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.

Foam injection molding of poly(lactic) acid: Effect of back pressure on morphology and mechanical properties

“…These values correspond to the back pressure in the hydraulic system. On the melt, the pressure was about 18 times larger . Being the gas pressure 100 bar, it was not possible to inject gas with back pressures higher than 5 bar (corresponding to about 90 bar on the melt.…”

Solidification behavior of high‐density polyethylene during injection molding process: Enthalpy transformation method

“…Computer‐aided engineering (CAE) technology has found wide application in material selection, mold design, analysis of the part defects as well as optimization of the processing parameters for injection molding 4–7. Attention has been attracted to investigate the heat transfer during the injection moldings of crystalline polymers since early 1990s,8–10 because the quality and performance of molded parts depend heavily on the selection of operational variables11–15 during the molding process, especially in the cooling period.…”

Monitoring of Injection Moulding of Thermoplastics: Adopting Pressure Transducers to Estimate the Solidification History and the Shrinkage of Moulded Parts

“…For amorphous polymers, it can be demonstrated [9] that, after the local solidification time t sol , the local pressure profile follows an exponential law, namely:…”

“…Indeed, it can be easily demonstrated [9] that even the complete pressure curve cannot be adopted as a suitable parameter to fully describe shrinkage, and a criterion based on the reproducibility of the pressure profiles can cause the rejection of parts that are consistent with quality parameters. On the other hand, the local average solidification pressure Ps (the average over the thickness of the pressures at which each layer solidifies locally) was demonstrated to be a suitable parameter for quality part description in the injection moulding process [9]. Determining the local average solidification pressure Ps requires the determination of both the local pressure history and the local solidification history.…”