Analysis of Abrasion Mechanism for an Extruder Cylinder & Screw

1. Generally say, how many abrasion categories of mechanical components could be divided into? 

Generally say, abrasion of mechanical components could be mainly divided into four kinds in theory: Adhesive wear, Abrasion wear, Fatigue wear and Corrosion wear, as below:

Adhesive wear was the most common wear condition we have seen. This type of wear known as two different kind of alloy steel interactive surfaces touching or friction against each other. The two alloy surfaces sliding against each other and the contact under high surface pressure until the micro-geometric layers tear off. Therefore, the adhesive wear also could be named as a“Contact wear”, or a“Matching wear”.

Abrasion wear is the most important wear where individual with harder surface roughness peaks and drastically deform the softer substance under the action of high shear forces, therefore shearing off the softer surface roughness peaks. Normally the loose wear particles can name as microscopic “metal chips” on the two alloy touching surfaces layer. 

Fatigue wear causes by two different kind of alloy steel interactive surfaces or friction against each other. Both alloy surfaces under shear forces condition causes surface particles deterioration and dislodgement of microscopic until the surface particles break off. 

Corrosive wear is the mechanism components surrounding with chemical reactions or Tribochemical reactions. When the alloy surface contact with chemical reactions and friction against each other, the metal surfaces under the shear and chemical attacked condition will gradually wear off.

2. What kind of  abrasion categories the cylinder/screw has to bear when they in working state? 

Simply said, the polymer extruder cylinder in a working state with a screw, almost no one of the abrasion categories above can be avoid, the four types of wear is not imposed on cylinder and screw in a single form, but in form of "composite" and "Overlay" role to the cylinder & screw. To study the different, it is only based on the different extruder working condition, resulting in some differences in the performance of the above four kinds of wear.

Under a normal working condition, screw flights put a polymer forward and meanwhile, it squeezes and shears the polymer material, so that the polymer gets important heat, but meanwhile the screw is kept wearing off under the “shearing wear off”. Specially when some reinforcing agent or some fillers are added inside the polymer, such as the glass fiber, carbon fiber, boron fiber of reinforcing agent, the silicate, talc, calcium carbonate, metal composite with iron oxide or ferrites of filler, etc, the wear condition will be more serious result in worn off fast. This is directly related to the main and the first wear process characteristic between the cylinder and the  screw.

Because of the extruder screw is installed in a cylinder as a cantilever, and is rotated inside the cylinder, both interactive surfaces touching or friction each other. It is unavoidable that there is a screw flight surface friction against the cylinder inner surface, this is a “ Adhesive wear”, or could  be named as the “contact wear”. The longer the screw L/D ratio, and the faster of the screw rotating speed, the serious the above wear. In normal operation, the polymer is coated with the screw and plays the role of "lubricant"  and the reduction of grinding. This is directly related to the secondary wear process characteristic  by the screw under a specific working condition.

Polymer material get quantity of heat from the cylinder and from the screw shearing until it reaches the plasticizing and melting level. Some of high polymer material  will release certain amount of corrosive gases, such as in material of CPVC, CPE, PTFE and PAA process, certain degree of acid oxide will give off. When a cylinder & screw under this working condition, not only we have discussed on the previously wear process above, but also we have endure a corrosive wear because of the environment with chemical reaction effect. This is directly related to the thirdly wear process characteristic  by the cylinder and the screw under a specific extruder working condition. 

Because of the screw rotate inside the cylinder, if both hardness was absolutely too low will result in both screw and cylinder worn off badly. Alternatively, If the screw and cylinder have too far the hardness in-between differences, then either one will worn off badly. Then we have mentioned previously abrasive wear and material fatigue wear was combined above adhesive wear in “multilayer positioning”. In more detail practice, we have realized profoundly that the extrusion wear process, in actual fact, is combined from beginning to end, they replenish and strengthen against each other compatibility, and not individual alone. 

These kinds of wear off act on both cylinder and screw, when the material of these extruder heart parts are normal, the surface will wear off in particles form or in very small pieces form being pull off. Whereas, if act on bimetallic screw, which there is an alloy layer coating on the flight, the first manifestation is that the original microcrack on the alloy layer will expanded and seen clearly until its tear or pull off in a particles or metal chips form under a high shear stress.(More about the screw special wear discussed in another paper) 

Chief Engineer Office / Shanghai Omega Machinery Co., Ltd