The role of the thermal contact resistance in injection mold cooling of poly(ethylene terephthalate)

During a typical injection molding process for thermoplastic polymers, the cooling stage comprises up to 75% of the total cycle time. Factors controlling the rate and efficiency of cooling a plastic part are greatly dependent on the interface between the polymer melt and the inside mold surface. The resistance of this interface to heat flow is often described as thermal contact resistance (TCR). Very little data are currently available to provide accurate and reliable TCR values for modeling cooling behavior of polyethylene terephthalate (PET) during injection molding. A method was thus developed in order to calculate the TCR value between a PET part and the mold wall during an injection molding process. A non-contact optic fiber infrared thermocouple was applied to measure the polymer melt temperature during the cooling of the PET part within the close mold.; The effects of injection molding parameters on PET density and thermal diffusivity were accounted for in the current thermal analysis. The TCR value was found to increase with time during the injection molding process.; The effects of processing parameters on TCR values were also investigated. Holding pressure is a significant molding parameter found to decrease the TCR values by improving the interface contact. Higher holding pressures cause the PET melt to be more densely packed in the mold cavity. Part shrinkage is decreased and consequently the TCR values are reduced.; A mold was designed and built in order to mold PET parts of different thickness at equivalent molding conditions. At the same molding conditions thicker (3mm) parts were found to exhibit higher TCR values than the thinner (2mm) parts. The main reason is that the thinner parts have higher initial densities than the thicker parts. The thinner parts, therefore, shrink less, and their TCR values are smaller.; As an injection molded part cools, shrinkage occurs and the outer surface of the part moves away from the inner mold surface, forming a space or gap. This change modifies the nature of the interface and results in increased TCR values. An ultrasonic measurement system was developed to detect the time required for initial gap formation between the PET part and the mold wall during the injection molding process. The onset of the gap formation varies with the injection molding conditions. Gap formation was delayed as the holding pressure, melt temperature, injection flow rate and coolant temperature increased. (Abstract shortened by UMI.)...