Blow Moulded Bottle Weight Trimmed until the Drop Test Fails
Trimming blow moulded bottle weight reduces unit resin costs but narrows drop test safety margins along bottom pinch seams and heel radii.
Polymer Elasticity
Meaning
Viscoelastic recovery behavior causes molten plastics to store elastic energy under shear or elongational stress and release it as dimensional expansion upon exiting restrictive tooling orifices. Occurrence of polymer elasticity generates die swell, melt memory and normal stress differences that alter the parison diameter, wall thickness and shape during extrusion operations. The property is inherent to long-chain high molecular weight thermoplastics such as blow-moulding grade high density polyethylene and polypropylene, stopping where low molecular weight Newtonian fluids exhibit no elastic memory.
Swell Dynamics
Flow through narrowing die head channels forces tangled polymer chains to stretch and align along the extrusion stream. As the melt exits the die bushing into open air, the sudden release of confinement allows oriented macromolecules to recoil and contract axially, driving lateral cross-sectional expansion known as die swell. High molecular weight polymers with broad molecular weight distributions store extensive elastic energy, expanding significantly more than narrow distribution resins.
Processors must account for both diameter swell and thickness swell when designing die tooling and setting parison extrusion programs.
Process Complications
Excessive elastic stresses create severe processing instability and cosmetic surface defects. High extrusion velocities amplify shear rates near the die wall, generating high elastic recovery forces that can exceed the melt cohesive strength, causing sharkskin or gross melt fracture. In extrusion blow moulding, variations in polymer elasticity cause uneven parison sag and unpredictable container wall distribution.
Regrind incorporation from multiple thermal cycles introduces chain scission fragments that decrease bulk melt elasticity, causing parisons to drop thinner and narrower than virgin material streams. Operators counter these shifts by modifying extrusion temperatures and cycle timings.
Material Specification
Rheological characterization of elastic properties requires dynamic mechanical analysis, capillary rheometry or die swell ratio testing. Sourcing professionals cannot rely exclusively on standard melt flow rate metrics because two polymers with identical melt index values can exhibit vastly different elastic swell behavior due to molecular branching architecture. Verifying resin elasticity parameters ensures seamless transition between raw material suppliers without requiring expensive die re-machining.
Stable elasticity maintains predictable container wall thickness, seam fusion strength and baseline bottle weight.