PET macromolecules contain lipid groups and have a certain degree of hydrophilicity. Pellets are sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET decreases and the products are colored and brittle during processing.
Therefore, before processing, the materials must be dried with the temperature of 150 ℃ in more than 4 hours. Generally 170 ℃, 3-4 hours. The air shot method can be used to check whether the material is completely dry. The proportion of recycled PET preform materials generally shouldn’t exceed 25%. And the recycled materials should be thoroughly dried.
Selection of injection molding machine
PET has a short stabilization time after the melting point and a high melting point. Therefore, we should select an injection unit with more temperature control stages and less self-friction heat during plasticization. Besides, the actual weight of the product (includes sprue) cannot be less than the 2/3 of the shot weight for the machine.
Mold and gate design
PET preforms are generally formed by hot runner molds. It is best to have a heat insulated board between the mold and the injection molding machine platen. Its thickness is about 12mm and it must be able to withstand high pressure. The exhaust must be sufficient to avoid local overheating or chipping. But the depth of the exhaust port should generally not exceed 0.03mm, otherwise there would be flashing easily.
It can be measured by air-shot method, ranging from 270-295℃, and enhanced GF-PET can be set to 290-315℃.
Generally, the injection speed should be fast to prevent premature solidification during injection. But if too fast, the shear rate is high, making the material brittle. The shot is usually completed within 4 seconds.
The lower the better, to avoid wear. Generally, it does not exceed 100 bar and usually is unnecessary.
Don’t use too long a residence time to prevent the molecular weight from falling. Try to avoid temperatures above 300℃. If the machine is shut down for less than 15 minutes, only air shot is required. If it is more than 15 minutes, it should be cleaned with viscosity PE. And the barrel temperature should be reduced to PE temperature until it is turned on again.
- Recycled material should not be too large, otherwise, it will be easy to “bridge” at the cutting place and affect the plasticization;
- If the mold temperature or the material temperature is not properly controlled, it is easy to produce “white mist” and opacity. The mold temperature is low and uniform, with fast cooling speed and less crystal, the product is transparent.