What should we pay attention to when PET bottle preform injection molding?

September 21, 2022

1. Processing of plastics

Because PET macromolecules contain lipid groups and have a certain degree of hydrophilicity, the pellets are sensitive to water at high temperatures. When the water content exceeds the limit, the molecular weight of PET decreases during processing, and the products become colored and brittle. Due to this, the material must be dried before processing. The drying temperature is 150°C for more than 4 hours, generally 170°C for 3-4 hours. The air shot method can be used to check whether the material is completely dry.

Generally, the proportion of recycled materials should not exceed 25%, and the recycled materials should be thoroughly dried.

2. Selection of Injection Molding Machine

Because PET has a short stable time after melting point and high melting point, it is necessary to choose an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual weight of the product (water-containing material) should not be less than machine injection. 2/3 of the amount. Based on these requirements, a special plasticizing system for PET should be selected.

3. Mold and gate design

PET preforms are generally formed by hot runner molds. It is best to have a heat insulation board between the mold and the injection molding machine template, and its thickness is about 12mm, and the heat insulation board must be able to withstand high pressure. The exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port should generally not exceed 0.03mm, otherwise it is easy to produce flash.

4. Melt temperature

Can be measured by air-shot method. 270-295℃, the enhanced GF-PET can be set to 290-315℃, etc.

5. Injection speed

Generally, the injection speed should be fast, which can prevent premature solidification during injection. But too fast, the high shear rate makes the material brittle. Shots are usually completed within 4 seconds.

6. Back pressure

The lower the better, so as not to wear out. Generally not more than 100bar. Usually not needed.

7. Residence time

Do not use excessive residence times to prevent molecular weight loss. Try to avoid temperatures above 300°C. If the downtime is less than 15 minutes. It only needs to be air-shot; if it takes more than 15 minutes, it should be cleaned with viscosity PE, and the temperature of the barrel should be lowered to the PE temperature until it is turned on again.

8. Precautions

(1) The recycled material should not be too large, otherwise it is easy to "bridge" at the blanking place and affect the plasticization.

(2) If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opacity. The mold temperature is low and uniform, the cooling speed is fast, and the product is transparent with less crystallization.

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