Measurement of pressure–volume–temperature diagrams based on simulated melt temperature and actual cavity pressure

Abstract

Pressure–volume–temperature (pvT) diagrams illustrate the behavior of a polymer melt during injection molding and can be used to determine the optimal process parameter settings for producing quality parts. However, the measurements of pvT diagrams are costly and time-consuming and have limitations for process control. This study generated an economic pvT diagram based on sensor data and simulation analysis. To avoid the use of expensive infrared sensors and inaccurate pressure simulations, a pvT curve was constructed by combining the melting temperature, which was simulated using computer-aided engineering, and the actual cavity pressure. The specific volume of the polymer melt was then calculated using a modified Tait model. The pvT diagram could be used to investigate the changes in specific volume at various injection stages, which could be applied to quality prediction. In this study, a flat plate with uneven thickness was used as the research vehicle. The specific volume of each sensor position of the plastic component cooled to room temperature was calculated. Thereafter, a correlation analysis between the changes in specific volume and the shrinkage ratio of the plastic part was conducted. The experiment indicated that both variables were highly correlated. Therefore, monitoring changes in specific volume could predict part size. Additionally, the proposed measurements of the pvT diagram could be used to compute the cooling time, which greatly affects the quality and production efficiency of injection-molded parts. A 25-s cooling time setting demonstrated a stable molding quality compared with the traditional heat transfer formula (16 s) and the commercial simulation (17 s).