Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with nearly 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.
Screw extruder is the core equipment of plastic forming and blending modification. In the actual production process of blending modification, the screw of the extruder is in a harsh high-pressure and high-temperature environment, and bears great friction and shear force.
Due to the special working environment, the screw of extruder is not the common friction between metal and metal, but between metal and polymer. Therefore, the wear of screw surface is often serious.
The wear of the screw increases the distance between the screw and the barrel, affects the compression and shear of the screw to the material, and will lead to the decline of product quality. On the other hand, frequent replacement of worn out screws not only increases the cost, but also delays the production plan, resulting in lower production efficiency.
Generally, the screw is scrapped due to long-term wear, and the gap between the screw and the barrel is too large to be extruded normally. However, there are also cases where the working stress caused by improper design or operation exceeds the strength limit and is damaged. The following is a brief introduction to the wear causes of the extrusion screw:
Each kind of plastic has an ideal plasticizing processing temperature range. The processing temperature of the barrel should be controlled to make it close to this temperature range. When plastic particles enter the barrel from the hopper, they will first reach the feeding section, and dry friction will inevitably occur in the feeding section. When these plastics are not heated enough and melt unevenly, it is easy to cause increased wear on the inner wall of the barrel and the surface of the screw.
Similarly, in the compression section and homogenization section, if the melting state of the plastic is disordered and uneven, it will also cause accelerated wear.
The speed should be adjusted properly. Because some plastics are added with reinforcing agents, such as glass fiber, minerals or other fillers. The friction force of these substances on metal materials is often much greater than that of molten plastics.
If high rotation speed is used, it is easy to tear up the reinforced fiber while increasing the plastic shear force. The torn fiber contains sharp ends, which greatly increases the wear force. When inorganic minerals slide at high speed on the metal surface, their scraping effect is not small. Therefore, the speed should not be adjusted too high.
3、Clearance between screw and barrel
The screw rotates in the barrel, and the friction between the material and the two makes the working surface of the screw and the barrel gradually wear, the screw diameter gradually shrinks, and the inner hole diameter of the barrel gradually increases. In this way, the fit diameter gap between the screw and the barrel increases gradually with the gradual wear of the two.
However, since the resistance of the head and distributor plate in front of the barrel has not changed, this increases the leakage flow of the extruded material when it moves forward, that is, the flow of the material from the diameter gap to the feeding direction increases. As a result, the production of plastic machinery decreased. This phenomenon also increases the residence time of materials in the barrel, resulting in material decomposition. In the case of PVC, the hydrogen chloride gas generated by decomposition strengthens the corrosion of the screw and barrel.