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What are the injection molding processes of PET plastics and products?
May 11, 2018

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, plastic processing

Because PET macromolecules contain lipid groups and have a certain degree of hydrophilicity, the 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 products are colored and 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.


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2, injection molding machine selection

Because PET has a short stabilization time after the melting point and a high melting point, it is necessary to use injection systems with more temperature control sections and less self-heating during plasticization. The actual weight of the product (water-containing mouthpiece) must not be less than the machine injection. 2/3 of the amount. 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 are generally formed by hot runner molds. It is better to have insulation boards between the molds and the injection molding machine template. The thickness of the PET preforms is about 12 mm, 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

Available air-ejection measurements. Different from 270-295°C, the reinforced grade GF-PET can be set to 290-315°C.


5, injection speed

The general injection speed is faster and prevents premature coagulation during injection. But too fast, the high shear rate makes the material fragile. The shot is usually completed within 4 seconds.


6, back pressure

The lower the better, so as not to wear out. Generally does not exceed 100bar. Usually do not need to use.


7, stay time

Do not use excessive residence time to prevent the decrease in 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 should not be too large; otherwise, it will easily lead to "bridge" at the material feed 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.


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