Tip Clearance in Twin Screw Extruders | Technovel
What Is Tip Clearance
In the melt mixing process of an extruder, mixing is generally understood to occur primarily through the action of shear stress as material passes through the tip clearance region. Tip clearance refers to the gap between the crest of a screw flight and the inner surface of the barrel, or the gap between the crest of one screw flight and the body of the opposing screw. In the melt mixing of an extruder, the design of an optimal tip clearance is of considerable importance.
Shear Stress
A small clearance applies higher shear stress to the material. This improves dispersion, but with certain materials it may cause thermal degradation or localized heating. A large clearance reduces shear stress, so uniform dispersion of particles or fillers tends to be more difficult to achieve.
Plastic Deformation and Viscous Dissipation
In a twin screw extruder, the melting of resin depends not only on heat conduction from the heaters mounted on the barrel, but also significantly on the heat generated by plastic deformation during mixing, by viscous dissipation, and by friction. Plastic deformation occurs when the flowing material is subjected to high shear stress in the clearance region. Viscous dissipation becomes pronounced in fluid flow under high shear stress. As the tip clearance becomes smaller, heat generation from plastic deformation and viscous dissipation also increases.
Self Cleaning Performance
As the tip clearance becomes smaller, material adhering to the barrel and screws is scraped off more efficiently, which considerably improves self cleaning performance. Better self cleaning prevents thermal degradation caused by long term resin adhesion and also raises the efficiency of heat conduction from the barrel heaters.
Inflow Rate
When the tip clearance is large, a greater amount of material flows into that clearance. By contrast, when the tip clearance is small, the amount of material entering the clearance is reduced.
Summary
In the melt mixing of an extruder, tip clearance design is a key factor that strongly influences dispersive mixing performance, thermal history, and even self cleaning performance. When the tip clearance is small, high shear stress is applied to the material between the screw tips and the barrel, which promotes the breakup of agglomerates and the dispersion of fillers, resulting in high dispersive mixing performance. Excessive shear, however, increases heat generation from plastic deformation, viscous dissipation, and friction, which can cause localized temperature rise and thermal degradation of the material.
Tip clearance cannot simply be judged as “narrower is better” or “wider is better.” It must be optimized in light of the properties of the target material, its thermal stability, the required dispersion performance, and operating conditions such as throughput and screw rpm. Designing an appropriate tip clearance that matches material properties and mixing requirements leads to an efficient and stable extrusion process.
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Technovel Corporation — Extrusion Machinery Specialists
Osaka based Technovel specializes in extrusion machinery. We built the world’s first horizontally multi screw extruder, and our Quad and Octa screw extruders now serve diverse industries. Our twin screw range runs from the world’s smallest 6 mm lab unit, through our best-selling 15 mm model, to large production machines. This column shares the know how behind them.