Optimization of heat transfer in injection molds and its impact on process efficiency and part quality

In injection molding, process efficiency and part quality are the most crucial parameters for a successful production. Economic efficiency is influenced by both of them, often with opposed effects. Polymer engineers and mold makers have to find the most economical process configuration. This includes part and mold design, machine and polymer selection and process settings. Mold temperature control is an essential part of the injection molding process having an impact on cooling time, surface quality and inner structure [1]. Beside mold temperature, thermal conductivity of the mold material is a relevant factor. Mold materials for serial production are available with thermal conductivity in the range of 15 W/mK to above 150 W/mK [2,3]. Materials with high thermal conductivity are limited when it comes to processing of reinforced polymers, due to low mechanical properties. Therefore, mold steels with high thermal conductivity in the range of 40 W/mK to 65 W/mK come into consideration. As these mold steels are priced about 7 times higher than conventional mold steels, industry has doubts regarding its application. Therefore, a mold steel of high thermal conductivity is compared to a conventional mold steel. In the study, cycle time using conventional mold temperature control and dynamic mold temperature control [4] was investigated. Furthermore, part quality of semicrystalline thermoplastics such as polyamide, were analyzed. It can be observed that cycle time can be reduced significantly in every mode of mold temperature control. The influence of mold steel thermal conductivity on part properties is relatively low when processing semicrystalline thermoplastics. Therefore, it is worth performing a calculation of part costs in detail, taking mold costs and cycle time into account.