What should we pay attention to injection machine make preform?
The PET chemical is called polyterephthalate, or polyester.Currently, gf-pet is the most commonly used product among customers, mainly for bottling preform.The rheological property of PET in the melting state is good, and the influence of pressure on the viscosity is greater than that of temperature. Therefore, the melt fluidity is mainly changed from pressure.
- Treatment of plastics
Because PET macromolecules contain lipid groups and have a certain hydrophilicity, the particles are sensitive to water at high temperature. When the water content exceeds the limit, the molecular weight of PET will decrease during processing and the products will become colored and brittle.Therefore, it is necessary to dry the materials before processing. The drying temperature is 150℃ for more than 4 hours, generally 170℃ for 3-4 hours.Air – shot method can be used to test whether the material is completely dry.Generally, the proportion of recycled materials should not exceed 25%, and the recycled materials should be completely dry.
2.Injection molding machine selection
Due to the short stable time after melting point and high melting point of PET, the injection system with more temperature control section and less heat generated by self-friction during plasticizing should be selected. Moreover, the actual weight of the product (water-bearing material) should not be less than 2/3 of the injection volume of the machine.
3.Mold and gate design
PET bottle embryo is generally formed by hot runner mold. It is better to have heat shield between mold and injection molding machine template. Its thickness is about 12mm, and the heat shield must withstand high pressure.Exhaust must be sufficient to avoid local overheating or fragmentation, but the depth of the vent generally should not exceed 0.03mm, otherwise easy to produce flash.
4.Melt temperature
It can be measured by air injection.270-295℃, enhanced gf-pet can be set as 290-315℃, etc.
5.injection speed
General injection speed to fast, can prevent premature coagulation when injection.But too fast, high shear rate makes the material fragile.The shot is usually finished in 4 seconds.
6.Back pressure
The lower the better to avoid wear and tear.Usually no more than 100bar.Usually no use.
7.Detention time
Do not use too long retention time to prevent molecular weight drop.Try to avoid temperatures above 300℃.If the downtime is less than 15 minutes.Only air shot;If more than 15 minutes, use viscosity PE cleaning, and the machine barrel temperature to PE temperature, until the boot.
- Recovery material can not be too large, or easy to produce under the material “bridge” and affect the plasticization.
- If the mold temperature control or improper control of the material temperature, easy to produce “white fog” and opaque.The mold temperature is low and uniform, the cooling speed is fast, the product is transparent if the crystallization is less.
