Parison Generation

The constant pressure of the continuous extrusion enables the manufacture of materials with different viscosities and compressibilities into an optimal plastic tube laminate. In the accumulator head system (intermittent parison generation), pressure differences of materials with different viscosities and compressibilities would lead to layer thickness problems. 

A parison of variable thickness can be generated by moving the mandrel vertically during extrusion as shown in Fig. 3.58. A thinner wall not only results in material savings but also reduces the cycle time due to the shorter required cooling times. 

Figure 3.58 Moving mandrel used to generate a programmed parison.

Parison cooling is an integral part of the process that has been treated by a number of researchers (103-105). The principles are based on contact solidification without deformation, as discussed in Chapter 5. A special complication is the frictional heat generation in injection blow molding between the rod and the parison. 

In order to generate a better balance of mechanical properties it is necessary to create biaxial orientation in the part. Stretch blow molding is used to accomplish this. In essence the parison is stretched along the axial direction before being inflated. Biaxial orientation is specifically required in large containers for fluids. For example, bottles for carbonated beverages are typically processed by means of stretch blow molding. 

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