Twin Screw Extruder Working Principle

The basic principle of a twin-screw extruder involves driving solid plastic particles to be transported along the barrel through two rotating screws. The rotation of the screws generates pressure and shear force. The mechanical energy from the rotating screws, combined with the heat from external heaters along the barrel, fully melts and evenly mixes the material. The screws can rotate in the same or opposite directions and can be intermeshing, partially intermeshing, or non-intermeshing. Different designs cater to various materials, ensuring the molten plastic is thoroughly mixed to achieve the desired outcome.To understand the detailed working principle of the extruder, it’s crucial to comprehend the extrusion process within a twin-screw extruder. As material moves forward along the screws, it undergoes changes in temperature, pressure, and viscosity among other factors. The screw is typically divided into three sections over its effective length to cater to the changing material characteristics: the feeding section, the transition or compression section, and the metering or pumping section.

feeding section compression section metering section

1、Feeding Section

The plastic granular material is introduced into the extruder via the hopper, where it meets the screw elements. As the twin screws rotate, the granules are dispersed along the screw threads, initiating mixing and ensuring uniform delivery to the transition section. Simultaneously, the material advances along the extruder barrel. It is heated by an external heater on the barrel and begins to soften due to friction generated by the rotating screws. This process creates relative movement between the screw grooves and the barrel’s inner wall, forming a gelatinous mass.

At this stage, the extruder must overcome the friction of the solid particles against the barrel wall and their mutual friction within the first few turns of the screw (in the feed zone).

Non-intermeshing twin screw extruders feature screws that rotate without touching each other. Because of this design, they do not create a closed or semi-closed cavity, hence lacking the ability to convey materials via positive displacement. Instead, these types of screws primarily rely on the frictional forces between the material and the screw and barrel surfaces to move the material forward, much like the mechanism in a single screw extruder. This process makes non-intermeshing twin screw extruders more suitable for gentle mixing and less aggressive processing of materials, offering advantages in applications where shear sensitivity is a concern.

On the other hand, intermeshing twin screw extruders have screws that engage with each other, effectively forming closed or semi-closed cavities that trap and convey material down the barrel. This interlocking design provides positive displacement transport conditions, where the material is positively and consistently pushed or pulled through the extruder. The degree to which the screws mesh — the closeness of their interlocking — determines the extent of positive displacement. The more closed the cavity, the higher the degree of positive displacement, resulting in improved conveying efficiency, mixing, and self-wiping action between the screw flights and the barrel. This makes intermeshing twin screw extruders highly effective for compounding, devolatilization, and reactive extrusion processes where precise material control and intensive mixing are required.

2、Transition or Compression Section

The compression section receives the gelatinous mass from the feeding section, compacts it, and further softens it as the channel depth gradually decreases. It also expels air entrained in the gelatinous mass. The material is subjected to increasing stress during this process. This compression is crucial for consolidating the material, ensuring the expulsion of all trapped gases, and aiding in the melting process. For instance, the compression here changes from volume 3 in the conveying section to volume 1, indicating a compression ratio of the screw of 3:1. The combination of heat from the barrel and mechanical shear from the screw further melts the material. The transition section is where the majority of the melting should be completed, involving processes such as color mixing and the addition of additives. 

Different Screw Designs Have Unique Effects On Materials

In a screw extruder, the screw threads include forward threads, reverse threads, and special threads.

Forward threads promote the forward flow of material along the extruder, aiding in its conveyance and ensuring consistent feeding into the extruder. Reverse threads serve as mixing elements and create pressure and shear forces, aiding in degassing and improving the homogenization of the melt. Special threads are often customized to perform specific tasks, such as enhancing mixing, generating shear, or facilitating specific reactions during reactive extrusion.

The gap between the two screws plays a crucial role in determining the shear rate and throughput

Smaller gaps result in higher shear rates, which increase the heat generated by friction, potentially improving mixing and homogenization. This can be beneficial for materials that require high shear forces for proper melting and mixing. However, excessive shear might degrade heat-sensitive materials.

Larger gaps lead to lower shear rates, which are gentler on the material and suitable for heat-sensitive or shear-sensitive polymers. Although this allows for greater throughput, it may not be as effective in thoroughly mixing or melting materials.

Screw Rotation Direction Influences Material Processing

Co-rotation promotes material flow and effectively fulfills its functions, while reverse rotation creates a backflow effect, prolonging material residence time, enhancing plasticizing ability, and improving mixing effectiveness.

In a co-rotating twin-screw extruder, within the meshing area, the screw edges are inserted into the screw groove, forming a “C” shaped chamber. This “C”-shaped chamber is not completely enclosed, allowing for a continuous channel between the two screws as the “C”-shaped chambers connect. In this formation, material enters the “C”-shaped chamber along the groove of one screw and transfers to the “C”-shaped chamber of the other. As the material advances, it moves forward until it reaches the meshing area, where it cycles back to the “C”-shaped chamber of the preceding screw channel.

 In co-rotating twin screw extruders, the “C” shaped chambers help create a more uniform and efficient material flow. The material moves in a figure-eight pattern or through overlapping paths, allowing for continuous mixing and forward movement. This results in highly effective mixing and dispersion of materials and additives.

1material flow in meshing co-rotating twin screws

3、Metering or Pumping Section

By this stage, the material is thoroughly mixed and melted. The metering section ensures the mixture achieves a uniform and consistent composition. Its main function is to build up the pressure in the molten polymer necessary for extrusion through the die. The channel depth in this section is shallow, which limits material flow and thereby increases the extruder’s internal pressure. The temperature of the molten polymer must be carefully controlled here to make it suitable for extrusion, involving either cooling or additional heating based on the material’s properties and the requirements of the final product.

If not already addressed in the previous section, any remaining volatiles or gases should be removed from the melt at this point. This step is crucial to ensure the quality of the final product, as trapped gas can lead to defects.

Furthermore, the metering section regulates the flow of molten material to the mold, guaranteeing consistent output. This precise control is essential for manufacturing products with uniform size and performance. Finally, the homogenized, pressurized, and temperature-controlled melt is delivered to the die, ensuring that the material exits the extruder in the optimal condition for molding.

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