Meaning
Extensional strain hardening describes a physical phenomenon where a polymer melt exhibits an increase in resistance to further stretching as the material is elongated at a constant rate. In this condition, extensional strain hardening acts as a self-stabilizing mechanism during high speed processing tasks like film blowing or fiber spinning. The property arises when polymer chains are sufficiently long and branched to become entangled under uniaxial tension, which prevents local thinning or necking during drawdown.
A resin with high resistance to stretching maintains a uniform thickness across the entire geometry of a part.
Melt Rheology
Molecular architecture dictates the intensity of this response in the melt state. Branched polymer chains resist flow more effectively than linear chains when pulled in a single direction because the side chains catch on adjacent molecules. If a material lacks this resistance, the polymer flows uncontrollably under tension and results in rupture or uneven wall thickness.
Consistent performance in high speed extrusion depends on the degree of long chain branching present in the specific grade.
Processing Dynamics
Excessive drawdown forces during moulding or extrusion lead to material failure if the strain hardening effect does not counterbalance the tension. Moulding operations rely on this property to ensure that thin sections do not break while the melt fills the cavity at high pressures. Regrind material often shows reduced performance compared to virgin resin because the reprocessing cycles degrade the chain length necessary for effective entanglement.
A moulder compensates for this loss by adjusting process temperatures to shift the viscosity range.
Cost Impact
Scrap rates increase when the melt fails to demonstrate sufficient strain hardening during high speed production. Parts with inconsistent thickness require higher quality control tolerances and often lead to rejection after initial inspection. Manufacturers select specific grades with high strain hardening potential to minimize machine downtime caused by film breakage or wall thinning.
Maintaining this material property prevents excessive waste during long production runs.
