Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 27

Induction heating is a method that allows obtaining a rapid thermal cycle, so the overall molding cycle time is not increased (34).

Finite Element Model. To analyze the heating and cooling phase of an induction heated injection molding tool accurately, the temperature-dependent magnetic properties, namely the nonlinear B-H curves, an induction heating simulation has been performed (35).

A finite element model has been developed, including the nonlinear temperature-dependent magnetic data described by a three parameter modified Fröhlich equation fitted to the magnetic saturation curve, and solved with an iterative procedure.

The numerical calculations were compared with experiments conducted with two types of induction coils built into the injection molding tool. The model shows a very good agreement with the experimental temperature measurements (35).

It could also be shown that the nonlinearity can be used without the temperature dependency in some cases, and a method was proposed for estimating an effective linear permeability to use with simulation codes that are unable to utilize a nonlinear solver (35).

Multi-turn Induction Heating Coil. An integrated multi-turn induction heating coil has been developed and was assembled into an injection molding tool. This tool contained a glass window, so the effect of induction heating can directly be captured by a high speed camera. In addition, thermocouples and pressure sensors are also installed, and together with the high speed videos, the induction heating and filling of the cavity is compared and validated with simulations.

Two polymer materials, i.e., ABS and high viscosity PC, were utilized during the injection molding experiments. A nonlinear electromagnetic model was used to establish an effective linear magnetic permeability. The three-dimensional transient thermal field of the mold cavity was then calculated and compared with the experiments. This thermal field was transferred to an injection molding flow solver to compare the simulations and experimental results from the high speed video, both with and without the effect of induction heating.

A rapid thermal cycle was proved to be feasible in a mold with an integrated induction coil. Furthermore, it was shown that the process can be modeled with good accuracy, both in terms of the thermal field and in terms of the flow pattern (34).

High-Frequency Induction Heating. The recent trends of miniaturization and multifunctionality in electrical parts have driven the development of molded interconnect devices (MIDs) that contain conductive tracks on a nonconductive base (36).

A polymer/metal hybrid molding technology was developed to fabricate MIDs in a single manufacturing process, without an additional assembly procedure. For this purpose, injection molding was performed to fabricate a thermoplastic carrier that contained negative circuit channels, and die casting was used to fill the circuit channels with metal alloy of low melting point. To increase the flow length of the molten metal through the narrow circuit channel, high-frequency induction heating was used prior to the die casting stage.

The effect of heating conditions on the mold temperature was investigated numerically, and the relevant induction heating conditions were determined accordingly. Induction heating was then applied to the die casting process to increase the flow length enough to be used as a circuit path for fabrication of MIDs (36).