Detailed introduction

08

2018

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08

Fracture Causes and Maintenance Methods of Extruder Screw


  
Modern plastic extruder widely usedCombined screw, distribution of mixed screw, and the addition of barriers or pins on the screw to improve plasticization, improve production capacity and product quality. The combined quality of the two parts of the screw and the barrel has an important impact on the plasticization of the material, the quality of the product and the production efficiency. Their quality is related to the manufacturing accuracy and assembly clearance of the two parts. When the two parts are seriously worn and the output of the extruder is reduced, the maintenance of the screw and barrel should be arranged.
The screw runs under high load stress, but the running speed is low, so it runs under high stress and low alternating frequency. Each start-up run produces a certain amount of plastic strain. Every time the screw is blocked by hard objects, a certain plastic deformation will also occur. This plastic deformation will cause much more serious damage to the screw than when it is started. In addition, due to the plastic strain caused by extrusion and friction for a long time, under the action of repeated plastic strain, the internal damage of the screw will accumulate to a certain extent to form cracks. When the cracks expand to a critical size and the remaining section of the screw is not enough to bear the running load of the screw, crack instability propagation leads to sudden brittle fracture.
Analysis of the reasons for the fracture of the extruder screw:
Cause Analysis of Macro Fracture of 2.1 Screw
From the macroscopic fracture morphology analysis, the screw fracture is a low-cycle fatigue failure. Three areas can be clearly seen on the section: fatigue crack source; fatigue crack propagation zone; instantaneous fracture zone. Fatigue crack sources can occur at the screw root where stress is concentrated, surface cracks, and internal inclusions. Screw fatigue crack source is very small, which is the core area of fatigue crack. It can be clearly seen from the photos that the fatigue crack of the screw is located at the stress concentration at the root of the screw. The circumferential length of the root of the defective screw is 12mm, the axial width is 3mm, the depth is greater than 2mm, and the total surface exceeds 20mm2, but it is less than 10% of the fracture area. Cracking of nitrided hardened layer. Fatigue cracks spread slowly in this area. At first, the fracture surface was rubbed smooth and bright due to repeated alternating stress, and then shell pattern appeared. The shell grain is generally perpendicular to the propagation direction of the crack and advances to the entire hollow circumference. The spacing between the shell lines (fatigue arcs) varies in size. Under the repeated action of alternating stresses of different sizes, the macro traces left by the discontinuous changes in the crack propagation process are caused by the load changes such as the driving of the extruder, parking, and hard objects blocking the screw. Usually, the area of fatigue crack propagation area accounts for 98% of the fracture area, which conforms to the basic characteristics of fatigue crack. When the shell grain (fatigue arc) on both sides of the circumference is close to tangent, the stress of the crack extending to the net section reaches the fracture stress of the screw, and the screw is cut off. The section has obvious steps, and the area of this part accounts for about 1% of the whole fracture area.
Cause Analysis of Micro Fracture of 2.2 Screw
The fatigue crack propagation zone and instantaneous fracture zone of the screw fracture were analyzed by scanning electron microscope.
2.2.1 Fatigue crack growth zone fracture micro-morphology analysis
The entire section of the fatigue crack growth zone is the result of fatigue crack growth. Usually, it can be clearly seen that the fatigue stripes are directional continuous or intermittent distribution, the fatigue stripes with steps extend parallel to the front edge of the crack, and the fatigue stripes are perpendicular to the propagation direction of the crack. These are microscopic traces of crack propagation during failure.
One fatigue stripe corresponds to one stress cycle. The fatigue stripes in the fatigue crack propagation zone of the screw fracture are basically brittle stripes, and the interval between the stripes is equivalent to the length of a stress cycle extension. The small cracks extending from the fracture surface to the inside are called secondary cracks, and the secondary cracks are the hole defects developed from the primary cracks.
2.2.2 Microscopic opening profile analysis of fracture in instantaneous fracture zone
The instantaneous fracture zone is a brittle cleavage fracture, and on the section, steps, river patterns, cleavage tongue and secondary cracks can be found, and no dimple can be found.
Extruder screw repair:
The twisted screw should be considered according to the actual inner diameter of the barrel, and the outer diameter deviation of the new screw should be given according to the normal clearance with the barrel. After the thread surface of the reduced diameter of the wear screw is treated, the wear-resistant alloy is thermally sprayed, and then ground to size. This method is generally a professional spraying plant processing repair, the cost is relatively low. A wear-resistant alloy is surfacing the threaded portion of the wear screw. According to the degree of screw wear welding 1~2mm thick, and then grinding the screw to size. This wear-resistant alloy is composed of materials such as C, Cr, Vi, Co, W and B, which increases the wear resistance and corrosion resistance of the screw. Professional surfacing plant for this kind of processing cost is very high, in addition to the special requirements of the screw, generally rarely used. Repair screw can also be used on the surface of hard chromium plating method, chromium is also a wear-resistant and corrosion-resistant metal, but the hard chromium layer is easier to fall off.
The hardness of the inner surface of the barrel is higher than that of the screw, and its damage is later than that of the screw. The obsolescence of the barrel means that the inner diameter increases due to time wear. Its repair method is as follows: due to wear and tear to increase the diameter of the barrel, if there is a certain nitriding layer, the barrel hole can be directly boring, grinding to a new diameter size, and then according to the diameter of the preparation of new screw. The inner diameter of the barrel is machined, trimmed and re-cast with a thickness of 1 to 2mm, and then finished to size. Under normal circumstances, the homogenization section of the barrel wears faster. This section (5 ~ 7D long) can be trimmed by boring, and then equipped with a nitrided alloy steel bushing. The inner hole diameter refers to the screw diameter, and stays in the normal fit clearance for processing and preparation. It is emphasized here that the two important parts of the screw and the barrel, one is a slender threaded rod, and the other is a relatively small and long hole in diameter. Their machining and heat treatment processes are more complicated, and the accuracy is more difficult to guarantee. Therefore, whether to repair or replace the new parts after the wear of these two parts must be comprehensively analyzed from an economic point of view. If the cost of repair is lower than the cost of replacing the screw, it is decided to repair. This is not necessarily the right choice. The comparison between the cost of repair and the cost of renewal is only one aspect. Also look at the cost of repair and repair after the use of screw time and update the cost and update the screw use time ratio.