The Integration Optimization Of Molding And Service For Injection-molded Product

The manufacture and service process of the injection-molded product can be divided into three stages:molding, assembly and service. The different settings of process parameters in injection molding can directly influence the internal residual stress of injection parts, while the internal residual stress field, to a certain degree, will in turn affect the performance of parts during assembly and service process. Therefore, for most parts which will be subjected to external loads as one part of a system, it has practical significance to improve their performance through integration optimization of molding and service. However, present researches mainly concentrate on the optimization approach development to eliminate the molding defects, and there are still lacks of integration studies about the influence of the injection molding process parameters on the stress of the injected product in the practical usage. In order to make a more comprehensive analysis of the plastic parts, this paper integrates the molding-assembly process and molding-service process by combining injection molding simulation and finite element analysis software, and accomplishes the residual stress optimization, assembly stress optimization and service stress optimization using the sequential optimization algorithm. The whole paper can be roughly divided into the following several parts:Firstly, for some plastic products which are non-assembly and non-load carrying, their maximum Mises residual stress after ejection is treated as the objective function, and the sequential optimization algorithm based on the Kriging surrogate model and expected improvement sampling criterions are employed for the residual stress optimization of the polycarbonate material parts. Results show that this optimization method could effectively reduce the maximum Mises residual stress of parts. Based on the analysis of the process parameters, further study is taken on the filling stage. With the filling method changed into Bezier curve control, absolute ram speed and ram position are used to control the filling process. Finally, the optimized injection rate curve is given and the residual stress of injected part is further reduced.Secondly, many plastic products need to be assembled onto other structure after ejection. The inevitable warpage deformation after ejection will induce new stress in the assembly process. Finally, the assembled stress overlaps the residual stress, which will form a new stress filed. Both warpage and residual stress are determined in the molding process, so the process parameters have greatly impact on the assembly stress of parts. Therefore, the molding technology optimization is quite necessary for reducing the assembly stress in practice. In this optimization work, we import the warped structure into the finite element analysis software, with the residual stress treated as prestress loading and the opposite value of warpage treated as the displacement loading, and therefore the molding simulation software is connected with the finite element analysis software. The maximum Mises stress is regarded as the objective function of the assembly stress optimization problem. The optimization effect is found to be remarkable. After this constant rate filling, curve injection rate is also considered and optimized, and results show that curve control of the injection rate further reduces the assembly stress.Finally, for some structural plastic parts, they will bear various external loading after assembly. The working load usually results in greatly change of the internal stress. Consequently, it is necessary to build the optimization model between process parameter and service performance of parts. This optimization issue integrates three processes including molding, assembly and service. Therefore, it is called the molding-service optimization. Based on the further analysis of the filling stage, the curve control mothed of the injection rate in this service process is also proved to be better than the traditional constant control method.This work is supported by the national key basic research program project "polymer molding process and mold design optimization" (No.2012CB025905).