Larger Length-Diameter Ratio Barrel & Screw Wear Characteristic

Because the extruder function is completely different from the injection machine, the extruder barrel & screw length is generally longer the injection machine. This working length is often compared with the diameter, that is, the length to diameter ratio (L/D) as the working length of the value. Injection machine length/diameter ratio L/D generally in the 17~23 range, ordinary single screw extruder length/diameter ratio L/D generally in more than 25. For some special purposes, such as reactive extrusion of a single screw extruder, its extreme length/diameter ratio L/D can even reach 55, and ordinary parallel twin screw extruder used for polymer materials mixing and modifying , the length/diameter ratio L/D range is generally in the range of 32~48.

Machine barrel & screw in a working state, the main wear is the two categories, one is the relative motion between the screw and the material, a “Shear Wear” caused by the screw shearing. Another is when a screw rotates and pushes the material forward, the metal which on top of screw thread touch the metal of the barrel inner wall because of their mutual movement, that is the "Contact Wear", or can be called as the "Matching Wear", or "Adhesive Wear”.

Due to an extruder screw is installed inside the barrel as a cantilever, so in the following two cases, the above two types of wear will be even worse, that is the length/diameter ratio L/D is larger, and the screw speed is higher. The screw with larger length/diameter ratio is in the working state, there inevitably is contact wear between the outer diameter surface on the screw end section and the barrel inner wall surface strongly, this is one. Once a screw with the large length/diameter ratio is subjected to the high back pressure, because the material strength is limited, the strong torque force will lead to the distortion of the screw, the screw is actually in a "non-straight line" working state, Lead to the diameter surface of the screw middle part, "scraping" the barrel inner wall surface one side, aggravate the contact wear between these two kinds of metal, this is two. The higher the screw rotating speed is, the more serious the contact wear is. This is the most important wear characteristic of the barrel & screw with large length/diameter ratio L/D.

In view of this wear characteristic above, our countermeasures should be:

First, if the wear rate of "lost worn weight" is used to quantify the "contact wear" between two metallic materials, our conclusion must be that "The contact wear between the same kind of metal materials is the worst" (see figure below). Two kinds of different materials, which are the best of the matching coefficient, should be our best choice as to be a protection layer for a bimetallic barrel and for a bimetallic screw separately, such as the iron-based boron cast iron Ω101 alloy as to be the protective layer for a barrel, together with a bimetallic screw on top of which there is a nickel FX4 alloy protective layer, as the extrusion machine paired core components.

Second, High torque steel can be selected as the body material for making the larger length/diameter ratio screw, for example the strong toughness of medium carbon steel AISI 4340, the thermal refining of the screw body must reach around 300HB and should be through the stress relieving process.

Chief Engineer Office / Shanghai Omega Machinery Co., Ltd