What Should I Pay Attention to When Injecting PET Preforms?

Category: Media coverage

Release time:2017-11-01

The chemical name of PET is polyethylene terephthalate, also known as polyester. Now the most used in customers is GF-PET, mainly for preforms. PET has good rheology in the molten state, and the influence of pressure on viscosity is greater than that of temperature, so the fluidity of the melt is mainly changed from the pressure.

1. Treatment of plastics

Because PET macromolecules contain lipid groups, with certain hydrophilicity, pellets are sensitive to water contrast at high temperatures, and when the moisture content exceeds the limit, the molecular weight of PET is reduced during processing, and the finished product is colored and brittle. Trapped in this, it is necessary to dry the material before processing, and its dry temperature is 150 °C, more than 4 hours, generally 170 °C, 3-4 hours. It is possible to check whether the data is completely boring by air-firing methods. The share of recovered material should generally not exceed 25%, and the recovered material should be completely boring.

2. Injection molding machine selection

Because PET has a short stable time after the melting point and a high melting point, it is necessary to choose an injection system with more temperature control sections and less self-conflict heat generation during plasticization, and the practical amount of the finished product (aqueous mouth) cannot be less than 2/3 of the machine injection amount. Based on these requirements, FORAY has developed a small and medium-sized series of PET-specific plasticizing systems in recent years. The clamping force is selected according to more than 6300t/m2.

3. Mold and gate planning

PET preforms are generally molded with a warm runner mold, and the best between the mold and the injection molding machine template is to have a heat insulation board, whose thickness is about 12mm, and the heat insulation board must be able to accept high pressure. The exhaust must be sufficient to prevent part of the display from overheating or chipping, but the depth of the exhaust port generally does not exceed 0.03mm, otherwise flash simply occurs.

4. Melting temperature

It can be measured by air-launched method. 270-295°C, enhanced grade GF-PET can be set to 290-315°C, etc.

5. Injection speed

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

6. Back pressure

The lower the better, to prevent wear. Generally, it does not exceed 100 bar. Generally does not need to be used.

7. Stay time

Do not use excessive residence time to prevent molecular weight reduction. Try to prevent temperatures above 300°C. If the downtime is less than 15 minutes. Only air-fired treatment is required; If it exceeds 15 minutes, it should be cleaned with viscosity PE and the barrel temperature should be reduced to PE temperature until it is stopped again.

8. Precautions

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

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

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