Numerical Simulation Of Warpage Prediction In Injection Molded Strip Products

With the increasing requirement of dimensional accuracy to the injection parts,warpage as molding defects gains more and more attention, many of the causes andeffects of mutual coupling make analysis very difficult. Due to long dimension ofstrip products, warpage, which is difficult to control, is the most prominent moldingdefects.The polymer still has plastic deformability below the solidification temperature,but it is small compared to the plastic deformation of the molten state. It is ignored orused by the error of stress relaxation model in the conventional approach, which doesnot affect the analysis accuracy of ordinary plastic products. However, strip products,its large size in the longitudinal direction, in which direction will produce obviousdeformation after accumulation of plastic deformation. Therefore, in order to considerthis issue, this paper using numerical methods for a long strip of products moldingprocess packing and cooling stages warpage analysis.The warpage is caused by uneven deformation of shrinkage resulting fromdifferent distribution of residual stress in the cross-section. The residual stress can bedivided into the flowing residual stress and thermal residual stress, the latter is moreten times than the former, so it can be considered that the thermal residual stress is themain cause of warpage.The thermal stress state of development in packing and cooling stage wasAnalyzed by numerical simulation to predict the warpage in this article. First of all,Assumptions was proposed based on the mechanism of injection molding. The stripparts was discreted in the direction of length and the discrete section units waschoosed as model to simplify the calculations. Secondly, time calculation step, theinitial and boundary conditions were determined reasonably, and two-dimensionaltransient temperature field was calculated by the heat conduction theory and finiteelement method. Once more, four-component model improved on the basis ofMaxwell was established to characterize the mechanical and plastic behavior of thepolymer at the glass transition temperature, and the change of stress, strain and creep was simulated in the mold cavity. Then, curvature was obtained by stress torque whenthe plastic parts drops to room temperature, structured fitting the curvature equationof the parts, getting the warpage equation of the parts. In addition, viscoelasticproperties was considered when polymer cured. Creep parameters, which wasinversed by experimental data, to characterize the plastic deformability after thematerial curing, so that guarantee the accuracy of stress calculation in the curing stage.Finally, Warpage is predicted for the strip products by writing the C++code of thetheory. The numerical methods were proved feasibility by comparing the warpage ofactual production. The effect was analyzed on creep and warpage by packing pressure,mold temperature and cooling time in the injection mold.