Extrusion Processing Technical Process: Common Operational Problems of Single-Screw Extruders During Operation
Causes of Material Backflow to Extruder Feed Inlet
Several factors may cause material to backflow into the extruder feed inlet, all of which ultimately stem from a reduction in the extruder's conveying capacity. The specific factors are as follows:a) Feed rate exceeds the maximum throughput capacity of the extruder screwIf the feed rate exceeds the screw’s handling capacity, material will eventually backflow to the extruder feed inlet. For single-screw extruders, theoretically, increasing the screw speed can enhance this throughput. However, the extruder’s transmission components, such as the gearbox, limit a significant increase in screw speed. Before drastically changing the screw speed, the adjustable range should always be verified.b) Excessive steam injectionWhen the steam injection rate is 1-4% of the dry material handling rate, it has no impact on the screw’s conveying capacity. If the injection rate exceeds this range, the injected steam may block material passage through the steam injection zone, leading to backflow to the feed inlet. Reducing the steam injection volume resolves this issue.c) Excessive resistance during extrusion processingHigh resistance is often caused by structural resistance of the screw before the die head or excessively small die plate perforation area. A sudden increase in resistance can be observed through the pressure reading before the die head, which causes over-filling of the extruder barrel and thus backflow. Increasing the die plate opening area solves this problem.d) Wear of screw elements and barrel linerWorn screw elements severely reduce conveying capacity. For example, a bald leading edge of the screw flight increases leakage flow, which raises barrel filling and eventually causes backflow. A worn or partially worn barrel liner also lowers screw conveying capacity, possibly leading to feed section blockage.
