With the rapid development of injection molded parts in different fields, products such as injection molded part with metal-insert are usually requested to have high precision and complicated shape。 During producing process, residual stresses and deformations such as warping, local bending and shrinkage are produced due to temperature change, pressure and cooling regime during packing and cooling stages。 In this research, numerical analysis of residual stress generated during the packing and cooling stages of injection molding process was carried out。 And the magnitude and distribution of final distortions were predicted, which were consistent with experimental measurement。 So we came to the conclusion that the residual stresses were formed due to different temperature gradient, causing thermal stresses become uneven distribution。 After ejection, the uneven residual stress caused local distortion。 Furthermore, the residual stresses were formed due to the process parameters。 Besides finding the reason of distortion, another task was to minimize the distortion。 So that optimal design was taken to design important parameters such as melt temperature, die temperature, packing pressure and packing time。 The results by optimal design showed that it could reduce the deformation of product effectively。72680

1。 Introduction

With the rapid development of injection-molded parts in different fields, the application of CAE analysis is becoming popular in the recent years, especially for physical design and molding process optimization。 Injection molding simulation is capable of simulating the filling, packing, and cooling processes, as well as product distortion after ejection。 It has been widely applied in industry and earned a fine reputation。 Insert molding is a branch of injection molding, which is also applied widely。 The inserted metal has a significant effect on final product。 Usually, the hot polymer is injected into the mold and heat is removed from the polymer in the mold until it is rigid and stable enough to be ejection。 The metal inserts are used to increase the performance of transferring heat from the polymer, increasing the cooling effect。 Thus, the injection simulation with metal-insert parts is a little complex compared with general process which is only considers polymer injection。

After long-time development, the CAE analysis technology has been quite mature and stable, which is good at the analytic speed and

efficiency。 Besides, for plastic part, especially the simple and thin shell parts, it could obtain accurate results。 However, when product especially with metal insert is requested to have high precision and complicated shape, defects still appear that may affect the accuracy and the efficiency。 On the other hand, residual stress and distortion induced by it for injection molded part with metal-insert cannot be predicted accurately。 Residual stress is the stress left in the molded part under the condition of no external loads after ejection, which includes the flow- induced residual stress and thermal induced residual stress。 The flow- induced residual stress is caused by freeze off packing pressure and the thermal-induced stress is caused by non-uniform cooling of the molding part。 In most cases, thermal-induced residual stress is the major residual stress。 Thermal stress may still be introduced into the product after ejection because of further cooling to the ambient temperature。 After ejection, the residual stress redistributes and causes the part distortion。1,2 Because of the non-isothermal steady flow of polymer melt at the filling and packing stages, normal stress and shear stress are generated。 If the cooling speed is too fast, the macromolecular chain cannot relax completely and then the stress is frozen in the cavity, turning into flow-

induced residual stress。 The stress is related to the molecular orientation and the relaxation of flow stress during filling, packing and cooling stages。 Isayev and Hieber3 analyzed the shear stress and normal stress during filling stage and calculated the stress relaxation at the cooling stage by using Leonov viscoelastic model。 Kim, et al。4 applied incompressible Leonov constitutive equation calculating the flow residual stress of center-gated disk during the injection and compression processes。 Baaijens5 calculated the flow residual stress in the thin- walled cavity during filling and packing stages。 Meanwhile, because of the fast and non-uniformed cooling of polymer melt, thermal-induced residual stress is generated。 Non-uniform thickness of product and non- uniform temperature distribution are the main reasons of the thermal residual stress。 Non-uniformed temperature field is generated between the layer near the die surface and the center part。 At the same time, the temperature difference is generated between the thin part and the thick part。 The polymer at different location faces diffident thermal stress。 Bartnev6 treated the material as pure elastomer and used thermoelastic theory calculating the thermal residual stress。 Lee and Rogers7 showed that Laplace transforms can be effectively used for viscoelastic stress analysis only for a restricted class of problems。 Isayev and Crouthamel8 referred inorganic glass quenching thermal stress model to thermal residual stress calculation of polymer。 Boitout9 used elastic model during calculation, considering the influences of thermal shrinkage and packing pressure。 Up to now, the residual stress of injection molded part has been researched deeply。 Some commercial software such as Moldflow can predict the residual stress of plastic  part。