Homogeneity And Stability Of Injection Molding

In order to mold high-quality parts, the melt homogeneity, and injection stability during the molding process must be guaranteed. The present thesis studied the above two aspects by three-dimensional nonisothermal simulations and experiment. Meanwhile, the improvement for two-stage injection molding machine was presented. The main results are as follows.1. The three-dimensional nonisothermal melt flow fields in metering section during plasticizing process were simulated with the melt temperature and melt viscosity as target parameters for analysis of the effects of different plasticizing parameters on melt homogeneity. The results showed that lower rotation speed, higher temperature and shorter plasticizing stroke improved the homogeneity in melt temperature. The melt viscosity homogeneity was improved by increasing rotation speed and temperature while the effect of back pressure on melt homogeneity is not obvious.2. The melt homogeneity in nozzle flow field was simulated for study of effects of injection parameters on melt homogeneity. The results showed that when injection velocity increased, the axial temperature difference and viscosity fluctuation decreased indicating the melt homogeneity improved. The increase of nozzle temperature can improve temperature homogeneity while the viscosity fluctuation was obviously increased.3. The residual stress and optical performance of parts were tested for studying the effects of different process parameters on homogeneity of product quality by part weight repeatability, double refraction and optical angular deviation. The results showed that increase of injection velocity, screw rotation speed and barrel temperature can properly improve weight repeatability and reduce residual stress. Increasing plasticizing stroke will make weight repeatability decrease and residual stress increase. Back pressure increases can improve weight repeatability. The effect of nozzle temperature on weight repeatability is not obvious. The direct relationship between melt homogeneity and angular deviation does not exist. Indeed, deviation angle decreases with the increase of injection velocity and nozzle temperature.4. The quantitative relationship between part weight and injection power was analyzed so that the injection stability can be analyzed by monitoring injection power. Single-factor experiment was designed to analyze effects of different process parameters on injection stability. The influence of different process parameters on injection stability were obtained by orthogonal experiment. By using the linear fitting, the relationship between injection power and part weight under different process parameters were obtained.5. The two-stage injection molding machine was studied based on homogeneity and stability. The results of numerical simulation and experiment showed that the melt homogeneity was better when the screw did not move in axial direction during the plasticizing process as in screw-reciprocation injection molding machine. The plasticization stability was illustrated by less pressure fluctuation such that the precision of measuring position must be insured. As observed, the injection stability is better in two-stage injection molding machine than screw-reciprocation injection molding machine.