What is Tip Clearance?

In extrusion melt kneading, dispersive mixing is primarily influenced by the shear stress exerted as materials pass through the tip clearance. Tip clearance refers to the gap between the screw crest and the inner surface of the barrel or the gap between the crest of one screw and the other screw. Designing the optimal tip clearance is crucial for effective melt kneading in an extruder.

Shear Stress

Smaller clearances result in higher shear stress, enhancing dispersion. However, this can also lead to heat degradation or localized overheating in some materials. Conversely, larger clearances reduce shear stress, making it harder to achieve uniform dispersion of particles or fillers.
　

Plastic Deformation and Viscous Dissipation

In twin screw extruders, the heat generated during resin melting arises not only from barrel heaters but also from plastic deformation, viscous dissipation, and friction during kneading. Plastic deformation occurs when materials experience high shear stress in the clearance. Viscous dissipation is prominent in high-shear fluid flows. Smaller tip clearances lead to greater heat generation from plastic deformation and viscous dissipation.
　

Self-Cleaning Performance

Smaller tip clearances enhance the self-cleaning ability of the extruder, effectively scraping material off the barrel and screw surfaces. Improved self-cleaning reduces heat degradation caused by prolonged material adherence and enhances heat transfer efficiency from barrel heaters.
　

Material Flow Rate

Larger tip clearances allow more material to flow through, while smaller clearances restrict material flow into the gap.。

Summary

The design of the optimal tip clearance in extrusion melt kneading significantly affects dispersive mixing performance, thermal history, and self-cleaning efficiency.

• Small Tip Clearance:
  • Generates high shear stress, improving dispersion.
  • Risks include heat generation from plastic deformation, viscous dissipation, and friction, potentially causing material heat degradation or localized overheating.
  • Enhances self-cleaning, reducing material adherence and minimizing heat degradation over long operation times.
• Large Tip Clearance:
  • Allows higher material flow into the gap.
  • Results in lower shear stress, which may compromise dispersion performance.

Thus, designing tip clearance according to the material properties and kneading requirements is key to achieving an efficient and stable extrusion process.

　

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