Abstract：In order to reduce weight and improve the dimensional accuracy of plastic products, microcellular injection molding technology is applied more and more in automotive. There are several defects are paid special attention by vehicle manufacturers, such as warpage and cell radius. This research investigated the effects of processing parameters in microcellular injection molding. The study investigated the influence of two kinds of gas foamers (CO2 and N2 ) on microcellular injection molding process, and the following processing parameters such as temperature, time, pressure and gas controlling were considered to research. Design of experiments was employed to perform the experiments. According to experimental analysis, not only the significant processing parameters to warpage were found out, but also the interactions between each factor were investigated. The results of this study could provide microcellular injection molding process design with theoretical basis and practical guidance.

Keywords: Microcellular injection molding; Factorial analysis; Warpage; Processing parameters

1. Introduction

Polymer raw material composes about 70% of the total cost of a plastic product, there are many scholars have done research about how to reduce the weight of plastic products, especially in automotive. The automotive plastic products demand both lightweight and high dimensional accuracy. The microcellular injection molding is employed to reduce the products weight, because the presence of foam cells in the polymer decreases the density of the material, which ensures using less row material. Furthermore, the morphological structure of foam cells can be arranged via controlling the porosity ratio in polymer, so different microcellular foam polymer with varying properties can be generated by using microcellular injection molding, which are proper to be used in different industrial area.

In an attempt to understand the effect of processing parameters, Hong-Bin Wu applied the MuCell  technology to produce an open porous polyurethane structure to identify the key processing parameters and try to quantify the main processing parameters which govern the open pore structure and morphology. They found that the gas content and weight reduction makes a more profound effect on the pore morphology [1]. Jaime Francisco analyzed the effects of several typical injection molding parameters on the foam morphology, apparent density and thermo- mechanical properties of PETG, trying to provide knowledge on microcellular injection molding parameters to control PETG and establish a correlation between the cellular structure and thermal properties and Bo Xie investigated the influence of processing parameters on the foam morphology and its mechanical properties on the parts [3-5]. In 1999, using a pressure vessel, Handa investigated the influence of variables (foaming temperature, foaming time, saturating pressure) on the resulting microcellular foam properties on PETG film samples, also in a batch process [6]. In 2010, Kanehashi analyzed the solubility of gases (CO2, O2 and N2) in crystalline polymers (PET among them), which tends to decrease with increasing crystallinity [7]. That same year, Sorrentino investigated the foam ability of two high molecular weight PETs, performing sorption  tests  to determine the solubility and diffusivity of CO2 and N2 in these two polymers [8]. Salih Hakan designed a series of experiments to study influence of injection pressure and melting temperature on the average cell dimension, cell number, skin layer thickness, foam density and mechanical properties of investigated foam materials were evaluated. It can be learned from the results that cell density is increased by the increment of injection pressure [9]. Chien, using a chemical foam agent, produced PP foams. They investigated the effects of injection speed, the temperature of molten material and molding, and feedback pressure onto the part thickness, and mechanical properties [10].