Design of an injection-molding system for manufacture of microcellular foams.

摘要

An innovative microcellular foam injection-molding system is systematically designed, constructed, and experimentally verified in this research. The basic approach is to form and maintain a homogeneous, single-phased polymer/gas solution, to induce a microcellular cell-nucleation rate, and to mold the nucleated polymer/gas mixture into a desired shape while preventing deterioration of cell density and achieving a desired void fraction. Because cell density strongly depends on the concentration of gas dissolved in the polymer, the formation of a homogeneous polymer/gas solution with a high gas concentration is the primary requirement. To maintain a constant gas-to-polymer weight ratio despite the stop-and-flow molding behaviors, a positive-displacement gear pump is located between the extrusion barrel and the accumulator. The polymer/gas solution is accumulated under a pressure higher than the solubility pressure to prevent the formation of a gas phase in the solution. To nucleate myriad cells, a rapid pressure drop is utilized. The cell-nucleating behaviors of polymer/gas solution are separately investigated using an extrusion-foaming system with nine deliberately designed nucleating dies with various pressure-drop rates and die pressures. These studies indicate that the pressure-drop rate is the most critical for the PS-CO₂ system, whereas the CO ₂ and nucleating-agent contents contribute to cell nucleation. Based on these results, effective strategies to achieve a desired cell density during injection molding are established. The cell-nucleating behaviours in injection molding are investigated in several stages of calibration experiments for each section of the system. Experience demonstrated that the microcellular-nucleating strategies in injection molding should be different from those in extrusion. Finally, manufacture of microcellular foams in injection molding is successfully demonstrated through critical experiments. The cell-deteriorating phenomena are also identified and studied at various melt temperatures.; To determine solubility accurately, a preliminary investigation is conducted to model and fit the pressure-volume-temperature (PVT) data of PS/CO ₂ solutions using the Simha-Somcynsky (S-S) equations of state (EOS). The PVT data of PS/CO₂ mixtures obtained from the literature are used as a case example to determine the characteristic parameters of S-S EOS.