Experimental and numerical investigation of jetting phenomenon in injection molding

• Published in: Journal of the Brazilian Society of Mechanical Sciences and Engineering 2020-04, Vol.42 (4), Article 193
• Main Author: Hua, Shaozhen
• Format: Article
• Language: English
• Subjects: Computer simulation; Engineering; Injection molding; Internal friction; Mechanical Engineering; Melt temperature; Molds; Newtonian fluids; Reproducibility; Shear rate; Shot; Technical Paper; Viscosity; Wetting
• ISSN: 1678-5878; 1806-3691
• DOI: 10.1007/s40430-020-02278-6

Jetting phenomenon is an injection error without thoroughly understanding by far. In this study, short shot experiments and numerical simulations were performed to investigate the trigger factor inducing jetting phenomenon in injection molding. Polycarbonate was prepared for short shot experiments that carried out in a typical end-gated rectangular mold under different injection speed, mold temperature and melt temperature. It was found that repeatability of jetting was poor and with higher melt temperature came more frequency of folding. Then, three repeatable evolutions of jetting were successfully simulated. Through analyzing the quantities during the initial stage of jetting and comparing with those in simple filling pattern, it has been numerically revealed that velocity kept constant before melt impinged on the wall opposite to gate. The pressure did not increase with the increase in melt flow length and was extremely small, which turned out jetting was not a pressure-driven flow any more. Shear rate in the initial stage of jetting was less than 10 s −1 , while shear rate in front of the pillar can be as low as 10 −4  s −1 , leading that viscosity approached zero-shear-rate viscosity and melt can be seen as a Newtonian fluid. Due to without wetting mold wall enough, friction between mold wall and melt was weak as well as the internal friction among layers of melt, which resulted in melt moving without deformation. Based on the results, a reasonable mechanism was proposed that jetting occurred for lack of shear rate. Jetting could be effectively depressed by changing the magnitude or direction of velocity, which provides a theoretical guidance for mold design and products.