Amorphous Thermoplastic Rheology and Extruded Sheet Stretch Dynamics
Amorphous sheet stretch dynamics depend on strain hardening and thermal saturation to prevent localized necking and corner blowout during deep thermoforming.
Catenary Sheet Sag
Meaning
Deflection behavior characterizes the gravitational drooping of a thermoplastic sheet during the oven preheating phase of thermoforming. In this step, catenary sheet sag describes the distance the center of the sheet drops below the clamping frame. It measures the thermal softening of the polymer before vacuum or pressure is applied to shape the part.
This behavior is bounded by the point where the sheet tears or touches the heater elements.
Gravity Deflection
Gravity pulling on a softened polymer sheet creates a catenary curve during the heating cycle. Because the sheet is clamped only along its perimeter, the central area behaves like a suspended cable. Rising temperatures reduce the tensile strength of the material, which accelerates the rate of catenary sheet sag.
Excessive droop causes uneven wall thickness in the formed part because the central area stretches before reaching the mold.
Material Resistance
Melt strength determines how well a polymer resists stretching under its own weight while hot. High molecular weight resins yield a high melt strength that opposes catenary sheet sag. Amorphous polymers like polystyrene exhibit a wider processing window with slow sag rates.
Semi-crystalline resins like polypropylene soften rapidly when the melting point is reached, leading to sudden sag.
Process Consequence
Excessive sagging leads to sheet tearing during preheating. In such cases, catenary sheet sag triggers photoelectric sensors to start the forming step. Proper timing ensures the sheet remains intact.