Meaning
Ratio of the stretched length of a polymer element to its initial, undeformed length along a principal axis of deformation defines the magnitude of extension experienced by the material during thermoforming. Evaluating the principal stretch ratio is necessary for predicting the final wall thickness of formed sheets. This parameter allows engineers to model how much the sheet must stretch to conform to the mold cavity.
Strain Calculation
Mathematical models use these ratios to calculate the deformation of the polymer during the forming process. The ratio is calculated by dividing the final length by the initial length, and a value of one indicates no deformation has occurred.
Sheet Deformation
Local deformation varies across the surface of the heated sheet as it conforms to the contours of the mold. In regions of high drawing, the ratio increases, which corresponds to localized thinning and a reduction in the barrier performance of the polymer film.
Orientation Effect
High stretch ratios align the polymer chains along the direction of extension, which increases the tensile strength of the material in that direction. This molecular orientation also improves the impact resistance of the formed part but can introduce anisotropy that makes the component susceptible to splitting along the perpendicular axis when subjected to multi-axial mechanical loads in service.
