Molecular dynamics study of nonisothermal interfacial bonding in polymer extrusion injection integrated molding

Abstract

For the high-precision fabrication of medical plastic products featuring both large aspect ratios and complex microstructures, extrusion-injection integrated molding (EIIM) has emerged as an effective manufacturing strategy. However, the interfacial bonding mechanism underlying EIIM remains insufficiently understood. In this study, a microscopic model consisting of an injection melt layer and an extruded polymer layer was established. The interfacial bonding energy of EIIM was systematically investigated. Furthermore, the molecular mean square displacement (MSD), the diffusion coefficient of the injection melt layer, and the density evolution of the overall model during the bonding process were analyzed. The simulation results demonstrate that when the stretching orientation is parallel to the bonding direction, increasing the draw ratio from 1.8 to 2.9 leads to an increase in the interfacial bonding energy, from approximately 1,000 kcal/mol to about 1,800 kcal/mol. In contrast, when the stretching orientation is perpendicular to the bonding direction, the draw ratio exhibits a negligible effect on the interfacial bonding energy. Notably, the interfacial bonding energy in the perpendicular orientation is consistently higher than that in the parallel orientation. This difference is mainly attributed to the orientation-dependent interfacial chain arrangement: chains parallel to the bonding direction can more readily penetrate the injection melt under pressure, while chains perpendicular to the bonding direction maintain a higher and relatively stable interfacial contact density, leading to consistently higher bonding energy. This work is expected to provide theoretical guidance for parameter optimization in EIIM and to offer valuable insights into the interfacial bonding mechanisms of other polymer integrated molding processes.