process parameters and improve shrinkage and warpage [14–17]。 Indi- vidually adjusting mold temperatures on the male and female mold plates facilitates efficiently improving warpage and reducing the cooling time [18–20]; they involve the dynamical control of mold and cooling temperatures。

This study focused on adjusting local mold temperature settings to eliminate the warpage in thin-walled molding。 First, a commercial sim- ulation software was utilized to predict the filling, cooling, shrinkage, and warpage corresponding to initial design cooling channels。 A neutral axis theory was considered to analyze the warping direction based on the information of temperature distribution along the part thickness, enabling the local mold temperature settings to be identified。 A new design cooling system that facilitates local mold temperature setting is then tested to verify its ability to reduce warpage。

3。 Product geometry and warpage simulation using an initial cooling channel design

Fig。 2 depicts the geometrical dimensions of the thin-walled injec- tion molded part investigated in this study。 The frame was 164-mm long, 98-mm wide, and had a rectangular hole in the center (131-mm long, 80-mm wide)。 The top and bottom edges were symmetrical; the right edge was wider than the left, and both edges had different cross- sectional structures。 The thickness ranged from 1  mm  to  3。5  mm and the overall average thickness was 2 mm。 Fig。 3 shows the sprue– runner–gate system in the injection mold, in which the four   fan-gates

Fig。 8。 Neutral axis on a bending structure。

with identical dimensions are positioned symmetrically to enable the molten polymer to flow in a balanced manner during mold filling。 Consequently, both uniform and acceptable amounts of shrinkage at the end of the flow paths can be achieved。 Fig。 4 depicts the simulated melt front used during mold filling, satisfying the traditional criteria of filling all flow paths of the same distances in a given time; thus, four pairs of melt fronts convene nearly simultaneously (i。e。, 1% filling time difference)。 The processed polymer was a PC and ABS blend polymer (PC/ABS 385) made by the Chi-Mei Company (Taiwan)。 A high-speed entirely electric injection molding machine with 100-tonnage maximal clamping force (Robotshot i100β) made by the Fanuc Company (Japan) was employed in the experiment。 Fig。 5 shows the injection-molded plastic part。

The initial cooling channel design for the thin-walled frame used in portable 3C devices is shown in Fig。 6。 The system includes three 8- mm diameter cooling channels that surround the cavity and core at dis- tances of 26 mm and 16 mm, respectively。 The cooling layouts designed for female and male mold plates differed; a pair of wrapped channels was placed on the four edges of the female mold plate, whereas one wrapped channel was used in the male mold plate。 To predict the defor- mation of the thin-walled part produced by injection molding, the Moldex3D commercial simulation software was employed to analyze the warpage。 When the temperature setting for the entire mold was 70 °C, the simulation results (Fig。 7) indicated that the left and right edges severely warped upward, whereas the top and bottom edges warped downward, a phenomenon that is consistent with actual mold- ing conditions。

4。 Neutral axis theory

To control the warpage defects frequently occurring in thin-walled molding, the neutral axis theory was considered to determine possible warping direction。 According to literature, the neutral axis is    defined

Fig。 7。 Warpage analysis of the initially designed cooling channels。

Fig。 9。 Cross-sectional temperature profile of initially designed cooling channels: (a) partial C-C section; (b) A-A section。