Experimental investigation and numerical simulation of the microinjection molding process through an expanding flow configuration

Abstract

The dependence of the induced morphological layer variations on the processing conditions and parameters during injection molding of polymers is analyzed through a robust numerical framework of the complete microinjection molding cycle. Predicted temperature, heat transfer and viscous dissipation, spherulite diameters, and shear rates provide sufficient clarifications to develop a deeper understanding of the complex evolution of the induced thicknesses of layers. The evolution of the structure of polyoxymethylene (POM) under strong strain rates and high thermal gradients is investigated while flowing along an expanding flow configuration composed of three steps of increasing thickness. High and low mold temperatures and injection velocity levels are tested according to the design of the experiment method (DOE). Morphological development in each zone was examined to provide the induced crystalline layer thickness in the longitudinal as well as the transverse directions using polarized light microscopy (PLM). The thickness of the layers strongly depends on the local thickness of the stepped‐part and on the abrupt dimensional changes. The variation of bulk tensile properties obtained by dynamic mechanical analysis (DMA) is related to the thermomechanical history experienced by the melt.

Microinjection molding of polyoxymethylene stepped‐parts: Morphology, crystallinity, shrinkage, and thermal characterization

“…The thermal crystallization kinetics of polyoxymethylene (POM) is achieved through Schneider equations, coupled and solved in tandem with the dynamic flow equations using the finite element method (FEM). 28 We considered the thermal joint resistance approach to simulate the intricate heat transfer along the melt-mold interface. 29 Furthermore, the rheological behavior of polymer is modeled through the Cross-Arrhenius model, while physical and thermal properties of homopolymer POM were integrated within the numerical framework to perform our simulations.…”

“…We therefore think that thermal conditions are the main driver for the core layer crystallinity, as material transformation in the core region is mainly controlled by thermal gradients. 49 Likewise, the crystallinity level of the core layer increases with thickness. 50 The skin-layer crystallinity displays an opposite trend, where it is relatively reduced for POM2 and POM3 micromoldings.…”

“…The continuity and complete Navier–Stokes equations in the presence of surface tension force were coupled with a conservative Level‐set method to predict the flow front interface movement. The thermal crystallization kinetics of polyoxymethylene (POM) is achieved through Schneider equations, coupled and solved in tandem with the dynamic flow equations using the finite element method (FEM) 28 . We considered the thermal joint resistance approach to simulate the intricate heat transfer along the melt‐mold interface 29 .…”

Bonding strength calculation in multicomponent plastic processing technologies

“…Simulations in process design require a strong mathematical background for each phenomenon that occurs during the studied process. On the one hand, injection molding is a well-known field for simulation [23][24][25] , and several CAE packages are available for modeling injection molding. On the other hand, the simulation of the bonding strength between the substrate and overmolded elements is missing in all available simulation software.…”

“…Moreover, the temperature of the base plate and that of the overmolded part are also different. Several studies [21,25] proposed a method that combines diffusion theory with the 1D temperature field simulation to assess adhesion at the welding interface. However, none of them provides a relationship between the processing parameters and bonding strength.…”