Determination of the Depth of the Screw Grooves in the Exhaust Screw and the Pump Ratio
Release Date:
2026-01-04
The screw groove depth and the pump ratio are critical parameters in screw design, directly influencing venting efficiency, plasticization quality, and extrusion stability. The pump ratio is defined as the ratio of the groove depth in the second metering (or second homogenizing) section to that in the first metering (or first homogenizing) section, expressed as Ω = H?/H?. By adjusting the distribution of flow resistance between these two sections, this parameter optimizes venting performance and conveying capacity.
The determination of screw channel depth requires a comprehensive consideration of conveying capacity, plasticization requirements, and mechanical strength. In the feeding section, a deeper screw channel is employed to enhance material conveyance; however, excessive depth can compromise screw strength and impair mixing uniformity. Conversely, in the metering section, the screw channel gradually becomes shallower to increase shear rate, thereby improving heat transfer and plasticization; yet an overly shallow channel reduces volumetric capacity and lowers extrusion output. The optimal depth must be tailored to the material properties—such as thermal sensitivity and viscosity—and to the processing requirements; for instance, thermally sensitive plastics necessitate a shallower channel to minimize residence time and prevent degradation.
Optimizing the pump ratio requires balancing venting efficiency with conveying stability. The pump ratio directly influences the pressure distribution and shear heating within the two metering zones, and is typically determined through experimentation or numerical simulation. A common range is 1.0 to 2.0, though the exact value must be tailored to the material’s venting requirements and plasticization quality. For instance, stringent venting demands may call for a higher pump ratio to enhance material backmixing; however, excessive pressure fluctuations must be avoided to maintain extrusion stability.
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