what’s PET ?
PET chemical name is polyethylene terephthalate, also known as polyester. At present, GF-PET is the most used among customers, mainly for production of PET bottle preform.
The rheology of PET in molten state is better, and the effect of pressure on viscosity is greater than temperature. Therefore, the fluidity of the melt is mainly changed from pressure.
plastic processing
Since the PET macromolecule contains a lipid group and has a certain hydrophilicity, the pellet is sensitive to water at a high temperature. When the moisture content exceeds the limit, the molecular weight of the PET decreases during processing, and the product is colored and brittle. In this case, the material must be dried before processing, and the drying temperature is 150 ° C, more than 4 hours, generally 170 ° C, 4-6 hours. Air injection methods can be used to verify if the material is completely dry.
The proportion of recycled materials should generally not exceed 25%, and the recycled materials should be thoroughly dried.
injection molding machine selection
Since PET has a short stable time after melting point, and a high melting point, it is necessary to use an injection system with more temperature control sections and less self-friction heat during plasticization, and the actual weight of the product (aqueous sprue material) cannot be less than 2/3 of injection capacity of the injection molding machine.
mold and gate design
The PET preform is generally formed by a hot runner mold. It is preferable to have a heat insulation plate between the mold and the injection molding machine‘s template. The thickness of the plate is about 12 mm, and the heat plate must withstand high pressure. Exhaust gas must be sufficient to avoid local overheating or chipping, but the depth of the exhaust port should not exceed 0.03mm, otherwise it will easily produce flash.
Melt temperature
It can be measured by air-to-air method. Between 270 and 295 ° C for production of PET preform, the enhanced grade GF-PET can be set to 290-315 ° C .
Injection speed
Generally, the injection speed need to be fast, which prevents premature coagulation during injection. But too fast, the high shear rate makes the material brittle. The shot is usually completed in 4 seconds for production of PET preform.
Back pressure
The lower the better, to avoid wear. Generally no more than 100bar. Usually no need back pressure for production of PET preform.
Residence time
Do not use excessive residence times to prevent molecular weight degradation. Try to avoid temperatures above 300 °C. If the shutdown is less than 15 minutes. Only need to be air-injection; if it is more than 15 minutes, clean it with viscosity PE and reduce the temperature of the barrel to PE temperature until it is turned on again.
Precautions
(1) The recycled material should not be too much, otherwise it will easily cause “bridge” at the blanking and affect plasticization.
(2) If the mold temperature is not well controlled or the material temperature is not properly controlled, it is easy to produce “white fog” and is opaque. The mold temperature is low and uniform, the cooling rate is fast, and the crystallization is less, the product is transparent.