The most important part of extruder-screw

1. What is screw of extruder?

The screw of extruder is the first factor to influence extruder’s every part, such as capacity, the plasticizing quality, the temperature of material after being melted, and power consumption, etc..

2. The kinds of screw

Generally, the screw of extruder can be divided into two kinds, gradual screw and mutant screw.

2.1 What is gradual screw?

It is mostly used to process non-crystalline plastics. It has little shearing effect on materials and low mixing characteristics. It is suitable for heat-sensitive plastics and some crystalline plastics.

2.2 What is mutant screw?

Its compression section is short. And it has strong cutting effect for materials. It is suitable for plastic material whose melting point is mutant and whose viscosity is low, such as PA, PP, PE. It can not use to process heat sensitivity plastic such as PVC.

3. Three stages of screw and their application

Full thread three-stage screw consists of feeding section, compression section, and homogenization.

3.1 The core of feeding section is conveying capacity. And its shortage is low capacity for solid material.

3.2 Compression section is further used to compress and melt material.

Its shortage is that there are solid bed and molten bath in the screw groove. So its melting efficiency is low. And it also exists high pressure. What’s more, the extruding speed and temperature will also show volatility.

Homogenization section is used to extrude plastic material after being melted into die keeping pressure,quantity and temperature immobile. Its disadvantage is that it still has the function of melting solid materials, and has little effect on the mixing of fillers and coloring materials.

4. What should be considered when design screw?

4.1 We should consider the characters of material. At the same time, we should consider the shape, size, and temperature of raw material before being fed into extruder.

4.2 We should think about the structural shape and drag characteristic of die.

4.3 And the structural style and heating and cooling condition of feeding barrel should be considered.

4.4 The rotate speed of screw we should consider too.

4.5 We also consider what you will do use extruder.

5. The material of screw

The screw must be made of high-quality steel with high temperature resistance, wear resistance, corrosion resistance and high strength, and the selected material should also have the characteristics of good cutting performance, small heat treatment residual stress, and small thermal deformation.

Commonly used materials: 45, 40Cr, nitriding steel 38CrMoAl, and other alloy structural steels, such as 42CrMo, etc. Or add (spraying, surfacing) wear-resistant alloy to the threads of commonly used materials, or use hard chrome plating and other techniques.

6. How to confirm the diameter of screw?

• How to confirm the diameter of screw: generally, we confirm the diameter of screw based on the cross-section size needing to be processed, raw material’s type,and the quantity of raw material.
  • L/D: The length of the screw is increased, and the plasticization quality is high. The extrusion capacity will be improved if the speed of screw can be increased. The applicability of the extruder is strengthened, and the processing range is expanded. However, it is difficult to process and assemble the screw and barrel, and its cost also will increase. If the screw is too long, it is easy to deform and wear, which will affect the service life of the extruder.