The extruder screw is the core component of plastic extrusion molding equipment, responsible for key tasks such as material transportation, melting, plasticization, and homogenization. The rationality of its design directly affects the extrusion quality, production efficiency, and energy consumption level. This article will analyze the structure, working principle, design optimization, and maintenance of screws.

1、 Basic structure and classification of screws

The extruder screw is usually composed ofFeeding section, compression section (melting section), metering section (homogenization section)Composed of three parts, each section has the following functions:

1. Feed Zone
   • functionTransport solid materials (such as granules or powders) from the hopper to the compression section.
   • Design featuresThe depth of the screw groove is relatively large (usually 15%~20% of the screw diameter) to accommodate more materials; The narrow width of the screw edge reduces the frictional resistance with the material.
   • Typical ApplicationsInitial transportation of thermoplastic materials such as PE and PP.
2. Compression Zone
   • functionBy gradually shallowing the depth of the spiral groove, the material is compressed, sheared, and melted to discharge volatile substances (such as moisture or additives).
   • Design featuresThe depth of the screw groove decreases linearly or nonlinearly, and the compression ratio (the ratio of the screw groove volume in the feeding section to the screw groove volume in the metering section) is usually 2:1~5:1.
   • key parametersThe compression angle (rate of change in groove depth) affects the melting efficiency. If it is too large, it will cause the material to overheat and decompose, while if it is too small, it will result in insufficient plasticization.
3. Metering Zone
   • functionTransport the molten material evenly to the machine head and establish a stable pressure and flow rate.
   • Design featuresThe depth of the screw groove is constant and relatively shallow (usually 5% to 10% of the screw diameter), and the width of the screw edge is increased to improve the shear rate.
   • optimization directionImprove homogenization effect by increasing screw length or using barrier screws.

Screw type：

• Conventional screwSuitable for most general plastic processing (such as PVC, PE).
• Separable screwSet up secondary threads in the measuring section to separate molten and unmelted materials and improve plasticization quality (commonly used in engineering plastics such as PA and PC).
• Exhaust screwInstall an exhaust section after the compression section for processing materials containing volatile substances (such as recycled materials).
• Double stage screwCombining the advantages of single screw and twin-screw, it is suitable for high filling or high viscosity materials (such as wood plastic composites).

2、 Working principle of screw

The extrusion process can be divided into three stages:

1. Solid conveying stage
   • The material is driven by the rotation of the screw in the feeding section and moves forward along the screw groove, forming a "solid plug".
   • The conveying efficiency is affected by the screw speed, groove depth, and friction coefficient between the material and the barrel.
2. Melting stage
   • The material is subjected to dual effects of barrel heating and screw shearing in the compression section, gradually transforming from a solid state to a molten state.
   • The melting rate depends on the geometric parameters of the screw (such as compression ratio) and process conditions (such as temperature and speed).
3. Melt conveying stage
   • The molten material is uniformly transported to the machine head in the measuring section and molded into the desired cross-sectional shape through a mouth mold.
   • The conveying pressure is regulated by the screw speed, head resistance, and back pressure valve.

3、 Key points of screw design

1. Length to diameter ratio (L/D)
   • Definition: The ratio of effective length to diameter of a screw, typically ranging from 20:1 to 36:1.
   • Impact: The larger the aspect ratio, the longer the plasticization time, and the better the homogenization effect, but the energy consumption and cost increase.
   • Selection suggestion:
     • General plastics (such as PE, PP): L/D=20:1-125:1
     • Engineering plastics (such as PA, POM): L/D=25:1~30:1
     • High filling material (such as calcium plastic): L/D ≥ 30:1
2. compression ratio
   • Definition: The ratio of the screw slot volume in the feeding section to the screw slot volume in the measuring section reflects the degree of material compression.
   • Impact: If the compression ratio is too large, it can easily cause the material to overheat and decompose, while if it is too small, it will result in insufficient plasticization.
   • Typical value:
     • Heat sensitive plastics (such as PVC): compression ratio=1.5:1~2:1
     • Crystalline plastics (such as PE, PP): compression ratio=2.5:1~3.5:1
3. screw speed
   • Range: Usually 30-120rpm, high-speed screws can reach over 300rpm.
   • Optimization direction:
     • Increasing the rotational speed can increase production, but it is necessary to simultaneously improve cooling efficiency (such as using air-cooled or water-cooled barrels).
     • Avoid excessive rotation speed that may cause material shear overheating (such as PVC processing where the rotation speed should be less than 60rpm).
4. Material selection
   • Screw body：
     • Conventional material: 38CrMoAlA (hardness ≥ 900HV after nitriding treatment, excellent wear resistance).
     • Material: Bimetallic screw (with 42CrMo substrate and Stellite alloy deposited on the screw edges, with a lifespan increase of 3-5 times).
   • Cylinder liner：
     • Conventional material: 45 # steel (chrome plated, surface roughness Ra ≤ 0.4 μ m).
     • Wear resistant material: Nickel based tungsten carbide coating (thickness 0.2~0.5mm, suitable for processing glass fiber reinforced plastics).

4、 Application scenarios of screws

1. Pipe Extrusion
   • Requirement: High output, low melt temperature fluctuation.
   • Optimization plan: Adopt barrier type screw and pin barrel to improve plasticization uniformity.
2. Film extrusion
   • Requirements: high melt strength, low gel content.
   • Optimization plan: Increase the length of the measuring section (L/D ≥ 30:1) and use it in conjunction with a static mixer.
3. Profile Extrusion
   • Requirement: Low residual stress, high dimensional stability.
   • Optimization plan: Use low compression ratio screws (1.8:1~2.2:1) to reduce shear heat.
4. Recycling material processing
   • Requirement: High wear resistance and strong exhaust capacity.
   • Optimization plan: Double stage screw+exhaust section design, hard chrome plating on the surface of the screw.

5、 Maintenance and optimization of screws

1. routine maintenance
   • Regularly clean the residual materials on the surface of the screw (especially rinse with polyolefin cleaning material before shutdown).
   • Check the gap between the screw and the barrel (the standard gap is 0.1~0.3mm, and it needs to be adjusted or replaced after wear).
   • Monitor the main motor current, abnormal increase may indicate screw wear or foreign objects entering the barrel.
2. fault diagnosis
   • production declineCheck if the feeding section is blocked or if the screw is worn, resulting in a decrease in conveying efficiency.
   • Surface defects of the productIf there are focal spots or bubbles, it may be due to overheating or poor exhaust in the measuring section.
   • Abnormal noiseFriction between the screw and the barrel or damage to the bearings require immediate shutdown for maintenance.
3. Upgrading and transformation direction
   • energy-saving retrofitAdopting variable frequency speed regulation technology, dynamically adjusting the screw speed according to the load.
   • intelligent upgradeInstall temperature and pressure sensors to achieve closed-loop control of the extrusion process.
   • modular designDevelop screw components that can be quickly replaced to meet the diverse production needs.