Computer simulation of transport processes during injection mold-filling and optimization of the molding conditions

The first part of this paper deals with the computer simulation of injection mold-filling at constant flow rate during the production of a cylindrical part under isothermal and non-isothermal conditions. The material of the object is low density polyethylene (LDPE) following power-law viscosity model for non-zero shear rate zone. However, where shear rate becomes zero, “zero-shear viscosity” value has been used. For the non-isothermal fillling the viscosity of LDPE is also a function of temperature. The material of the mold is steel. The concept of melt–mold thermal contact resistance coefficient has been incorporated in the model for the non-isothermal filling conditions. The length and diameter of the body have been taken as 25.4 and 0.508 cm, respectively. The finite-difference method has been used to solve the governing differential equations for the processes. The results show excellent agreement with the analytical solutions for isothermal conditions showing the correctness of the numerical method. The simulation results for the non-isothermal filling indicate physically realistic trends and lend insight into various important aspects of mold-filling including frozen skin layer. The final part of the paper presents the optimization of the molding conditions based on the simulation results for the non-isothermal mold-filling and an empirical relation (published in literature) predicting cooling time. An exhaustive search method has been used for the optimization study. Based on the concept of the injection moldability index (IMI) it is also inferred that between LDPE and polypropylene, the latter is a better material for the production of this object.