The process section of the extruder
An extruder comprises a displacement part with a housing, inside of which at least one screw shaft is housed and which is provided, at one end, with a material entry opening for the material to be processed and, at the other end, with a material outlet opening for the processed material, and with an intermediate opening between the material entry opening and the material outlet opening for discharging gas from the displacement part. A material retaining device is connected to said intermediate opening and has at least one screw shaft inside an intermediate housing. This screw shaft extends from the intermediate opening to a gas discharge opening in the intermediate housing and conveys material entering the intermediate housing back into the displacement part.
Sheet production extruder is often used for conveying and processing substances. To separate gases during processing of plastics in an extruder, a gas outlet opening is provided in the housing of the process section of the extruder in which the solid or viscous plastic is processed. To prevent plastic from being discharged from the process section through said opening, the material retaining device is provided, also known as the “stuffer”. It has one or two screw shafts whose screw has a direction of thread over the total length by which it forces plastic entering said opening back into the process section, while gas passing through the screw can be drawn off.
As has turned out, the formation of black specks is due to the fact that the screw shaft and the inside wall of the retaining device have the same high temperature as the process section. In the known extruder, plastic residues adhering to the hot screw shaft or inside wall of the retaining device can thus decompose. Since the decomposition depends on the duration at which a plastic residue adheres to the hot metal surface of the screw or the inside wall of the retaining device, a plastic residue transported by the screw over its total length as far as the gas outlet opening in the intermediate housing of the material retaining device will naturally undergo particularly great decomposition. The solid products of decomposition are then, in the known extruder, conveyed by the material retaining device screw backfeeding over its full length back to the process section and mixed with the plastic in the process section to form the black specks in the extruded plastic product. Starting out from this finding, the invention is based on giving only the portion of the material retaining device screw adjacent the process section a backfeeding design.