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.
Bubble Inflation Rheometry
Meaning
Experimental characterization provides data on how a molten polymer behaves when stretched in multiple directions simultaneously. Researchers use bubble inflation rheometry to simulate the stresses found in blown film extrusion and vacuum forming. This method applies gas pressure to a circular specimen to create a hemispherical dome.
Stress Analysis
Pressure transducers and optical sensors track the growth of the expanding surface over time. Accurate bubble inflation rheometry relies on measuring the height and radius of curvature of the thinning film. These readings allow for the calculation of biaxial stress and strain.
Deformation Mechanism
Molecular chains resist expansion through entanglements that define the melt strength of the resin. During bubble inflation rheometry the material undergoes equibiaxial extension which differs from the linear flow seen in capillary tests. Strain hardening becomes evident when the resistance to stretching increases as the film gets thinner.
Material Validation
Processors use these measurements to select resins that can maintain a stable bubble during high speed production runs. Effective bubble inflation rheometry identifies which batches will fail due to draw resonance or melt fracture. Consistent results ensure that virgin resin performs identically to regrind blends.
Laboratory data obtained through this technique guides the blending of recycled content with virgin stock to maintain processability. The resulting viscosity maps prevent the selection of grades that would cause thickness variations in the final part.