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, ηSH. Four different rubber compounds are investigated. The ηSH results are compared to minimum torque (ML) from routine rheometric laboratory measurement. The calculated ηSH 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 ηSH 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.
This work aims to get new insights into the “process monitoring tool” proposed by the authors for the online monitoring of the shear heating phenomenon during injection molding of technical rubber parts. The online monitoring is based on direct measurement of the surface rubber temperature (shear heating temperature, TSH) by an infrared thermal camera of the rubber as it leaves the extruder barrel of the injection molding machine. The measured rubber temperature is a process indicator giving the thermal history of the rubber compound injection and process safety. Therefore, this fast process control is applied to industrial applications to investigate the processing behavior of ethylene acrylate (AEM) rubber compounds. In particular, the relationships between TSH vs injection pressure, vs injection speed, vs screw speed rotation and vs screw length over diameter ratio (L/D ratio) and vs AEM rubber compound properties variation due to exceeding the shelf life are investigated. The results show that TSH is manly influenced by the screw L/D ratio, followed by injection pressure and screw speed rotation (especially if are set to higher levels), whereas the injection speed is the least effective parameter to reduce TSH. Furthermore, a previously found robust correlation between the shear heating parameter (ηSH) and the minimum torque measured in rheometric laboratory tests (ML) is used to show the noteworthy deviation from the proportional trend when the AEM rubber compound exceeded it shelf life. Therefore, the coefficient of determination of $${\text{log}}\eta_{{{\text{SH}}}}$$ log η SH vs ML curves provides a good indication of process stability, while it is running.
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