Standard PET can become hazy or opaque when crystallization develops during heating or cooling. The risk depends on resin grade, intrinsic viscosity, prior thermal history, sheet structure, nucleation, and the time spent in the crystallization range.
This mechanism is distinct from PETG overheating haze. PETG is modified to suppress crystallization under normal processing, while PET processes may deliberately control crystallinity for heat resistance or minimize it for clarity.
Process control
The material specification and intended final structure must be established first. Heating should move the sheet through sensitive temperature ranges in a controlled way, and forming and cooling must match the product requirement. Long uncontrolled dwell can allow crystallization before forming.
Incoming sheet history matters. Reprocessed material, contamination, or varying crystallinity can change the response even when the machine recipe is unchanged. Optical acceptance should be tied to the correct PET grade and end-use requirement.