Twin Screw Geometry
Comprehensive Analysis in Extrusion Machinery Design
The design and classification of twin screw extruders represent a critical aspect of modern extrusion machinery technology. These sophisticated systems play a vital role in various industrial processes, from polymer processing to food production. The geometry of twin screws directly influences their performance, efficiency, and suitability for specific applications in extrusion machinery.
Twin screw extruders are versatile pieces of extrusion machinery that offer significant advantages over single screw designs, particularly in terms of mixing capability, throughput, and process control. The geometric configurations of these screws determine how materials are transported, mixed, and processed within the extruder barrel. Understanding the various classification methods and geometric parameters is essential for optimizing extrusion machinery performance.
Classification Methods of Twin Screw Extruders
Twin screw extruders, a key component in modern extrusion machinery, can be classified through numerous methods. These classification systems help engineers and operators select the appropriate equipment for specific processing requirements. The primary classification methods include:
1. Intermeshing vs. Non-intermeshing
Based on the relative positions and distances between the axes of the two screws, twin screw extruders in extrusion machinery can be categorized as intermeshing or non-intermeshing types, as illustrated in Figure 1-50. Intermeshing types are further divided into fully intermeshing and partially intermeshing configurations.
Figure 1-50: Geometric configuration diagram of relative positions in twin screw extrusion machinery
- Fully intermeshing (closely intermeshing): The center distance between the two screws A = r + R, as shown in Figures 1-50(a) and 1-50(d). This configuration is common in high-performance extrusion machinery.
- Partially intermeshing (incompletely intermeshing): The center distance satisfies 2R > A > r + R, as demonstrated in Figures 1-50(b) and 1-50(e). This design offers a balance between mixing and throughput in extrusion machinery.
- Non-intermeshing (outer diameter contact or tangent type): The center distance between the two screws A > 2R, as depicted in Figures 1-50(c) and 1-50(f). This configuration is often used in specialized extrusion machinery applications.
In these configurations, r represents the root radius of the screw, R denotes the tip radius, and A is the center distance between the two screws. When describing intermeshing twin screws in extrusion machinery, two additional concepts are important: conjugate and non-conjugate designs.
Conjugate refers to a design where the flight of one screw and the channel of the other screw have similar geometric shapes that fit closely together with only minimal (manufacturing and assembly) clearance. This precision is crucial for high-performance extrusion machinery. In contrast, non-conjugate designs feature screws where the flight of one screw fits loosely into the channel of the other, with significant intentional clearance around the perimeter.
It's important to distinguish between conjugation and intermeshing in extrusion machinery. Partial intermeshing can be considered non-conjugate, while full intermeshing does not necessarily mean conjugate. This distinction significantly impacts the performance characteristics of the extrusion machinery.
2. Open vs. Closed Types
The terms open and closed refer to whether there are possible channels (excluding manufacturing and assembly clearances) for material flow along or across the screw channels in the intermeshing zone of extrusion machinery. This classification can be further divided into longitudinal open/closed and transverse open/closed configurations, as shown in Figure 1-51.
Figure 1-51: Schematic diagram of closed forms in twin screw extrusion machinery
Longitudinal open type: If there is a conveying channel from the feeding zone to the end of the screws, allowing material to flow from one screw to the other (flow along the channels), it is referred to as longitudinally open, as shown in Figure 1-51(a). This design enhances mixing capabilities in extrusion machinery.
Longitudinal closed type: In contrast, longitudinally closed types feature two screws that form several mutually isolated chambers with no material exchange between them, as illustrated in Figure 1-51(b). This configuration provides better process control in extrusion machinery.
Transverse open type: In the intermeshing zone of the two screws, if there is a channel across the flights (i.e., material can flow from one channel to an adjacent channel on the same screw, or from a channel on one screw to adjacent channels on the other screw), it is called transversely open, as shown in Figure 1-51(a). This design promotes superior mixing in extrusion machinery.
Transverse closed type: Conversely, transversely closed types prevent such cross-flow, as depicted in Figure 1-51(b). This design is beneficial for applications requiring minimal back-mixing in extrusion machinery.
3. Relative Rotation Directions of the Two Screws
Twin screw extruders in extrusion machinery are categorized based on the direction of screw rotation into two main types: co-rotating and counter-rotating. Counter-rotating twin screw extruders can be further divided into inward-rotating and outward-rotating configurations, as shown in Figure 1-52. Among counter-rotating twin screws, the configuration shown in Figure 1-52(c) is most commonly used in industrial extrusion machinery.
Co-rotating
Figure 1-52(a)
Inward Counter-rotating
Figure 1-52(b)
Outward Counter-rotating
Figure 1-52(c)
Figure 1-52: Schematic diagram of twin screw rotation directions in extrusion machinery
The rotation direction significantly affects material processing in extrusion machinery. Co-rotating screws tend to provide better mixing due to the complex flow patterns created as materials are transported between the screws. This makes them particularly suitable for compounding applications in extrusion machinery where homogeneous mixing is critical.
Counter-rotating screws, on the other hand, typically generate higher pressure and are often preferred for extrusion processes requiring precise dimensional control, such as pipe and profile extrusion. The outward counter-rotating configuration is especially prevalent in extrusion machinery for its balance of pressure generation and material handling capabilities.
