PVC material is inexpensive, inherently possesses anti-inflammatory properties, is strong and durable, has excellent chemical resistance, and a shrinkage rate of 0.2-0.6%. Its applications in electrical appliances, machinery, construction, daily necessities, toys, and packaging are increasingly widespread. Based on the characteristics of PVC material, the injection molding process is analyzed as follows:

I. Characteristics of PVC Material

PVC has poor thermal stability, its molding temperature is close to its decomposition temperature, its fluidity is poor, and it is easy to form defects in appearance. PVC material has poor heat resistance and is most likely to burn, produce acidic gases, and thus corrode the mold. Plasticizers can be added during processing to increase its fluidity. Generally, additives are required for its use. Its strength, electrical insulation and chemical resistance are good.

II. Mold and Gate Design

To shorten the injection molding cycle, the injection port should be as short as possible, with a round cross-section. The nozzle diameter should be at least 6 mm, and it should be conical with an inner angle of 5 degrees. It is best to add a cold slug well, which can prevent poorly melted semi-solid materials from entering the mold cavity, as such materials can affect the surface finish and the strength of the product.

The draft angle should be between 0.50 and 10 to ensure sufficient venting within the mold cavity. Commonly used vent hole dimensions are 0.03-0.05 mm deep and 6 mm wide, or a clearance of 0.03-0.05 mm around each ejector pin. The mold should be made of stainless steel or hard chrome plated.

III. PVC Molding Process

PVC is a heat-sensitive plastic. Overheating or excessive shearing can cause it to decompose and spread rapidly. One of the decomposition products (such as acid or HCl) can catalyze the process, causing further decomposition. Acidic substances can also corrode metal, causing it to become pitted, and can also cause the protective layer of the metal to peel off, leading to rust, which is even more harmful to the human body.

1. Screw gasket: The screw gasket is between 2 and 3 mm, with a higher chance of larger screws.

2. Injection volume: The actual residence time in the syringe should not exceed 3 minutes.

3. Barrel temperature setting:

The provided temperatures are for reference only and should be adjusted according to the actual conditions of the machine and raw materials. They may also exceed the recommended range.

4. Barrel residence time: Under temperature control at 200°C, the maximum barrel residence time (for the rubber compound) can not exceed 5 minutes.

5. Injection speed: The injection speed should be slow, otherwise excessive shearing will degrade the material. When producing extremely smooth, thick-walled products using UPVC, multi-stage injection speeds should be used. If light brown streaks radiate from the gate, it indicates that the injection speed is too fast.

6. Back pressure: The commonly used surface value is 5 bar. Increasing the back pressure is beneficial for color mixing and venting, but the lower the back pressure, the better.

7. Shutdown: Due to the low overheat stability of PVC, the shutdown procedure is very important. The barrel must be thoroughly cleaned, leaving no trace of PVC. Other thermoplastics with high overheat stability that do not resist PVC (such as PMMA, PP, LDPE, or GPPS) can be used. Never mix POMT and UPVC in the same barrel, otherwise it will cause an excessively strong chemical reaction and cause serious damage to the machine.

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

9. Typical products: UPVC is mainly used to manufacture drainage pipes, or the casings of computers and televisions, water filter tanks, photocopier casings, electric meters and gas casings, transparent covers for printing presses, ventilation windows, and various electronic parts, etc.