost option for experi- menting with technologies such as induction heating, enabling molders to evaluate the potential benefits for individual Induction heating can be focused on specifc areas of the part by applying magnetic inserts in only those locations, allowing for parts with areas of high and low gloss to be molded on the same surface. FIGURE 2 The heating capacity of water and steam tapers over time, while induction heating has no practical limit to the temperature that can be reached on the mold surface. FIGURE 3 Time (sec.) 300 250 200 150 100 50 0 Temperature (°C) 0 5 10 15 20 25 30