An Experimental Investigation of Capillary Extrudate Swell in Relation to Parison Swell Behavior in Blow Molding

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1 T An Experimental Investigation of Capillary Extrudate Swell in Relation to Parison Swell Behavior in Blow Molding DILHAN >l. KALYON Department of Chemistry and Chemicd Engineering, Steuens Institute of Technology Hoboken, New Jersey and MUSA R. KAMAL Department of Chemical Engineering McGiU Uniuersity Montreal. Quebec H3A 2A7 Canada An experimental study was carried out to study and characterize the capillary e.xtrudate swell and parison swell behal-ior in extrusion blow molding of two commercial blow molding grade high density polyethylene resins. The capiiia~ extrudate swell behavior of these resins were determined employing a capillary rheometer and a special thermostatting chamber. Parison swell behavior was determined using an Impco Al3-R12 reciprocating screw blow molding machine in conjunction with cinematography and pinch-off. The experimental conditions under which capiiiaq extrudate and parison swell data can be related are elucidated. Excellent agreement is found between the area swell values determined on the basis of capillary and parison swell experiments. he extrusion blow molding process invoh.es uniaxial elongational stress growth data (7) of three consecutive stages i.e.. Lhe formation the resin are available. The formidable problem of the parison, clamping, and inflation of the that the blow molder faces is to determine the parison and the cooling of the aflicle (11. In this dimensional changes of the parison induced by sequence, the dynamics of the parison forma- swell. which strongly depends on the thermotion stage is of fundamental importance, as the mechanical history in the die and on the propresulting dimensions of the parison before erties of the resin. clamping determine the final thickness distri- Ideally. the dimensional changes of the exbution of the blow molded article [ 1-5). trudates emerging from capillaries and dies The dynamics of the development of the par- should be predicted on the basis of the simulison are basically influenced by the swell and taneous solution of the set of equations of consag phenomena. During the formation of the tinuity. motion, and energy in conjunction with parison. swelling induces an increase in the a suitable constitutive equation and an equation diameter and thickness and a corresponding of state. In this rigorous approach, analytical decrease in the length of the parison. whereas solutions cannot be generated due to the sinthe sag or -draw-down* induces an opposite gularity of the stress at the point of exit from change in the dimensions of the parison. The the die and due to the difficulties associated competition of these two opposing phenomena with the correct location of the free surface (8). dictates the final shape of the parison before Generally. various numerical methods i.e., ficlamping. nite differences (9). ftnite elements (10). and The effect of gravity on the length of the boundary element methods are employed to parison can be accounted for by various meth- tackle the problem. The numerical methods ods.if either the creep compliance [5. 6) or have so far been successful in solving problems 508 POLYMER ENGINEERING AND SCIENCE, MID-APRIL. 1986, Vol. 26, No. 7

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