In the entire production process of PFA pipes, process control serves as the core link connecting source control and finished product inspection, directly determining the forming quality, dimensional accuracy, appearance quality and service performance of PFA pipes. It covers multiple key processes including extrusion forming, cooling and shaping, traction and cutting, and in-process inspection. The parameter setting, operating specifications and equipment status of each process all have a direct impact on product quality. Therefore, establishing a scientific and strict process control system, standardizing the operating procedures of each process, monitoring production parameters in real time and troubleshooting process hidden dangers in a timely manner are the keys to ensuring the stable quality and batch consistency of PFA pipes.
I. Core Objectives and Principles of Process Control
(I) Core Objectives
The core objectives of process control are to ensure that each process in the production process complies with preset standards, avoid the transfer of quality defects between processes, reduce quality defects caused by operational errors, parameter deviations, equipment failures and other factors, and make the produced PFA pipes meet the design requirements and national standards in terms of dimensional accuracy (outer diameter, wall thickness, length), appearance quality (smooth inner wall, no cracks, no air bubbles), mechanical properties, temperature and corrosion resistance performance, while improving production efficiency and reducing rework and scrap rates.
(II) Core Principles
Real-time Monitoring Principle: Conduct real-time monitoring of the key parameters of each process (such as extrusion temperature, traction speed, cooling temperature, etc.) to ensure that the parameters remain stable within the preset range. Any deviation shall be adjusted immediately to prevent the expansion of defects.
Standardized Operation Principle: Formulate standardized operating procedures (SOP) for each process, specifying operating steps, parameter requirements and precautions, and require operators to strictly follow the procedures to eliminate irregular operations.
Early Hidden Danger Troubleshooting Principle: Regularly inspect the operating status of production equipment, identify potential hidden dangers in processes, and timely handle equipment failures, mold wear, operational errors and other problems to prevent accidents before they occur.
Full-process Traceability Principle: Record information such as production parameters, operators, production time and in-process inspection results of each process, and establish a process traceability system to ensure that when quality problems occur, the problematic process can be quickly located and the causes can be identified.
II. Key Process Control Points of Each Critical Procedure
(I) Extrusion Forming Process: The Core Link of PFA Pipe Forming
Extrusion forming is the process of heating and melting PFA resin and extruding it into a tubular shape through an extrusion die, which is the core of PFA pipe forming. Its control points mainly include three aspects: temperature control, mold control and screw speed control.
Temperature Control: Temperature is a key parameter for extrusion forming, directly affecting the melting quality, fluidity and forming effect of PFA resin. The temperature of each section of the extruder (feeding section, melting section, metering section, head section) shall be reasonably set according to the type and melt flow index of PFA resin to ensure uniform and stable temperature without obvious fluctuations.
Specific Control Standards: The temperature of the feeding section is controlled at 280-300℃, mainly for preheating raw materials to avoid caking and facilitate feeding; the temperature of the melting section is controlled at 320-340℃ to ensure complete melting of PFA resin and form a uniform melt without unmelted particles; the temperature of the metering section is controlled at 330-350℃ to adjust the fluidity of the melt and ensure stable output; the temperature of the head section is controlled at 340-360℃, matching the mold temperature to avoid solidification of the melt at the head and affecting extrusion forming. At the same time, the temperature of each section shall be monitored in real time, with the temperature deviation controlled within ±5℃. Excessively high temperature will cause decomposition of PFA resin, generate impurities, and result in pitting and cracks on the inner wall of the pipe; insufficiently low temperature will lead to inadequate melting of the resin, causing uneven wall thickness, rough surface and even failure in forming of the pipe.
Mold Control: The precision, finish and assembly quality of the mold directly determine the dimensional accuracy and appearance quality of PFA pipes. First, molds meeting the design requirements shall be selected, with the inner and outer diameter deviation of the mold controlled within ±0.05mm. The inner wall of the mold shall be polished to a finish of Ra≤0.2μm to avoid scratches and unevenness on the pipe surface. Second, the mold assembly shall be precise to ensure the concentricity of the mold core and mold sleeve and uniform gap, avoiding uneven wall thickness and eccentricity of the pipe caused by assembly deviation. In addition, the mold shall be cleaned regularly during production to remove residual melt and impurities on the inner wall, preventing residual substances from affecting the forming quality of subsequent pipes. After cleaning, the mold precision shall be rechecked and production can be resumed only after confirmation of conformity.
Screw Speed Control: Screw speed determines the extrusion speed, output and shear strength of the melt. It shall be reasonably set according to the pipe specifications and resin melting state to ensure stable speed without obvious fluctuations. The screw speed is usually controlled at 10-30r/min. Excessively high speed will lead to excessive shear strength of the melt, generate excessive heat and cause resin decomposition; meanwhile, too fast pipe forming speed is prone to uneven wall thickness and rough surface. Excessively low speed will result in low output, and the melt will stay in the extruder for too long, easily causing resin aging and affecting pipe performance. During production, the screw speed shall be adjusted in real time in combination with extrusion temperature and melt fluidity to ensure a stable extrusion process.
