Description of the Related Twin screw extruder
Description of the Related Twin screw extruder
Twin screw extruder is continuously operating screw presses with two meshing screws and are used to plasticize and extrude plastics. Twin screw extruders are used preferably for the continuous processing of PVC powder formulations.
Twin screw extruders may be divided into different zones which are distinguished. A melting zone, a mixing zone, a degassing zone and a discharge zone may be arranged in line from an extruder inlet to the outlet. As a rule, the screws have different configurations adapted to the particular zone, over their length. The single screw extruder turns may have different pitches and spacing in the zones and may be provided with interruptions, baffles or the like. The screw turns may be replaced by meshing toothed disks or kneading blocks particularly in the melting and/or the mixing zones in order to obtain a particularly intensive mixing of the powder in the melt and to homogenize the melt..
Twin extruders of any type, equipped with cylindrical, conical or double conical screw extruder, have the following problem in common: the melt is exposed to a very high shear force in the wedge or lens shaped overlapping area of the sheet production extruder, in which the screws turns mesh with slight roll and flank clearances. The melt will be plasticized stronger in this zone and be at a higher temperature and have a lower viscosity than in the other cross-sectional areas of the extruder. Furthermore, the material is driven forward stronger in this overlapping area. The material therefore enters the tool attached to the extruder from the overlapping area at a higher temperature, lower viscosity and a higher driving force, than from the other cross-sectional areas. This results in a loss of quality in the extruded products.
Attempts were made to resolve the problem by special adaptations or corrections of the molding tool attached to thetwin screw extruder, in particular so that the hotter melt from the overlapping area is distributed in different cross section areas of the mold. Also, various eating devices have been mounted on the mold in order to obtain a more uniform temperature distribution of the melt by supplemental heating. All of these measures on the side of the tool were found to be unsatisfactory.
Twin screw extruder is continuously operating screw presses with two meshing screws and are used to plasticize and extrude plastics. Twin screw extruders are used preferably for the continuous processing of PVC powder formulations.
Twin screw extruders may be divided into different zones which are distinguished. A melting zone, a mixing zone, a degassing zone and a discharge zone may be arranged in line from an extruder inlet to the outlet. As a rule, the screws have different configurations adapted to the particular zone, over their length. The single screw extruder turns may have different pitches and spacing in the zones and may be provided with interruptions, baffles or the like. The screw turns may be replaced by meshing toothed disks or kneading blocks particularly in the melting and/or the mixing zones in order to obtain a particularly intensive mixing of the powder in the melt and to homogenize the melt..
Twin extruders of any type, equipped with cylindrical, conical or double conical screw extruder, have the following problem in common: the melt is exposed to a very high shear force in the wedge or lens shaped overlapping area of the sheet production extruder, in which the screws turns mesh with slight roll and flank clearances. The melt will be plasticized stronger in this zone and be at a higher temperature and have a lower viscosity than in the other cross-sectional areas of the extruder. Furthermore, the material is driven forward stronger in this overlapping area. The material therefore enters the tool attached to the extruder from the overlapping area at a higher temperature, lower viscosity and a higher driving force, than from the other cross-sectional areas. This results in a loss of quality in the extruded products.
Attempts were made to resolve the problem by special adaptations or corrections of the molding tool attached to thetwin screw extruder, in particular so that the hotter melt from the overlapping area is distributed in different cross section areas of the mold. Also, various eating devices have been mounted on the mold in order to obtain a more uniform temperature distribution of the melt by supplemental heating. All of these measures on the side of the tool were found to be unsatisfactory.