Understanding Extruder Screws: Types, Functions, And Design

1. What is an Extruder Screw?

An extruder contains a screw housed inside a heated cylindrical barrel. By rotating the screw with a motor, resin is melted within the cylinder and transported to the die at the tip, which shapes the material into various product forms.

There are two types of extruders: single-screw extruders, which have one screw, and twin-screw extruders, which have two screws. By adjusting the depth and angle of the screw's grooves, various mixing and kneading processes can be performed for different purposes.

2. Single-Screw Extruder and Its Screw

In a single-screw extruder, the screw is typically designed for a straightforward process—melting resin by heating and processing it into pellet form. The screw consists of four main sections: the feed zone, compression zone, melting zone, and metering zone.

Some screw designs do not include a mixing zone. However, there are also screws with a kneading section, known as "darmage" (mixing zone), where the screw is specially designed with notches to improve resin blending. The design varies depending on the manufacturer.

A screw without a mixing zone is called a "full-flight" screw. More than 90% of the screws used for recycling processes are single-screw types.

3. Twin-Screw Extruder and Its Screw

In contrast, twin-screw extruders generally provide better mixing than single-screw extruders, resulting in improved quality in terms of homogeneity and dispersion. Therefore, they are commonly used to mix and blend different resins, additives, colorants, pigments, or color masterbatches to achieve a uniform dispersion.

This entire mixing process is called compounding.

The screw in a twin-screw extruder consists of different segments designed for specific functions. The term "compounding" refers to thoroughly mixing resin to achieve uniform dispersion.

Twin screws can rotate in the same direction (co-rotating) or in opposite directions (counter-rotating):

Co-rotating: Both screws rotate in the same direction, providing better mixing.

Counter-rotating: The screws rotate in opposite directions.

When melting and pelletizing a single type of resin, a single-screw extruder is often chosen. However, when mixing different resins or adding color masterbatches for uniform blending, a twin-screw extruder is typically preferred.

Apart from the classification of single-screw and twin-screw extruders, screws can also be categorized by their length, diameter, or specific design features. These variables significantly impact the physical properties of the produced pellets.

4. Extruder Screw Design

Another important perspective is the design of the screw itself. The screw must be selected based on the intended application.

Full-Flight Screw A full-flight screw has a design that continuously pushes the resin forward without any mixing structures along the way. Since the material is not halted during processing, this type of screw enables higher production rates.

Darmage (Mixing Section) On the other hand, if additional mixing is needed in a single-screw extruder, a mixing section called "darmage" can be introduced. In this design, the resin is temporarily held back within the cylinder to enhance mixing. However, this process slightly reduces production output.

Thus, screw design directly impacts production rates. The choice of design depends on whether better resin mixing, higher production volume, or a balance between the two is the primary requirement.

Selecting the right extruder and screw design ultimately depends on the specific needs of pellet manufacturing. Clearly defining the intended application will naturally lead to the best choice.

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