PVC Stabilizer can be used only for increasing thermal decomposition temperature in processing because PVC Resin is a kind of heat-sensitive material. It means PVC Stabilizer will help extend heat stability time but cannot prevent PVC Resin from thermal decomposition. Therefore, temperature for PVC pipe processing should be strictly controlled. Especially for UPVC pipe, its processing temperature is close to its thermal decomposition temperature and it’s easily decomposed due to failure of temperature control. So extrusion temperature should be determined by factors as such: product formula, extruder properties, extruder head structure, screw speed, location of temperature test point, deviation of temperature test equipment and depth of temperature test point.
1. Temperature Control
Temperature control is a critical part in the extrusion of UPVC pipe. Temperature for the extrusion of UPVC pipe needs to be controlled include barrel temperature, screw temperature and mould temperature. If temperature is too low, it will cause a series of quality problems like bad plasticization, bad pipe surface gloss poor mechanical properties. If temperature is too high, thermal decomposition of materials and discoloring of pipe will occur.
2. Screw Speed
If screw speed increased, extrusion production will increase accordingly, but it will easily cause some bad results, including rough pipe inner wall and loss of strength. Therefore, extruder head pressure shall be adjusted to get highest quantity and quality. Screw temperature control will affect material transmission rate, material plasticization and melts quality. Cold water is needed to lower the screw temperature for better plasticization of pipes. Cooling water for screw shall range from 50 to 70 centigrade.
3. Pulling Speed
Pulling speed adjustment in extrusion process is very important. Materials are plasticized after being extruded and melting. It will come into molding equipment, cooling equipment and pulling equipment respectively, thus pulling speed shall be the same as that of extrusion speed in theory. Under normal processing condition, pulling speed can be 1%~10% higher than extrusion speed.
4. Air Compression and Pressure
Compressing air will inflate the pipe and make pipe a certain roundness, thus requiring proper pressure. If the pressure is too low, pipes won’t keep round as designed, but if too high, mandrel will be cooled and some quality problem will occur to pipe inner wall, like rough pipe inner wall and cracking. The pressure shall keep stable, otherwise pipes will have embossments like “bamboo joints”.
5. Temperature Control of Sizing Equipment and Cooling Equipment
Different sizing method and cooling method shall be taken for extrusion of different plastic products. Cooling medium can be air, water or similar liquid. Temperature needs to be controlled. Temperature relates to production efficiency and product inner force, etc.