Soft roller calendering consists of a combination of hard and soft rollers. The effect of soft calendering In operation, the paper sheet is in contact with the heat roller. In a short time, the heat roller transfers heat energy to the paper and goes deep into a certain critical depth (usually considered to be at 0.007 μm). At this time, the temperature of the paper surface reaches or approaches. The softening point of the glass, when the paper surface is plastically reinforced, and the soft roll is elastically deformed when pressed, and the nip forms a surface contact. The mirror-like hard roller acts like a stamped, textured paper, smooth and shiny, and retains a good thickness. The advantages of soft calendering Compared with hard calendering, it has the following advantages: And super calender ratio: 1 The maximum allowable temperature of super calendering cannot exceed 85 degrees, and the temperature of soft calender calendering can reach 160-200 degrees. 2 Soft calendering can be at the same speed as a paper machine or a coater, and it is not necessary to decelerate when pulling. The main disadvantage of soft roller calendering The biggest disadvantage of calender calendering is that once its working width is determined, it cannot be changed at will. Because the soft roll surface outside the paper web is directly in contact with the hard (hot) roll, the soft roll surface layer is easily damaged. At the same time, the roll surface outside the paper web is in contact with the hot roll, causing a large local temperature, if the temperature difference exceeds the soft roll material. Acceptable values ​​(generally controlled within 5 degrees) are easily deformed. Therefore, in the production, it is necessary to ensure the width of the web in the press-in zone.
Hard roll: hot roll, chill roll, high surface hardness, smooth and smooth like a mirror surface, heated with oil or water outside the machine and sent to the roll, also useful for induction heating, roll surface temperature from 120 degrees to 200 degrees, the highest Up to 230 degrees.
Soft roll: Controlled high roller, the surface is made of special polymer, requiring high temperature and high pressure, good elasticity, able to withstand frequent changes in load. Currently used materials are mainly epoxy, nylon, synthetic rubber, polyester urethane and so on.
1 Pressed paper tightness is consistent with a higher elastic modulus of the calender roll, generally 500-50 times softer than the steel roll, when the elastic roller is pressed with a certain line to a hard roll or a cold-quenched cast iron roll, it The nip width becomes much wider than when steel-steel rollers are in contact. The peak pressure decreases with the reciprocal of the nip width, the contact surface is no longer a plane, and along the radius of curvature of the steel roll, the total radial compressive deformation of the steel roll is small (0.2-2%).
In the steel-steel nip, the pressure is not uniform if there are uneven patches in the sheet. When the sheets of paper that enter the nip area are too thick or with high flocculation, the pressure here will be the highest and the surrounding area will be lower. The overall result is that the high fiber density zone of the web is manifested in the low fiber density zone of the same sheet thickness, which is subjected to a lower pressure impact force. When the sheet is pressed in the calender roller of the elastic surface layer, the hard roller presses in the depth of the surface layer so that the thickness of the sheet does not change so much that the peak pressure does not change (the peak pressure is the center line of the nip The highest pressure).
2 Even if the formation of the paper is not good, the opacity can be improved and the stain can be eliminated.
Since the soft roll surface material rebounds, the roll surface can be adapted to any unfavorable formation and quantitative changes in the sheet. The thin area of ​​the paper is in contact with the heat roller, and at the same time, the thickness does not have to be greatly reduced for the thick area. Such thick and thin areas can increase the subtle smoothness and eliminate the stain.
3 The printing ink absorbs well due to the tightness of the paper sheet, and the thickness reduction is not so great, so the pressed paper is good and uniform.
4 It is possible to make the finished product have higher moisture because the paper is blackened and stains are eliminated in the soft roller nip. The paper sheet can enter the nip under high moisture content.
5 Improves paper bulk and maintains stiffness stability.
6 The stability of soft calendering and the range of line pressure adjustment are large, which can adapt to the requirements of calendering for different paper types.
7 Due to the fact that the fibers are not strongly squeezed, the strength of the paper sheet is good, which improves the operating rate of the company.
8 Get higher smoothness and eliminate both sides.
3 The soft roller has a longer time interval and service life than the paper roller.
4 Overpressure is difficult to control the thickness of the paper. 5 Overpressure covers an area that is large. The cost of the plastic surface and roller is high, and the investment is high.
Soft calendering also has other drawbacks when operating at higher temperatures, such as a drop in paper moisture; failure due to high temperatures, especially on a dual soft calender calender.
Source: Zhiqiao