The movement of material within the extruder screw is studied in three segments, and therefore, screw design is often segmented accordingly. Since each segment is a continuous channel, in actual production, as long as requirements are met, it's not always necessary to divide the screw into three segments. In fact, some screws only have two segments, and some don't have any segments at all. For example, when extruding highly crystalline materials like nylon, only a feeding segment and a homogenizing segment are needed. For general screws extruding soft polyvinyl chloride (PVC), a single compression segment can be used, without the need for feeding and homogenizing segments.
The segmentation of the screw is derived from experience and mainly depends on the properties of the material. The feeding segment length can range from 0 to 75% of the total screw length. Generally speaking, it is longest when extruding crystalline polymers, followed by rigid amorphous polymers, and shortest for soft amorphous polymers. The compression segment length is usually 50% of the total screw length, with exceptions for nylon and soft PVC mentioned above. When extruding polyethylene, the homogenizing segment length can be 20-25% of the total length. However, for certain heat-sensitive materials (such as polyvinyl chloride), the material should not remain in this section for too long, and the homogenization section may be omitted. Some high-speed extruders even use a homogenization section length of 50%.