Introduction to PET injection molding process

PET chemical name is polyethylene terephthalate, also known as polyester. The hygroscopicity of PET is relatively large, and the pellets are sensitive to water at high temperatures. When the water content exceeds the standard, hydrolysis occurs during molding, and the quality is greatly reduced. Therefore, the raw materials must be dried before the molding process.

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

Pay attention to the following eight points when PET is injection molded.

First, drying treatment

Since the PET macromolecule contains a lipid group and has a certain hydrophilicity, the pellet is sensitive to water at a high temperature. When the moisture content exceeds the limit, the molecular weight of the PET decreases during processing, and the product is colored and brittle. In this case, the material must be dried before processing, and the drying temperature is 150 ° C, 4 hours or more, generally 170 ° C, 3-4 hours. Airborne methods can be used to verify that the material is completely dry.

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

Second, the selection of injection molding machine

Injection molding is mainly used to enhance the molding of PET. PET can only be formed by screw injection molding machine under normal circumstances.

It is preferable to use a mutant screw with a check ring at the top, which has a large surface hardness and wear resistance, and the aspect ratio is not L/D = (15 to 20): 1 compression ratio is about 3:1.

The L/D too large material stays in the barrel for too long, and excessive heat is likely to cause degradation, which affects the performance of the product. The compression ratio is too small, the shear heat is small, the plasticization is poor, and the product performance is poor. On the contrary, the glass fiber is broken more and the mechanical properties are degraded. When processing glass fiber reinforced PET, the inner wall of the barrel is worn more, and the barrel is made of wear-resistant material or lined with wear-resistant material.

The nozzle is short, the inner wall is required to be polished, and the aperture is required to be as large as possible. It is better to use a hydraulic brake valve type nozzle. The nozzle should have insulation and temperature control measures to ensure that the nozzle does not freeze and block. However, the temperature of the nozzle should not be too high, otherwise it will cause runny. The low pressure PP material must be used before starting the molding to clean the barrel.

Third, the mold and gate design

The PET preform is generally formed by a hot runner mold. It is preferable to have a heat shield between the mold and the injection molding machine template. The thickness of the preform is about 12 mm, and the heat shield must withstand high pressure. Exhaust gas must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should not exceed 0.03mm, otherwise it will easily produce flash.

Fourth, the melt temperature

It can be measured by air-to-air method. Between 270 and 295 ° C, the enhanced grade GF-PET can be set to 290-315 ° C and the like.

Five, injection pressure

PET melt has good fluidity and is easy to form. It is usually medium pressure, pressure is 80~140MPa, and glass fiber reinforced PET injection pressure is 90~150MPa. The injection pressure should be determined by considering the viscosity of PET, the type and quantity of filler, the location and size of the gate, the shape and size of the plastic part, the mold temperature, and the type of injection molding machine.

Sixth, the injection speed

Generally, the injection speed is fast, which prevents premature coagulation during injection. But too fast, the high shear rate makes the material brittle. The shot is usually completed in 4 seconds.

Seven, back pressure

The lower the better, to avoid wear. Generally no more than 100bar. Usually no need to use.

Eight, staying time

Do not use excessive residence times to prevent molecular weight degradation. Try to avoid temperatures above 300 °C. If the shutdown is less than 15 minutes. Only need to be air-blasted; if it is more than 15 minutes, clean it with viscosity PE and reduce the temperature of the barrel to PE temperature until it is turned on again.

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