| When predicted fill pressures were compared with machine pressures, the predicted values did not match the measured values. Therefore, the flow rate, machine response time, ram inertia, and compression of the melt during injection molding were examined as possible causes. The flow rates in the machine varied significantly with shot size and did not match those in the simulation package. Response time contributed little to the flow rate discrepancy because machine response occurred during decompression. In contrast, inertia of the ram (overshoot) produced a significant contribution to the filling, with the overshoot accounting for most of the discrepancy between the predicted and machine fill pressures for polyacetal. The compression in the nozzle produced a pressure drop of 3 to 12 MPa. Since incorporation of elongational viscosity and juncture losses into the models did not affect the predicted pressure, this suggests that three-dimensional models may be required for the simulation of pressure drops through the nozzle region. A large difference between the measured and predicted part densities, however, indicates material is compressed in the nozzle and then expands in the mold. In general, polyacetal was more sensitive to input flow rates than polystyrene. |
