Using P(Pressure)-T(Temperature) Path to Control the Foaming Cell Sizes in Microcellular Injection Molding Process

Abstract

Microcellular injection molding technology (MuCell) using supercritical fluid (SCF) as a foaming agent is one of the important green molding solutions for reducing the part weight, saving cycle time, and molding energy, and improving dimensional stability. In view of the environmental issues, the successful application of MuCell is becoming increasingly important. However, the molding process encounters difficulties including the sliver flow marks on the surface and unstable mechanical properties that are caused by the uneven foaming cell sizes within the part. In our previous studies, gas counter-pressure combined with dynamic molding temperature control was observed to be an effective and promising way of improving product quality. In this study, we extend this concept by incorporating additional parameters, such as gas pressure holding time and release time, and taking the mold cooling speed into account to form a P(pressure)-T(temperature) path in the SCF PT diagram. This study demonstrates the successful control of foaming cell size and uniformity in size distribution in microcellular injection molding of polystyrene (PS). A preliminary study in the molding of elastomer thermoplastic polyurethanes (TPU) using the P-T path also shows promising results.

A Review on Microcellular Injection Moulding

“…Although the combined use of GCP and a mould temperature control system can successfully eliminate silver marks, the use of GCP alone increases the solid skin thickness, which compromises the part weight reduction, while the individual use of a mould temperature control system increases the size of the cells and contributes to an uneven distribution of cells in the MuCell ® parts. Furthermore, it was found that the critical parameters in the P-T control system, such as gas pressure holding time, relief time, and the mould cooling speed result in different cell morphologies [ 67 ]. Longer relief times and faster cooling speeds, which can be induced by controlling the holding time, originated thick skin thicknesses and small cell sizes.…”

“…Chen et al [59] developed a novel pressure-temperature (P-T) contro bining a GCP and mould temperature control system together in the mi tion moulding process, obtaining parts with small and uniform cells and Although the combined use of GCP and a mould temperature control sys fully eliminate silver marks, the use of GCP alone increases the solid skin compromises the part weight reduction, while the individual use of a mo control system increases the size of the cells and contributes to an uneve cells in the MuCell ® parts. Furthermore, it was found that the critical par T control system, such as gas pressure holding time, relief time, and th speed result in different cell morphologies [67]. Longer relief times an speeds, which can be induced by controlling the holding time, originated nesses and small cell sizes.…”

Effect of Gas Counter Pressure on the Surface Roughness, Morphology, and Tensile Strength between Microcellular and Conventional Injection-Molded PP Parts

“…Microcellular injection technology [ 24 , 25 , 26 ] is a special process to overcome some problems that the conventional injection molding cannot. Most of the microcellular foam plus GCP technology [ 27 , 28 ] emphasizes improving surface roughness problems.…”

Using Gas Counter Pressure and Combined Technologies for Microcellular Injection Molding of Thermoplastic Polyurethane to Achieve High Foaming Qualities and Weight Reduction

“…Foaming also plays a significant role in the packing stage, resulting in shrinkage reduction, warpage minimization, and the elimination of residual stresses, etc. Studies on the process characteristics of MuCell ® and the associated molded-part properties have been reported earlier [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. Despite its advantages, MuCell ® also faces technological difficulties leading to hesitation in its further application.…”

“…A high foaming density, fine cell size, and uniform size distribution seem to be required to in order to fulfill the formed sole properties. Thus, in this study we extend our foaming control experiences in MuCell ® of thermoplastics [ 17 , 18 , 19 , 20 , 21 ], namely, gas counter pressure and mold-temperature control constituting the so-called P (pressure)-T (temperature) path [ 21 ], to see if it also works for thermoplastic elastomers (such as TPU). Relevant processing parameters including SCF dosage, injection speed, mold temperature, gas counter pressure, and gas holding time on foaming cell size and the associated size distribution were investigated in detail.…”