(1) Different thickness of insert metal as the major reason for distortion。 The fundamental reason is that different thickness leads to difference in the thin part and the thick part changes slowly, thus causing a huge temperature difference in cross section。 So the thermal stress is quite different between thin part and thick part。 After ejection, this difference of residual stress leads to distortion。

(2) To overcome the defect of Moldflow in metal-insert mold injection calculation, an analysis method is proposed。 By using elastic-plastic model, the influence of thermal stress of insert metal at the packing and cooling stages is considered。 Final distortion could be predicted, through mapping Moldflow analysis results and employing thermal-viscoelastic-plastic model in Abaqus。 By comparison, the distortion results between actual experiment and numerical analysis were quite similar, so the method could use to predict the distortion for other metal- insert injection molded products。

(3) Optimal design has been taken by Taguchi method and the optimal process condition has also been found through response surface method。 Mold temperature is the most significant influence factor。 Melt temperature and packing pressure also have some effects as surface plot shown in Fig。 16。 To minimize the distortion, the deeper action should be taken to get uniform temperature field during packing stage and cooling stage that leads to the temperature of all parts changes as a same temperature gradient。 Also, parameter study is necessary to modify the structure。

ACKNOWLEDGEMENT

This research was supported by the Research Grant of Sogang University (No。 201010042) and LG Electronics Co。 Ltd (No。 2013 70035)。 The authors would like to thank these supports。

REFERENCES

1。 Zoetelief,  W。,  Douven,  L。,  and  Housz,  A。,  “Residual    Thermal

Stresses in Injection Molded Products,” Polymer Engineering & Science, Vol。 36, No。 14, pp。 1886-1896, 1996。

2。 Wang, T。 H。 and Young, W。 B。, “Study on Residual Stresses of Thin- Walled Injection Molding,” European Polymer Journal, Vol。 41, No。 10, pp。 2511-2517, 2005。

3。 Isayev, A。 I。 and Hieber, C。 A。, “Toward a Viscoelastic Modelling of the Injection Molding of Polymers,” Rheologica Acta, Vol。 19, No。 2, pp。 168-182, 1980。

4。 Kim, I。 H。, Park, S。 J。, Chung, S。 T。, and Kwon, T。 H。, “Numerical Modeling of Injection/Compression Molding for CenterGated Disk: Part I。 Injection Molding with Viscoelastic Compressible Fluid Model,” Polymer Engineering & Science, Vol。 39, No。 10, pp。 1930- 1942, 1999。

5。 Baaijens, F。 P。 T。, “Calculation of Residual Stresses in Injection Molded Products,” Rheologica Acta, Vol。 30, No。 3, pp。 284-299, 1991。

6。 Chen, X。, Lam, Y。 C。, and Li, D。 Q。, “Analysis of Thermal Residual Stress in Plastic Injection Molding,” Journal of Materials Processing Technology, Vol。 101, No。 1-3, pp。 275-280, 2000。

7。 Lee, E。 and Rogers, T。, “Solution of Viscoelastic Stress Analysis Problems using Measured Creep or Relaxation Functions,” Journal of Applied Mechanics, Vol。 30, No。 1, pp。 127-133, 1963。

8。 Isayev, A。 I。 and Crouthamel, D。 L。, “Residual Stress Development in the Injection Molding of Polymers,” Polymer-Plastics Technology and Engineering, Vol。 22, No。 2, pp。 177-232, 1984。