Compression/Transition Section

The compression/transition section is a critical component of an extrusion screw, located between the feed and metering sections. Its primary function is to compress the solid polymer material, remove entrapped air, and gradually melt the material through shear heating and conduction from the barrel. This section features a decreasing channel depth that creates the necessary pressure for proper melting and homogenization before the material enters the metering section. The design of this section is crucial for the overall performance of the extrusion process, as it directly influences melt quality, throughput, and energy consumption. The compression ratio, defined as the ratio of the channel depth in the feed section to that in the metering section, typically ranges from 2.0 to 3.5, affecting melting efficiency and pressure buildup. The screw diameter, ranging from 20 to 300 mm, determines throughput and machine size, while the length-to-diameter ratio (L/D) of 20 to 30 improves mixing and melting. Channel depths vary, with feed sections typically 5–20 mm deep and metering sections 2–10 mm, allowing for gradual compression. Surface roughness is maintained at Ra 0.2–0.8 μm (ISO 4287) to reduce material sticking, and hardness is typically 58–62 HRC (ISO 6508) for wear resistance. Operating temperatures range from 150 to 400 °C, and pressures from 10 to 60 MPa, requiring robust screw design. Screw speeds of 20–150 rpm affect shear rate and output. Materials commonly used include nitrided steel (e.g., 38CrMoAlA per GB/T 3077), bimetallic alloys, and tool steel. The weight of the section varies from 50 to 2000 kg depending on size and material. These parameters are reference ranges; actual values must be confirmed with the manufacturer for specific applications.

As the extrusion screw rotates, material moves from the feed section into the compression/transition section where the decreasing channel depth creates increasing pressure. This pressure, combined with shear forces from screw rotation and heat transfer from the barrel, compresses the material, removes air voids, and initiates melting. The tapered design ensures gradual compression and controlled melting to prevent material degradation. The compression ratio and channel depth profile are key design parameters that determine the pressure buildup and melting efficiency. Proper design ensures complete melting and homogenization before the material reaches the metering section, which is essential for consistent output and product quality.