Because PET macromolecules contain fat groups and have a certain degree of hydrophilicity, the pellets are more 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.

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8 issues to be noted when injection molding PET performs
1. Plastic processing
Because PET macromolecules contain fat groups and have a certain degree of hydrophilicity, the
pellets are more 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.
Therefore, before processing, the materials must be dried, the drying temperature is 150 ℃,
more than 4 hours; generally 170 ℃, 3-4 hours. The air shot method can be used to check
whether the material is completely dry. The proportion of PET preform recycled materials should
not exceed 25%, and the recycled materials should be thoroughly dried.
2. Selection of injection molding machine
Because PET has a short stabilization time after melting point and a higher melting point, an
injection system with more temperature control sections and less self-friction heat generation
during plasticization needs to be selected, and the actual weight of the product (water-containing
mouth material) cannot be less than that of machine injection 2/3 of the amount.
3. Mold and gate design
PET preforms are generally formed by hot runner molds. It is best to have a heat insulation board
between the mold and the injection molding machine template. Its thickness is about 12mm, and
the heat insulation board must be able to withstand high pressure. The exhaust must be
sufficient to avoid local overheating or cracking, but the depth of the exhaust port should
generally not exceed 0.03mm, otherwise flash is likely to occur.
4. Melt temperature

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It can be measured by the air-shot method, ranging from 270-295 ℃, and enhanced grade
GF-PET can be set to 290-315 ℃, etc.
5. Injection speed
Generally, the injection speed should be fast to prevent premature coagulation during the
injection. But too fast, the shear rate is high, making the material brittle. The shot is usually
completed within 4 seconds.
6. Backpressure
The lower the better, to avoid wear. Generally no more than 100bar, usually no need to use.
7. Residence time
Do not use too long residence time to prevent molecular weight from decreasing. Try to avoid
temperatures above 300 ℃. If the machine is shut down for less than 15 minutes, only air shot is
required; if it is more than 15 minutes, it needs to be cleaned with viscosity PE, and the barrel
temperature is reduced to PE temperature until it is turned on again.
8. Matters needing attention
Recycled material should not be too large, otherwise, it is easy to "bridge" at the cutting place
and affect plasticization;
If the mold temperature control is not good, or the material temperature is not properly
controlled, it is easy to produce "white mist" and opaque; the mold temperature is low and
uniform, the cooling rate is fast, and the crystal is less, the product is transparent.