a b s t r a c t   Currently, 3C products are required to be lightweight, portable, and convenient。 Injection molding is among the most used techniques for mass production in plastic processing industries; however, producing thinner parts that do not warp is challenging。 Although plastic components warp for numerous complicated reasons, warpage primarily is caused by variations in shrinkage during the injection process of plastic part manufacturing。 Material properties, part design, mold design, and processing conditions are factors influencing variations in the part shrink- age。 For example, inconsistent thickness in component geometry, poor sprue–runner–gate or cooling design in the injection mold, and improper molding condition settings may cause plastic parts to warp excessively。 Warpage causes unpredictable component shapes, which may cause poor assembly quality。 Although mold cooling achieved by adjusting mold temperatures improves warpage, the conventional single mold temperature setting for each male or female mold plate limits the cooling capability。 Therefore, this paper describes local mold temperature set- tings for a cooling system that can prevent severe warpage in an asymmetric plastic cover for handheld communi- cation devices。 The neutral axis theory is introduced to analyze the temperature distribution in the cross section of a part, and then predict the warping trend。 Through simulation and experiments conducted in this study, the feasi- bility of using an effective local mold temperature setting in a cooling system to reduce part warpage was verified。76192

1。 Introduction

Plastic parts that are thin (b 1。5 mm) and feature a high ratio of flow length to thickness (N 100) form quickly solid layers when molten poly- mer enters mold cavities, thereby facilitating a short shot caused by the sharp decrease in the flowing channel。 Thus, a high injection speed is required to complete filling and packing during the injection molding process [1]。 This high-speed injection molding requires using high injec- tion pressure to force molten polymer into mold cavities and overcome flowing resistance, and thus considerable changes inner pressures, par- ticularly those of polymeric materials located near and far from the gates。 This phenomenon may induce uneven shrinkage of plastic parts that warp easily after mold release [2–4]。 Thin-walled plastic parts are particularly prone to severe warping because of their weak mechanical structures; nevertheless, the effects of improper molding condition settings and uneven cooling result in sectional shrinkage variations。 Therefore, warpage control is crucial in manufacturing industries to pre- vent quality problems during the successive assembly process, and warpage  must  be minimized  within  dimensional tolerance。

The main cause of warpage in injection-molded parts is the uneven volumetric shrinkage from high to low temperatures。 The volumetric shrinkage level of an injection-molded part can be described using the pressure–volume–temperature (PVT) diagram depicted in Fig。 1。 The pattern from Point 1 to 2 represents the filling stage, in which molten polymer enters mold cavities, and cavity pressure is gradually increased corresponding to the degree of enforcing injection pressure。 Point 2 is the end of the volumetric mold filling, which is followed by packing; the molten polymer pressure in cavities achieves maximum at Point 3。 The pattern from Point 3 to 4 is the transition of packing to static holding that is frequently set at a relatively lower value than the injection pres- sure; the cavity pressure is slightly reduced by the back flow of molten polymer during the transition。 The static holding stage (Point 4 to 5) is performed at constant pressure, compensating for the specific volume reduced by cooling。 Notably, compensation achieved by the holding pressure is effective only when molten polymer at the gates is not fro- zen。 The pattern from Point 5 to 7 represents the cooling stage, during which the pressure decreases continually with the degree of volumetric shrinkage during mold cooling; Point 5 to 6 and Point 6 to 7 indicate the constant specific volume and constant pressure conditions, respectively。 At Point 7, the molded parts are ejected from mold cavities and then cooled to room temperature (Point 8) under atmosphere pressure。 The travel distance between Point 6 and 8 determines the degree of vol- umetric shrinkage in an injection-molded part。