Monitoring of injection molding of thermoplastics: Average solidification pressure as a key parameter for quality control

Abstract

The quality of the molded parts depends on the processing conditions, which creates a continuous demand for advanced techniques for monitoring and controlling the process. Temperature and pressure transducers are being used increasingly in industry. Therefore, the use of their measurements to obtain indications concerning the product quality could overcome the traditional resistance of industry to introduce new sensors in their production. The aim of this paper was to select a parameter, which can be identified by adopting pressure and temperature transducers, to correlate with the quality. To this goal, several series of injection molding tests were conducted by adopting a general purpose PS, changing the holding pressure and time, injection time, mould and injection temperature, nozzle length, gate and cavity thickness. The pressure histories and temperature evolution at the interface with the mould at several positions along the flow-path were measured by means of pressure-temperature transducers. The samples were measured after molding, and dimensional accuracy was assumed to be the target quality parameters. The mean solidification pressure, namely the average along the thickness direction of the pressures at which each layer solidifies, is a key parameter for quality control. A procedure not requiring simulation of the entire injection molding test and based on the knowledge of both the local pressure evolution, mold temperature and the polymer thermal diffusivity has been developed to evaluate the local solidification profile.