Application of single screw extruder in pipe production

The traditional single screw extruder is mostly designed in three stages. Its main function is to make the granular or powdery plastics plasticized, homogenized and continuously pressed to the extruder head. The plastics undergo different physical states during the extrusion process, but no chemical reactions occur. In the production of silane-crosslinked polyethylene pipes, the raw materials in the extruder have to undergo the above-mentioned physical process, but also complete the polyethylene-silane grafting reaction. The traditional three-stage single screw extruder obviously can not fully meet the requirements of reactive extrusion molding, the main drawbacks are that the residence time distribution of the material is too wide and The relative length of the melting zone is short, and the conveying efficiency of the solid-liquid mixing system is low. The specific requirements for reactive extrusion are:
(1) The moisture content of resin in raw material drying system is required to be more than 200 mg/kg. If the ordinary hopper dryer can not meet the drying requirements, a raw material drying system with air dehumidification device should be equipped.
(2) Measuring and feeding system The single screw reactive extruder of Nissron Company of Switzerland is equipped with advanced weighing and feeding system. Solid and liquid are measured separately. The ratio is controlled by computer. The accuracy can reach 0.25%.
The precision of domestic precision metering pump can reach 0.5% 1 1%.
Because vinylsilane absorbs moisture easily in the air and converts to silanol, metering and feeding systems adopt strict moisture-proof measures. At the same time, the flash point of vinylsilane is low, the metering pump should be equipped with explosion-proof motor, and the equipment and containers in contact with the silane should have reliable ground wire to prevent the electrostatic spark from detonating the silane.
(3) Plastic extrusion components as mentioned above, in addition to plasticizing the single screw reactive extruder, but also to ensure the smooth grafting of polyethylene and silane, so the basic principle of screw design:
A. must have a long feeding section.
B. Screw runner should avoid dead angle and improve the surface finish of screw and barrel to prevent the accumulation of reactive materials and shorten the continuous operation period of extruder.
The groove depth of C. screw metering section should not be too shallow to avoid excessive shearing of graft.
The length diameter ratio of D. screw can be longer, usually (28 – 30):1;
E. can prolong the residence time of melt in extruder and make the residence time of the melt as uniform as possible.