PVC Injection Molding
PVC injection molded products have the characteristics of light weight, thermal insulation, moisture resistance, flame retardancy, and easy construction. They are also flexible in processing and can add patterns and various colors to the products. They are widely used in daily life and industrial fields.
Injection Molding Process: PVC Material
PVC material has strong heat sensitivity, so special attention is needed during the injection molding process. As long as it is exposed to too high a melt temperature or too long a heating time, it will easily cause PVC to decompose or be damaged. Therefore, controlling the melt temperature is the key to PVC injection molding.
For the factory to manufacture plastic molds, the raw materials need to be melted before plastic injection molding. The heat source for melting PVC raw materials mainly comes from two aspects, namely, the shear heat of the plastic generated by the screw movement and the resistance wire heating of the outer wall of the barrel. Normally, it mainly comes from the shear heat of the screw movement, and the external heating of the barrel is only the heat source provided when the machine starts.
Product Design and Mold Design for PVC Injection Molding Process:
- Generally, in the production and manufacturing of plastic products, factories generally recommend that there should be no sharp corners or sudden changes in design to prevent PVC from degradation and deterioration.
- The mold should have a draft angle of more than 10 degrees, leaving about 0.5% shrinkage.
- Several points should be paid attention to when designing the runner of the mold:
1.The injection port of the mold should be slightly larger than the nozzle hole and larger than the diameter of the intersection of the main flow channels so that the PVC material can flow into the mold cavity without staying and the pressure can be balanced.
2. Use intercept gates as much as possible to prevent molten slag from flowing into the product and runner, resulting in lower temperatures and easier molding.
3.The gate should be designed at the thickest wall of the product, with sufficient width and preferably a length of 6-8mm, so that the PVC material can flow smoothly.
4.The mold temperature should have a cooling water control device so that the mold temperature can be controlled between 30°C and 60°C.
- The mold should have a smooth surface and be chrome-plated to prevent corrosion.
Use a Special PVC Injection Molding Machine for Production:
- The screw length-to-diameter ratio should be 20:1, and the compression ratio should be between 1:1.6 – 1:1.2.
- The screw and nozzle should be specially made of PVC with chrome plating on the surface.
- Injection pressure, speed, back pressure, and temperature can be controlled at multiple levels and precisely.
Raw Material Preparation for the PVC Injection Molding Process
- In injection molding, engineers must avoid the presence of other plastic PVC raw materials, which may cause the PVC raw materials to deteriorate. Especially when the machine has used other raw materials, the operator must first clean the plastic in the barrel and screw it before re-injecting PVC products. (Use PE\PP\PS materials to clean the screw and barrel)
- Before stopping the machine, the PVC material must be completely cleaned. If the machine is shut down for a long time, to prevent PVC from corroding the machine, the screw and barrel should be cleaned with cleaning materials.
- When the PVC material has been left for more than 6 months and the environment is high and humid, it should be dried at 60-80°C for 2-5 hours before use.
Common Problems in the Production of PVC Injection Molded Products:
- Burnt Streaks Flow out From the Gate:
Solution:
- The operator needs to reduce the injection speed of the material;
- Expand the gate or nozzle diameter;
- Reduce injection pressure;
- Increase the material temperature (that is, the temperature of the material, such as a mixture of resin powder, calcium carbonate, PVC composite stabilizer, etc.) to reduce the viscosity.
- There are Burnt Streaks in the Center of the Main Channel:
Solution:
- Reduce screw speed or back pressure;
- Reduce the temperature of the barrel head;
- Polish the inner surface of the screw head or barrel head;
- Use eccentric screw head.
- Discoloration or Burning Occurs at a Certain Point in the Mold or Along the Seam Line:
Solution:
- Reduce the injection speed;
- Improve mold exhaust;
- Polish or round the sharp corners of the runner and gate.
- There are Scattered Black Particles on the Product:
Solution:
- Clean the screw and barrel;
- Reduce the injection speed.
- The Material Changes Color or is Partially Burned:
Solution:
- Check whether the temperature controller is qualified;
- Preliminarily clean the barrel until the color is normal. If it is still abnormal, clean the screw and barrel;
- Reduce screw speed or back pressure;
- Reduce the temperature of the electric heating coil;
- Check the gap between the screw and barrel.
- Red Spots Appear Around the Gate:
Solution:
- Increase the size of the main gate and nozzle;
- Reduce the nozzle temperature;
- Reduce the injection speed, polish the runners and gates, and increase the mold temperature;
- Change the gate position or increase the number of gates.
- Umbrella-shaped Stripes or Silver Streaks Appear on the Surface Near the Gate of the Product:
Solution:
- Reduce the material temperature;
- Reduce the injection speed;
- Reduce the nozzle temperature;
- Dry materials.
- Poor Seam Lines:
Solution:
- Increase the material temperature appropriately;
- Increase injection speed;
- Increase the holding pressure m;
- Increase the holding pressure;
- Change the mold temperature;
- Improve exhaust;
- Change the position of each gate or the size of each gate.
- Layering:
Solution:
- Ensure that the screw has been thoroughly cleaned;
- Increase the material temperature;
- Increase injection speed;
- Expand gate size.
- Product Warpage:
Solution:
- Reduce the mold temperature;
- Reduce injection pressure and holding pressure;
- Check the uniformity of the wall thickness of the product;
- Use anti-shrinkage shaping fixtures.
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