Bubble morphological evolution and surface defect formation mechanism in the microcellular foam injection molding process

Abstract

The filling stage of the Microcellular Foam Injection Molding (MFIM) process is a three phase flow process of polymer melt, super critical fluid (SCF) and air. It not only includes the nucleation and growth of spherical bubbles, but also the morphological evolution of the bubbles such as deformation, bursting, and vanishing. There are usually silver marks, spiral lines, pits and other defects on the product surface. In order to effectively control the surface quality, it is significant to reveal the morphological evolution law of bubbles and the formation mechanism of surface defects in the filling stage of MFIM. This paper established an incompressible, non-isothermal, and unsteady three-dimensional mathematical model of multiphase flow. A new setting method of the boundary conditions with the exhaust function on the mold cavity walls was proposed. The problem of temperature solution divergence on the interface between the two phases with a high viscosity ratio was solved through the coupling algorithm of energy equation and PIMPLE loop. The tracking accuracy of micron grade bubbles interface in macroscopic scale flow field was improved though the Adaptive Meshing Refining (AMR) technique. Based on the abovementioned mathematical model, the influence law of the temperature field and velocity field on the bubble morphological evolution in the thickness cross-section of the injection flow field was obtained. The deformation, bursting and vanishing process of bubbles with different initial sizes and locations in the shear and fountain flow field was predicted. Combined with a short shot experiment, the formation mechanism of pits, silver marks and collapses on the product surface manufactured by MFIM was also revealed.