Continuous extrusion blow molding of semicrystalline resins

Two blow molding grade polyethylene resins (polymerized with chromium versus Ziegler-Natta catalyst) and three blow molding grade nylon resins (neat, glass filled, and polyolefin modified) were thoroughly characterized (shear and extensional rheology, P-V-T behavior, extrudate swell, and thermal behavior). Significant improvements were made on a rotary-clamp type extensional rheometer which permitted the accurate characterization of the extensional behavior of the melts. Fiber orientation distributions in the drawn samples were determined via microradiography as a function of strain. A custom designed capillary die, which permitted an unobstructed view of the entire extrudate, was employed for extrudate swell characterization. Fiber orientation distributions in the extruded samples were characterized as a function of shear rate in the die and time allowed for swelling in a density matched thermostatting chamber.; Samples were molded on a commercial Krupp Kautex continuous extrusion blow molding machine. A pinch-off mold and a cylindrical transparent blow mold were used in conjunction with cinematography to characterize the dimensional changes of the parison during formation, clamping and inflation. For the first time, pressure transducers located in the mold wall as well as in the tip of a custom designed blow pin permitted simultaneous measurement of the transient pressure inside and outside the inflating parison. An "effective viscosity" was calculated for the inflating parison and compared to the extensional viscosity of the resin. The contact temperature and rate of heat transfer from the parison to the mold were measured during the cooling stage. A mathematical model of parison cooling was developed and tested. The calculated thermal histories were used to explain the crystallinity and birefringence distributions found in the molded parts.; Molded specimens were prepared over a range of systematically varied operating conditions. The molded specimens were extensively characterized in relation to thickness, density, crystallinity, and birefringence distributions, fiber orientation, morphology and ultimate tensile properties. The distributions were analyzed in relation to the inherent resin properties and thermo-mechanical history imparted during molding. The collected experimental data were interrelated and compared. The results of this extensive study should facilitate a better understanding of the dynamics of the continuous blow molding process and the interrelationships between the resin properties and moldability versus the microstructural distributions on the one hand and the microstructural distributions versus the ultimate properties of the molded articles on the other hand.