Biaxial Strain Hardening Metrics for High Draw Ratio Amorphous Sheet Thermoforming
Biaxial strain hardening metrics dictate wall thickness uniformity in high draw thermoforming; matching plug dynamics to material hardening prevents blowout.
Considere Criterion
Meaning
Mathematical condition defining the onset of necking in a material under tensile load. Engineers apply the considere criterion to predict when a polymer part will begin to deform locally rather than uniformly. This point marks the maximum engineering stress on a stress strain curve.
Necking Threshold
True stress and strain hardening rates determine whether a material can withstand further drawing. Under the considere criterion the plastic flow becomes unstable when the slope of the true stress curve matches the stress value itself. This transition leads to a localized thinning that eventually results in failure.
Tensile Stability
High molecular weight resins often show a delayed onset of instability compared to lower weight alternatives. Implementing the considere criterion allows designers to calculate the safe limits for cold drawing or solid state orientation. Stable drawing requires that the material hardens faster than the area reduces.
Processing Limit
Manufacturing operations like fiber spinning or film orientation rely on staying below this instability point. Utilizing the considere criterion prevents the formation of thin spots that compromise the barrier properties or mechanical strength of the product. Variations in resin temperature can shift this limit during a production shift.
If the temperature rises too far the hardening effect diminishes and the material necks prematurely. Monitoring this limit is a standard part of evaluating new polyolefin grades for high speed extrusion lines.