Single Screw Geometry in Plastic Extrusion
The design and geometry of a single screw in an extruder plastic machine play a critical role in determining the efficiency, output quality, and processing capabilities of the extrusion system. This comprehensive guide explores the fundamental aspects of single screw geometry and its impact on extruder plastic performance.
General Screw Geometry
The conventional geometry of a single screw, as illustrated in Figure 1-2, is divided into three distinct zones based on the movement of material in the screw channel and the changes in its physical state. These zones work sequentially to transform raw plastic material into a homogeneous melt ready for extrusion.
When material enters the extruder plastic, it contains voids and air pockets. As it progresses through the screw, it undergoes a transformation from a solid to a viscous flow state. This transformation requires a gradual reduction in the screw channel volume from the solid conveying zone to the metering zone, known as the compression ratio – a critical parameter in single screw design for an extruder plastic system.
Figure 1-2: Single Screw Geometry Diagram
Three-Zone Division of Single Screw
Feed Zone (Solid Conveying Zone)
First section of the extruder plastic screw
Function:
Material introduced through the hopper is conveyed forward in this zone and begins to be compacted. Gases within the material are expelled back through the hopper, a critical function in any extruder plastic system.
Location:
Extends from the hopper to the start of the compression zone.
Characteristics:
- • Constant channel depth (h₁)
- • Designed for efficient solids conveying
- • Critical for consistent feeding in extruder plastic
- • Must handle material with varying bulk densities
Key Consideration:
Proper design prevents bridging of material in the hopper and ensures consistent feeding into the extruder plastic system.
Compression Zone (Melting Zone)
Middle section of the extruder plastic screw
Function:
Material continues to be compacted and begins its transformation to a molten state. This zone achieves melting and basic distributive mixing, a critical transition in the extruder plastic process.
Location:
Middle portion of the screw, between the solid conveying zone and metering zone.
Characteristics:
- • Gradually decreasing channel depth
- • Primary melting occurs in this zone
- • Generates significant pressure in extruder plastic
- • Critical for proper fusion of polymer particles
Key Consideration:
The rate of channel depth reduction directly affects melting efficiency in the extruder plastic process.
Metering Zone
Final section of the extruder plastic screw
Function:
Material exists in a viscous flow state here, completing both distributive and dispersive mixing. This zone provides consistent pressure, volume, and temperature control for extrusion through the die, the final critical stage in the extruder plastic process.
Location:
Section of the screw closest to the die end.
Characteristics:
- • Constant shallow channel depth (h₂)
- • Material is fully molten
- • Provides pressure build-up for extrusion
- • Ensures uniform melt temperature
Key Consideration:
Channel depth and length directly influence pressure stability and output rate in the extruder plastic system.
Comparison of Screw Zones in Extruder Plastic Systems
| Characteristic | Feed Zone | Compression Zone | Metering Zone |
|---|---|---|---|
| Material State | Solid pellets/powder | Transitioning from solid to melt | Fully molten |
| Channel Depth | Deep (constant) | Gradually decreasing | Shallow (constant) |
| Primary Function | Conveying and initial compaction | Melting and compression | Mixing and pressure generation |
| Temperature Profile | Moderate heating | Significant heating | Controlled, uniform temperature |
| Pressure Development | Low pressure | Increasing pressure | High, stable pressure |
| Typical Length Ratio | 20-30% of total length | 40-50% of total length | 20-30% of total length |
Table 1-3: Comparison of Single Screw Zones in Extruder Plastic Systems
Compression Ratio in Extruder Plastic Design
The compression ratio is a fundamental design parameter in extruder plastic technology, representing the volumetric reduction of material as it progresses from the feed zone to the metering zone. This reduction is necessary to remove entrapped air, compact the material, and facilitate melting.
The geometric compression ratio (ε) is typically defined as the ratio of the volume of the first flight in the feed zone to the volume of the last flight in the metering zone. For design purposes and to simplify calculations, the influence of the flight itself is often neglected.
In extruder plastic design, the compression ratio can be approximated using the following formula, which relates the screw diameter (D) and channel depths in the feed (h₁) and metering (h₂) zones:
Typical compression ratios in extruder plastic applications range from 2:1 to 4:1, depending on the material being processed. Powdery materials or those with low bulk density generally require higher compression ratios, while granular materials can often work with lower ratios.
The compression ratio directly affects the amount of work done on the material, the pressure developed, and the quality of melting. An improperly chosen compression ratio in an extruder plastic system can result in poor mixing, incomplete melting, or excessive shear that degrades the polymer.
