What should be paid attention to when injection molding PET preforms?

The chemical name of PET is polyethylene terephthalate, also known as polyester. At present, GF-PET is the most used by customers, mainly for preforms. PET has better rheology in the molten state, and the pressure has a greater effect on viscosity than temperature. Therefore, the pressure is mainly used to change the fluidity of the melt.

Treatment of plastics Because PET macromolecules contain fat bases, which have a certain degree of hydrophilicity, pellets are more sensitive to water at high temperatures. When the moisture content exceeds the limit, the molecular weight of PET will decrease during processing, and the products will become colored and brittle. . Because of this, the materials 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. The proportion of recycled materials should generally not exceed 25%, and the recycled materials must be thoroughly dried.

The injection molding machine uses PET due to its short stabilization time after the melting point and high melting point. Therefore, it is necessary to select 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) cannot be Less than 2/3 of the injection volume of the machine.

Mold and gate design PET preforms are generally formed by hot runner molds. It is better to have a heat shield between the mold and the injection molding machine template, the thickness of which is about 12mm, and the heat shield 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 it is easy to produce flash.

The melt temperature can be measured by the air shot method. The temperature ranges from 270-295°C, and the enhanced GF-PET can be set at 290-315°C.

Injection speed Generally, the injection speed should be fast to prevent premature coagulation during injection. But too fast and high shear rate make the material fragile. Injection is usually completed within 4 seconds. 

The lower the back pressure, the better to avoid wear. Generally not more than 100bar. Usually don't need to be used. 

Retention time Do not use too long a retention time to prevent the molecular weight from decreasing. Try to avoid temperatures above 300°C. If the downtime is less than 15 minutes. Only need to do air injection treatment; if it exceeds 15 minutes, clean with viscosity PE, and reduce the barrel temperature to PE temperature until the machine is turned on again.

Precautions

⑴Recycled material should not be too large, otherwise it is easy to cause "bridging" at the feeding place and affect plasticization.

⑵If 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 rate is fast, and the product is transparent with less crystallization.