I. Introduction: Difficulties and key breakthroughs in the production of large-diameter PVC pipes
Large-diameter PVC pipes are important in infrastructure construction, but there are many production challenges. Raw material characteristics, molding process, mold design and other links restrict quality and efficiency. Understanding the difficulties and solving them is the key to industrial development.
II. Analysis of the core difficulties in the production of large-diameter PVC pipes
(I) Raw material characteristics: the inherent challenges of PVC materials
Thermal stability hazards
The PVC processing temperature (160-200℃) is close to the decomposition temperature (about 140℃). When large-diameter pipes are molded, the material stays in the mold for a long time and is easy to decompose. Hydrogen chloride gas causes the pipe to discolor, deteriorate its performance and corrode the equipment, affecting production continuity and quality.
The dilemma of high melt viscosity
PVC melt viscosity is high. When large-diameter pipes are extruded, the flow resistance in the mold cavity containing the core rod is large, making it difficult to fill evenly, resulting in defects such as uneven wall thickness.
(II) Molding process: the dual challenges of cooling and pressure
The difficult road of cooling and shaping
Large-diameter PVC pipes have thick walls and large heat capacity. Slow and uneven cooling will produce internal stress, causing deformation and warping. For example, local cooling differences will cause the pipe to bend, affecting straightness and performance.
Challenges and risks of high extrusion pressure
Large-diameter pipes require high extrusion pressure to fill the mold. Excessive pressure causes wear of the screw and barrel, unstable melt flow and rupture, making the pipe rough in appearance and reducing dimensional accuracy, which does not meet engineering requirements.
(III) Mold design: the dual test of complexity and precision
The design dilemma of complex structure
Large-diameter PVC pipe molds must take into account both hollow structure and wall thickness uniformity. The core rod dimensional accuracy and concentricity deviation lead to uneven wall thickness. The runner system design is complex, and it is difficult to ensure uniform distribution of the melt, increasing the difficulty and cost of design and manufacturing.
Precision Challenges of Large-Size Molds
Large-diameter pipe molds are large in size, and require high manufacturing precision and rigidity. They are easily deformed by pressure and temperature during processing. For example, elastic deformation under high pressure causes errors in pipe size and shape, and the qualified rate is reduced.
3. Breakthrough for Large-diameter PVC Pipe Production: Equipment and Process Optimization
(I) Extruder Optimization: Improving Material Handling Capacity
Innovative Screw Design
Barrier Screw: Barrier ribs are set to separate the solid bed and the melt pool, so that the PVC material is plasticized more fully. When large-diameter pipes are extruded, the melt evenly fills the mold and reduces uneven wall thickness.
Separation Screw: Multi-channel screw groove promotes mixing of PVC resin and additives, improves plasticization quality, and ensures stable flow of melt.
Barrel improvement strategy
Zoned heating and cooling system: Zoned temperature control according to the characteristics of large-diameter PVC pipe extrusion materials, low temperature near the feed port (140-160℃) to prevent feeding difficulties, and high temperature in the middle and front sections (160-180℃) to promote plasticization. Accurate temperature control reduces the risk of PVC decomposition and improves production stability.
Application of high-performance lining materials: Large-diameter pipe extrusion pressure is high and friction is large. Wear-resistant and corrosion-resistant lining materials (such as high-quality alloy steel) are selected to extend the life of the barrel, reduce equipment maintenance costs, and ensure stable production.
(II) Upgrade of cooling device: Achieve efficient and uniform cooling
Integration of multiple cooling methods
Internal and external collaborative cooling: A cooling mandrel is installed in the pipe to pass circulating water to take away the internal heat, and spray or immersion cooling is applied externally. For example, the spiral channel design in the cooling mandrel increases the internal cooling effect, cooperates with external cooling, accelerates cooling, reduces internal stress, and improves pipe quality.
Multi-stage cooling system construction: multi-stage cooling is set according to the temperature change law of pipe cooling. A high temperature cooling medium is used at the beginning of extrusion to prevent surface sudden cooling defects. The temperature is gradually reduced as the temperature drops to accelerate cooling, which meets the cooling needs of large-diameter PVC pipes and improves production efficiency and quality.
Automated cooling control realization
Equipped with an automated temperature control system, the sensor monitors the temperature of the pipe, and automatically adjusts the cooling medium flow and temperature according to the preset cooling curve to ensure stable and uniform cooling. When a local temperature abnormality is detected, the cooling medium flow in the corresponding area is adjusted to accurately control the temperature and avoid cooling problems that cause pipe quality defects.
