Principles of Screw Configuration in Different Process Sections of A Twin-Screw Extruder

As a core piece of equipment in plastic processing, the twin-screw extruder's key lies in the configuration of its screw elements. Different types and arrangements of screw threads result in significantly varied processing effects and final product quality.

Types of Screw Elements

Based on their function, screw elements can be categorized as conveying elements, kneading elements, and mixing elements, each with different shapes and purposes in design.

Among kneading blocks, the pitch of the screw plays an essential role. Wider-pitch kneading blocks result in less leakage and exert stronger dispersive shear on the material. Narrow-pitch screw elements, on the other hand, promote better material distribution as they allow side overflow during compression.

The screw configuration of a twin-screw extruder is generally divided into five functional zones:

1.Feeding Zone 2.Melting Zone 3.Mixing Zone 4.Venting Zone 5.Metering (Homogenizing) Zone

Each functional section requires a tailored screw combination to meet specific processing goals:

1. Feeding Zone

Transports materials while preventing backflow or overflow.

2. Melting Zone

Ensures thorough melting and homogenization through heat transfer and shear friction.

3. Mixing Zone

Further reduces the size of material components and enhances their uniform distribution. It enables both distributive and dispersive mixing for ideal material structure.

4. Venting Zone

Removes moisture, low-molecular-weight volatiles, and other impurities.

5. Metering (Homogenizing) Zone

Builds up pressure and densifies the melt to ensure consistent extrusion and pelletizing while also aiding in final mixing.

Principles of Screw Configuration

Screw layout must consider material properties, feeding sequence and position, venting port location, etc. The key configuration principles include:

1.Feeding Section

Use large-pitch conveying elements to prioritize material throughput, reduce residence time, and minimize degradation.

2.Melting Section

Incorporate reverse-thread elements or forward kneading blocks combined with reverse-thread elements to facilitate melting.

3.Kneading and Dispersive Section

Reducing the gap between screw flights and increasing the number of threads boosts the average shear rate, enhancing mixing performance. The stagger angle between kneading blocks is a critical factor influencing their efficiency.

4.Metering (Homogenizing) Section

Use small-pitch threads that gradually decrease in size. These improve pressure buildup, melting quality, and extrusion stability.

The configuration of screw elements in twin-screw extruders significantly impacts processing performance and product quality. It requires accumulated experience and practical knowledge to optimize these combinations effectively.

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