8 points need to know about PET preform injection molding
1.Processing of the plastic material
Since the PET macromolecule contains aliphatic and has a certain hydrophilicity, the particle is sensitive to water at a high temperature.
When the moisture content exceeds the limit, the molecular weight of the PET decreases during processing, then the product may be colored and brittle.
Therefore, the PET resin must be dried before processing, and its drying temperature is 150 ° C, more than 4 hours; generally 170 ° C,4-6 hours. Generally,the proportion of recycled PET preforms should not exceed 25%, and the recycled materials must be enough dried.
2.Choose of the PET preform injection molding machine
Since PET material has a high melting point and short stable time after melting point,it is necessary to choose an injection system with more temperature control section of the barrel and less self-friction heat during plasticization.
The actual product weight (plus sprue ) can’t be less than 2/3 of the injection capacity of the PET preform injection molding machine.
Generally, PET preform mold is with hot runner system.It is better to add a heat shield plate between the mold and the injection machine,the thickness is about 12mm.Air exhaust is also very important to the quality of the final preform.
4.Heating temperature of the barrel
It can be test by the empty-injection method.Generally,270-295℃. If it is GF-PET,the temperature can be 290-215℃.
Generally,the injection speed need to be fast,which prevents premature coagulation during the injection.But if too fast,the shear rate is high, may make the material brittle.
usually,the injection should be finished within 4 seconds.
6.Back pressure of charging
The less back pressure, the better. Generally, no need back pressure for making PET preform.
Do not use excessive residence time to prevent molecular weight degradation and try to avoid temperatures above 300 °C.
If the shutdown is less than 15 minutes, only air-to-air injection treatment is required; if it is more than 15 minutes, it should be cleaned with viscosity PE and the barrel temperature of injection machine need to lower to PE temperature until machine is turned on again.
- don’t add to much recycled material. otherwise,it may affect the plasticizing.
- If the mold temperature is not well controlled, or the material temperature is improperly controlled, it is easy to produce “white fog” and opaque; the mold temperature is low and uniform, the cooling rate is fast, and the crystals are less,then the product will be transparent.