Guide to Twin-Screw Extruder Screw Element Design

Getting to Know: Intermeshing Co-Rotating Twin-Screw Extruders

The screws in these popular extruders have some key characteristics that make them so effective:

1. Parallel & Same Direction Rotation: The two screws spin side-by-side in the same direction. This creates a consistent shearing action between the screw threads (flights) and the barrel wall. The intensity of this shear can be fine-tuned by choosing specific screw element combinations and adjusting the spacing.
2. Superb Mixing Action: Thanks to their geometric design and co-rotating movement, these screws excel at distributing and mixing materials – perfect for compounding! As material enters and softens, the screws work together in a unique way. At the point where they intermesh, one screw pulls material into the gap while the other pushes it out, causing the material to move between the screws in a figure “∞” pattern. This creates high relative speeds, ideal for thorough mixing and homogenization. Plus, the very tight gap in the intermeshing zone ensures high shear, leading to uniform plasticization.
3. Reactive Processing Powerhouse: These extruders aren’t just mixers; they can act as dynamic chemical reactors! Once materials melt inside the barrel, they can undergo various chemical reactions like polymerization or grafting. This capability, known as reactive extrusion, is used for creating new polymers, modifying existing ones (like polyolefins), functionalizing polymers for better properties or compatibility, and blending different materials. It also covers physical modifications like filling, compounding, toughening, and reinforcing plastics.
4. Modular “Building-Block” Design: A wide variety of screw elements are available – conveying elements, kneading blocks (for shearing), mixing elements, reverse-thread elements (to build pressure or increase mixing time), and more. Like toy building blocks, these elements can be arranged and combined on the screw shaft according to the specific needs of the material and process. This modularity allows for optimized screw designs tailored to diverse formulations.

Meet the Building Blocks: Types of Twin-Screw Elements

Generally, based on their structure and function, common screw elements fall into these main categories: conveying elements, shearing elements, and mixing/dispersing elements. Let’s look at each one.

(1) Conveying Elements: The Movers

Conveying elements are primarily designed to move material along the extruder barrel. They come in two main flavors: forward conveying (moving material towards the extruder exit) and reverse conveying. Reverse elements push material backward, acting like a temporary dam. This increases the material’s residence time, improves how full the screw channels are (fill degree), builds pressure, and significantly boosts mixing effectiveness.

Key design features for conveying elements include channel depth, lead (the distance the screw thread advances in one rotation), flight thickness, and clearance (gap between screw and barrel), as illustrated in Figure 1. Their main job is transport, so material usually passes through them relatively quickly. The lead is arguably the most critical factor here. A larger lead means higher throughput (more material extruded per hour) and shorter residence time, but potentially less thorough mixing.

Note: D = screw outer diameter, d = screw root diameter, P = pitch, L = lead

Here’s when different lead sizes are typically used:

• Large Lead Elements: Used when high output is the priority, for heat-sensitive materials requiring minimal barrel time (to prevent degradation), and often placed near vent ports (degassing zones) to maximize material surface area for better volatile removal.
• Medium Lead Elements: Chosen when a balance between conveying and mixing is needed. Often used in sequences where the lead gradually decreases to gently build pressure.
• Small Lead Elements: Primarily used in the melting zone to build pressure, enhance melting efficiency, increase mixing intensity, and improve overall extrusion stability.

(2) Shearing Elements: The Kneaders

Shearing elements, commonly known as kneading blocks, are the powerhouses for intensive mixing. They apply high shear forces and are excellent at both distributing (spreading components apart) and dispersing (breaking down agglomerates) materials. Key parameters are the number of lobes or “heads,” the thickness of each block, and the staggering angle between adjacent blocks (see Figure 2). They are typically used in groups. The angle between blocks influences how material flows through them. The tight interaction between the blocks on the two screws creates a “grinding disk” effect, forcing material mixing and exchange. When multiple blocks are combined, they can form a net spiral angle, helping to move material axially while intensely mixing it.

