PVC Injection Molding Process


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


First, the characteristics of PVC materials
PVC thermal stability is poor, the molding temperature is close to the decomposition temperature, the fluidity is not good, the appearance is easy to form bad defects, the PVC material has poor heat resistance, the most easy to burn, the acid gas is generated and the mold is corroded, and the plasticizer can be added during processing. The agent increases its fluidity and generally needs to be added with additives, and its strength, electrical insulation and chemical resistance are good.


Second, the mold and gate design
In order to shorten the injection molding cycle, the shorter the nozzle, the better, the cross-section should be circular, the diameter of the nozzle mouth is at least 6 mm, the cone is rounded, and the inner angle is 5 degrees. It is best to add cold wells and cold materials. The well prevents the poorly melted semi-solid material from entering the mold cavity, and the materials affect the surface modification and the strength of the product. The draft angle should be between 0.50 and 10 to ensure that there are enough exhaust equipment in the cavity. The common vent size is 0.03-0.05mm deep, 6mm wide, or the gap between each thimble is 0.03-0.05. Mm. The mold is made of stainless steel or hard chrome plated.


Third, PVC molding process
PVC is a heat-sensitive plastic. Excessive or excessive shearing will cause decomposition and spread quickly, because one of the decomposition products (such as acid or HCI) will cause catalysis, causing the process to further decompose, and the acidic substance will corrode the metal and make it change. Forming a depression will cause the protective layer of the metal to peel off, causing rust and being more harmful to the human body.

The common screw length to diameter ratio is 18 to 24:1, the three ratio is 3:5:2, and the compression ratio is 1.8 to 2. The recommended groove depth of the feed section is as follows:
Screw diameter (mm) 35 50 60 75 100
Groove depth (mm) 2 3 4 5 6
The tip of the screw should have an internal angle of 25 to 30 degrees. When the screw is in place, the distance between the tip and the nozzle should be 0.7 to 1.8 mm. The screw must be made of stainless steel or chrome plated.

1) Screw padding: The screw padding is between 2 and 3 mm, and the large-scale opportunity is larger.

2) Injection volume: The actual cylinder retention time cannot exceed 3 minutes.

3) Barrel temperature setting: The temperature provided can only be used as a reference. It can be adjusted according to the actual conditions of the machine and raw materials, and may exceed the recommended range. Position mold nozzle front section intermediate feed section temperature (0C) 30-60 170-190 160-180 150-170 140-160 The injection volume used is 20-85% of the theoretical injection volume of the machine. The actual injection volume is actually used. Small, the longer the residence time of the material, the greater the risk of degradation after heating.

4) Cylinder retention time: Under the temperature control of 2000C (rubber), the cylinder retention time can be more than 5 minutes.

5) 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-stage injection speed should be used. If light brown stripes are emitted from the gate, Indicates that the shot speed is too fast.

6) Back pressure: The commonly used surface value is 5 bar. Increasing the back pressure is good for color mixing and exhausting, but the lower the back pressure should be.

7) Shutdown: Due to the low thermal stability of PVC, the shutdown part is very important. The barrel should be clean and free of PVC. It can be used with other thermoplastics with high stability and resistance to PVC (such as PMMA, PP, LDPE, or GPPS), should not mix POMT UPVC in the same barrel, otherwise it will lead to excessive chemical reaction and cause serious damage to the machine.

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

9) Typical products: The main purpose of UPVC is to manufacture drainage pipes, or the casing of computer and television, water filter groove, photocopying machine casing, electric meter and gas casing, transparent cover of printing machine, communication style and various electronic parts. 


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