On-line measurement of cavity pressure during injection molding via ultrasonic investigation of tie bar

The injection molding process is one of the most important procedures in the production of plastic products. Cavity pressure history during the injection molding process is a factor that dramatically affects the properties of the product. This study proposes a non-destructive method for measuring cavity pressure by evaluating stress on the tie bars of the injection molding machine using ultrasonic technology. Both theoretical discussion and experimental results are presented in this study, and the correlation between ultrasonic signals and stresses on the tie bars are further determined by a magnetic-type clamping force detector. The method is then precisely calibrated with an R-squared value up to 0.99962 in average. Following this, it is applied to measure the cavity pressure and proves feasible, with relative errors within 4.3%. The influence of the clamping force on the measured cavity pressure curve is also investigated and results indicate an appropriate clamping force should be selected for more precise results. This method can be applied to online monitoring in the injection molding process to detect parameter variations and indicate product properties. It has the advantage of high stability, being non-destructive, online and low cost, and can be widely promoted in injection molding industries. (C) 2018 Elsevier B.V. All rights reserved.