Method for preparing biaxially oriented polyester film

The raw materials primarily used for standard BOPET films are masterbatch chips and bright chips; masterbatch chips consist of PET chips containing additives such as silica, calcium carbonate, barium sulfate, or kaolin. Bright chips are mixed with a specific proportion of masterbatch chips using a metering mixer before proceeding to the next stage of the process.

The production of BOPET film involves several core process steps: following the mixing of ingredients, the PET chips undergo pre-crystallization and drying-typically at 150–170°C for approximately 3.5–4 hours-to ensure the moisture content is controlled within the 30–50 ppm range. The material is then melt-extruded at temperatures between 210°C and 280°C. The melt is cast into a sheet using a coat-hanger die (maintained at 275–280°C) and cooled on a chill roll to form a glassy cast sheet. Longitudinal stretching follows, conducted at 115–125°C with a stretch ratio typically ranging from 3.3 to 4 times. This is followed by transverse stretching, usually at a ratio of 3.5 to 4 times. Finally, the film undergoes edge trimming, thickness measurement, and corona treatment before being hauled off, wound, and slit.

Heat-sealable BOPET films are typically produced using copolyester resins (such as APET or PETG) via single-layer extrusion or three-layer co-extrusion (A/B/C structure). Key process considerations include the lower drying temperature required for the copolyester resin (approximately 70°C) and an extrusion temperature range of 230–270°C. Longitudinal stretching is performed at 75–80°C, while transverse stretching is controlled at 90–100°C. Corona treatment should be applied to the non-heat-sealable surface.