Development of a rapid surface temperature variation system and application to micro-injection molding

In conventional injection molding, the mold temperature control is obtained by a continuous cooling method, in which a coolant with constant temperature is circulated in the cooling channels to cool the mold and the polymer melt. During the filling stage, this causes an abrupt polymer solidification close to the mold surface, which reduces the section open to flow and, due to the viscosity increase, causes a decrease of the ability of the polymer melt to fill the cavity. This issue is particularly significant for micro-injected parts in which high aspect ratios are precluded because of premature solidification. In this work, a system for rapid surface temperature control was designed, built and applied to a cavity for micro-injection molding. The system consists in an electrical resistive thin component and an insulation layer and can increase the mold surface temperature of some tenths of a degree Celsius in a time of the order of one second. The system is versatile enough to allow the control of thermal histories during the whole process and at different positions inside the cavity. Injection molding tests were then carried out with this system by using a general purpose isotactic polypropylene and a cavity 200 mm thick in order to check the effect of surface heating on reachable flow length and morphology of the molded parts. The effect of mold temperature on the flow length was as expected dramatic on both the flow length and the obtained morphology: the samples molded with a high mold temperature presented a spherulitic morphology in the whole cross-section, while with a low surface temperature the spherulitic morphology was detectable only at the positions close to the midplane whereas the layer closer to the surface present a very oriented structure due to flow taking place at low temperature.

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