Plastic processing
Because PET macromolecules contain lipid groups and have a certain degree of hydrophilicity, the particles are more sensitive to water at high temperatures. When the water content exceeds the limit, the molecular weight of PET decreases during processing and the product becomes discoloured and brittle. Therefore, the material must be dried before processing, the drying temperature is above 150°C for 4 hours, generally 170°C for 3-4 hours. The material can be checked for complete drying by air injection. In general, the proportion of recycled material should not exceed 25% and the recycled material should be thoroughly dried.
Mould and gate design
A PET preform generally consists of a hot runner mould. It is advisable to have a heat shield between the mould and the injection moulding machine template, which must be approximately 12 mm thick and must be able to withstand high pressures. The exhaust air volume must be sufficient to avoid localised overheating or cracking, but the depth of the exhaust air opening should generally not exceed 0.03 mm, otherwise flash can easily occur.
Melt temperature
The melt temperature can be measured by air injection. 270-295°C, 290-315°C for reinforced GF-PET, etc. 5.
Injection speed
The injection rate should be very fast to prevent premature solidification during injection. However, the high shear rate makes the material brittle if it is too fast. Shots are usually 4 seconds.
Back pressure
The lower the better to avoid wear. Normally no more than 100 bars. 7.
Dwell time
Do not use excessive dwell times to prevent molecular weight loss. Try to avoid temperatures higher than 300°C. If the dwell time is less than 15 minutes. If it takes more than 15 minutes, it should be cleaned with viscous PE and the barrel temperature should be lowered to PE temperature until the plastic injection molding machine is started again.
Preventive Measures
1) Recycled material should not be too big, otherwise it will be easily “stuck” in the dropout and affect the plasticization.
2) If the mould temperature is not controlled properly or the material temperature is not controlled properly, it is easy to produce “white mist” and opacity. Low and uniform mould temperature, fast cooling speed, transparent and crystalline products. Daoben is a professional manufacturer of china injection molding machine.
