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What should we pay attention to when injecting PET bottle embryo?
- Dec 11, 2018 -

PET chemical name is polyethanol terephthalate, also known as polyester. At present, GF-PET is the most widely used in customers, mainly bottled embryos. The rheology of PET is better in melting state, and the influence of pressure on viscosity is greater than that of temperature. Therefore, it is necessary to change the fluidity of melt mainly by pressure.

1. Plastics treatment

Because PET macromolecule contains lipid group, it has certain hydrophilicity. Particles are sensitive to water at high temperature. When the moisture content exceeds the limit, the molecular weight of PET decreases and the products become colored and brittle. Therefore, the material must be dried before processing. The drying temperature is 150 C, more than 4 hours, generally 170 C, 3 - 4 hours. It can be used to check whether the material is completely dry.

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

2. Injection molding machine selection

Because of the short time of stabilization after melting point and the high melting point of PET, the injection system with more temperature control section and less self-friction heat generation during plasticization should be selected, and the actual weight of the product (water-containing orifice) should not be less than 2/3 of the injection volume of the machine. Based on these requirements, Huameida has developed a small and medium-sized series of special plasticizing system for PET in recent years. The clamping force is selected according to the force greater than 6300t/m2.

3. Design of Mould and Gate

PET bottle embryos are usually formed by hot runner moulds. It is better to have a heat insulation plate between the moulds and injection moulding machine templates. The thickness of the heat insulation plate is about 12 mm, and the heat insulation plate must be able to withstand high pressure. Exhaust must be sufficient to avoid local overheating or fragmentation, but the exhaust outlet depth should not exceed 0.03mm, otherwise it is easy to produce flash.

4. Melt temperature

It can be measured by air-firing method. Enhanced grade GF-PET can be set to 290-315 (?)

5. 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. The projectile is usually completed in 4 seconds.

6, back pressure

The lower the better, so as not to wear. Usually not more than 100 bars. Usually no need to use.

7. Detention time

Do not use too long residence time to prevent molecular weight decline. Avoid temperatures above 300 degrees Celsius as far as possible. If the downtime is less than 15 minutes. Only air-jet treatment is required; if it lasts more than 15 minutes, it should be cleaned with viscous PE and the barrel temperature should be lowered to PE temperature until the machine is restarted.

8, matters needing attention

(1) Recycled material should not be too large, otherwise it is easy to build a bridge at the discharging site and affect plasticization.

(2) If the mould temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opaque. The mould temperature is low and uniform, the cooling speed is fast, and the product is transparent if the crystallization is small.