Injection Mold Manufacturer's Perspective: Manufacturing Challenges and Solutions for PVC Pipe Fitting

PVC pipe fitting is an important part of building drainage system. Its quality and performance are directly related to the stability and safety of the whole system. Therefore, in the manufacturing process, we must strictly control every link to ensure the quality and reliability of the product. However, this process is not easy. We are faced with many challenges such as mold design complexity, material fluidity, dimensional stability, etc.

Challenge: PVC pipe fittings have various structures, including elbows, tees, crosses and other shapes and specifications. This complexity places high demands on injection mold design. Mold designers need to accurately calculate the size and shape of each part and ensure that the mold can operate smoothly during the injection molding process.

These pipe fittings only have concentric and linear characteristics. There are no bumps, steps or threads inside the product. So we only need to design a side core pulling mechanism that can run in a straight line. The mold structure includes angle pin + slider, hydraulic cylinder + slider and other fstructures.

Because of the long sweep of these pipe elbows, the mold core needs to be pulled out smoothly along the sweep. Its structural features convert the linear output of the hydraulic cylinder into circular motion through mechanisms such as connecting rods.

Because the female threads are inside the product, they require a special mold core to be molding. However, tapping can be formed by simply machining the structure on the mold cavity. There are two injection mold structures suitable for parts with female threads, namely the collapsible core mechanism and the automatic unscrewing thread mechanism.

For this type of accessories, the groove at the hub requires the core of the mold to shrink in order to be demolded smoothly, so the collapsible core mechanism is still applicable here.

PVC melt viscosity is high, especially in large pipe joints or complex structures, requiring higher injection pressure and holding pressure. And the holding time needs to be extended to 2-3 times that of ordinary plastics, resulting in a longer molding cycle

PVC has a narrow processing temperature range, usually 170-190°C. Excessive back pressure or screw speed can easily generate shear heat, causing material degradation and release of hydrogen chloride gas, corroding equipment and affecting product performance.

When the moisture content of the material exceeds 0.3%, silver threads, bubbles or reduced transparency are likely to occur during injection molding. Dark-colored thick-walled pipes are more sensitive to moisture.

Add processing aids (such as ACR, CPE) to improve fluidity, and the amount of lubricant should be controlled at 0.5-1.5phr (based on PVC).

Use multi-stage injection (such as 3-stage speed control), slow filling in the first stage (10mm/s), speed up in the middle stage (30mm/s), and slow down in the last stage (15mm/s). Use mold temperature controller to achieve gradient temperature control, and the temperature difference between the main runner area and the cavity area is 10-15℃.

The PVC material is dried by drying equipment to remove the moisture. The plastic particles are dried by hot air circulation, the temperature is controlled at 80-90℃, and the time is 2-4 hours.

The machine selects an electric injection molding machine with large clamping force and large injection volume, combined with closed-loop control. These are generally equipped with a pressure holding system (pressure holding time > 60 seconds).

The gate size needs to take into account both filling efficiency and quality: too small will lead to large resistance, and too large will easily produce cold material spots. It is recommended to use fan-shaped gates or multi-point side gates, and the thickness should be controlled at 1.5-2mm.

The runner design needs to reduce dead corners to avoid melt retention and decomposition, and the volume of the cold material well needs to reach 3-5% of the injection volume.

The depth of the exhaust groove needs to be precisely controlled (such as 0.02-0.05mm). Too deep may cause flash, and too shallow will cause poor exhaust.

The cooling water channel needs to be evenly distributed, and the temperature difference should be controlled within ±2℃ to avoid warping and deformation.

PVC will produce hydrogen chloride gas during the molding process, which is seriously corrosive to the equipment. The machine needs to use nickel-plated screws or bimetallic barrels, and clean the barrels regularly (such as cleaning with PE).

Challenges: The dimensional stability of PVC pipe fittings is crucial for the installation and use of products. However, during the injection molding process, factors such as mold shrinkage, thermal expansion, and the precision of the injection molding machine may lead to unstable product dimensions.

When designing the mold, it is necessary to accurately calculate the shrinkage of the material and adjust the size and shape of the mold according to the shrinkage. Through testing and verification, ensure that the shrinkage of the mold matches the actual shrinkage of the material.

Regular maintenance and care of the mold, such as replacing worn parts and adjusting the precision of the mold, can ensure the stability and durability of the mold. At the same time, the use of advanced mold materials and heat treatment processes can improve the wear resistance and deformation resistance of the mold.

By optimizing the injection molding process, such as using multi-stage injection and segmented cooling, the impact of mold shrinkage and thermal expansion on product size can be reduced. In addition, the use of advanced temperature control systems and monitoring devices can monitor the temperature changes of molds and plastics in real time to ensure the dimensional stability of the product.

As an injection mold manufacturer, we face many challenges in the manufacturing process of PVC Pipe Fitting. However, by adopting advanced mold manufacturing technology, optimizing formula and injection molding process parameters, and strengthening the maintenance and care of molds and injection molding machines, we can effectively solve these problems and improve product quality and production efficiency. In the future, we will continue to be committed to technological innovation and process improvement to provide customers with more high-quality and reliable PVC pipe fitting molds and products.