Extrusion screws are generally divided into several sections. What is the function of each section? For the extrusion molding of crystalline plastics, what kind of screw should be selected?

An extrusion screw, also known as a screw barrel, is a critical component in the extrusion process. It conveys plastic raw materials from the hopper to the mold while simultaneously melting, plasticizing, homogenizing, and compacting them for final shaping through the mold.

The extrusion screw can be divided into three sections based on the changing characteristics of the material within it, such as temperature, pressure, and viscosity:

Feeding Section: Located at the screw's front, this section's primary function is to transport plastic from the hopper into the screw channel. The feeding section's groove depth is typically deeper to accommodate a larger volume of plastic, and the plastic remains solid throughout this stage.

Compression Section: Also referred to as the phase transfer section, this section acts by squeezing and shearing the material delivered from the feeding section. Here, the material continues to be heated, facilitating its transformation from a solid state to a molten one. Additionally, this section expels air and other volatile components from the plastic, resulting in increased density. Ideally, the plastic should emerge from the compression section in a fully plasticized, viscous flow state.

Metering Section: Sometimes called the homogenization section, this section's role is to further refine the uniformity of the plasticized material. The screw channel design and back pressure from the machine head work together to further mix and plasticize the material. Finally, the thoroughly homogenized material is extruded through the die of the machine head under consistent pressure.

Considerations for Crystalline Plastics

When it comes to extruding crystalline thermoplastics, specific adjustments are needed:

Feeding Section Length: A longer feeding section is necessary for crystalline thermoplastics. This extended section allows the plastic more time to gradually soften, typically occupying 60% to 65% of the total screw length.

Compression Section Length: Crystalline plastics have a narrower melting temperature range, so the compression section can be shorter, around 35% of the screw diameter (Ds).

Metering Section Length: To ensure stable material flow, the metering section for crystalline plastics should have a sufficient length, generally between 20% and 25% of the total screw length.

The design and type of extrusion screw significantly impact the quality and efficiency of the extruded product. When selecting an extrusion screw, careful consideration should be given to the type of plastic, performance requirements, and the specific processing technology employed.