Extruder is the main equipment for mixing and plasticizing plastics. According to the experience of the engineers in Guangdong Blesson Precision Machinery Co., LTD, the screw is subjected to strong friction and shear under harsh conditions such as high pressure and high temperature in the actual production process.
Due to its special working conditions, the screw of extruder will not only bear the ordinary metal-to-metal friction, but also metal-to-polymer friction, which might easily cause severe wear on the screw surface.
It is known to all that the wear of the screw will gradually increase the distance between the screw and the barrel, which affects the compression and shearing of the material by the screw, resulting in poor product quality. On the other hand, frequent replacement of worn and defective screws not only increases costs, but also delays production schedules, resulting in reduced production efficiency.
No matter it is for single screw extruder or for twin screw extruder, the screw has to be replaced due to prolonged wear after long period of running, which makes the gap between the screw and the barrel too large and cannot cook the plastic material normally.
Here is a summary of reasons about the wear of extruder screw presented by Mr Lin, the chief engineer of Guangdong Blesson Precision Machinery Co., LTD.
Each kind of plastic material has a processing temperature range that is ideal for plasticizing, and the operator must control the barrel temperature to approximate this temperature range. When the plastic material enter the barrel from the hopper, they firstly reach the feeding section. Dry friction is unavoidable in the feed section. When the plastic material is not heated enough and melt unevenly, it might easily result in increase of wear between the inner wall of the barrel and screw surface.
Similarly, if the melting state of the plastic in the compression and homogenization section is not ideal, it will also make the wear of the screw worse.
The speed must be set correctly. Due to the addition of substances such as fiberglass, additives, or other fillers to some plastic materials, the frictional force on metallic materials is usually greater than on molten plastics materials.
In the case of increasing the screw speed, the shear force of the screw on the plastic will be higher, but it might easily tear the reinforced fiber, which results in sharp ends of the fiber and, hence increase the friction. When inorganic mineral additives slide over a metal surface at high speed, the scraping effect can not be ignored. Therefore, the speed should not be too high.
The screw rotates in the barrel, the friction between the material and the two causes gradual wear of the working surface of the screw and the barrel, the diameter of the screw gradually decreases, and the diameter of the inner hole of the barrel decreases. gradually increased. Thus, the corresponding diameter difference between the screw and the barrel increases progressively as they wear.
Since the resistance pressure of the extrusion die against the barrel has not changed, the leakage rate of extruded material during feed will increase as well as the flow of material from the diametral gap in the feed direction, which ultimately leads to a decrease of output of the extruder. This phenomenon increases the residence time of the material in the barrel, which leads to the decomposition of the material. In the case of PVC extruder, the hydrogen chloride gas generated during decomposition can also increase screw and barrel corrosion.
Because the material is plasticized unevenly or metallic foreign materials are mixed in the material, the torque of the screw suddenly increases and the torque exceeds the tensile strength of the screw, causing it to break. This is non-standard accidental damage which we should try to avoid.
Above all, these are the main reasons for screw wear happened on extruders. Reducing screw wear is beneficial to prolong the service life of screw, thus reducing the maintenance cost of equipment, ensuring the quality of products, and bringing higher economic benefits to factories who produce extrusion products like PVC pipes, HDPE pipes, PVC profiles and panels, etc. As a national high-tech enterprise, Blesson adopts high engineering standards and the highest quality machine components, to offer the reliable and high performance machines.