Meaning
Dimensionless ratio that quantifies the increase in extensional viscosity of a polymer melt relative to its linear viscoelastic limit. The strain hardening factor functions as a primary metric for assessing the melt strength of resins used in processes where the material is stretched, such as film blowing or thermoforming. It indicates how well the polymer can resist localized thinning during deformation.
Operating Stability
Operating stability is enhanced when a material exhibits strong resistance to localized thinning. High values for this ratio suggest that the material will stabilize itself during stretching, preventing the formation of holes or tears. In film production, this property is what allows the bubble to remain stable even at high blow-up ratios.
Branched polymers like low density polyethylene naturally exhibit higher factors than linear polymers like high density polyethylene. Stable behavior reduces the frequency of line breaks and improves the overall yield of the extrusion process.
Selection Criterion
Engineers use this data to determine if a resin is suitable for deep-draw thermoforming applications. If the factor is too low, the part may exhibit excessive thinning at the corners or fail to capture the detail of the mold. It is a necessary specification for any application involving biaxial or uniaxial stretching.
Measurement Method
Extensional rheometers are used to measure the stress response of the melt at different Hencky strain rates. This laboratory data is then compared to the predicted viscosity based on shear measurements. The resulting ratio provides a clear indication of how the material will behave under real-world processing conditions.
