In the PVC extrusion process, the burnt problem is relatively common, which will affect the product quality and production efficiency. Here are some ways to avoid the burnt problem of PVC during extrusion:
Resin selection: Select PVC resin with good quality and good thermal stability. For example, the resin with moderate degree of polymerization has relatively good thermal stability and processing performance. Like SG-5 type PVC resin, it is widely used in extrusion processing such as pipes and profiles, and its thermal stability can meet the general extrusion process requirements.
Additive matching: Reasonable use of additives such as heat stabilizers and lubricants. Heat stabilizers can delay the thermal degradation of PVC. For example, lead salt heat stabilizers have good long-term thermal stability, and organic tin heat stabilizers have good initial coloring. Lubricants can reduce the friction between materials and equipment and reduce the paste caused by frictional heat generation. Internal lubricants such as stearic acid glyceryl ester and external lubricants such as paraffin should be accurately proportioned according to the formula and process requirements.
Raw material drying: Ensure that the PVC raw materials are fully dried. Excessive moisture content will affect the plasticization effect of the material and may also cause local overheating. Generally speaking, the moisture content of PVC resin should be controlled below 0.3%.
Extruder cleaning: Regularly clean the screw, barrel and other parts of the extruder to prevent the material from accumulating and aging in the dead corners and producing paste. Generally, a comprehensive cleaning is carried out every certain production cycle, such as 1-2 weeks.
Temperature control: Accurately control the temperature of each section of the extruder. Excessive temperature is one of the main causes of paste. Different PVC products and formulations require different extrusion temperatures. Taking PVC pipe extrusion as an example, the general feeding section temperature is 160-170℃, the compression section temperature is 170-180℃, and the homogenization section temperature is 180-190℃. At the same time, ensure the accuracy of temperature monitoring equipment such as thermocouples and calibrate them regularly.
Screw speed: Choose a suitable screw speed. Too fast speed will cause excessive shear heat of the material and cause paste; too slow speed will affect production efficiency and plasticizing effect. For example, when producing PVC profiles, the screw speed is usually controlled at 30-60 rpm.
Feeding speed: Maintain a uniform and stable feeding speed to avoid feeding too fast or too slow. Feeding too fast will prevent the material from plasticizing in the extruder in time, resulting in excessive local pressure, temperature rise and paste; feeding too slowly will cause the screw to run idle and generate too much heat.
Cooling system: Optimize the cooling system to ensure that the material can be cooled in time during the extrusion process. For pipe extrusion, spray cooling or vacuum sizing cooling can be used to control the cooling water temperature and water flow rate. Generally, the cooling water temperature is controlled at 15-25℃.
Production environment: Keep the temperature and humidity of the production environment stable. Excessive ambient temperature or excessive humidity will affect the extrusion of the material. Generally, the production environment temperature is controlled at 20-30℃, and the humidity is controlled at 40%-60%.
Startup and shutdown: Strictly follow the startup and shutdown operating procedures. Before starting, the equipment should be preheated so that the temperature of each component reaches the set value; when shutting down, the feeding should be stopped first, and the screw should continue to run for a period of time to empty the material in the barrel to prevent the material from staying in the barrel for a long time and becoming sticky.
Quality monitoring: Strengthen quality monitoring during the production process, promptly detect early signs of sticking, such as changes in material color, rough surface of extrudates, etc., and take corresponding measures to adjust process parameters or equipment status.
