Bimetallic Screw and Barrel
The most significant problem in connection with the screw extruders is the rapid wear of both screw and barrel, especially in the case of WPC and formulations with high percentage of calcium fillers. The wear out of the screw barrel will lead to many production problems and increase the maintenance cost.
In order to extend the service time of screw barrels, we are continuously researching solutions and have done so for many decades.
Increasing demands of screw extruders
Recently, the demands of extrusions are becoming more and more various and strict. The extrusion end users tend to use better and high performance polymers with various additives to produce products with better transparency or gloss. Meanwhile, they require the extrusion machine featured with high output and low energy consumption and low process waste. Sometimes recycled polymers are used in many applications. All these demands of end products and machines performance lead to higher abrasion and corrosion of the screw barrel.
Signs and bad results of screw wearing
When screw barrels wear, the gap between them widens, which increases the back flow of melt polymer inside the barrel. The melting temperature and polymer residence time will rise. Also, the productivity and plasticizing capacity will become worse. How we can know that the screw barrels have been worn. When you find that you need to increase the speed of the screw to achieve same amount of output. Degraded particles and some un-melted particles appear. You will find polymer has come out from the venting hole.
Three types of wear are observed with screw and barrel
There are three types of screw and barrel wear; they are adhesive wear, abrasive wear and corrosive wear. Adhesive wear is the wear which occurs due to metal to metal contact of screw and barrel while the screw is rotating. The reasons can be because of the buckling of the screw due to back pressure, or the straightness and concentricity of the screw and barrel. As for abrasive wear, some additives are very aggressive to metal wear, such as CaCo3 and glass fibers. Corrosive wear occurs when screw and barrel come in contact with polymers or addictive which are corrosive in nature.
Solutions of screw and barrel wear
A. | Choose proper steel for screw barrel |
Before manufacturing, we quench the steel to the hardness of HRC28. The hardness of a finished nitride screw barrel can be HV900~HV1000 and the depth is 0.5mm-0.7mm. This is easy and the most economical applications. We have strict quality control on the nitride process to ensure the hardness and depth is correct. | |
B. | Deep nitride treatment |
Before manufacturing, we quench the steel to the hardness of HRC28. The hardness of a finished nitride screw barrel can be HV900~HV1000 and the depth is 0.5mm-0.7mm. This is easy and the most economical applications. We have strict quality control on the nitride process to ensure the hardness and depth is correct. | |
C. | Bimetallic screw barrel |
However, the service life of a nitride screw barrel can’t meet many applications. A bimetallic screw and barrel is the best option. It has 3 to 7 times higher abrasion resistance compared to a nitride screw barrel depending on the grade of bimetallic parts. A bimetallic screw has two metals with anti- abrasive and anti- corrosion hard facing layer. The thickness of the hard facing layer can be up to 1.5mm. |
By clicking here you can get to know more about detailed solutions of bimetallic screw and barrel.
Twin screw barrel