Simulation and verification of filling and postfilling stages of the injection molding process.

摘要

Although the existing incompressible cavity-filling flow-simulation and mold-cooling-channel analysis programs provide an immediate help to the injection-molding industry, there is a definite need for proceeding further into the modelling of the post-filling stage. This is particularly important due to the fact that the final part quality is strongly affected by the post-filling stage of the process.; In this thesis, a numerical method for simulating the unified filling and post-filling stages of the entire injection-molding process is presented. The analysis is based upon a hybrid finite-element/infinite-difference solution of the generalized Hele-Shaw flow of a compressible and viscous fluid. A complex, thin-wall, injection-molded part is modelled and discretized as flat finite-elements which can have any orientation in a three-dimensional space. On the other hand, the delivery system (runner) and possibly portions of a part (such as bosses) can be represented as circular tubes or rectangular-strip elements.; The predicted pressure variation at various locations in the delivery system and cavity over the entire filling and post-filling stages for both amorphous and semi-crystalline materials indicates fairly good agreement with corresponding experimental pressure traces for two test molds. Whereas the assumptions of constant thermal properties and density as well as an Arrhenius-type temperature sensitivity of the shear viscosity are considered adequate for the cavity-filling simulation, more accurate representations in all these respects are essential for the post-filling stage. In particular, the compressibility of the polymer becomes a critical ingredient in modeling the material behavior during the latter stage as additional material is packed into the cavity under high pressure in order to compensate for shrinkage and an increased density under continuous cooling. As a result of substantial cooling, it becomes crucial to incorporate the temperature dependence of the thermal properties and shear viscosity over a larger temperature range in the simulation. Further, the latent heat released during the crystallization process for semi-crystalline materials could have significant effect to the prediction of the onset time of cavity pressure decay or the gate-freeze-off time. As illustrated in one of the case studies, the compressibility of the polymer melt can have significant effect on the predictions even during the filling stage if one portion of the cavity gets filled and undergoes a packing-type flow while the remaining portion of the cavity is still unfilled.