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The main production equipment of the cardboard factory is the single-sided machine, and the core component of the single-sided machine is the corrugated roller. However, the problem of corrugated roller cracks has been constantly troubling the manufacturers and users, especially for the manufacturers who repair the corrugated rollers, as it often causes the rollers to suddenly break into teeth during the grinding process, which may even harm the operators. This article aims to analyze how the corrugated roller cracks are formed.
Improper heat treatment
The crack problem caused by improper heat treatment is the most serious, and it inevitably causes losses during the renovation process. Corrugated materials are usually made of 35CrM and 42CrM. Their heat treatment method is surface induction medium-frequency quenching, with a maximum hardness of HRC 55-60. The higher the hardness, the longer the service life. If the heat treatment process is improper, two types of quenching cracks will occur: longitudinal cracks and transverse cracks.
There is another factor contributing to the formation of these two types of cracks - delayed cracking: this occurs when the tempering process is not timely or insufficient, resulting in an increase in the stress due to the transformation of the structure with a large amount of residual austenite.
Solution: As long as the heat treatment process is correct and the hardness of the corrugated roller tooth surface is within the range of HRC 55-60, cracks can be completely avoided.
2. Incorrect installation
The installation of the corrugating rolls and the pressing rolls on the single-sided machine, as well as the correctness of the operation, will directly determine whether the corrugating rolls will develop cracks or lose teeth or break teeth.
Detailed analysis: Due to the imbalance or excessive pressure at both ends of the pressing roller, it will cause local detachment and tooth breakage at both ends of the lower corrugating roller. Regardless of whether the corrugating roller is newly manufactured or refurbished, there are high parameters in the upper corrugating roller. If the axis of the upper corrugating roller is not parallel to the axis of the lower corrugating roller, local cold work hardening will occur (uneven force on the entire tooth width area causes stress concentration), and under the combined effect of lattice distortion stress and heat treatment defect stress, cracks will form, which appear in an S-shaped pattern on the corrugating roller. At this time, the two sides of the corrugating roller paperboard show opposite-direction asymmetrical corrugations.
3. Incorrect grinding process
The grinding cracks that are entirely caused by burning are those formed by grinding burns and are of secondary martensite origin. Their characteristic is perpendicular to the grinding marks, manifested as irregular fine, dense, shallow and sparse or dense network-like cracks beneath the surface layer of the tooth surface. Some grinding cracks do not occur on the outer surface but are defects that are difficult to be detected by the naked eye beneath the surface layer. The direction of the cracks is often perpendicular to the grinding direction or in a network pattern. The generation of cracks often occurs simultaneously with the burning. Grinding burns are difficult to avoid because the heat generated during grinding is 800-1200℃, and the heat absorbed by the grinding wheel accounts for 12%, while the heat absorbed by grinding accounts for 4%, and the remaining heat is all absorbed by the corrugated rolls.
Solutions to avoid burns:
Improve the cooling effect
(a) Use high-pressure and large-flow cooling. This not only enhances the cooling effect but also enables the flushing of the grinding wheel surface, preventing the gaps from being clogged by the cutting process.
(b) To reduce the effect of the high-pressure adhering airflow on the surface of the high-speed rotating grinding wheel, air baffles are installed and coolant is sprayed from multiple directions towards the grinding area, ensuring that the coolant can smoothly reach the grinding zone.
(c) Use alkaline coolant.
2. The selection of grinding parameters involves increasing the rotational speed of the grinding wheel and the corrugator. During the rough grinding process, the grinding depth and the speed of the corrugator should be increased, which will enhance the degree of plastic deformation and thereby increase the roughness. Finally, using a small grinding depth or sparkless grinding can improve the surface finish.
3. Improving the grinding performance of the grinding wheel: The abrasive and binder of the grinding wheel are extremely important in the grinding process of the corrugated roller. The rubber-bonded grinding wheel can prevent surface burnout because this adhesive has certain elasticity. When the abrasive particles are subjected to excessive cutting force, they will automatically retreat, reducing the grinding depth. Additionally, the correction of the grinding wheel is also very important. That is, the sharpness of the grinding wheel depends on the grinding depth and the speed of the corrugated roller.
In summary, the cracks produced by the corrugated rolls are primarily a problem related to the heat treatment process. Whether they are longitudinal or transverse cracks, they are often penetrating cracks that cause the corrugated rolls to lose or break teeth. Secondly, incorrect installation and operation can lead to the superposition of cold-worked hardening lattice distortion stress and quenching stress, resulting in tooth loss or breakage. Incorrect grinding process may cause burn cracks, but they are shallow and sparse and will not result in penetrating cracks, and thus will not cause tooth loss or breakage.
