[Technology] Understanding PET preform
PET bottles have strong applicability and are widely used in daily necessities, daily chemical packaging and other fields. The preform is used as the intermediate semi-finished product of the subsequent two-way stretch blow molding. Some factors of the preform affect the quality of the injection molded product.
Under a certain temperature and pressure, the mold is filled with raw materials and processed by the injection molding machine to form a bottle preform with a certain thickness and height corresponding to the mold. PET preforms are reprocessed by blow molding to form plastic bottles, including bottles used for packaging cosmetics, medicines, health care, beverages, mineral water, reagents, etc. This bottle making method is called a two-stage method, that is, the preform is formed through injection molding, then PET bottles form through blow molding.
PET raw material characteristics
The transparency is as high as 90% or more, the surface gloss is excellent, and it has a glass-like appearance; it has excellent fragrance retention and good air tightness; it has excellent chemical resistance, and it can resist almost all organic drugs, led by acid; it has good hygiene; it burned will not generate Toxic gas; the strength characteristics are excellent, and various characteristics can be further improved by biaxial stretching.
8 factors in injection molding
Because PET macromolecules contain fat bases and have certain 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 product becomes colored and brittle.
Therefore, before processing, the materials must be dried. 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 PET preforms should not exceed 25%, and the recycled materials must be thoroughly dried.
Injection molding machine selection
Since PET has a short stabilization time after the melting point and high melting point, it is necessary to select an injection system with more temperature control sections and less self-friction heat generation during plasticization, and the actual weight of the product (including injection gate material) cannot be less than 2/3 of the machine injection shot weight.
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. The air vent must be sufficient to avoid local overheating or chipping, but the depth of the air vent should generally not exceed 0.03mm, otherwise it is easy to produce flash.
It can be measured by air shot method, ranging from 270-295℃, and for enhanced GF-PET can be set at 290-315℃, etc.
Generally, the injection speed should be fast to prevent premature solidification during injection. But too fast, the shear rate is high, making the material fragile. Injection is usually completed within 4 seconds.
The lower the better to avoid wear. Generally, it does not exceed 100bar and usually does not need to be used.
Melt keeping time
Do not use too long keeping time to prevent the molecular weight from decreasing, and try to avoid temperatures above 300°C. If the machine is shut down for less than 15 minutes, only air injection is required; if it exceeds 15 minutes, it must be cleaned with viscosity PE and the temperature of the barrel is lowered to the PE temperature until the machine is turned on again.
The recycled material should not be too large, otherwise, “bridging” will occur at the blanking place and affect the plasticization; if 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, If the cooling rate is fast and the crystallization is less, the product is transparent.
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