Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 22

The modulation of the mold temperature during injection molding is a strategic issue since it allows modulating and calibrating interesting properties of the moldings (27).

Thin heating devices (28) were layered on the cavity surface allowing its fast temperature evolution between injection and cooling channels temperatures.

The heating device was made up of four layers (27): an 80 µm thick electrically conductive layer of carbon black loaded poly(amide imide), which induces the cavity surface heating by Joule effect, two Kapton electrically insulating layers, one on each side of the electrically conductive layer. A steel layer, 100 µm thick, protects the heating device from the incoming melt. The heating power was chosen such that, when the polymer reaches the cavity, the cavity surface temperature was intermediate between the mold temperature, as held by the cooling channel, and the injection temperature.

The heating devices were made by a conductive layer between two insulating layers with thicknesses selected in order to realize a heating/cooling cycle as fast as possible.

Several tests were performed, injecting iPP, using different heating powers and heating times to analyze the effect of the fast cavity surface temperature evolution on the molding morphology and properties. The heat transfer through the mold was modeled, accounting for the Joule effect in the conductive layer of the heating devices (27).

The injection molding process coupled with the system designed to achieve a fast evolution of the cavity surface temperature can be considered a multiphase process (because during the process the polymer in the cavity changes from the molten to the solid state) during which there is a transient heat transfer in the mold (including the multilayer heating device) that has to be considered simultaneously and combined with the transient heat transfer in the flowing polymer. Commercial software can be adopted to analyze this combination of transient processes but the solution can be obtained only adopting a serial combination of the analysis of the two different processes (27):

1 The injection molding process (the transient temperature distribution inside the flowing polymer for a given mold temperature), and

2 The evolution of the mold temperature distribution for a given polymer temperature distribution.

The solution of one of the two processes can be used as a boundary condition for the other process and therefore the solution of the whole process can be obtained by alternatively considering the transient of one of the two processes, while the other is held at steady state, and vice versa.

To validate the proposed modeling of the heat exchange during the process, the simulated temperature evolutions at the polymer-cavity and the heating device-mold interfaces were satisfactorily compared with the experimental ones recorded during the tests conducted adopting different mold temperature evolutions. Furthermore, pressure evolutions during the process recorded at different positions along the flow path were satisfactorily compared with the simulated ones to validate the predictions of the thermomechanical histories experienced by the polymer (27).