EFFECT OF DIE GEOMETRY AND FLOW CHARACTERISTICS ON VISCOELASTIC ANNULAR SWELL

Commercial extrusion blow moulding operations typically employ tapered or ovalized annular dies. The geometry of the tool and the theologica l properties of the molten polymer determine the swelling characterist ics and resulting dimensions of the extruded parison. A parison that i s too thick will result in excessive part weight and an unnecessary wa ste of resin. A parison that is too thin may blow out during the infla tion stage or yield a part possessing inferior mechanical properties. The thickness swell of the annular extrudate is a consequence of the m olecular orientation which develops as a result of the shear and exten sional stresses that result from the deformation history experienced b y the material in a specific die configuration. The thickness swell wi ll also be influenced by the diameter swell of the parison. The diamet er swell and parison shape are primarily influenced by the circumferen tial (hoop) stresses and gravitational forces acting on the parison. O ptimization of the swelling behaviour can be accomplished through appr opriate die design with the aid of a computer-based simulation. The ef fect of die inclination angle, die gap opening, die contraction ratio, and diverging section length on the swelling characteristics of a hig h-density polyethylene melt are systematically examined. Numerical com putations of the profile shape and stress distributions have been obta ined using a commercial finite element software package (POLYFLOW). Th e material behaviour has been represented by an integral-type K-BKZ co nstitutive equation. Whenever possible, the predicted results have bee n compared with experimental data obtained on a commercial extrusion b low moulding machine. It was found that the thickness swell increases with increasing die contraction ratio and decreases with increasing in clination angle from the vertical as well as with increasing length of the tapered section. The diameter swell is sensitive to changes in th e die geometry. In the absence of gravitational body forces, the circu mferential stresses cause contraction of the outer diameter of the par ison downstream from the die in the case of diverging die geometries. However, a pronounced extrudate expansion was observed in the case of extrudates emerging from converging annular dies. A detailed examinati on of the stress field by means of computer simulation is particularly helpful in understanding the extrudate swell behaviour associated wit h geometrically-complex dies.