Study of Foam Sheet Formation: Part III — Effects of Foam Thickness and Cell Density

Abstract

The existing cell growth model in the literature has been modified to include the gas loss effects due to surface evaporation experienced in a high surface to volume ratio of this foam sheet formation. As it exits from the high pressure die, nucleation occurs due to sudden pressure drop and then the sheet experiences natural cooling, expansion due to gas diffusion and gas loss from its surface. Rheological experiments were performed to supply simulation parameters for the modeling using the Haake rheometer. Both HCFC-22 and -142b were studied with low density polyethylene (LDPE) in generating the rheological data. They were also used to make foams to generate experimental data discussed in this paper using a 70 mm counter-rotating twin screw extruder. The theoretical and experimental results are compared in terms of foam density and foaming efficiency. Foaming efficiency is defined as the actual expansion over maximum possible theoretical expansion dictated by the ideal gas law for a particular blowing agent. The agreement is good when the foam expansion is under ten times and the deviation increases when the foam expands further, in which bubble-bubble interaction effects become significant.