Abstract
Microcellular injection-molded parts have surface defect problems. Gas counter pressure (GCP) is one of the methods to reduce surface defects. This study investigated the effect of GCP on the surface roughness, morphology, and tensile strength of foamed and conventional injection-molded polypropylene (PP) products. GCP is generated by filling up the mold cavity with nitrogen during the injection-molding (IM) process. It can delay foaming and affect flow characteristics of microcellular and conventional injection-molding, which cause changes in the tensile strength, flow length, cell morphology, and surface quality of molded parts. The mechanism was investigated through a series of experiments including tuning of GCP and pressure holding duration. Surface roughness of the molded parts decreased with the increase in GCP and pressure holding duration. Compared to microcellular IM, GCP-assisted foaming exhibited much better surface quality and controllable skin layer thickness.
Highlights
Microcellular injection molding (Mucell®) is a popular process that can solve the shrinkage/warpage problem for parts made by injection molding (IM) [1,2]
The results show that the cell size in MuCell®, MuCell®+gas counter pressure (GCP), and MuCell®+GCP+precision core back (PCB) processes are 47–85, 23–68, and 1.9–10 μm, respectively
Our investigation tried to correlate the interactions between processing, morphology, and properties of PP and the foamed specimens manufactured in MuCell® and traditional injection-molding processes with/without GCP
Summary
Microcellular injection molding (Mucell®) is a popular process that can solve the shrinkage/warpage problem for parts made by injection molding (IM) [1,2]. In GCP IM the gas exists in the cavity before and throughout the whole process, which can avoid polymer melt bubbling and serve as packing pressure. Bledzki et al [9] utilized the GCP in microcellular IM, which reduced the product’s external roughness from 23 to around 1 μm. Except for the pressure method, in mold decoration [12,13] and co-injection molding techniques [14], which lead to unfoamed skin with a foamed core, there are other options to improve the surface quality. Llewelyn et al [16] used PBA and CBA methods to study the surface roughness improvement by processing parameters of injection molding. Melt temperature was found to be the most effective parameter to reduce the surface roughness. Free foaming will be constrained if GCP is between 1 bar and the critical pressure, which leads to single phase melt. Polymers 2022, 14, 1078 tensile property and flow length were comprehensively discussed, which is rarely reported based on previous work regarding PS molding [22]
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