Meaning
Critical stress level at which a material begins to plastically deform under a state of pure shear or constrained planar tension defines the onset of permanent deformation. For polymer films and sheets, planar yield stress is a fundamental material property that determines the force required to initiate flow during forming operations.
Forming Limit
Exceeding this deformation limit is necessary to shape the polymer sheet permanently into the desired mould profile. An accurate measure of planar yield stress prevents thermoforming equipment from being underpowered for the processed polymer grade. If the sheet is heated inadequately, the material will not reach this yield point uniformly, resulting in incomplete part definition.
This lack of definition is particularly common in high-speed packaging lines where the heating cycle is brief, making accurate knowledge of the yielding behavior essential for maintaining quality.
Simulation Input
Reliable process simulation of sheet stretching relies on accurate mechanical properties under planar deformation. Engineers use the planar yield stress to define the yield surface of the thermoplastic in the simulation software. This prevents overestimating the required forming pressures, which helps optimize the clamping and heating cycles of the thermoformer.
Part Quality
Consistent thickness across the formed part depends on uniform yield behavior during stretching. Variations in planar yield stress, often caused by resin blending issues or inconsistent regrind ratios, lead to uneven wall thickness in the finished parts. This variation can cause parts to fail top-load tests or exhibit localized optical distortion.
