Extruder Screw Barrel The Solution Of Fracture Reason

Single screw extruder is a rotating screw in the heating barrel, and the outer section of the barrel is provided with a heating and cooling device, one end has a radial feeding port and the other end is connected with the head. Screw is the key part of Extruder. The main technical parameters of single screw extruder are screw diameter, length-diameter ratio (ratio of screw length to diameter), screw speed range, driving motor power, barrel heating power and machine production capacity. Screw diameter Determines the extrusion machine's production capacity and the maximum allowable section of the product. Different materials require different geometrical shapes of screw and length-diameter ratio. Modern plastic extruder is widely used in combination screw, distribution of hybrid screw, and on the screw to add barriers or pins, etc. to improve plasticizing, improve production capacity and product quality. The combination of the two parts, screw and barrel, has an important effect on the plasticizing of materials, the quality of products and the efficiency of production. Their working quality is related to the manufacturing accuracy and assembly clearance of two parts. When 20 pieces of wear are serious, extruder production decline, should be arranged on the screw, barrel maintenance.

1. Operation of Screw Rod

Production of 1.1 screw extruded polyester Film

The equipment produces two-way tensile polyester film, the raw material is mixed in a certain proportion, and sent to the extruder material barrel, the raw material is plasticized under the barrel heating and screw extrusion, and continuously mixed extrusion under the action of screw rod.

1.2 Screw operation Parameters

The running speed of screw is 30~100r/min, the operating temperature of screw is 270~290℃, the extrusion pressure of screw is 0~18mpa, and the screw extruder is polyester chip.

Analysis of operation load of 1.3 screw rod

The principle of the polyester film extruder screw is that the granular slices are mixed evenly in the spiral groove, the polyester slices are plasticized by the barrel heating and the screw shearing, and finally the blending and plasticizing materials are extruded into the die. The screw temperature is 270~290℃, considering that the heating temperature is low and the thermal strain is negligible. Screw to the internal rotation, in the production of screw forces under the circumstances, the extruder in every start-up process, due to the previous left in the spiral groove of the remaining materials in the reasonable process temperature, the hardness is greater, need a larger starting torque to make the screw operation. In addition, there are obvious pits on the surface of the screw, the alloy layer on the top of the screw is also scarred, which indicates that the raw material contains hard sundries to make the screw running condition deteriorate.

To sum up, the screw is operating under higher load stress, but the running speed is low, so it is operated under the action of high stress and low alternating frequency. Each start-up operation will produce a certain amount of plastic strain. Every time a screw is obstructed by hard objects, it also produces a certain plastic deformation, this plastic deformation on the screw damage than the starting time is much more serious, coupled with the screw long time under compression, friction caused by plastic strain, under the repeated plastic strain, the screw internal damage accumulated to a certain degree to form cracks, when the crack expansion to the critical size, When the residual section of the screw is not enough to withstand the running load of the screw, the crack instability expands to cause a sudden brittle fracture.

2. Cause analysis of screw breakage of extruder

Cause analysis of macroscopic fracture of 2.1 screw

From macroscopic fracture morphology analysis, the screw fracture is low cycle fatigue damage. There are three areas in the section: fatigue crack source, fatigue crack growth area and instantaneous fault zone. The source of fatigue crack can occur in the screw tooth root, surface cracking and internal inclusions in stress concentration. The fatigue crack source of screw is very small, it is the core area of fatigue crack. From the photos can be clearly seen, the screw fatigue crack in the root stress concentration, defect screw root week length of 12mm, axial width of 3mm, depth greater than 2mm, the total surface of more than 20mm2, but less than 10% of the fracture area. Nitriding hardened layer cracking. The fatigue crack in the area slowly expands, the initial fracture surface is subjected to the repeated alternating stress effect, the friction is more smooth, the brightness, then appears the shell pattern pattern. The shell pattern is generally perpendicular to the direction of the crack propagation, advancing toward the entire hollow circumference. The spacing between the shell lines (fatigue arcs) varies in size. Under the repeated action of the alternating stress of unequal size, the macroscopic traces left by the discontinuous change of the crack propagation process are caused by the driving of the extruder, the parking, the hard object hindering the screw and so on. Generally, the area of the fatigue crack growth area is 98% of the fracture area, which accords with the basic characteristics of fatigue crack. When the circumferential shell (fatigue arc) close to the tangent, the crack expands to the net section of the stress to the fracture stress of the screw, the screw is cut off. The section has obvious steps, which account for about 1% of the total fracture area.

Cause analysis of micro fracture of 2.2 screw rod

The morphology of the fatigue crack propagation zone and the instantaneous fracture zone of the screw fracture was analyzed by scanning electron microscope.

2.2.1 Fatigue crack growth area fracture microscopic appearance analysis

The whole section of fatigue crack growth zone is the result of fatigue crack propagation. It is generally clear that the fatigue stripes are directional, continuous or discontinuous, and that the fatigue fringe with steps extends parallel to the crack leading edge, and the fatigue fringe is perpendicular to the crack propagation direction. These are microscopic traces of the crack propagation in the process of destruction.

A fatigue stripe corresponds to a stress cycle. The fatigue stripes in the fatigue crack growth area of the screw fracture are basically brittle stripes, and the interval between the stripes is equivalent to the length of a stress cycle. Small cracks, which extend from the surface of the fracture to the interior, are called two cracks, and two cracks are developed by a crack.

2.2. Analysis of micro-opening appearance of fracture zone in 2 moment

The instantaneous fracture zone is brittle cleavage fracture, the section can view the steps, the river pattern, the cleavage tongue and two cracks, and cannot find the dimple.

3, extruder screw Repair

The twisted screw is to be considered according to the actual diameter of the barrel, and the outer diameter deviation of the new screw is manufactured according to the normal clearance of the barrel. Wear screw diameter reduced the thread surface after treatment, thermal spraying wear-resistant alloy, and then grinding to size. This method generally has a professional spraying factory processing repair, the cost is relatively low. The wear-resisting alloy of the threaded part of the worn screw is surfacing. According to the degree of screw wear, surfacing 1~2mm thickness, then grinding screw to size. The wear-resisting alloy is composed of C, Cr, Vi, Co, W and b materials, which increase the abrasion resistance and corrosion resistance of the screw. Professional surfacing plant for this processing costs 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, chromium is also wear-resistant and corrosion-resistant metal, but hard chrome layer is easier to fall off.

The inner surface hardness of the barrel is higher than the screw, and its damage is later than that of the screw rod. The scrap of the barrel is the increase of the diameter due to time wear. Its repair method is as follows: Because the wear increases 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 this diameter to prepare new screw. The inner diameter of the barrel is machined and repaired, the alloy is 1~2mm, and then refined to size. In general, the uniform section of the cylinder wear faster, can this section (take 5~7d long) through boring, and then with a nitriding alloy steel bushing, the diameter of the hole reference screw diameter, left in the normal coordination gap, processing preparation. Here it is emphasized that the screw and barrel are two important parts, one is a slender thread rod, one is a small diameter and long hole, their mechanical processing and heat treatment process are more complex, the accuracy of the guarantee is more difficult. Therefore, the two parts of the wear after the repair or replacement of new pieces, must be from a comprehensive economic perspective analysis. If the cost of repairs is lower than the cost of a new screw, it is decided that it is not necessarily the right choice, but the comparison between the cost of repairs and the cost of renewal is only one aspect. In addition to the repair costs and repair after the use of screw time and update the cost and update the ratio of screw use time.