Exploring The Structure And Functions of Extruder Screws

A screw can be divided into three sections: the feeding zone, which transports material forward; the compression zone, where the resin melts; and the metering zone, which ensures uniform delivery of the molten resin. Additionally, a mixing section (darmage) may sometimes be included, but this discussion focuses on the three primary zones.

The screw serves three key functions:

Plasticization – Converting solid resin into a molten state.

Resin Conveyance – Transporting the resin from the hopper to the extruder head.

Resin Mixing – Ensuring homogeneous blending of the resin.

The screw must efficiently plasticize the material while maximizing the amount of resin delivered to the head.

1) Feeding Zone

This is the section where resin enters from the hopper. At this stage, the resin remains in a solid state and undergoes preheating before reaching the compression zone.

If higher resin intake is required, the screw grooves can be deepened to accommodate more material. However, it is crucial to balance this with the compression section. The feeding zone’s primary role is to move unmelted resin forward.

Just below the hopper, it is important to prevent premature melting to ensure smooth material intake. If the resin starts melting too early, intake efficiency decreases, leading to reduced extrusion output.

Additionally, this zone preconditions the resin for effective melting in the compression zone. Preheated resin enters the compression zone more smoothly, whereas insufficient preheating causes retention issues and delays in melting.

The primary heat source in the feeding zone is the cylinder heater.

Considerations for Preheating Before the Compression Zone:

Screw Groove Depth:

Shallower grooves improve heat absorption efficiency, accelerating preheating.

However, shallower grooves reduce material intake per unit time, lowering output.

Deeper grooves allow greater material intake but reduce heat transfer efficiency due to increased distance from the cylinder.

A balanced design is necessary based on material properties.

Feeding Zone Length:

A longer feeding zone allows for more effective preheating.

If the groove depth is shallow and heat transfer is efficient, the feeding zone can be shorter.

If the groove depth is deep and heat conduction is poor, a longer feeding zone is necessary for adequate preheating.

Additional Information:

Preheating the resin before it enters the screw is known to improve production efficiency. This is because preheating in advance reduces the thermal load on the feeding section, making the melting process smoother.

2) Compression Zone

In this zone, the depth of the screw grooves gradually decreases, causing the resin to absorb heat and melt. The melting process occurs through two mechanisms:

Heat from the Cylinder Heater – External heating promotes melting.

Shear Heat from Screw Rotation – Mechanical energy from screw rotation generates shear heat, enhancing melting efficiency.

Increasing screw speed generates more shear heat, allowing for energy-efficient operation with reduced reliance on external heating.

There are two types of compression in this zone:

Rapid Compression – Generates high shear heat, accelerating melting.

Gradual Compression – Relies more on external heating.

The two primary heat sources in this zone are:

External Heat Source – The cylinder heater wrapped around the barrel.

Shear Heat – Heat generated by the mechanical energy of resin compression and shear.

Rapid compression leads to higher shear heat generation, while gradual compression depends more on external heating.

During melting, moisture and gases may be released, which are expelled through a vent. If gas production is high, a vacuum pump can be used to enhance degassing efficiency.

In practice, shear heat from screw rotation is the dominant heat source inside the extruder, rather than external heating. Certain cylinder heater sections may have minimal heating functions during operation.

3) Metering Zone

The groove depth in this section remains the same as in the compression zone. This is the final zone before extrusion through the die. By this stage, the resin is fully melted and is pushed forward at a stable, controlled rate.

The metering zone ensures uniform mixing and prevents material retention before reaching the die.

A single-screw extruder consists of a feeding zone, compression zone, and metering zone. If additional mixing is required, a special mixing structure called darmage can be incorporated.

4) Darmage (Mixing Section)

Darmage is a specialized groove design in a single-screw extruder that enhances resin mixing. However, incorporating a darmage section typically reduces production output slightly.

5) Screw Elements

In twin-screw extruders, removable screw elements (segments) are attached to optimize extrusion performance and mixing. These elements can be adjusted based on processing needs.

Various elements serve different purposes. For instance, to prolong the resin’s retention time in the barrel, counter-rotating screw elements can be used.

6) Compression Ratio

The compression ratio refers to the ratio of the space volume between the feeding zone and the metering zone per screw turn.

For standard extruders, a typical compression ratio is 2:1 to 3:1. However, for low bulk density materials like film scraps, the feed zone volume can be increased, setting the compression ratio to 4:1 to 5:1.

The compression ratio determines the groove depth difference between the feeding and compression zones:

Lower Compression Ratio – Produces lower resin pressure and extrusion output.

Higher Compression Ratio – Increases shear heat and pressure but can cause overheating and resin degradation.

The optimal compression ratio is determined by balancing screw diameter and length-to-diameter (L/D) ratio.

7) Barrel (Cylinder)

The barrel is a cylindrical steel casing that houses the screw. External heaters are wrapped around it to transfer heat to the resin and facilitate melting.

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