Meaning
Thermal melt processing of polyester resin converts amorphous or semicrystalline pellets into continuous sheet, film, monofilament, or strap through a shaped die. Executing PET extrusion requires strict moisture control and precise temperature management to prevent hydrolytic degradation during processing. The scope of this manufacturing process covers virgin resin processing, post-consumer flake reprocessing, and co-extrusion bottle sheet production.
This processing method stops at the die land exit, where downstream vacuum forming or biaxial orientation sets final part geometry. Devolatilizing twin-screw extruders process un-dried post-consumer bottle flake by stripping moisture and volatile organic compounds under high vacuum.
Hydrolytic Control
Water content above fifty parts per million cleaves ester linkages in molten polyester, causing rapid drop in intrinsic viscosity. Desiccant drying systems heat resin to elevated temperatures for hours before PET extrusion to ensure structural integrity. Vacuum devolatilization zones inside co-rotating twin-screw extruders extract water vapor directly from the melt without requiring resin pre-drying.
Lower intrinsic viscosity reduces melt strength, leading to web sag or sheet tearing in downstream processing equipment.
Thermal Degradation
Excessive melt temperature triggers thermal cleavage of ester bonds, generating acetaldehyde and yellowing the clear matrix. Processing temperatures above two hundred and eighty degrees Celsius accelerate degradation rates while reducing melt viscosity. Barrel residence times remain brief to preserve molecular weight distribution in optical grade packaging sheet.
Crystallinity Management
Rapid quenching of the extruded melt against temperature-controlled chill rolls produces amorphous sheet suitable for thermoforming. Slow cooling allows polymer chains to organize into crystalline domains, producing opaque and brittle sheet stock. Temperature settings on chill rolls control sheet haze and downstream orientation behavior.
Recycled flake content alters crystallization kinetics, requiring melt pressure adjustments to maintain uniform sheet gauge across long runs.
