Shear heating parameter of rubber compounds useful for process control in injection molding machine

Control of the injection molding process of rubber is made complex by the influence of many parameters. Shear heating, heat generation due to viscous dissipation, is a phenomenon largely exploited in rubber to lower compound viscosity, particularly in the extrusion phase, but it is impossible to predict it from laboratory tests. With the aim of providing useful tools for process control, a parameter for on-line monitoring of shear heating phenomenon is proposed. This parameter is based on direct measurement of rubber surface temperature by infrared thermal camera at the nozzle outlet of the injection molding machine extruder. The measured rubber temperature is a process indicator to give the thermal history of the rubber injection and process safety. Measured temperature increase is then converted into a parameter having dimensions of viscosity, the shear heating parameter, gSH. Four different rubber compounds are investigated. The gSH results are compared to minimum torque (ML) from routine rheometric laboratory measurement. The calculated gSH values increase with increasing ML values. The ML value is a rough indication of the rubber compound viscosity, where its relevant variations affect the rubber processability. Meanwhile gSH is more sensitive to small processability variations and more accurate because it takes into account the thermal history during the injection stage. The found relationship provides deeper insights into the behavior of rubber, compared to laboratory tests, and how it can be used in the industrial practice to improve the process control by monitoring the shear heating effect during the injection stage.