Let’s look at the parameters:

• Staggering Angle (α): Common angles are 30°, 45°, 60°, and 90°. For forward-staggered blocks (helping material move forward), a larger angle generally means lower conveying capacity. This increases residence time and enhances mixing quality. Reverse-staggered blocks hinder forward flow, significantly increasing pressure and mixing intensity.
• Thickness (t): Typically ranges from 7 to 19 mm, chosen based on the application. Thickness impacts the shearing intensity and mixing style. Thicker blocks generate more shear but might have slightly less distributive mixing efficiency compared to thinner blocks, which often provide better distributive mixing.

Both conveying and shearing elements also vary by the number of heads (or lobes) – typically single, double, or triple-head designs (Figure 3).

The number of heads influences performance: For forward-acting elements, more heads generally mean lower conveying capacity, lower torque generation per unit volume, potentially less distributive mixing, but increased shear intensity. For reverse-acting elements, more heads can mean *greater* backward conveying capacity (stronger barrier) and less distributive mixing.

Single-head Screw Element

Offers the highest conveying efficiency (per channel) and its thicker flight minimizes material leakage (backflow). Has a smaller overall open volume compared to multi-head designs.

Double-head Screw Element

The standard, versatile choice for co-rotating twin-screws. Generates less shear than triple-head elements. Commonly used for solid feeding, melt conveying, and degassing zones. Known for uniform heating and good self-cleaning properties.

Triple-head Screw Element

Provides higher shear, making it ideal for melting, dispersion, and intensive mixing. Allows for more flexible control over pressure and temperature distribution in the barrel. Can produce excellent degassing effects, but typically results in lower output compared to double-head elements with the same lead.

(3) Mixing Elements: The Homogenizers

Mixing elements often refer to toothed elements (which can have straight or helical teeth/grooves cut into the screw flight tips), as shown in Figure 4. The primary purpose of these grooves is to create connections between adjacent screw channels, encouraging material to exchange between them. This promotes melt homogenization and enhances longitudinal (along the screw axis) mixing. Because the screw flight is interrupted by grooves, these elements have slightly reduced conveying and pressure-building capabilities. However, this also increases the fill level within the screw channels and extends the material’s residence time in that section.

The number, shape, and arrangement of the teeth (or grooves) are key design parameters. The tooth shape primarily serves to disrupt the material flow, speeding up homogenization. More teeth generally lead to a more pronounced mixing effect. However, it’s crucial during design and operation to avoid excessive shear that could damage the polymer molecules.

Putting It All Together: Combining Screw Elements

A complete extruder screw configuration is typically divided into functional sections or zones, each designed for a specific task. A common layout includes five main sections (Figure 5):

1. Conveying Section (Feed Zone)

Goal: Reliably transport solid material (pellets or powder) from the feed hopper into the extruder and prevent material from backing up into the feed opening.

Typical Elements: Large lead conveying elements (often double-headed).

2. Melting Section (Transition Zone)

Goal: Melt the solid material completely and uniformly through heat transfer from the barrel and frictional heat generated by shearing.

Typical Elements: Small lead conveying elements, often combined with kneading blocks (shearing elements) to efficiently input energy.

3. Mixing Section

Goal: Ensure thorough mixing and homogenization of single or multiple components (e.g., polymer + additives, different polymers). Can involve distributive mixing (spreading components evenly) and dispersive mixing (breaking down agglomerates or droplets).

Typical Elements: Combinations of kneading blocks (various thicknesses and staggering angles) and specialized mixing elements (like toothed elements). Reverse conveying elements might be used here to increase residence time and mixing intensity.

4. Degassing Section (Venting Zone)

Goal: Remove moisture, trapped air, monomers, oligomers, or other volatile impurities from the melt to improve final product quality. Requires the screw channels to be only partially filled in this zone.

Typical Elements: Large lead conveying elements (often with deeper channels) to maximize surface exposure under vacuum. Often preceded by a melt seal (e.g., a reverse element or small lead element) to prevent vacuum leakage.

5. Homogenizing & Metering Section (Die Zone)

Goal: Convey the fully mixed melt towards the die, build sufficient and stable pressure for extrusion, further homogenize the melt temperature, and ensure a consistent output rate.

Typical Elements: Small lead conveying elements to build pressure reliably. Sometimes includes gentle mixing elements for final temperature homogenization.

In Summary