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Single Screw Extruder Vs Twin Screw Extruder

Plastic industry typically categorizes extruders into two main types: single-screw extruders and twin-screw extruders. Each type boasts distinct characteristics, making them widely utilized across the plastic industry.So, what are the unique advantages of these two types of extruders, and how do they differ?

This article will delve into the disparities between single-screw and twin-screw extruders, to help you better understand these essential pieces of equipment.

Single Screw Extruder

As the name suggests, a single-screw extruder has a screw inside the extruder barrel. Generally, the effective length is divided into three sections. The effective length of the three sections is determined according to the screw diameter, pitch, and screw depth. Generally, each section is divided into one-third.

single screw extruder

Conveying Section

The first section, starting from the last thread of the feed hopper, is called the conveying section. Materials in this section are required not to be melted but need to be preheated and compressed. In the past, conventional extrusion theory considered the materials here as loose bodies. However, it has been demonstrated that the materials in this section are actually solid plugs. In other words, when subjected to pressure, the materials here behave like solid plugs. Therefore, completing the conveying task is its primary function.

Compression Section

This segment is known as the compression section. Here, the volume of the screw groove gradually diminishes, and the temperature must reach the level necessary for material melting. Compression takes place in this section, where material can be compressed from section three of the conveying section to section one. This is referred to as the screw compression ratio, typically 3:1, although ratios may vary depending on the machine. Once the material is completely melted, it advances to the third section.

Metering Section

The third segment is known as the metering section. In this stage, the material retains its plasticizing temperature and is precisely and quantitatively conveyed, resembling a metering pump, to supply molten material to the die head. During this phase, the temperature should not drop below the plasticizing temperature, typically slightly higher.

Application

Single-screw extruders primarily serve for extruding pipes, sheets, plates, and profiles, along with granulating certain modified materials.

Twin Screw Extruder

A twin-screw extruder comprises two screws and several systems. The screw system plays a key role in the plasticizing and conveying process of materials, significantly impacting the performance and quality of the end product.

Twin Screw Extruder
  1. Feeding system: This comprises a hopper, mixing motor, and feeding motor. Its purpose is to prevent material buildup and ensure smooth entry into the feed inlet.
  2. External heating system: It primarily employs heating rods and the barrel to effectively heat the material, facilitating the plasticization process.
  3. Cooling system: It employs a heat exchange system consisting of thermal oil or water to lower the temperature inside the barrel, thereby effectively regulating the barrel temperature.
  4. Hydraulic screen changer system: It uses replaceable filter screens to capture impurities, enhance plasticization, and guarantee uniformity and stability in the quality of the output materials.
  5. Vacuum system: It extracts moisture and other low-molecular-weight volatile substances from the material.
  6. Electrical control system: It oversees and regulates the main and auxiliary material systems’ relevant equipment.
  7. Screw system: The extruder’s pivotal component, it comprises the conveying section, melting section (venting port), plasticizing section (vacuum port), and discharge section.

Screw System

The screw system is typically divided into four sections (illustrated using parallel twin-screw extruders as a reference): conveying section, melting section (venting port), plasticizing section (vacuum port), and discharge section.

  1. Conveying section: Its role is to transport materials and prevent reverse flow.
  2. Melting section: In this segment, materials undergo complete melting and mixing through heat conduction and frictional shear.
  3. Plasticizing section: This segment further melts and mixes the material components, facilitating distributive and dispersive mixing functions.
  4. Discharge section: This section transports and pressurizes the materials, creating a specific pressure to compact them more tightly. Additionally, it continues to mix the materials to accomplish extrusion granulation.

Conveying Elements

  1. Types: Large pitch and small pitch.
  2. Effect of pitch usage: The more pitches used, the higher the extrusion output, and the shorter the residence time of the material, resulting in weaker mixing ability.
  3. Typical usage of small pitch: Gradually decreasing in combination, used in the conveying section, melting section, and plasticizing section, to increase pressure, enhance melting, improve the homogenization of the mixture, and stabilize the conveying capacity.

Shearing Elements

  1. Direction classification: There are two categories: forward and reverse. Forward direction facilitates material flow and fulfills its function, while reverse direction, also known as reverse rotation, causes material to flow backward, extending material residence time, enhancing plasticization capability, and improving mixing effect.
  2. Angle classification: Generally divided into 30°, 45°, 60°, and 90°.
  3. Function and effect: In the forward direction, increasing the staggered angle decreases conveying capacity, extends residence time, and improves plasticization. Concerning dispersion, a larger angle yields a more pronounced effect; for dispersion mixing, 45° is typically the optimal angle, followed by 30°, with 60° being the least effective.
  4. Effect of the number of elements: In the forward direction, fewer elements result in higher extrusion conveying capacity, torque, and mixing effect, albeit with reduced shear action. Conversely, in the reverse direction, fewer elements lead to lower extrusion conveying capacity, but improved mixing effect.

