Meaning
Expansion of the molten polymer tube as it exits the die head in an extrusion blow molding process. Managing parison swell is necessary for achieving the correct wall thickness and weight in the finished hollow part. It results from the elastic recovery of the polymer chains after they have been compressed and sheared inside the tooling.
Dimensional Control
Adjustments to the die gap and extrusion speed are the primary methods for compensating for this expansion. Because different resin grades exhibit different levels of recovery, the molder must recalibrate when switching materials. High molecular weight high density polyethylene typically shows more substantial growth than lower viscosity grades.
Operators monitor the weight of the pre-blown tube to ensure the programmed wall thickness profile matches the mold geometry.
Production Defect
Excessive swell leads to the formation of heavy flash at the pinch-off areas or uneven cooling in the mold. Conversely, insufficient swell can result in thin spots that compromise the structural integrity of the container. Proper control ensures that the parison properly fills the mold cavity without wasting material.
Thermal Variable
Temperature plays a decisive role in determining the magnitude of the expansion, as hotter melts tend to relax more quickly before exiting the die. Processors must balance the melt temperature to manage both the swell and the cycle time. Consistent monitoring of this behavior is a standard part of set-up for any blow molding operation.