(II) Cooling and Shaping Process: Ensuring Stable Pipe Dimensions
After extrusion forming, PFA pipes have a high temperature (about 200-250℃) and are soft and prone to deformation. Therefore, the cooling and shaping process is required to rapidly reduce the pipe temperature, solidify and shape the pipes, and ensure stable dimensions and regular shape of the pipes. The control points of cooling and shaping mainly include three aspects: cooling method, cooling temperature and shaping time.
Cooling Method: Water cooling (immersion cooling or spray cooling) is usually adopted for the cooling and shaping of PFA pipes, with immersion cooling as the priority due to its uniform cooling effect, which can ensure simultaneous cooling of the inner and outer walls of the pipes and avoid deformation and cracking caused by uneven cooling. The cooling water in the cooling water tank shall be kept clean without impurities and oil stains to prevent contamination of the pipe surface. At the same time, the stable operation track of the pipe in the cooling water tank shall be ensured to avoid friction with the inner wall of the water tank, guide wheels and other components, which may cause scratches on the pipe surface.
Cooling Temperature: The cooling temperature shall be controlled reasonably to rapidly cool the pipes for solidification and forming, while avoiding excessive cooling speed that may cause a large temperature difference between the inner and outer walls of the pipes, generate internal stress and lead to cracks and embrittlement. The cooling water temperature is usually controlled at 20-30℃, with the water temperature in the cooling water tank kept uniform and the temperature deviation controlled within ±3℃. A temperature control device can be installed to monitor the water temperature in real time and adjust the supply of cooling water in a timely manner to ensure stable water temperature.
Shaping Time: The shaping time shall be adjusted according to the pipe specifications (wall thickness, outer diameter) to ensure that the pipes are completely solidified and their dimensions are stable before entering the subsequent traction and cutting process. Generally, the thicker the wall thickness and the larger the outer diameter, the longer the shaping time, which is usually controlled at 5-15min. Insufficient shaping time will result in incomplete solidification of the pipes, prone to deformation and dimensional deviation; excessively long shaping time will affect production efficiency and increase production costs. During production, the shaped pipes shall be inspected regularly to confirm that their dimensions and shapes meet the requirements before adjusting the shaping time.
(III) Traction and Cutting Process: Ensuring Pipe Dimensional Accuracy and Cut Quality
After cooling and shaping, PFA pipes are pulled to the cutting equipment by a traction device and cut into finished products of preset length. The control points of this process mainly include three aspects: traction speed, cutting precision and cut quality.
Traction Speed Control: The traction speed shall match the extrusion speed and cooling and shaping speed to ensure stable traction speed without fluctuations, with the traction speed deviation controlled within ±0.5m/min. Excessively fast traction speed will cause stretching of the pipes, resulting in reduced wall thickness, smaller outer diameter, length deviation and other problems; excessively slow traction speed will lead to pipe accumulation and deformation, affecting the subsequent cutting process. During production, the extrusion speed and traction speed shall be coordinated in real time to ensure their synchronization. At the same time, the pressure of the traction device shall be checked to avoid damage to the pipe surface caused by excessive pressure and unstable traction caused by insufficient pressure.
Cutting Precision Control: Cutting precision directly determines the length deviation of PFA pipes. The cutting length shall be preset according to customer requirements, and the parameters of the cutting equipment shall be adjusted to ensure that the cutting precision meets the requirements. Generally, the length deviation is controlled within ±0.1mm for precision PFA pipes and within ±0.5mm for ordinary PFA pipes. During production, the length of the cut pipes shall be spot-checked regularly. If deviation occurs, the parameters of the cutting equipment shall be adjusted immediately, and the sharpness of the cutting blade shall be checked to avoid cutting deviation caused by blade wear.
Cut Quality Control: The cut shall be flat and smooth without burrs, chipping and cracks to avoid leakage and damage of the pipes during subsequent installation and use due to poor cut quality. During cutting, the cutting blade shall be kept perpendicular to the pipe and the cutting speed shall be uniform to avoid violent cutting. After cutting, the cut shall be slightly polished to remove burrs, and at the same time, the cut shall be inspected for cracks, chipping and other defects. Unqualified cuts shall be recut and processed.
