The Extruder Screw: The Core Component of Plastic Extrusion

Screw Body

Screw Shaft

• Function: The screw shaft is the foundational part of the extruder screw. It's a metal rod with a specific diameter and length that supports and holds other components. It determines the overall length and diameter of the screw, which in turn affects the amount of plastic processed and the extruder's production capacity.
• Design Considerations: The diameter and length of the screw shaft are tailored to meet the specific requirements of the extrusion process, ensuring optimal performance and efficiency.

Threads

• Function: The threads are machined onto the screw shaft and are divided into different sections based on their function and position. These include the feed section threads, compression section threads, and metering section threads.
  • Feed Section Threads: These threads transport plastic granules from the hopper to the screw.
  • Compression Section Threads: These threads gradually compress and compact the plastic, initiating the plasticization process.
  • Metering Section Threads: These threads further mix and homogenize the plastic melt, ensuring uniform quality.
• Design Parameters: The pitch, depth, and helix angle of the threads are designed based on the specific plastic material and processing requirements to optimize performance.

Screw Head

Screw Tip

• Function: Located at the front end of the screw, the screw tip significantly influences the flow of the plastic melt and the extrusion outcome. Common shapes include conical and rounded tips, each suited to different plastics and extrusion processes.
  • Conical Tip: This shape is ideal for concentrating the flow of the plastic melt, making it suitable for extruding thin-walled products.
  • Rounded Tip: This shape ensures smoother flow of the plastic melt, perfect for applications where the appearance of the final product is critical.

Check Ring

• Function: Installed at the head of the screw, the check ring prevents the backflow of the plastic melt during injection or extrusion. When the screw moves forward, the check ring opens, allowing the plastic melt to pass through. When the screw retracts, the check ring closes, preventing backflow and ensuring stable extrusion and precise metering of the plastic melt.

Connecting Components

Keys

• Function: Keys connect the screw to the drive shaft, transmitting torque to ensure the screw rotates synchronously with the drive shaft. Common types include flat keys and semicircular keys, selected based on the torque transmission requirements of the screw.

Nuts

• Function: Nuts secure the screw and other components, preventing them from loosening during operation. The tightening torque of the nuts must be carefully controlled to ensure a reliable connection.

Other Auxiliary Components

Heating Bands

• Function: Typically installed on the barrel outside the screw, heating bands heat the plastic to its plasticization temperature. The power and distribution of the heating bands are designed based on the length of the screw and the processing requirements of the plastic to ensure even heating.

Thermocouples

• Function: Thermocouples measure the temperature inside the screw and barrel, feeding temperature signals back to the control system. This allows for real-time adjustment of the heating bands' power to achieve precise temperature control.

Lubricant Nozzles

• Function: Some screw designs include lubricant nozzles that inject lubricating oil into the bearings and other moving parts of the screw. This reduces friction and wear, enhancing the screw's lifespan and operational stability.