Brief introduction of PET plastics and injection molding processing

PET chemical name is polyethylene terephthalate, also known as polyester. At present, GF-PET is the most used among customers, mainly for bottle preforms.

The rheology of PET in the molten state is good, and the pressure has greater influence on the viscosity than the temperature. Therefore, the fluidity of the melt is mainly changed from the pressure.

1. Treatment of plastics PET macromolecules contain a lipid group 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 during processing and the product is colored. Become brittle. For this reason, the material must be dried before processing. The drying temperature is 150°C, more than 4 hours, usually 170°C, 3-4 hours. Air ejection can be used to check if the material is completely dry.

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

2. The injection molding machine uses PET because of the short time after the melting point is stable and the melting point is higher. Therefore, it is necessary to use more injection molding systems with more temperature control sections and less self-heating during plasticization, and the product (water-containing mouth material) is actually used. The weight should not be less than 2/3 of the injection volume of the machine. Based on these requirements, Ramada has developed a small and medium-sized PET special plasticizing system in recent years. The clamping force is greater than 6300t/m2.

3, mold and gate design PET preforms generally use hot runner mold, mold and injection molding machine template between the best insulation board, the thickness of about 12mm, and the insulation board must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the exhaust port generally should not exceed 0.03mm, otherwise it is prone to flash.

4. Melt temperature can be measured by air injection method. Different from 270-295°C, the reinforced grade GF-PET can be set to 290-315°C.

5, the injection speed is generally faster, can prevent premature coagulation during injection. But too fast, the high shear rate makes the material fragile. The shot is usually completed within 4 seconds.

6, the lower the back pressure, the better, so as not to wear. Generally does not exceed 100bar. Usually do not need to use.

7. Do not use long residence time to prevent the decrease of molecular weight. Try to avoid temperatures above 300°C. If the stoppage is less than 15 minutes. Only air shot processing is required; if it exceeds 15 minutes, it is cleaned with viscosity PE and the barrel temperature is lowered to PE temperature until it is turned on again.

8. Matters needing attention (1) Recycling materials can not be too large, otherwise it is easy to produce "bridging" at the blanking area and affect plasticization. (2) If the mold temperature control is not good or the material temperature is not properly controlled, it is easy to produce "white fog" and not transparent. The mold temperature is low and uniform, the cooling rate is fast, and the product is transparent when the crystal is low.