Views: 0 Author: Site Editor Publish Time: 2022-04-26 Origin: Site
In all kinds of plastic industry production process, the screw of extruder are in high pressure high temperature environment, and under a lot of friction force and the shear stress, the surface of the screw is seriously abrasion.
The increase space between screw and screw barrel because of the abrasion. On the one hand, it affects the screw compression and shear of material which led to the decrease of the quality of the product; on the other hand, frequent replacement of wear and failure screw not only increases the cost, but also delays the production plan, resulting in reduced production efficiency.
Therefore, reducing screw abrasion and extending screw life will greatly reduce the maintenance cost of equipment, ensure the quality of products, and bring higher economic benefits to enterprises.
There are four common types of wear:
Wear is the process of surface material damage or fall off in the relative movement of objects in contact with each other. It is the inevitable result of friction. Wear phenomenon in the process of mechanical equipment is very common, generally divided into four common basic types: abrasive wear, adhesive wear, fatigue wear and corrosion wear. The analysis of wear phenomenon is very complicated. The following will be combined with the specific working environment of the screw extruder to talk about the various wear of the screw.
Two friction surfaces rolling or sliding each other, under the cyclic change of contact stress, lead to material fatigue and surface cracks and deformation, and finally spalling to form pits, which is known as fatigue wear.
Screw extruder extrusion techniques applied in the process of extrusion, and its material internal pressure will gradually increase, the internal pressure reached top at the when near the discharging mouth, the friction between the screw surface and material at this time also is very big. Because the strength and hardness of the feedstock in clean condition is far less than the screw material, it will not produce direct damage to the screw. When the shear force on the surface of the screw repeatedly exceeds the fatigue strength of the material, it will cause fatigue on the surface of the screw and crack initiation, the cracks then spread gradually along the sliding direction inside the surface. When the crack was extended to the surface, the root of the crack will break again under stress beyond the strength of the material, eventually, the surface is stripped and pits are formed.
In the process of friction, the surface damage caused by chemical or electrochemical reaction between metal and the surrounding medium is called corrosion wear.
Corrosion wear phenomenon in the processing of plastic screw extruder is particularly prominent, the following will be from chemical corrosion wear, electrochemical corrosion wear two aspects to analyze the corrosion wear of screw.
Chemical corrosion wear is one of the important causes of screw wear in the screw extruder for processing wood plastic composite materials. Chemical corrosion and wear process of screw is forward loop, corrosion compounds are formed on the surface of screw, and then be wear off soon in the process of extrusion friction materials, the new organization of exposed screw materials, and exposed in the new organization will be corrosion. This cycle, screw wear is gradually serious. It can be seen that chemical corrosion wear occurs when friction and chemical corrosion exist simultaneously, and friction will accelerate the progress of chemical corrosion.
In the process of producing wood plastic composite materials, electrochemical corrosion will occur when the screw contacts with acid compound in the material. Screw material of iron and other metal elements constitute a galvanic anode, dissolved ions form and release the electrons, corrosive liquid as the transmission medium will electronic migrated to predominantly cementite cathode, visible electrochemical corrosion equivalent materials produced electric potential difference two area, form a short circuit batteries, and anode metal loss.
When the surfaces of two objects slide relative to each other, the microconvex body of the contact interface will shear, and the material to be cut will migrate from one surface to another or fall off into free debris, which is collectively referred to as adhesive wear.
When the screw is fed unevenly, the material will be filled in some positions of the screw and vacant in other positions. Because the solid material is not easy to slide on the wall of the machine, the filled area produces greater pressure to the screw, while the vacant area is not, which causes the radial direction of the screw to produce unbalanced force. Due to the limited stiffness of the screw, the screw may be deflated after being subjected to a large local force, resulting in direct contact friction between the screw and the barrel. The friction surface produces adhesive wear under the action of high temperature and high shear force.
Hard rough surface or hard particles sliding on the soft surface of the plastic cause the surface damage like deformation or fracture, which is known as abrasive wear.
With the continuous progress and innovation of the plastic industry, the new plastic has been endowed with more functions and changes. In the manufacturing process of some high-strength plastic products, various fiber auxiliary materials are often added into the plastic raw materials to increase the strength of the product. On the other hand, in order to control the cost of plastic products and improve some properties, some high hardness fillers are often added, such as calcium carbonate, talcum powder, barite powder, wollastonite powder, clay, mica and metal powder. In the working environment of screw extruder with high pressure, the abrasive with high hardness is pressed into the friction surface by normal load. After relative sliding, the abrasive will cause cutting and plough wrinkle on the surface, resulting in furrow effect and abrasive wear.
So how to reduce screw wear, prolong screw life, in order to reduce production costs, to ensure the quality of the results?
I. The natural life of the extruder is long, and its service life mainly depends on the wear of the machine and the wear of the reducer. The selection of good design and material extruder and reducer is directly related to the performance. Although the equipment investment increases, but the service life is prolonged, from the overall economic benefit, more reasonable.
II. The normal use of screw extruder can give full play to the efficiency of the machine and maintain a good working state. It must be unremittingly careful maintenance, prolong the service life of the machine.
III.The main faults of screw extruder are abnormal wear, foreign body stuck, link blocking, transmission parts wear or damage, poor lubrication or oil leakage. In order to avoid the occurrence of faults, it is necessary to strictly manage the drying, mixing and feeding operations and the setting of process temperature, and carry out daily maintenance.
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