Advancements in Screw Design Boost Extrusion Efficiency

From the snack foods we enjoy to the plastic items we use daily, extrusion processing plays a crucial role in manufacturing. At the heart of this technology lies the screw - an engineering marvel that determines product quality through its precise design and configuration.

Twin-screw extruders are favored for their self-cleaning screw geometry, often referred to as "Erdmenger profiles." Critical screw parameters include diameter, pitch, flight thickness, and channel depth. The pitch - the distance between adjacent flights - influences shear force on materials, with larger pitches increasing shear. Channel depth (from flight tip to channel bottom) and channel width (space between flights) similarly affect shear when reduced.

Screw elements assemble onto shafts whose dimensions are constrained by extruder specifications. This modular design allows for customized configurations tailored to specific material processing requirements.

Screw elements serve as the extruder's operational core, with different types performing specialized functions in material transport, mixing, and shaping.

Featuring deeper channels, these components efficiently move materials through the extruder without excessive shear or degradation. Channel depth typically follows this progression: feeding zone > compression zone > metering zone.

Kneading elements primarily facilitate mixing through shear and elongational forces. Their geometry enables two mixing mechanisms:

• Distributive mixing: Evenly disperses particles without breaking them
• Dispersive mixing: Breaks up agglomerates through intense shear

Wider kneading elements favor dispersive mixing, while narrower versions enhance distributive mixing. Element orientation also affects performance:

• Forward elements generate mild shear
• Neutral elements increase residence time without conveying
• Reverse elements maximize shear by countering material flow

Understanding element nomenclature is essential for proper screw configuration. The following classification system (based on Brabender extruder screws) details various element types and their functions.

SE = Screw Element, SK = Erdmenger profile-based thrust edge. First number = pitch in threads, second number = segment length (mm). R = Right rotation.

L = Left rotation.

KP = Kneading block with half-disks, KBW = Kneading block with full disk width. First number = disk displacement angle, second = disk count, third = segment length (mm). Z = Toothed mixer (first number = teeth count, second = tooth rows, third = length in mm).

Optimal screw configuration requires thorough understanding of material properties, particularly behavior under heat and shear. Shear-sensitive materials often fail to form stable products when processed with high-shear profiles. The ideal screw design must:

• Efficiently pump materials
• Break down agglomerates
• Homogenize and melt materials
• Generate proper melt temperature and pressure at the die

This precision engineering enables manufacturers to develop unique products while maintaining consistent quality in high-volume production.