(IV) In-process Inspection Process: Timely Troubleshooting of Process Quality Hidden Dangers
In-process inspection is an important guarantee for process control. Targeted inspections shall be carried out after the completion of each critical process to timely identify quality hidden dangers and prevent unqualified products from flowing into the next process. The in-process inspection follows the principle of "combining random inspection and 100% inspection", with the specific inspection points as follows:
Inspection after Extrusion Forming: Conduct random inspection on the state of the melted melt to confirm no unmelted particles and impurities; conduct random inspection on the appearance of the newly extruded pipes to check whether the surface is smooth, scratch-free and bubble-free, the wall thickness is uniform, and the eccentricity meets the requirements (eccentricity ≤5%).
Inspection after Cooling and Shaping: Conduct 100% inspection on the dimensions (outer diameter, wall thickness) of the pipes to confirm compliance with preset standards; conduct random inspection on the appearance of the pipes to check for deformation, cracks, pitting and other defects; conduct random inspection on the hardness and flexibility of the pipes to confirm compliance with mechanical performance requirements.
Inspection after Traction and Cutting: Conduct 100% inspection on the length and cut quality of the pipes to confirm compliance with requirements; conduct random inspection on the appearance and dimensions of the pipes to reconfirm no quality defects; for PFA pipes used in high-end fields, additional random inspection on the temperature and corrosion resistance performance (samples sent to the laboratory for testing) shall be conducted to ensure compliance with special industry requirements.
Inspection Record and Disposal: Keep detailed records of in-process inspection results, marking the inspection time, inspectors, inspection results, quantity of unqualified products and disposal methods. Unqualified products detected shall be isolated and stored immediately with an "unqualified" label, and the causes of non-conformity shall be analyzed. If rework is feasible (such as cut burrs and slight dimensional deviation), rework shall be carried out and the products shall be re-inspected after rework, and only qualified products can flow into the next process. If rework is not feasible (such as cracks and severe uneven wall thickness), the products shall be scrapped directly, and the relevant processes shall be traced to identify the causes and take corrective measures to avoid the recurrence of similar problems.
III. Auxiliary Safeguard Measures for Process Control
(I) Equipment Management and Control
The operating status of production equipment is the foundation of process control. A full-process management and control mechanism of "daily maintenance, regular overhaul and fault troubleshooting" for equipment shall be established. Before daily production, inspect the operating status of extruders, cooling water tanks, traction devices, cutting equipment, etc., to confirm no equipment failures and correct parameter settings. During production, monitor the operating sound, temperature, pressure and other indicators of the equipment in real time, and stop the machine immediately for troubleshooting if any abnormality is found. After daily production, clean the residual melt and impurities inside the equipment, and inspect and maintain the vulnerable parts of the equipment (such as screws, molds, cutting blades). Conduct a comprehensive overhaul of the equipment regularly (once a month), calibrate the equipment precision and replace worn vulnerable parts to ensure the long-term stable operation of the equipment.
(II) Personnel Management and Control
The professional level and operational standardization of operators directly affect the effect of process control. It is necessary to strengthen personnel management and control and improve the comprehensive quality of operators. First, recruit operators with relevant professional knowledge and experience in PFA pipe production. Second, conduct pre-job training for operators, covering production processes, equipment operating specifications, quality control points, safety operating procedures and other content, and operators can only take up their posts after passing the assessment. In addition, carry out on-the-job training regularly to update the professional knowledge of operators and improve their ability in fault troubleshooting and emergency response. Establish an operator assessment mechanism, incorporate operational standardization, product qualification rate, completeness of inspection records and other indicators into the assessment, and link the assessment results with performance to motivate operators to strictly follow the specifications.
(III) Environmental Management and Control
The cleanliness, temperature and humidity of the production environment will have a certain impact on the quality of PFA pipes. It is necessary to strengthen environmental management and control and create an environment meeting production requirements. The production workshop shall be kept clean and ventilated, and cleaned regularly to prevent dust, oil stains and impurities from mixing into the production process and contaminating raw materials and pipes. The workshop temperature shall be controlled at 18-32℃ and the relative humidity at 40%-60% to avoid the impact of excessively high or low temperature and humidity on resin melting and pipe cooling and shaping. The storage of acids, alkalis, flammable and explosive hazardous substances in the workshop is prohibited to prevent pipe contamination. At the same time, fire prevention, explosion prevention and anti-static measures shall be taken to ensure production safety.
IV. Summary
Process control is the core link of quality control for PFA pipes, running through the entire production process, and its core lies in "real-time monitoring, standardized operation and early hidden danger troubleshooting". By strictly controlling the parameters and operating specifications of key processes such as extrusion forming, cooling and shaping, traction and cutting, implementing in-process inspection requirements, and strengthening the auxiliary management and control of equipment, personnel and the environment, the transfer of quality defects between processes can be effectively avoided, quality defects reduced, and the stable quality and batch consistency of produced PFA pipes ensured. Process control and source control complement each other. Only by organically combining the two and building a complete quality control system can the high quality of PFA pipes be guaranteed from the source, the application requirements of various downstream fields be met, and the high-quality development of enterprises be promoted.
