Effects of shear stress and pressure drop rate on microcellular foaming process

The effect of shear stress and pressure drop rate on cell nucleation density during the foaming process has been studied using a foaming process simulator. The major part of this simulator is a test chamber that can be temperature controlled and pressurized with gas. Shear stress can be applied to the polymer melt through a rotor. With this simulator, the effects of shear stress and pressure drop rate can be investigated separately. The three materials tested were PS, unfilled HDPE, and HDPE filled with 5% talc. It was found that both shear stress and pressure drop rate have significant effect on cell nucleation density, although shear stress is generally dominant. The effect of shear stress or pressure drop rate becomes more critical when the saturation pressure is low. The transition pressure is about 1500 psi with PS or unfilled HDPE. The transition pressure reduces to about 500 psi with the filled HDPE, since the filler acts as a nucleation agent and reduces the requirement on saturation pressure for sufficient nucleation.