Figure 1-3: Screw Channel Volume for One Pitch Length
Typical Compression Ratios for Extruder Plastic Applications
Screw Helix Parameters in Extruder Plastic Design
The helical configuration of an extruder plastic screw is primarily defined by two key parameters: the pitch (S) and the helix angle (φ). These parameters significantly influence the conveying efficiency, mixing characteristics, and pressure generation capabilities of the extruder plastic system.
The pitch is the axial distance advanced in one complete rotation of the screw. For manufacturing simplicity and effective material conveying in extruder plastic applications, a constant pitch is typically used, often equal to the screw diameter (D).
The helix angle varies at different diameters within the same screw. When specifying the helix angle for an extruder plastic screw, it is standard to reference the angle measured at the outer diameter of the screw. This angle can be calculated using the formula:
Where:
- φ is the helix angle at the outer diameter
- S is the screw pitch
- D is the screw outer diameter
For a standard extruder plastic screw with a pitch equal to the diameter (S = D), the helix angle calculates to approximately 17.7 degrees. This angle represents a balance between conveying efficiency and pressure development capabilities in most extruder plastic applications.
Helix angles typically range from 10° to 30° in extruder plastic design. Smaller angles generally provide better pressure development but reduced conveying capacity, while larger angles increase throughput but may compromise pressure stability.
Helix Formation in Extruder Plastic Screws
Figure 1-4: Screw Helix Formation Diagram
Key Helix Relationships:
- • Helix angle increases with larger pitch
- • Helix angle decreases with larger diameter
- • Constant pitch = angle decreases toward core
- • Standard extruder plastic screw: S ≈ D, φ ≈ 17.7°
Single Screw Extruder Plastic Machine Parameters
The design and performance characteristics of an extruder plastic machine are heavily influenced by its dimensional parameters. Standard specifications, such as those outlined in JB/T 8061-1996, provide guidelines for screw dimensions and performance characteristics across different machine sizes.
| Screw Diameter (D) [mm] | Length/Diameter Ratio (L/D) | Max Screw Speed [r/min] | Max Output [kg/h] | Motor Power [kW] | Heating Power [kW] |
|---|---|---|---|---|---|
| 30 | 28~30 | 170 | 18.4 | 7.5 | 6 |
| 65 | 28~30 | 150 | 46 | 18.5 | 10 |
| 90 | 28~30 | 125 | 100 | 40 | 18 |
| 120 | 28~30 | 108 | 154 | 60 | 30 |
| 150 | 20~25 | 74 | 200 | 75-160 | 40-80 |
Table 1-4: Basic Parameters of Single Screw Extruder Plastic Machines (JB/T 8061-1996)
Length/Diameter Ratio (L/D) Significance
The L/D ratio is one of the most critical parameters in extruder plastic design, representing the total screw length divided by its diameter. This ratio directly impacts:
- Material residence time in the extruder plastic
- Melting efficiency and uniformity
- Mixing capability and homogenization
- Pressure development potential
- Ability to process difficult materials
Higher L/D ratios (28-30:1) are typically used for materials requiring extensive melting and mixing, while lower ratios (20-25:1) suffice for simpler extruder plastic applications.
Screw Speed Considerations
Screw speed in an extruder plastic machine directly influences throughput, shear rates, and processing temperatures:
- Higher speeds increase throughput but raise shear rates
- Lower speeds provide better mixing but reduce output
- Optimal speed varies by material type and viscosity
- Smaller extruder plastic machines typically operate at higher speeds
- Speed must be balanced with motor power and heating capacity
Modern extruder plastic machines often feature variable speed drives to optimize processing conditions for different materials and applications.
Summary of Single Screw Geometry
The geometry of a single screw in an extruder plastic machine is a sophisticated balance of design parameters that work together to transform solid plastic material into a homogeneous melt suitable for extrusion. The three-zone design (feed, compression, and metering) addresses the different processing requirements as material progresses through the extruder plastic system.
Key geometric parameters—including channel depth, compression ratio, pitch, and helix angle—determine the extruder plastic's performance characteristics, including throughput, melting efficiency, mixing capability, and pressure generation. These parameters must be carefully selected based on the specific material properties and processing requirements of the extruder plastic application.
Understanding the relationship between screw geometry and processing behavior is essential for optimizing extruder plastic performance, product quality, and process efficiency. Proper screw design ensures that materials are conveyed, melted, mixed, and pumped efficiently while maintaining the desired temperature profile and avoiding material degradation.