Process parameters and production problems of hard polyvinyl chloride (UPVC) injection molding

June 14, 2019

 

Process parameters and production problems of hard polyvinyl chloride (UPVC) injection molding

 

Process characteristics:

The raw materials to be mixed have different process conditions for the molded articles. The raw materials mixed with various additives and polyvinyl chloride resins usually start to soften at 60 ° C, and gradually become highly elastic at 100 ° C or higher, and exhibit a viscous flow state at 150 ° C or higher, starting at about 200 ° C. break down. Produces hydrogen chloride gas during decomposition, which is corrosive and irritating. The melt has poor thermal stability, poor fluidity, and is easily decomposed when the temperature inside the barrel is too high or the residence time is long. Low-speed high-pressure injection should be used during molding.

 

Selection of raw materials:

The polyvinyl chloride resin used for injection molding is commonly used as SG5~SG8 (GB/T5761-1993). Since this resin molded article allows a water content of 0.4%, the resin does not need to be dried during production. If the water content of the resin is greater than 0.4%, the raw material can be dried in a hot air circulating oven at 90~100 ̊C for 1~1.5h.


Process parameter selection:

1. Plasticizing injection temperature
The plasticizing injection temperature of the polyvinyl chloride resin is generally controlled within the range of 160 to 190 ̊C.
Since the melting plasticization temperature of this resin is close to its decomposition temperature, special attention should be paid in the production process: the residence time of the raw material in the barrel should not be too long, and the rotation speed of the screw should not be too high to avoid improper control due to process conditions. And cause the raw materials to decompose. Note that the plasticizing temperature of the raw material should be as low as possible; the temperature control of the barrel should be gradually increased from 160 to 190 °C from the feeding end.
The temperature control of the nozzle is about 15 ̊C lower than the maximum temperature of the barrel. At the same time, a larger nozzle aperture (4~10mm), such as a straight-through nozzle, should be used.
2, the injection pressure of the melt
The selection of the injection pressure when the polyvinyl chloride resin melt molding product has a large influence on the molding quality of the product. Generally controlled at 90MPa or more. The pressure used for holding pressure can be lower than the injection pressure, generally 60~80MPa.
Note: The injection speed that is taken too fast during injection is also very likely to cause the decomposition of the melt. Therefore, the injection molding of the PVC melt should be carried out at a medium to low injection speed and a high injection pressure.
3, molding temperature
The temperature of the molding die for cooling and setting the PVC melt is generally controlled within the range of 30 to 60 ̊C. For parts with simple structure and shape, lower temperature is required; if the shape of the product is more complicated, the wall thickness is smaller or larger, the temperature of the forming mold is higher, and it can be controlled between 40~60 °C.


Precautions:

1. PVC injection molding products must use screw injection molding machines.
2. The screw structure should be a gradual type. The aspect ratio is L/D=l2~20, the compression ratio is 2~2.5, and the screw head is tapered.
3. The length distribution of the threaded part of the screw: 40% of the feeding section, 40% of the plasticizing section (compression section), and 20% of the homogenization section (counting section).
4. In order to avoid the decomposition of the melt, the process time of each process should be shortened as much as possible, so that the product can be demoulded smoothly without deformation, and the general molding cycle is controlled between 40 and 80 s.
5. If the product is found to have discoloration stripes during production, it should be immediately stopped to clean the barrel.
6. When replacing the raw material production, if the barrel is made of polystyrene or ABS resin, the temperature of the machine should be adjusted to the temperature for plasticizing the polyvinyl chloride resin, and then the polyvinyl chloride resin cleaning barrel is used for production; The plasticizing temperature of the residual material in the cylinder is higher than the plasticizing temperature range of the polyvinyl chloride resin. The cleaning machine can be cleaned with polyethylene or polystyrene resin, cleaned and then cleaned with polyvinyl chloride resin.
7. When the PVC resin is put into the production after cleaning, the melt is bright and completely plasticized before it can be formally filled.
8. After the injection molding of the polyvinyl chloride resin is stopped, the barrel should be cleaned immediately, and then the barrel should be cleaned with polystyrene or polyethylene resin.

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