Plastic injection molding plays a key role in the production of high-quality plastic parts. Shrinkage is one of the most significant problems of a plastic part in terms of quality in the plastic injection molding. This article focuses on the study of the modeling and analysis of the effects of process parameters on the shrinkage by evaluating the quality of the plastic part of a DVD-ROM cover made with Acrylonitrile Butadiene Styrene (ABS) polymer material. An effective regression model was developed to determine the mathematical relationship between the process parameters (mold temperature, melt temperature, injection pressure, injection time, and cooling time) and the volumetric shrinkage by utilizing the analysis data. Finite element (FE) analyses designed by Taguchi (L27) orthogonal arrays were run in the Moldflow simulation program. Analysis of variance (ANOVA) was then performed to check the adequacy of the regression model and to determine the effect of the process parameters on the shrinkage. Experiments were conducted to control the accuracy of the regression model with the FE analyses obtained from Moldflow. The results show that the regression model agrees very well with the FE analyses and the experiments. From this, it can be concluded that this study succeeded in modeling the shrinkage problem in our application.70012

1. Introduction

Injection molding is one of the most efficient processes in mass production of manufactured plastic part with thin-shell features. The quality of the plastic part depends on the material characteristics, the mold design, and the process parameters, one of which is more important (Ref 1-3). Several studies found that the injection molding process parameters have crucial effects on the quality of the plastic parts (Ref 4-6). They investigated the problems of the injection molding part such as the shrinkage, warpage, weld line, sink marks, and residual stress generated by the process parameters. Their studies also show that the most important parameters affecting the quality of the plastic parts are packing pressure, melt temperature, and mold temperature. However, these studies did not examine in sufficient depth the effect of the other process parameters including injection time and cooling time. Further, Demirer et al. (Ref 7) have conducted an experimental research that evaluates the shrinkage and the warpage causing to the problems of the part quality. This research also explained that the shrinkage and the warpage increased with increasing the

H. O¨ ktem, Gebze Vocational School, Department of Industrial Molding, University of Kocaeli, 41410 C¸ ayırova, Kocaeli, Turkey. Contact e-mail: hoktem@kocaeli.edu.tr.

process temperature, decreased with increasing the injection pressure. In this research, although experimental conditions are enough to provide the valuable results, the major influential process parameters consisting of the injection time and the cooling time are not considered.

In the injection molding of the plastic parts as thin-shell features, many published papers have indicated that a statistical relationship can be built between the process parameters and the problems associated with the shrinkage and the warpage affecting the quality of the plastic parts (Ref 8-10). In the prior studies, a number of experiments were performed to measure the values of the shrinkage and the warpage under the process parameters. The mathematical models to determine the opti- mum process parameters were developed by exploiting the measured values. In the similar manner, the regression analysis is utilized to derive the relationship between the process parameters and the shrinkage based on the experimental through  the  injection  molding  of  thin-shell  plastic  part (Ref 11, 12). Second-order generalized polynomial regression equations were created to derive this mathematical relationship on the shrinkage by the means of the process  parameters. From these, it has also been found that the process plays statistically a key role in determining the quality of the plastic part. On the other hand, in the studies above, it is not employed any molding simulation tool (Moldflow analysis) for comparing the experimental results.