Parison weight control

The weight and diameter of the parison is controlled by the geometry of the annular die opening and the resin being extruded. As the molten polymer is extruded through the die, it swells to a thickness and diameter that is greater than the dimensions of the die opening. The amount of swell is dictated by a complex relationship between the internal shape of the die and the viscoelastic properties of the polymer. 

In regard to parison control, a compromise is necessary between the desired net weight and the need to maintain a sufficient safety margin over a set of minimum specifications, which include minimum wall thickness, drop speed, drop strength, dimensional stability, and fluctuations in net weight. Most of these parameters can be directly affected by the molder s ability to control the parison wall thickness. The most common and practical way of doing this has been to adjust the gap between the die and mandrel (Table 4-2). 

The annular die can be designed to incorporate a hydraulic mechanism to vary or programme the annular gap size in both extrusion- and plunger-type blow moulding. With extrusion blow moulding, a specific wall thickness distribntion or controlled weight of the parison can be programmed. 

The viscosity-shear rate curves of the samples are shown in Figure 1. The branched PVDF samples have a similar viscosity profile to those of the reference samples across the entire shear rate range. The change in the viscosity is mostly controlled by the molecular weight of the samples. The melt viscosity of the resin plays an important role in the first step of the extrusion blow molding process where the resin is processed under moderate shear rates well below the onset of melt instabilities. This is extremely important for the formation of a smooth parison and ultimately in a quality product. 

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