Twin Screw Extruder is an important piece of equipment used in various industries for mixing, compounding and extruding materials such as plastics, food and pharmaceuticals. The versatility of the twin-screw extruder makes it an essential piece of equipment in modern manufacturing. However, not all extruders are created equal; they come in different types, each suited to specific needs.

In this article, we'll explore the different types of twin-screw extruders and their unique capabilities.

Categorized by Screw Configuration 
Based on screw configuration, twin-screw extruders can be categorized into parallel twin-screw extruders and conical twin-screw extruders. In a parallel twin-screw extruder, the screws are parallel to each other throughout their length. In conical twin-screw extruders, the screws are tapered and the diameter decreases gradually from the feed end to the die end.

Parallel Twin Screw Extruder
Parallel twin-screw extruders have screws with the same diameter throughout their length. They are commonly used for processing polyolefins (such as PE and PP), nylon, PC, polyester and other materials.

Advantages
The adaptability of the L/D ratio remains an advantage of parallel twin screw extruders. They can be adjusted to increase or decrease the L/D ratio in response to changes in molding conditions to meet the requirements of plastics processing technology. This flexibility expands the range of applications for parallel twin-screw extruders, something that is difficult to achieve with conical twin-screw extruders.

Disadvantages:
Due to the small center distance between the two screws and the limited space available, the radial and thrust bearings supporting the two output shafts and the associated drive gears face significant challenges in the drive gearbox of a parallel twin-screw extruder.

Despite the efforts of the designers, it is still difficult to solve the real problems of limited bearing capacity, small modulus and diameter of the gears as well as the reduced diameter of the tail end of the two screws, resulting in an obvious lack of torsional capacity. Limited output torque and greatly reduced bearing capacity is a prominent drawback of parallel twin-screw extruders.

Conical twin-screw extruder
Conical twin-screw extruders have a larger diameter at the feed end of the screw and a smaller diameter at the die end. These extruders are mainly used for processing PVC and their unique design is optimized for specific material requirements.

Advantages
The two conical screws are arranged horizontally with their axes at an angle when inserted into the barrel. As the distance between the shafts is gradually increased from the narrower end to the wider end, the drive gearbox can accommodate a larger center distance between the two output shafts. This arrangement provides ample space for mounting gears, gear shafts, radial and thrust bearings, thereby increasing transmission efficiency.

This flexible design allows for the installation of larger radial and thrust bearings with shafts sized to withstand high torque transmission. As a result, conical twin-screw extruders offer significant features, including high operating torque and load carrying capacity, that distinguish them from parallel twin-screw extruders.

Disadvantages
The disadvantage of conical twin-screw extruders is their limited adaptability, as the screws have varying diameters along their lengths, which can make it challenging to achieve uniform material processing. In addition, their design complexity and potentially higher construction costs are factors to consider.

Similarities between the two types
They both have mechanisms for forced plastic propulsion, good mixing and plasticizing capabilities, and dewatering capabilities. Their adaptability to materials and plastic product molding processes is essentially the same.

Differences between the two types
● Diameter: Parallel twin screws have the same diameter, while conical twin screws have different diameters at the small and large ends.
● Center Distance: The center distance between parallel twin screws remains the same, while the axis of conical twin screws forms an angle, resulting in different center distance dimensions along the axis.
● L/D Ratio: The L/D ratio (L/D) of a parallel twin screw is the ratio of the effective length of the screw to its outside diameter, whereas the L/D ratio of a conical twin screw is the ratio of the effective length of the screw to the average of the diameters of its large and small ends.

It can be seen that the biggest difference between a parallel twin screw extruder and a conical twin screw extruder lies in the geometry of the screw barrel, which leads to various structural and performance differences. Although these two types of extruders have their own characteristics, but they have their own advantages.

Classification according to the direction of rotation of the twin-screw
Twin-screw extruders are categorized into co-rotating and counter-rotating types according to the direction of screw rotation.

Co-rotating twin-screw extruder
Co-rotating twin-screw extruders have two screws rotating in the same direction. These machines are efficient in mixing and formulating materials. With the ability to handle a wide range of viscosities and formulations, co-rotating extruders are ideal for processing polymer blends, masterbatches and reaction extrusion applications. They have excellent self-scrubbing and conveying characteristics to ensure that additives are uniformly dispersed and distributed throughout the material.

