The injection techniques of poly terephthalic acid alcohol ester (PET)
The chemical name of PET is poly terephthalic acid alcohol ester, also called polyester. At present, GF-PET is used by most clients. PET injection molding parts are mostly used in the bottle preform products
PET has a good rheological property in molten condition and pressure has more effects on viscosity than temperature does. Therefore, we change the rheological property mainly from the pressure.
Due to PET macromolecule contains an aliphatic base, it has certain hydrophilicity. The aggregate is very sensitive to water when it’s at a high temperature. When the water content exceeds the limit, the molecular weight of the PET will drop during the processing, so the product will be colored and become crisp. Therefore, we need to dry the material at 150 Celsius degree for more than 4 hours and 3 to 4 hours at 170 Celsius degree before processing. We can use air injection method to examine whether the material is completely dry or not. The proportion of recycled material is generally not more than 25%, and the recycled material needs to be completely dried.
Selection of injection molding machine
PET can only be stable for a short time after the melting point, and the melting point is so high, which needs to choose the injection system which has more temperature-control periods and less friction heat when plasticizing, and the product’s (the material which contains water) actual weight cannot be less than two-thirds of the amount of machine injection.
Mold and gate design
We generally use hot runner mold to design PET bottle preform. It is better to have an insulation board between mold and injection molding machine template. Insulation board’s thickness is about 12 mm, and can bear high pressure. There is enough exhausting effect for it so as not to appear local overheating or chipping, but the vent’s depth is generally not more than 0.03 mm, otherwise, it is easy to produce flashing.
It can be measured by the air injection method. The temperature is ranging from 270 Celsius degree to 295 Celsius degree and the enhanced GF – PET’s temperature can be set from 290 Celsius degree to 315 Celsius degree, etc.
Generally, injection speed must be fast, in order to prevent premature solidification. But if injecting too fast, the shear rate would be so high that the material would become fragile easily. The material injection is usually completed in 4 seconds.
In order to avoid wearing, the packing pressure shall be as low as possible. Generally, it’s not more than 100 bar. We usually don’t need to use it.
It should avoid overlong detention time to prevent from the molecular weight declining. Try to avoid the temperature above 300 Celsius degree. If the machine stops less than 15 minutes, we only need to make air injection; If the machine stops more than 15 minutes, we should not start the machine again until we use viscosity PE to clean the machine and make the feed cylinder’s temperature drop to PE temperature.
(1) Recycled material cannot be too much. Otherwise, it’s easy to form “bridging” at discharging chute which will influence the plasticizing.
(2) If we don’t control the mold temperature or the metal temperature properly, it’s easy to produce “white fog” and become opaque. When the mold temperature is low and even, the cooling speed shall be too fast with few crystallization and transparent products.