Research On Prediction Of Shrinkage For Injection Molded Parts

On account of light weight and good comprehensive mechanical properties, precision injection-molded parts get increasingly widely used in such industries as electronics, aviation, automobile, communication and so on. Industrial plastic parts are usually requested to have high precision and complicated shapes, so it is important to control the shrinkage rates of injection-molded parts. Aiming at prediction of the shrinkage rates distribution of injection-molded parts, this paper makes a systematic study including theoretical analysis, numerical simulation and experimental verification. The main work of this paper is as following:1. By discussing the problems that prevent existing prediction tools from being applied in practice, the general line of this paper is decided. Above all, shrinkage mechanism of injection-molded parts is analyzed in order to establish proper shrinkage rules. Based on these rules, assuming that the injection molding condition is reasonable, a program is developed to predict the shrinkage situation of a plastic part according to the part geometry and the distribution of runners and cooling channels. By means of combing the shrinkage situation gained from numerical simulation with the average value of shrinkage rate provided by the plastic manufacturer, the shrinkage rate of every computational point on the part is determined.2. On the ground of movement characteristics of macromolecules during the injection molding process, a thought is put forward that a point will shrink along its flow path. When melt marks exist, the shrinkage displacement of a point on a melt mark is decided by shrinkage forces from the two flow paths that produce this point. If the shrinkage movement of a point along its flow path is directly obstructed by the surface of the mold, this point will shrink along the surface of the mold.3. Basic equations of fluid mechanics are properly simplified in accordance with features of the injection molding process, and then approximate governingequations for the flow and temperature fields are obtained.4. Making use of the relaxation effect of polymers, the traditional Tait equation representing the steady pressure-volume-temperature relationships of polymer melt and solid is modified, and a non-steady P-V-T model expressing the dependence of specific volume on cooling rate is obtained. Then the P-V-T data measured under a constant cooling rate can be employed to calculate the specific volume in case of changeful cooling rates.5. A new method utilizing the equivalent distance is put forward to more easily determine the polymer melt front advancement during the filling stage. The equivalent distance is given by modifying the shortest flow distance with the changes of the flow direction between a point and the entrance of the filling domain where this point belongs.6. According to the melt front curves predicted by the equivalent distance method, the filling domain of an entrance is divided into a number of flow paths that originate from this entrance and stretch in the light of the profile of this filling domain. Intersection points of flow paths with melt front curves will be used as nodes for numerical simulation. The mesh generated by means of flow paths analysis has advantages of clear physical meanings, less computer-memory requirement and being convenient for programming.7. Referring to the concept of Green's Functions, the distribution trend of mold wall temperature is get from piling up the influence exerted by every point source, thus determining the boundary condition for the plastic part temperature field in the mold.8. The governing equations for the flow and temperature field of injection-molded parts are solved by the finite-difference method (FDM). The corresponding program is developed to numerically implement the integrated analysis of the filling, packing, in-mold cooling and out-mold cooling stages. For the part during the cooling stage, shrinkage will take place where the pressure becomes equal to normal atmos...