(III) Mold optimization: ensure high-precision molding
Flow channel structure optimization design
Hanger flow channel application: The hanger flow channel shuns the melt from the center to both sides, reduces the flow resistance in the mold, ensures uniform wall thickness in the circumferential direction of large-diameter pipes, improves the uniformity of melt distribution, and reduces quality problems caused by poor flow channels.
Exploration of spiral flow channels: The spiral flow channel makes the melt rotate and flow evenly distributed, optimizes the melt filling effect of large-diameter PVC pipes, improves molding quality, and reduces defects caused by uneven melt distribution.
Adjustable core rod and mold materials, precision improvement
Adjustable core rod technology: Use mechanical or hydraulic devices to adjust the core rod position in real time during production, ensure the concentricity of the core rod and the mold body, control the uniformity of the pipe wall thickness, adapt to different production conditions, and stabilize product quality.
Selection of high-quality mold materials: Select high-strength, high-hardness, and high-thermal conductivity mold materials (such as alloy steel) to make molds, withstand high pressure and high temperature for large-diameter pipe molding, fast heat transfer, facilitate cooling and shaping, shorten production cycle, and improve production efficiency.
High-precision processing guarantee: Use high-precision equipment and processes such as CNC machining centers to finely process mold cavities and flow channels, ensure mold dimensional accuracy and surface finish, reduce pipe molding dimensional deviations and surface defects, and improve product qualification rate and appearance quality.
IV. Key points for precise control of large-diameter PVC pipe extrusion process parameters
(I) Temperature parameters: Fine control to ensure melt state
Barrel temperature zone setting
The barrel is heated in multiple zones, and the temperature of each zone is precisely set according to the material state requirements. 140 - 160℃ near the feed port to ensure feeding, 160 - 180℃ in the middle to promote plasticization, 170 - 190℃ near the die head to ensure melt fluidity and uniform extrusion. For example, when producing a 600mm diameter pipe, the first section of the barrel is 150℃, the second section is 170℃, and the third section is 180℃.
The key role of die head temperature
The die head temperature is 170 - 190℃. If it is too high, PVC will decompose and cause the surface of the pipe to be rough and yellow. If it is too low, the melt viscosity will be high, causing flow marks or irregular shapes on the surface of the pipe.
Reasonable control of mold temperature
The mold temperature of large-diameter pipes is 40 - 60℃. Low temperature is conducive to rapid shaping, but too low temperature will cause internal stress and cracking. Precise control according to pipe specifications and process requirements to ensure shaping speed and quality.
(II) Pressure parameters: Balanced control to ensure stable extrusion
Reasonable range and regulation of extrusion pressure
Large-diameter PVC pipes require 10-30MPa extrusion pressure due to their large cross-sectional area. If the pressure exceeds 30MPa, the melt is prone to rupture and uneven wall thickness. In actual production, the screw speed and feeding speed are adjusted to control the pressure, such as increasing the screw speed or reducing the feeding speed to increase the pressure. Pay attention to the balance, ensure the stable flow of the melt, and avoid quality problems caused by pressure fluctuations.
Moderate adjustment of back pressure
Back pressure is 0.5-2MPa. Moderate back pressure allows the PVC material to be fully mixed and plasticized in the barrel. If it is too low, uneven material mixing will affect the quality. If it is too high, it will aggravate screw wear and increase the risk of material decomposition. Accurately adjust according to material characteristics and equipment conditions to ensure stable and efficient material processing.
(III) Speed parameters: Matching and coordination to achieve high-quality molding
Reasonable setting of screw speed
Screw speed is 10-30r/min. Faster speeds make the melt stay in the barrel for a short time and plasticization is incomplete, and increased extrusion pressure leads to quality problems. For example, if the screw speed is 40r/min, the PVC material is not plasticized enough, the strength of the pipe is reduced, and it does not meet the engineering strength requirements.
Accurate matching of traction speed
The traction speed is 0.5-2m/min, which is accurately matched with the extrusion speed. Faster traction makes the pipe thinner and the wall thickness smaller, while slower traction causes accumulation and deformation. For example, for pipes with an outer diameter of 800mm, the traction speed is about 1m/min and runs stably to ensure uniform wall thickness and good appearance of the pipe, which meets engineering standards.
V. Conclusion: Multiple links work together to promote industrial progress
The production of large-diameter PVC pipes is a complex system, and the raw material characteristics, molding equipment, extrusion process parameters and other links are related and affect each other. In-depth research and mastering of the technical points of each link, coordinated optimization of raw materials, improvement of equipment, and precise control of process parameters can produce high-quality, standard large-diameter PVC pipes to meet the needs of large-scale engineering construction, promote the development of the PVC pipe industry in the field of engineering applications, provide reliable pipe support for infrastructure construction, and promote stable development in the engineering field.
