The rheological property of pet is better in the melting state, and the effect of pressure on viscosity is greater than that of temperature. Therefore, the fluidity of pet is mainly changed by pressure.
1. Plastic treatment
Because pet macromolecules contain aliphatic groups and have certain hydrophilicity, the particles are more active to water at high temperature. When the water content exceeds the limit, the molecular weight of pet decreases during processing, and the products are colored and brittle. Therefore, the material must be dried before processing. The drying temperature is 150 ℃, more than 4 hours, generally 170 ℃, 3-4 hours. Air jet method can be used to check whether the material is completely dry.
2. Selection of injection molding machine
Because pet has a short time of stabilization after melting point and a high melting point, it is necessary to select a injection system with more temperature control sections and less self friction and heat generation during plasticization, and the practical component of the product should not be less than 2 / 3 of the injection volume of the machine. According to these requirements, in recent years, Huameida has developed small and medium series pet special plasticizing system. The clamping force shall be more than 6300t / m2.
3. Mold and gate planning
PET bottle embryo is usually formed by warm runner mold. It is better to have heat shield between mold and injection molding machine template, with thickness of about 12mm, and the heat shield must be able to withstand high pressure. The exhaust must be sufficient to avoid partial overheating or fragmentation, but the depth of the exhaust port should not exceed 0.03mm generally, otherwise it is easy to cause flash.
4. Glue melting temperature
It can be measured by air injection. 270-295 ℃, the enhanced gf-pet can be set as 290-315 ℃.
5. Injection speed
Generally, the injection speed should be fast to avoid premature coagulation during injection. But too fast, high shear rate makes the material fragile. The ejection usually ends in 4 seconds.
6. Back pressure
The lower the better to avoid wear. Generally, it does not exceed 100bar. Generally, it does not need to be used.
7. Stay time
Do not use too long residence time to avoid molecular weight reduction. Try to avoid temperatures above 300 ℃. If the shutdown is less than 15 minutes. Only air jet treatment is required; if it exceeds 15 minutes, clean it with viscosity PE, and reduce the barrel temperature to PE temperature until restart and stop.
If the mold temperature control is not good or the material temperature control is not proper, it is easy to have a white fog and not clear. Mold temperature is low and uniform, cooling speed is fast, less crystal products are clear.
Article source: Jiangmen pet food container manufacturerhttp://www.avalentin.com
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