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What should be paid attention to when injection molding PET preforms?

Views: 86     Author: Site Editor     Publish Time: 2022-03-25      Origin: Site

1. Treatment of plastics


Because PET macromolecules contain fat base and have certain hydrophilicity, the pellets are more sensitive to water at high temperature. When the water content exceeds the limit, the molecular weight of PET decreases during processing, and the product becomes colored and brittle. Because of this, the materials must be dried before processing. The drying temperature is 150°C for more than 4 hours, generally 170°C for 3-4 hours. The air shot method 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 must be thoroughly dried.


2. Selection of injection molding machine


Because PET has a short stabilization time after melting point and high melting point, it is necessary to choose an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual weight of the product (water cut material) cannot be less than the machine injection 2/3 of the amount. Based on these requirements, Ramada has developed a series of small and medium-sized PET special plasticizing systems in recent years. The clamping force is selected as greater than 6300t/m2.

pet bottle

3. Mold and gate design


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


4. Melting temperature


It can be measured by air shot method. The temperature ranges from 270-295°C, and the enhanced GF-PET can be set at 290-315°C.


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. Injection is usually completed within 4 seconds.


6. Back pressure


The lower the better, so as to avoid wear. Generally not more than 100bar. Usually not needed.

bottle

7. Detention time


Do not use an excessively long residence time to prevent the molecular weight from decreasing. Try to avoid temperatures above 300°C. If the downtime is less than 15 minutes. Only need to do air injection treatment; if it exceeds 15 minutes, clean with viscosity PE, and reduce the barrel temperature to PE temperature until the machine is turned on again.


8. Matters needing attention


⑴Recycled material should not be too large, otherwise it is easy to cause "bridging" at the feeding place and affect plasticization.


⑵If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce "white fog" and opacity. The mold temperature is low and uniform, the cooling rate is fast, and the product is transparent with less crystallization.

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