The traditional single screw extruder is mostly three-s […]
The traditional single screw extruder is mostly three-stage planning, its main function is to plasticize and homogenize granular or powdery plastics and then press them to the extruder head. During the extrusion process, the plastics have experienced different physical conditions, but there is no chemical reaction. In the whole process of silane crosslinked polyethylene pipe production, in addition to the above physical process, the material in the extruder has to complete the grafting reaction of polyethylene and silane. The traditional three-stage single screw extruder is obviously unable to completely adapt to the requirements of the reaction extrusion molding. Its main disadvantages are that the residence time of the material is too wide and the relative length of the complete melting zone of the material is relatively short At the same time, there is a problem that the conveying power of the solid-liquid mixing system in the solid conveying section is lower than that in the solid-liquid mixing system. The specific requirements of reaction extrusion are as follows:
① The moisture content of resin is required to be no more than 200mg / kg in the process of material drying system. If the drying requirement can not be met only by using general hopper dryer, a material drying system with air dehumidification equipment should be equipped.
② Metering and feeding system the single screw reverberation extruder of Switzerland's nislon company is equipped with advanced weighing and feeding system. The solid and liquid are separately measured. The ratio of solid and liquid is controlled by computer, and the accuracy can reach 0.25%.
Using domestic fine metering pump, the measurement accuracy can reach 0.5% - 1.0%.
Because vinyl silane simply absorbs moisture in the air and transforms into silanol, the metering and feeding system adopts a close moisture-proof method. In one case, the flash point of vinyl silane was low. The metering pump should be equipped with explosion-proof motor, and the equipment and container touching silane should have a firm ground wire to prevent the silane from detonating by electrostatic sparks.
③ As mentioned above, the single screw reaction extruder not only plasticizes the plastics, but also ensures the smooth grafting of polyethylene and silane. Therefore, the basic principles of screw planning are as follows:
a. There should be a long feeding section;
b. The screw channel should prevent dead angle, improve the surface finish of screw and barrel together, so as to prevent the accumulation of reaction materials and shorten the continuous working cycle of extruder;
c. The screw should not be too deep to be grafted;
d. The length diameter ratio of the screw can be appropriately longer, generally (28-30): 1;
e. The residence time of melt in extruder should be extended properly and the residence time of melt should be as uniform as possible.
However, both theoretical and experimental studies have shown that only adding the length of screw can not effectively improve the graft reaction power. At present, a four stage single screw reaction extruder has been proved to be able to significantly improve the reaction power of silane grafting and the output value of single machine.