PET — Polyethylene Terephthalate
Polyethylene Terephthalate
PET (polyethylene terephthalate) is a semi-crystalline polyester that can be processed two ways: amorphous and transparent for optical parts, or crystalline and rigid for reinforced technical grades (Rynite, Arnite). It combines good mechanical strength, chemical resistance and gas barrier properties.
- Mold temperature: 15-140°C
- Melt temperature: 260-290°C
- Shrinkage: 0.2-2.0%
- Density: 1.33-1.40
- Manufacturer grades: Rynite 530 (DuPont), Arnite A04 200 (DSM), Lighter C93 (Equipolymers)
Processing
PET is highly hygroscopic: thorough drying of about 4 h at 140 °C is mandatory to target less than 50 ppm moisture, otherwise residual water causes hydrolysis, a viscosity drop and very brittle parts. Run the melt between 260 and 290 °C. Mold temperature controls crystallinity: a cold mold (15 to 30 °C) freezes the material in an amorphous, transparent state, while a hot mold (120 to 140 °C) promotes controlled crystallization for rigid reinforced technical grades — hence the very wide mold temperature range (15 to 140 °C).
Advantages
- Dual behavior: amorphous (transparent) or crystalline (rigid) depending on mold temperature
- Good mechanical strength and rigidity, especially in glass-fiber reinforced grades
- Excellent chemical resistance and good gas barrier properties
- Good dimensional stability of crystalline technical grades
Limitations
- Highly hygroscopic: thorough drying (≈ 4 h at 140 °C, < 50 ppm) is mandatory
- Hydrolysis-sensitive: moisture degrades the material and embrittles parts
- Narrow processing window: overheating and shearing quickly degrade the polymer
- Crystallization must be controlled: a poorly regulated mold means warpage or lost transparency
Common injection molding defects
- Silver streaks — Residual moisture and hydrolysis: dry PET at 140 °C to below 50 ppm
- Brittle parts — Hydrolytic degradation from moisture: improve drying and limit residence time
- Warpage — Differential crystallization and shrinkage: homogenize mold regulation and cooling
Typical applications
Bottles, food packaging, textile fibers