Views: 1 Author: Site Editor Publish Time: 2021-09-14 Origin: Site
The calendering process in a paper machine is a process in which a calender is used to process paper or cardboard containing a certain amount of moisture to improve the smoothness and gloss of the paper, and to control the thickness of the paper or cardboard to a certain extent. The factors that affect the calendering effect of the paper machine are as follows:
When the moisture content of the paper increases, the plasticity increases, which is good for calendering, but when the moisture is too high, it will compress too tightly, which greatly increases the transparency, and the paper looks dark, that is, the phenomenon of blackening occurs. Therefore, the water content is generally controlled at 6-8%.
It is often found in production that the calendering effect is not good due to the excessive drying of the paper. Moreover, if the load of the drying part of the machine is not enough, when over-drying occurs due to strong drying, the steam pressure of the drying will often cause large fluctuations. At this time, a steam nozzle can be installed on the calender to make the paper sheet properly re-moisturized. , Can improve the calendering effect.
2. The nature of the paper
Generally speaking, the paper is relatively soft and plastic, and it is easy to produce a good transfer effect in the nip, which is beneficial to improve the calendering effect. For example, newsprint, although the fiber is relatively stiff, but because of its softness and porosity, plus more fine components, it is easy to move and deform each other, so its calendering effect is better than that of more brittle and hard straw pulp; moderately beaten paper, due to its plasticity enhancement , And the evenness of the finished paper is better, so the calendering effect is better than that of unbeaten paper. Similarly, the plasticity of soft pulp is better than that of hard pulp, so the calendering effect is also better. But the high beating degree and softer pulp are more prone to blackening; the plasticity of the filled paper is increased, so the calendering effect is improved.
The higher the temperature of the nip, the more conducive to the plastic deformation of the paper, which is conducive to improving the calendering effect. Practice has proved that at a higher temperature, a better calendering effect can be obtained under a lower linear pressure and a smaller number of passes through the nip. This can greatly reduce the side effects such as reduced paper strength caused by mechanical pressing.
Therefore, recently developed high-temperature calendering technology, through various types of hot calender rolls (steam heating, electromagnetic heating and hot water heating), the temperature of the roll can be raised up to 200°C, and the inside of the paper sheet is kept at a lower temperature. , And there is no large compression deformation inside the paper, so the strength loss of the paper is very small. However, this kind of high-temperature calendering technology requires that the roller has a uniform effect on the paper sheet, otherwise it is difficult to make the thickness uniform under low pressure, which will seriously affect the uniformity of the paper sheet calendering. Therefore, the processing and medium-to-high design of the roller require Very strict, and the paper itself requires very good evenness, which limits its application on general paper machines. This calendering technology can be used in conjunction with controllable middle and high rollers, and the use of a net-filled paper machine with a stepped diffuser and a collecting headbox with a flare.
4. Number of nip passing
The more nips passed, the more times the paper is subjected to calendering, and the higher the natural calendering effect. However, according to the structure of the calender, this seems to be the result of the high linear pressure due to the large number of nip, that is, the large number of rollers. In the case of the same line pressure, the number of nips passing through the paper sheet is different, and the obtained results prove that the number of nips is large and the effect is good. In fact, it is the effect of prolonging the calendering time.
5. Types of calender rolls
As mentioned above, the calender rolls of ordinary paper machine calenders are harder chilled cast iron rolls. For a sheet with an uneven surface, the roller surface will only touch the convex part of the surface, and not the low part. The tightness of the contact part is increased, and the strengthening pressure forms a light spot, which makes the surface characteristics uneven. Soft calendering technology uses the denaturation of the elastic roller to absorb the unevenness of the paper surface, so that the paper surface is completely in contact with the iron roller surface. After calendering, the tightness of the paper changes smoothly and continuously, and the surface characteristics of the paper are invalid; The area is wide, the calendering time is long and the specific pressure is relatively reduced, which can reduce the thickness (small strength loss) and improve the smoothness.
6. Line pressure
The greater the line pressure, the stronger the compression produced in the nip and the better the transfer effect. But it must be pointed out that the improvement of its effect is limited. When a certain limit is exceeded, the calendering effect is not improved much, and the mechanical strength of the paper will be greatly reduced.
7. Vehicle speed
Generally, the higher the speed, the lower the calendering effect. This is because the paper also has properties similar to so-called viscoelastic substances like asphalt, that is, under the action of force, the longer the action time, the softer and more easily deformed, and the shorter the action time, the harder it appears. And flexible. When the vehicle speed is high, the paper sheet passes through the nip instantly, so it is harder and more flexible, and the calendering effect is reduced. Therefore, the increase in vehicle speed often requires that the line pressure should be increased, and the number of rolls should also be increased accordingly. Some high-speed paper machines even set up two calenders to extend the calendering time.