Development of a rapid surface temperature heating system and its application to micro-injection

Abstract

During filling stage, the constant mold temperature control strategy by circling coolant through cooling channels used in conventional injection molding causes an abrupt polymer solidification close to the mold surface and consequently resulting in a frozen layer. As the viscosity increases, the mobility of the polymer to fill the cavity is largely decreased influencing the frozen orientation. This phenomenon affects greatly the dimensional stability of the part and its optical properties, especially micro-injected parts in which high aspect ratios are precluded because of premature solidification. Recently, several studies have been proposed in literature in an effort to obtain a fast and uniform heating and cooling through the mold surface during filling stage. Inspired in those studies, a new system for rapid surface heating was designed, built and applied to a cavity for microinjection molding. To prove his feasibility as well as attempting to improve results, preliminary tests were conducted comparing theoretical and practical studies. Furthermore, the system consists in a well-balanced electrical resistive thin component and an insulation layer and can increase the mold surface temperature of several tenths of celsius degrees in less than one second. Injection molding tests were then carried out comparing this and a conventional system using both amorphous and semi-crystalline materials as well as different surface temperature for two cavities with a thickness of 250 μm. Moreover, in order to check the effect of those surface heating test, specimens were characterized according to their optical (polarized light and bright-field microscopy), mechanical (tensile tests) and thermal properties (differential scanning calorimetry).