Selection Method for the Screw Length-to-Diameter Ratio in Extruders

Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with over 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.

Generally, the selection can be made based on material characteristics and production efficiency.

Material Characteristics

· Viscosity and Filling Properties

High-viscosity materials and those containing a large amount of filler or reinforcing material require greater shear force and a longer duration to achieve sufficient plasticization and homogeneous mixing. Therefore, a larger length-to-diameter (L/D) ratio is typically required, usually selecting a screw with an L/D ratio between 30 and 40. Conversely, low-viscosity materials flow and plasticize more easily during extrusion and can use a screw with a relatively shorter L/D ratio, such as 20 to 30.

· Thermal Stability

Materials with poor thermal stability require minimized heat exposure during processing to avoid degradation or other adverse reactions. For such materials, a screw with a shorter L/D ratio should be selected, generally within the range of 18 to 25. In contrast, materials with good thermal stability can withstand longer heating and shear times, allowing for the use of screws with larger L/D ratios to achieve better plasticization and mixing effects.

Production Efficiency

· Output Requirements

In large-scale production, to increase output, more material needs to be extruded per unit time. A larger L/D ratio can, to some extent, enhance the material conveying capacity and plasticization efficiency, thereby achieving higher output. However, an excessively large L/D ratio can also lead to issues such as prolonged production cycles and increased energy consumption. Therefore, the appropriate L/D ratio should be selected based on actual output requirements. For example, for producing common plastic products with high output demands, a screw with an L/D ratio of 25 to 35 can be chosen to improve production efficiency while ensuring a certain level of plasticization quality.

· Extrusion Speed

Higher extrusion speeds require the screw to have good material conveying and plasticization capabilities. Screws with larger L/D ratios can better maintain material uniformity and stability during high-speed extrusion, avoiding issues like poor plasticization caused by excessively fast extrusion speeds. For instance, in the continuous extrusion production of products such as pipes and sheets, screws with L/D ratios of 30 to 40 are typically selected to achieve higher extrusion speeds and output.

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