4. Cylindrical vs. Conical Twin Screws
When the axes of the two screws are parallel, they are referred to as parallel twin screws, also known as cylindrical twin screws [Figure 1-53(a)]. When the axes of the two screws intersect, they are called conical twin screws [Figure 1-53(b)]. This geometric distinction has significant implications for the performance characteristics of extrusion machinery.
Figure 1-53(a): Cylindrical twin screws in extrusion machinery
Figure 1-53(b): Counter-rotating conical twin screws in extrusion machinery
Cylindrical twin screws are the most common configuration in extrusion machinery due to their simpler manufacturing process and consistent processing characteristics along the entire length. They maintain a constant diameter throughout their length, allowing for uniform processing conditions which is advantageous in many extrusion applications.
Conical twin screws, with their tapered design and intersecting axes, offer unique advantages in specific extrusion machinery applications. The converging geometry naturally creates increasing pressure along the screw length, which is beneficial for certain processing requirements such as material compaction. Conical designs are often found in extrusion machinery used for PVC processing and other applications requiring high compression ratios.
1.4.1.1 Counter-rotating Twin Screw Geometry
The motion relationship between the two screws of counter-rotating twin screw extruders resembles that of a pair of meshing gears rolling past each other. Under the conditions of maintaining constant screw transmission and mutual self-cleaning, there are countless possible thread side curves for counter-rotating intermeshing twin screws. This gives counter-rotating intermeshing twin screws greater design flexibility in extrusion machinery.
From the perspective of meshing principles, regardless of the shape of their end face threads, they can mesh without interference. Additionally, unlike co-rotating intermeshing twin screws, counter-rotating designs are not restricted by center distance in extrusion machinery. This means that with a fixed center distance, single-start, double-start, or triple-start threads can be selected, and the channel depth can be relatively large, typically 15% to 21% of the screw outer diameter. Consequently, the corresponding conveying capacity in extrusion machinery is high.
Counter-rotating twin screws have opposite thread directions, with their flights embedding into each other parallel in the meshing zone. Their structure is shown in Figure 1-54, a critical reference for understanding counter-rotating extrusion machinery design.
Counter-rotating Twin Screw Geometric Structure
(a) 3D view of counter-rotating intermeshing twin screw structure in extrusion machinery
(b) Cross-sectional view of lateral clearance in the meshing zone
(c) Close-up view of radial clearance in the meshing zone
(d) Cross-sectional view of tetrahedral clearance in the meshing zone
Figure 1-54: Counter-rotating twin screw geometry in extrusion machinery
As shown in Figure 1-54, in addition to the clearance δ₁ between the inner wall of the barrel and the top of the screw flights, counter-rotating intermeshing twin screws form three specific clearances when meshed together. These clearances are critical in determining melt leakage flows during processing in extrusion machinery and thus significantly impact performance:
- Radial clearance (δᵣ): This clearance exists between the top of one screw's thread and the bottom of the other screw's channel. In extrusion machinery, radial clearance affects the backflow of material and thus influences pressure generation and mixing efficiency. Proper control of radial clearance is essential for optimizing the performance of extrusion machinery.
- Lateral clearance (δₗ): This refers to the clearance between the side of a flight on one screw and the corresponding side of a flight on the other screw within the meshing zone. Lateral clearance in extrusion machinery impacts the shear forces applied to the material and the degree of mixing achieved. It also affects the self-cleaning capability of the extrusion machinery, with smaller clearances generally providing better cleaning.
- Tetrahedral clearance (δₜ): This unique geometric clearance forms a tetrahedral shape in the meshing zone where the flights of the two screws intersect. The tetrahedral clearance plays a crucial role in material mixing and pressure development in extrusion machinery. It creates complex flow patterns that enhance distributive and dispersive mixing, making it a key design consideration in high-performance extrusion machinery.
The precise control of these clearances represents a key aspect of extrusion machinery design and manufacturing. These dimensions must be carefully calculated and maintained to ensure optimal performance, longevity, and processing efficiency in extrusion machinery. Too large a clearance can result in excessive leakage and reduced process efficiency, while clearance that is too small may lead to increased wear, higher energy consumption, and potential mechanical failure in extrusion machinery.
In modern extrusion machinery, computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies have greatly improved the precision with which these critical clearances can be achieved. Advanced simulation tools allow engineers to analyze the impact of different clearance configurations on material flow and mixing behavior before physical prototypes are built, significantly reducing development time and costs for new extrusion machinery designs.
The geometric characteristics of counter-rotating twin screws make them particularly suitable for certain applications in extrusion machinery, including profile extrusion, pipe extrusion, and certain compounding operations. Their ability to generate high pressures while maintaining good mixing capabilities makes them a versatile choice in the field of extrusion machinery, where process flexibility and product quality are paramount concerns.
Summary of Twin Screw Geometry in Extrusion Machinery
The geometry of twin screws represents a fundamental aspect of extrusion machinery design that directly impacts processing capabilities, material handling, and end-product quality. The various classification systems—based on intermeshing characteristics, openness/closedness, rotation directions, and overall screw shape—provide a framework for understanding the capabilities and limitations of different extrusion machinery configurations.
Counter-rotating twin screw designs, with their unique geometric characteristics and clearance configurations, offer specific advantages in certain extrusion applications. The careful engineering of these geometric parameters ensures that modern extrusion machinery can meet the diverse and demanding requirements of today's manufacturing industries.