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August 12, 2015
Extruder screw repair
In a well-designed extruder, the main wear must be on the screw, as it is easier to replace or repair it than the cylinder.
The reason why such services as repair of extruder screws are becoming popular is that repair is cheaper than complete replacement. Repairs are usually carried out with the use of surface hardening materials. With the right choice of reinforcing materials, the screw after repair can become better than the original one. But usually this can not be applied to the repair of small screws (with a diameter of less than 40 mm), since the cost of repairs may be the same (or higher) than the manufacture of a new screw. In addition, the application of hardening layer to the worn surfaces of small screws is very difficult from a process point of view. However, large-diameter screws can be hardened repeatedly and without any difficulties.
Our extruder screw repair technology is about the following:
- cleaning of the screw and cylinder (if required);
- identification of defects on the screw – material cylinder system and mapping of wear;
- getting final approval from the Customer of technical specifications for the restoration of the system;
- aligning geometry of the barrel and defining tolerances for repair of the screw;
- aligning the screw against outside diameter for deposition;
- deposition of semi-solid substrate;
- deposition of the extruder screw crests against outside diameter using various hard alloys based on iron, cobalt, nickel, molybdenum or tungsten (selected based on the operating conditions) with high wear resistance under conditions of dry friction in metal to metal contact, high corrosion resistance (also in the nitric acid environment) and high resistance to abrasive wear. Alloy hardness after deposition is 40 – 58 HRC. We draw your attention to the fact that high hardness of the deposited materials on the screw crests is not the determining and the only factor of high wear resistance of both the screw and the screw - sleeve system. Wear resistance of the system is determined in total by a combination of such factors as: scoring resistance (heat resistance), corrosion resistance, abrasive resistance of both surfaces in the friction couple;
- associated in-process control;
- grinding of the screw against outside diameter;
- processing the side surface of the screw crests manually with the necessary roughness level;
- outgoing inspection;
If you need to repair the screws, use the services of capital repair of extruder screws at our Company, which can carry out a full cycle of repair works in 15-45 days. Following the principles of "lean" production, our company analyzes the received equipment, identifies defects of the screw-and-barrel system, calculates costs, issues a quotation and repairs parts with the consent of the customer.
When is it necessary to start thinking about repairing the extruder screw?
When the screws are worn-out, the gap between the screw coil and the cylinder wall increases, especially intensively this process occurs in the high pressure zones of the cylinder. For example, a new screw with a diameter of 63 mm has a gap of 0.13–0.18 mm per diameter or about 0.08 mm per radius.
When the screw with a diameter of 63 mm is worn-out, there will be a noticeable decrease in productivity, as well as in melting point. In such cases, to compensate for the drop in productivity, the speed of screw rotation of the screw is usually increased. As a result, the melting point becomes even higher. Later on, to reduce the melting point, the cylinder temperature levels are reduced. At the same time, the extruder continues to work with increased screw rotation speed and lower temperature in the cylinder zones. Thus, the operator no longer controls the extrusion process. Extruder operator has to struggle with an obvious problem, although the screw can simply be handed over for technical expertise, to optimize the process and increase profit. Because of the worn-out screw, you will get much more substandard products than usual.
When the screw is worn-out, it is no longer possible to obtain acceptable quality of the finished product, and the productivity drops so rapidly that with each kilogram of the product being extruded, the losses only increase. The need for re-grinding increases, and the quality of finished products is reduced. Operators are not happy, the quality control department is forced to release substandard products, customers submit too many complaints. And then you have to stop your activities, because competitors are pushing you out of the market.
Note. In the considered example, we were talking about a new unworn cylinder. In practice, when assessing the total size of the gap, it is necessary to take into account not only the wear of the screw, but also the wear of the cylinder
When is it time to replace the extrusion cylinder?
When the cylinder is worn-out, the gap between the extruder screw coil and the cylinder wall increases, especially intensively this process occurs in the high pressure zones of the cylinder. Buying a new screw and installing it in a worn cylinder will not solve the problem of the increased gap. With the new screw, the extruder will work somewhat better, but a worn-out cylinder will still not allow you to achieve the previous performance properties.
Since the cylinders wearout mainly in high-pressure zones or in areas where polymer melting occurs, measuring the inside diameter of the cylinder from one side will not provide complete information about wear.
You should remember that to determine the total size of the gap, in addition to cylinder wear, the screw wear shall be taken into account.
NTV Scientific & Technical Firm has developed a method for measuring the inside diameter of the cylinder along its entire length. If you do not know what degree of wear your cylinders have, our company can offer defect identification on theinside diameter of the material cylinder with further provision of recommendations for its repair.