PVC is a multi-component plastic. Different additives can be added depending on the intended use. Due to the different components, PVC products exhibit varying physical and mechanical properties, tailored to specific applications. PVC pipes account for a large proportion of all plastic pipes. PVC water pipes are primarily made from a resin, which is called "PVC," hence the name "drain pipe."
PVC pipes are categorized into two types: hard and soft.
RPVC pipe is made by mixing PVC resin with additives such as stabilizers and lubricants, granulating it, and then extruding it. Powdered materials can also be extruded in one go. RPVC pipe offers excellent chemical resistance and insulation properties, primarily used for conveying various fluids and as a wire conduit. It is easy to cut, weld, bond, and bend with heat, making it very convenient to install and use.
SPVC pipe is made by adding a large amount of plasticizer, a certain amount of stabilizer, and other additives to PVC resin, granulating it, and then extruding it. SPVC pipe offers excellent chemical stability, superior electrical insulation, and good flexibility and colorability. It is often used as a replacement for a rubber hose to convey liquids and corrosive media. It is also used as cable conduit and wire insulation tubing. This article will detail the raw material selection, process flow, and process control for RPVC extrusion pipes, as well as common defects and potential improvements.
1. RPVC Raw Material Selection and Formulation
For rigid pipe production, the resin used should be SG-5 resin with a low degree of polymerization. A higher degree of polymerization improves its physical and mechanical properties and heat resistance, but poor resin fluidity can make processing difficult. Therefore, SG-5 resin with a viscosity of 1.7-1.8 × 10-3 Pa·s is generally preferred.
Rigid pipe generally uses a lead stabilizer, often tribasic lead, which offers excellent thermal stability. However, its inherent lubricity is poor, so it is often used in conjunction with lead or barium soaps, which offer superior lubricity.
The selection and use of lubricants is crucial in rigid pipe processing. Both internal lubrication (to reduce intermolecular forces and lower melt viscosity for easier molding) and external lubrication (to prevent adhesion between the melt and the hot metal, ensuring a glossy finish) are important considerations.
Metal soaps are generally used for internal lubrication, while low-melting-point waxes are typically used for external lubrication.
Fillers primarily include calcium carbonate and barium (barite powder). Calcium carbonate improves the pipe's surface properties, while barium improves formability and facilitates shaping. Both can reduce costs. However, excessive use can affect pipe performance. Pressure pipes and corrosion-resistant pipes are best left with minimal or no filler.
2. Process Flow
RPVC pipes are molded using SG-5 PVC resin, along with stabilizers, lubricants, fillers, and pigments. These raw materials are properly processed and kneaded according to the formula. If a single-screw extruder is used for extrusion, the kneaded powder should be pelletized before extrusion. If a twin-screw extruder is used, the powder can be directly molded. The RPVC pipe production process is shown in Figure 3-1.
Alternatively, the production process can be altered from the above by extruding the powder directly into pipes without pelletizing. However, two key points should be noted:
1) First, a twin-screw extruder is best used for direct powder extrusion. This is because powders, compared to pellets, require less mixing, shearing, and plasticizing. Therefore, using a twin-screw extruder can enhance shearing and plasticizing, achieving the desired effect.
2) Second, because pellets are denser than powders and therefore absorb and conduct heat poorly, the processing temperature for powders should ideally be approximately 10°C lower than that for pellets.
3. Process conditions and control
During the production process, since PVC is a heat-sensitive material, even if a heat stabilizer is added, it can only increase the decomposition temperature and prolong the stabilization time, but it is impossible to prevent decomposition. This requires that the molding temperature of PVC be strictly controlled. Especially for RPVC, because its processing temperature is very close to the decomposition temperature, decomposition is often caused by improper temperature control. Therefore, the extrusion temperature should be determined based on factors such as the formula, extruder characteristics, die structure, screw speed, temperature measurement point position, temperature measurement instrument error, and temperature measurement point depth.
(1) Temperature control
Temperature control is an important control factor in extrusion operations. The temperature control factors required for extrusion molding are barrel temperature, machine diameter temperature, and die temperature. If the temperature is too low, plasticization is poor, the pipe appearance is dull, the mechanical properties are poor, and the product quality does not meet the requirements; if the temperature is too high, the material will decompose, and the product will change color.
(2) Screw speed
Increasing the screw speed increases the extrusion volume, thereby increasing the output, but it is easy to produce poor plasticization, resulting in a rough inner wall of the pipe and reduced strength. At this time, the die pressure should be adjusted to achieve the best output and quality. The temperature control of the screw affects the material conveying rate, plasticization of the material, melting quality, etc. The extruded pipe needs to be passed through cooling water to reduce the screw temperature, which is conducive to improving the plasticization quality. The screw cooling water temperature is around 50~70.
(3) Pulling speed
The adjustment of the pulling speed is very important in the extrusion operation. The material is melted and plasticized by extrusion, and is continuously extruded from the die head and pulled to enter the shaping device, cooling device, pulling device, etc. The pulling speed should match the extrusion speed. Generally, in normal production, the pulling speed should be about 1%~10% faster than the extrusion speed of the pipe.
(4) Compressed air and pressure
Compressed air can inflate the pipe and keep the pipe at a certain roundness. The pressure should be appropriate. If the pressure is too low, the pipe will not be round. If the pressure is too high, the core mold will be cooled, cracks will appear on the inner wall of the pipe, and it will not be smooth, and the quality of the pipe will be reduced. At the same time, the pressure must be stable. If the pressure fluctuates, the pipe will easily become bamboo-like.
(5) Temperature of sizing device and cooling device
Different plastic products are extruded using different sizing methods and cooling methods. The cooling medium can be air, water, or other liquids, and the temperature needs to be controlled. The temperature is mainly related to production efficiency, product internal stress, etc.