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A. PVC material selection and formulation
- PVC resin. The injection molding type of PVC need the material with good MFI, suitable viscosity, easy-to-plastics, so normally use lighter and loose chemical structure PVC resin.
- PVC Stabilizer. The biggest shortcoming for PVC material is that PVC material is not stable and easily De-Hydrochlorination (HCL), this two shortcomings caused decomposition and cross-linking, the components become age color products, easily broken…After add the stabilizer into the material, all the Physical & Chemical Properties will be much improved.
- Plasticizer. You can add some plasticizer into the PVC resin, it can improve the PVC MFI, but the percentage should be controlled less then 5%.
- Lubricants: Due to PVC resin’s low MFI high viscosity, when it melted and flowing, the friction will cause high temperature. This high temperature will decomposition the PVC material. So if we add some lubricants, it will reduce the viscosity and increase the MFI.
- PVC molding processing modifier. Because the low stabilities of PVC material and short moulding temperature, we need to add some material to make the PVC resin easily melted and short plasticizing time. So we need to short the time of the melting PVC staying in the machine’s barrel or moulds’ runner.
- Impact modifier. As because the PVC resin with low impact capacity, we need to add the Impact Modifier to make it not easy broken. Normally, we use Chlorinated polyethylene to increase the impact of PVC moulding. The material Chlorinated polyethylene CL should be around 36%.
- Fillers. In order to save cost and reduce the shrinkage rate of PVC moulding, we add light Calcium Carbonate
B. PVC moulding equipments
- PVC raw material double screws extrusion machine, this machines are use for PVC material’s mixing and extrusions.
- PVC injection moulding machine, such like DAKUMAR PVC Series machines.
- PVC injection moulds
C. PVC moulding process.
- PVC moulding process diagram as blow:
- PVC material ingredients, before the ingredients please be sure that all kinds of the material are correct and pure enough, then be sure to dry all the material with correct weight.
- Mixing. Heat the hopper temperature into 90 centigrade degree, then stop the heating. We have to add all the different material in certain order. PVC resin (100), stabilizer, lubricants and fillers mixing for 6 minutes, then add plasticizer, after the plasticizer being absorbed, then add Chlorinated polyethylene for 5 minutes mixing. Then take out the mixed material from the mixer for cooling.
- Granulation. Please setup the heater temperature for your extrusion:
Heating Zone: Zone I Zone II Zone III Extruder
Temperature: 100-110 120-130 140- 145 140-150
D. PVC injection moulding, though you have improved the PVC material properties, but we need need to do very well controlling to
- PVC injection moulding machine’s temperature.
- PVC injection moulding pressure and injection speed. Though we have improved the PVC material properties, but it’s MFI is still very low. When melted PVC flowing, the friction will cause a lot of heat, this will caused a big damage to the material itself. For this reason, we need low pressure and speed. Noramally the moulding pressure should be 7.5 to 10MPa(M), the speed can be controlled by the cavities filling situations.
- PVC injection moulding machine screw RPM and back pressure. When we make the screw RPM too small,it will effect the production speed, while if too fast, there will be big heat occurred. To our experience, we suggestion the screw RPM 30/Minute. The back pressure should control between 0.5 to 2MPa.
- PVC mould temperature. PVC material has no obvious melting temperature point, 60 centigrade degree can make it soft, in order to be sure that the PVC part will not be deformed after ejection. Normally we should control the mould temperature below 60 degree. But mould temperature can not be too low, otherwise,it will effect the PVC material filling, for this reason, we need to control the PVC mould temperature within 35 to 55 centigrade degree.