Advantages of co-rotating twin-screw extruders:
● Enhanced Mixing: Intermeshing screws and customizable screw elements allow for precise control of mixing intensity and quality beyond single-screw extruders.
● Processing Flexibility: Metering feed allows independent control of throughput, enabling multiple processing functions on a single machine.
● Controlled Processing Parameters: Narrow residence time distribution and precise temperature control improves shear-time-temperature profiles for consistent product quality.
● Efficient production: Volumetric conveying allows for a wide range of materials to be processed with minimal downtime.
● COST EFFICIENCY: High flexibility and productivity for a wide range of end products with consistent quality, while screw wear can be compensated for by adjusting screw speed.

Based on the interlocking behavior of the screws as they rotate, co-rotating twin-screw extruders can be further classified into engaging co-rotating twin-screw extruders and non-engaging co-rotating twin-screw extruders.

Engaged co-rotating twin-screw extruder
In an Engaged Co-Rotating Twin Screw Extruder, the screws lock or engage with each other as they rotate, thereby increasing the efficiency of material processing and conveying. This design enhances the extruder's distributed mixing capability, which facilitates thorough dispersion of additives and fillers in the polymer matrix.

Intermittent extruders are typically used in compounding applications where precise control of particle size distribution and blend uniformity is critical. They offer excellent scalability and flexibility, allowing manufacturers to customize the extrusion process to specific product requirements.

Non-engaging co-rotating twin-screw extruders
On the other hand, the screws of a non-meshing co-rotating twin-screw extruder do not interlock or engage as they rotate. Instead, they rotate in close proximity to each other without engaging, allowing for a different type of material processing, often for fragile materials or shear-sensitive applications such as processing heat-sensitive polymers, biomaterials and food products. Non-engaging extruders provide gentle processing conditions while still offering efficient mixing and compounding capabilities.

Counter-rotating twin-screw extruders
In contrast to co-rotating extruders, counter-rotating twin-screw extruders have screws that rotate in the opposite direction. This configuration creates a significant kneading and shearing effect on the material, resulting in thorough mixing and dispersion. Counter-rotating extruders excel in applications that require intense shear, such as desolventization, reactive extrusion and blending of highly filled materials. They provide precise control of residence time and shear rate, making them suitable for demanding processing conditions.

Engaging reversing twin-screw extruders
In an engaged reversing twin-screw extruder, the two screws are symmetrically positioned but rotate in opposite directions. This configuration prevents the material from moving in an “∞” shape, as the helical path of one screw is blocked by the other. Instead, during solids conveying, the material is conveyed forward in a nearly closed “C” shaped cavity. However, a gap is maintained between the outside diameter of one screw and the root diameter of the other screw to allow material to pass through.

As the material passes through the radial gap between the two screws, it is subjected to intense shearing, mixing and compaction, resulting in effective plasticization. In addition, the compression ratio can be realized by gradually decreasing the screw pitch, making it suitable for processing a wide range of products.

Non-engaging counter-rotating twin-screw extruder
Less commonly used than the meshing extruder, the non-meshing counter-rotating twin-screw extruder operates differently from the single-screw extruder, but has similarities in that it relies on friction and viscous resistance to convey material. In addition to movement toward the head, the material can exhibit different flow patterns due to the large radial gap between the two screws, which can lead to significant leakage.

In addition, the relative positions of the screw flights can cause the material pressure on the thrust side of one screw to be higher than the resistance side of the other screw, resulting in material flow from the high pressure thrust side to the resistance side of the other screw. Rotation impedes material flow at point A, resulting in flow and a variety of other flow patterns that make it suitable for mixing, venting, and devulcanization applications.

Conclusion
Each type of twin-screw extruder offers unique benefits and capabilities to meet the varying processing needs of a wide range of industries. Understanding the differences between these extruder types is critical to selecting the most appropriate equipment for a particular application.

Whether it's to achieve optimal mixing efficiency, maintain product integrity, or improve processing flexibility, the right choice of twin-screw extruder can have a significant impact on product quality, productivity, and overall manufacturing success.