Heat transfer and microstructure in extrusion blowmolding

Experimental measurements were made to determine the contact temperature and the rate of heat transfer between the mold and the polymer during the cooling stage of the extrusion blowmolding process. The experimental data were compared to the results of a finite difference computer simulation of the heat transfer problem, allowing for the variation of the properties of the polymer in the solid and molten states. Good agreement was obtained between experimental and calculated results. Heat transfer results were subsequently employed to explain observed differences in the microstructure of the blowmolded articles, particularly in relation to the distributions of crystallinity and birefringence. Finally, it was demonstrated that much faster cooling rates and more uniform molded articles can be obtained by employing rapid internal cooling, as in the case of water injection into subzero air inside the inflated parison.