Single-Screw Extruder Vs. Twin-Screw Extruder

Item

Single screw extruder

Twin screw extruder

Screw length-to-diameter ratio

Most are between 7:1 to 11:1, with some at 18:1.

12:1~16:1

Screw cross-sectional thread shape

Rectangle

Arc-shaped

Barrel structure

There are pinch bolts on the barrel.

Smooth inner surface of the barrel

Screw temperature control

Internal cooling of the screw and temperature control are easy.

The cooling and sealing performance inside the screw is relatively poorer, making temperature control more difficult.

Ease of changing coating types and colors

Relatively convenient

average level

Difficulty of processing special powder coatings with rapid curing, etc.

Relatively easy

average level

Difficulty of equipment maintenance

Relatively difficult

average level

1. Cost

Single-screw extruders feature a straightforward design and affordable pricing, whereas twin-screw extruders boast a more intricate structure and higher cost. Generally, twin-screw extruders are approximately twice as expensive as their single-screw counterparts. However, prices may vary, depending on the manufacturer and model.

2. Formulation

In terms of operation, the disparity between the two is not substantial. However, single-screw extruders typically involve simpler processes and formulations, whereas twin-screw extruders may entail somewhat more complex processes and formulations.

3. Screw Combination

Twin screws offer the flexibility to combine threads based on material conditions and operator preferences. In contrast, single screws are more rigid and cannot be combined.

4. Production Efficiency

Twin-screw extruders exhibit high output, rapid extrusion speeds, and lower energy consumption per unit output compared to single-screw extruders. Twin-screw extruders typically achieve approximately twice the efficiency of single-screw extruders. However, actual efficiency may vary depending on the manufacturer and model.

5. Mixing and Plasticizing Capability

In simple terms, single-screw extruders are suitable for plasticizing and extruding polymers, as well as for extrusion processing of granular materials like molding, blown film, injection molding, etc., covering a broad range of materials.

Twin-screw extruders excel in mixing and plasticizing capabilities, making them ideal for plastic modification. Moreover, they are capable of producing dual-color products.

6. Ease of Maintenance

When it comes to maintenance, single-screw extruders are typically simpler to handle compared to twin-screw extruders due to their less complex structure, resulting in easier repair procedures.

7. Conveying Mechanism

In single-screw extruders, material conveying primarily depends on the frictional force between the material and the barrel. In twin-screw extruders (take co-rotating twin screw extruder for example), the conveying is forward, with a pushing effect to propel the material forward. Moreover, co-rotating twin-screw extruders apply a shearing action on the material at the intermeshing point of the two screws.

8. Velocity Field

In single-screw extruders, the velocity distribution is relatively straightforward and easy to characterize. However, in counter-rotating twin-screw extruders, the situation is significantly more intricate and challenging to delineate. This is mainly due to the presence of intermeshing regions in the screws, where complex flow occurs. This complexity gives counter-rotating twin-screw extruders many advantages, such as thorough mixing, uniform heat transfer, strong melting ability, and good exhaust performance. However, accurately analyzing the flow state in the intermeshing region is challenging. (Specific to counter-rotating twin-screw extruders)

9. Application

Twin-screw extruders: used for various applications such as fiberglass reinforcement, flame retardant pelletizing, high-filled pelletizing, thermosensitive material pelletizing, concentrated color masterbatch, antistatic masterbatch, alloys, coloring, low-fill blending pelletizing, cable material pelletizing, XLPE pipe material pelletizing, thermosetting plastic compounding extrusion, hot melt adhesives, PU reaction extrusion pelletizing, K resin, SBS devolatilization pelletizing, etc.

Single-screw extruders: used for various applications such as PP-R pipes, PE gas pipes, PEX cross-linked pipes, aluminum-plastic composite pipes, ABS pipes, PVC pipes, HDPE silicone core pipes, co-extruded composite pipes, as well as extruding profiles and sheets of PVC, PET, PS, PP, PC, and other plastics. They can also extrude filaments and rods, and adjusting the extruder speed and screw structure enables production of various plastic profiles, including PVC and polyolefin materials.

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