Meaning
Two dimensional stretching occurs when a flat specimen is pulled with the same force along both axes. Processing engineers study equibiaxial extension to understand the thinning behavior of polymer sheets during vacuum forming. This mode of flow is distinct from the uniaxial stretching found in simple tensile tests.
Rheological Response
Molten polymers exhibit significant strain hardening when subjected to this type of deformation. Measuring equibiaxial extension reveals how a resin will resist tearing when it is pushed into the corners of a mould. The resistance stems from the simultaneous orientation of chains in multiple directions.
Simulation Accuracy
Prediction of wall thickness in complex parts requires data from multi axial testing. Incorporating equibiaxial extension parameters into simulation software leads to more reliable results for blow moulding and thermoforming. It allows for the optimization of preform shapes and heater profiles.
Sheet Thinning
Uniformity of the final part depends on the material maintaining its integrity as the surface area increases. During equibiaxial extension the thickness of the sheet decreases as the square of the stretch ratio. Resins with high melt strength prevent the premature failure of the web during the expansion phase.
Maintaining a constant temperature across the sheet is necessary to ensure that the extension remains uniform. This deformation mode is the primary driver of molecular orientation in biaxially oriented polypropylene films.
