The length -diameter ratio is an important parameter of the extruder screw. There are certain skills in selection. To correctly select the length -diameter ratio, you need to understand the role of the length- diameter ratio (L/D) in the extrusion process. The selection of extruder screw should be considered according to the properties of the processed materials and the requirements for product quality.
1. For the processing of heat-sensitive materials, such as PVC, it is advisable to choose a smaller screw length-diameter ratio, because a screw with too large length-diameter ratio will easily prolong the residence time and cause decomposition. For materials requiring higher temperature and pressure, such as fluorine-containing plastics, it need be processed with one with a larger length-diameter ratio. When the product quality requirements are not too high (such as waste recycling and granulation), a smaller screw length-diameter ratio can be selected, otherwise a larger one is much more suitable. For materials with different geometric shapes, the requirements for screw L/D ratio are different. For example, for granular materials, it can be selected to be smaller due to plasticization and granulation; or it is required to be larger, the ranges of which generally is from 32-52.
2. As for the feeding method of the plastic extruder, the plastic extruder screw design should consider this: a large amount of heat will be generated under the action of the shearing force during the extrusion process due to the high viscosity of the rubber compound. Thus, shortening the L/D ratio can keep the temperature rise within a certain limit. Therefore, the required L/D ratio is relatively small.