Polyvinyl chloride PVC performance and injection molding process

Because of its low price, PVC material has inherent anti-inflammatory properties, is strong and strong, has good chemical resistance, and has a shrinkage rate of 0.2-0.6%. The products are increasingly used in electrical appliances, machinery, construction, daily necessities, toys, and packaging. According to the characteristics of PVC material, the injection molding process of the product is analyzed as follows:


1. Characteristics of PVC material

The thermal stability of PVC is poor, the molding temperature is close to the decomposition temperature, the fluidity is not good, and the appearance is easy to form defects. Additives are generally required to increase its fluidity. Its strength, electrical insulation and chemical resistance are good.

2. Mold and gate design

In order to shorten the injection molding cycle, the shorter the injection port, the better, the cross-section should be circular, the minimum diameter of the nozzle mouth is 6 mm, it is a circular cone, and the inner angle is 5 degrees. It is best to add a cold well and cold material. The well can prevent poorly melted semi-solid materials from entering the mold cavity, and these materials will affect the surface modification and the strength of the product.

The draft angle should be between 0.50 and 10 to ensure that there is enough venting equipment in the cavity. The commonly used vent size is 0.03-0.05mm deep and 6mm wide, or the perimeter clearance of each thimble is 0.03-0.05 mm. The mold should be made of stainless steel or plated with hard chrome.

3. the PVC molding process

PVC is a heat-sensitive plastic. Overheating or excessive shearing will cause decomposition and spread rapidly, because one of the decomposition products (such as acid or HCI) will produce a catalytic effect, causing the process to further decompose, and acidic substances will attack the metal and make it change. If it is dented, the protective layer of the metal will peel off, causing rust, which is even more harmful to the human body.

    The common screw length-to-diameter ratio is 18-24:1, the three-stage ratio is 3:5:2, and the compression ratio is 1.8-2. When the screw is in place, the distance between the tip and the nozzle should be 0.7-1.8mm, The screw must be made of stainless steel or chrome-plated.

    Screw gasket: The screw gasket is between 2mm and 3mm. Large opportunities are greater.

    Injection volume: The actual retention time of the cylinder should not exceed 3 minutes.

    Cylinder temperature setting: The temperature provided can only be used as a reference. Make appropriate adjustments according to the actual conditions of the machine and raw materials, or it may exceed the recommended range.


    Barrel residence time: under the temperature control of 2000C (compound), the barrel residence time can exceed 5 minutes at most

    Injection speed: The injection speed should be slow, otherwise excessive shearing will degrade the material. When UPVC is used to produce extremely smooth thick-walled products, multi-level injection speeds should be used. If light brown stripes radiate from the gate, it means injection The feed speed is too fast.

    Backpressure: The commonly used surface value is 5bar. Increasing the back pressure is conducive to color mixing and exhaust, but the backpressure should be as low as possible.

    Shutdown: Due to the low overheating stability of PVC, the shutdown part is very important. Clean the barrel without leaving any PVC. Other thermoplastics with high overheating stability but not resistance to PVC can be used (such as PMMA, PP, LDPE, or GPPS), do not mix POMTUPVC in the same barrel, otherwise, it will cause an excessive chemical reaction and cause serious damage to the machine.

    Mixing ratio: The maximum mixing ratio of the nozzle material is 20%, otherwise it will affect the product quality.