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A numerical algorithm is developed to simulate the injection-compression molding (ICM) process. A Hele-Shaw fluid-flow model combined with a modified control-volume/finite-element method is implemented to predict the melt-front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt filling, compression melt filling, and postfilling stages of the entire process. Part volumetric shrinkage was then investigated by tracing the thermal-mechanical history of the polymer melt via a path display in the pressure-volumetemperature (PVT) diagram during the entire process. Influence of the process parameters including compression speed, switch time from injection to compression, compression stroke, and part thickness on part shrinkage were understood through simulations of a disk part. The simulated results were also compared with those required by conventional injection molding (CIM). It was found that ICM not only shows a significant effect on reducing part shrinkage but also provides much more uniform shrinkage within the whole part as compared with CIM. Although using a higher switch time, lower compression speed, and higher compression stroke may result in a lower molding pressure, however, they do not show an apparent effect on part shrinkage once the compression pressure is the same in the compression-holding stage. However, using a lower switch time, higher compression speed, and lower compression stroke under the same compression pressure in the postfilling stage will result in an improvement in shrinkage reduction due to the melt-temperature effect introduced in the end of the filling stage.

ABSTRACT:Simulations of the injection-compression molding (ICM) process based on a Leonov viscoelastic fluid model has been employed to study the effects of processing conditions on the birefringence development and distribution in injection-compression molded parts. A numerical algorithm combined with a modified control-volume/finite-element method is developed to predict the melt front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt-filling, compression melt-filling, and postfilling stages of the entire process. Part birefringence was then calculated from residual stresses following the thermal-mechanical history of the entire molding process. Simulations of a disk part under different process conditions including compression speed, switch time from injection to compression, compression stroke, packing pressure, and postfilling time were performed to understand their effects on birefringence variation. The simulated results were also compared with those required by conventional injection molding (CIM). It has been found that an ICM part shows a significant reduction of part birefringence near the gate area as compared SIMULATION OF INJECTION-COMPRESSION MOLDING PROCESSwith CIM parts. However, ICM parts exhibit higher birefringence values near the rim of the disk. The minimum birefringence occurs around the location where injection is switched over to compression. Although longer postfilling time and higher packing pressure result in higher birefringence values, their effects are not very significant. On the other hand, higher compression speed, larger compression stroke, and shorter switch time exhibit greater effects on the increase of part birefringence. Flow-induced residual stress is the major origin of birefringence formation in the present case. The simulated birefringence for both ICM and CIM parts show good coincidence with those obtained from measurements by using a digital photoelasticity technique.

To increase the productivity of injection molding machines, we developed a smart injection part weight stability control system based on Cþþ programming and domain knowledge. The proposed system is meant to eliminate variability in the quality of injected parts by adjusting the changeover position. We developed a viscosity index based on melt pressure data related to guide the adjustment to the changeover position in accordance with material properties. This was achieved by mounting a pressure sensor on the nozzle of the injection molding machine to enable the on-line monitoring of pressure throughout the injection molding process. A series of experiments was conducted to characterize the relationship between viscosity index and injection-molded samples in order to validate the efficacy of the proposed injection stability system. Single-factor experiments were conducted with the changeover position and melt temperature as parameters. The quality of the molded samples obtained under different process parameters was evaluated in terms of weight. Experiment results revealed a correlation between changes in viscosity index and changes in the weight of the samples. The injection stability system can also be operated in self-adjusting mode, in which the changeover position is varied according to viscosity index. In experiments, abnormal machine operations prompted the adjustment of changeover position. Variation in the weight of parts was used to define an index to validate the efficacy of the proposed system.

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