Understanding the Screw Configuration of Parallel Co-Rotating Twin-Screw Extruders

In the selection of twin screw extruders, we often hear about various types such as intermeshing, parallel, and co-rotating twin screw extruders. Each type has distinct advantages and characteristics to meet the needs of different industries. So, what leads to the many classifications of twin screw extruders?

Actually, the terms ‘intermeshing,’ ‘co-rotating,’ and ‘parallel’ you mentioned all describe different types of screws in twin screw extruders.

Types of twin screw Extruders

1, Parallel or conical

According to the relative position of the two screws, it can be divided into conical and parallel twin screw extruders. Parallel twin screw extruders are highly suitable for applications requiring high-shear mixing and compounding, such as polymer processing, compounding, and masterbatch production. Conical twin screw extruders can provide better self-wiping and conveying capabilities, suitable for applications requiring gentle processing and material conveying, such as food extrusion, pharmaceutical granulation, and processing of heat-sensitive materials and cosmetics.

2, Co-rotating or counter-rotating

Based on the rotation direction of the two screws in twin screw extruders, they can be classified as co-rotating or counter-rotating twin screw extruders.

Co-rotating twin screw extruders typically provide high torque and throughput, making them suitable for applications requiring intense mixing, compounding, and dispersion. They excel in processing high-viscosity materials, reactive compounds, and formulations containing additives or fillers.

Counter-rotating twin screw extruders can generate more intense mixing and kneading action, usually suitable for applications requiring thorough dispersion, mixing, and reactive processing. They are suitable for processing materials of different viscosities, shear-sensitive formulations, and applications requiring precise temperature control.

For compounding plastics, intermeshing parallel co-rotating twin screw extruders are generally considered the best type. Consistent co-rotation and a constant screw configuration along their entire length help distribute and disperse plastic materials better, providing consistent shear and mixing action.

Meshing screws promote thorough mixing by providing multiple shearing zones and extensive particle interaction, resulting in effective mixing of additives, fillers, and other components with the base polymer.

3, Intermeshing or non-intermeshing

Twin screw extruders consist of two rotating screws, and the degree of meshing between the threads of these two screws has different effects on compounding.

  • Fully intermeshing twin screw extruders have the most extensive mixing and kneading action, suitable for applications requiring thorough dispersion and mixing.
  • Partially intermeshing twin screw extruders provide a balance between mixing efficiency and conveying capacity, achieving efficient material conveying and moderate mixing.
  • Non-intermeshing twin screw extruders are suitable for applications requiring gentle processing, such as food extrusion, pharmaceuticals, and specialty polymers.
Co Rotating

Co Rotating

Counter Rrotating

Counter Rrotating

Here, we’ll discuss in detail the screw characteristics of parallel co-rotating twin screw extruders.

Composition of the Screw in Parallel Co-Rotating twin screw Extruders

The screw of a co-rotating parallel twin screw extruder is composed of various modular elements, each designed to perform specific functions during the extrusion process. These elements can be configured and combined in different ways to optimize the processing of different materials. Here are the key parts of the screw in a co-rotating parallel twin screw extruder:

1, Feed Screw Elements

Located at the beginning of the screw, these elements are responsible for feeding the raw material into the extruder. They typically feature a deep, wide pitch to facilitate material intake.

2, Conveying Screw Elements

These elements transport the material along the screw. They have a helical shape and can vary in pitch and depth to control the flow rate and pressure of the material.

3, SK Elements (Segmented Kneading Elements)

SK elements are specialized components that provide intensive kneading and mixing action within the extruder. They typically consist of segmented discs with kneading pins that create high shear and elongational forces, enhancing mixing efficiency and dispersion.

4, Kneading Blocks

Kneading blocks are used for intensive mixing and homogenization. They consist of a series of staggered discs that create high shear and elongational forces, breaking up agglomerates and dispersing fillers or additives evenly.

Kneading blocks are arranged by multiple intermeshed pieces at certain angles (30°, 45°, 60°, or 90°) in a clockwise or counterclockwise direction. The conveying direction of kneading blocks can be divided into forward, reverse, and neutral. When the staggered angles of kneading blocks are the same, different effects can be produced with different thicknesses of kneading blocks.

  • 30° Kneading Blocks: Spaces between kneading pins may allow for some mixing; the flow gap between channels is very small; forward direction= minimal mixing; reverse direction= high pressure.
  • 45° Kneading Blocks: Greater mixing capability than 30° kneading blocks; conveying capability is less than 30°; forward direction= moderate mixing; reverse = moderate to high pressure.
  • 60° Kneading Blocks: Stronger mixing capability than 45°; conveying capability is less than 45°; forward direction= moderate, reverse direction = medium to low pressure.
  • 90° Kneading Block: No conveying capability; produces 100% filling (low pressure); very strong mixing capability
Kneading Blocks

The screw conveying elements have three key points: conveying direction, pitch, and number of threads.

Conveying Direction

Forward Conveying: Materials move towards the discharge end of the equipment.

Reverse Conveying: Materials move away from the discharge end of the equipment. Reverse conveying elements can be used as “restrictive” elements to accumulate materials inside the screw.

Pitch

The axial length of one cycle of blade rotation. Pitch can determine whether materials fill quickly or slowly to control the filling degree.
Characteristics of Large Pitch (≈ 1.5 to 2 times the diameter): Maximum capacity; Fastest conveying speed; Lowest filling degree; Used for feeding and exhaust.
Characteristics of Medium Pitch (≈ 1 times the diameter): Medium capacity; Medium conveying speed; Increased filling degree at downstream large pitch element.
Characteristics of Small Pitch (≈ 0.25 to 0.75 times the diameter): Minimum capacity; Slowest conveying speed; Increased filling degree at downstream medium pitch element.

Number of Threads

Single Head Screw Element

  • Wide blades minimize material leakage
  • Smaller capacity than double-headed screws
  • Maximum suction efficiency

Double Head Screw Element:

  • Standard part of co-rotating twin screw conveying elements
  • Lower shear force than triple head elements
  • Used for solid feeding, melt conveying, exhaust, and melt pumping.

Triple Head Screw Element:

  • Maximum shear force
  • Shallow channel depth
  • Used for melting, dispersion, and mixing
Number of Threads

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