Factors affecting the calendering effect of paper machine
March 23, 2023
"My paper is not glossy"
"Why is the paper surface is always eeaily folded after calendering?"
Uneven paper thickness.
Have you ever met these problems and how do you deal with it?
If you have no clue, don't be anxious, it may be a problem with the calendering.
Today I’m going to take you to learn factors affecting the calendering effect of paper machine：
- Number of pressure zones passed
- Linear pressure
- Work speed
- Moisture content
- Types of calender rolls
- The property of the paper sheet
What is calendering?
Calendering, is the process of processing paper or cardboard containing a certain amount of moisture with a calender to improve the smoothness, gloss and tension of the paper and to a certain extent the thickness of the paper or cardboard.
Why is calendering necessary?
Calender plays several functions.
One is to trim to improve the smoothness, glossiness and tightness of the paper, or to increase the smoothness and glossiness while maintaining a loose thickness, that is, the tightness does not increase much.
The second is to correct the thickness of the paper, or to maintain a relatively uniform tightness. The break length of calendered paper increases the number of folds.
Factors affecting the calendering effect of paper machine
Number of pressure zones passed
The more the number of pressing areas passed, the more times the paper is pressed, and the higher the natural calendering effect. According to the experiment, the number of pressure zones is large and the effect is good. In fact, it is the result of prolonging the calendering time.
The greater the linear pressure, the stronger the compression effect in the compression zone, and the better the transfer effect. However, it must be pointed out that the improvement of its effect is limited. When it exceeds a certain limit, the calendering effect will not be improved, and the mechanical strength of the paper will be greatly reduced.
On the calender, the paper tightness increases with the specific pressure, and the physical properties of the paper are related to the tightness it obtains on the calender. The smoothness and gloss of the paper is the result of the paper sliding between the rolls and the plastic deformation of the surface under pressure.
As the paper surface is not flat in the nip between the rolls, and the paper and the rollers only partial contact rather than contact over the full width.
When the pressure increases, the contact area increases, and thus the gloss and smoothness of the paper increases. When the pressure increases beyond a certain limit range (250 ~ 300kN / m), the pressure continues to increase on the paper smoothness of the impact is not significant.
The calendering effect will be reduced when the vehicle speed increases. This is because the paper also has similar properties to the so-called viscoelastic material like asphalt, that is, under the action of force, the longer the action time, the softer and easier to deform, and the shorter the action time, the harder and more elastic. When the vehicle speed is high, the paper sheet passes through the pressing area instantly, so it is hard and elastic, and the calendering effect is reduced.
Therefore, with the increase of vehicle speed, it is often required that the line pressure should also increase, and the number of rollers should also increase accordingly. You can even install two calenders to extend the calendering time.
When the moisture content of the paper increases, the plasticity increases, which is conducive to calendering, but when the moisture content is too high, it will be compressed too tightly, which will greatly increase the transparency, and the paper will look dark, which means that the phenomenon of blacking occurs. Therefore, the moisture content is generally controlled at 6-8%.
It is often found that the calendering effect is not good due to the drying of the paper. In addition, if the drying part of the machine is not loaded enough and over-drying occurs due to strong drying, adjusting the dry steam pressure often causes large fluctuations. At this time, a steam nozzle can be set on the calender to make the paper properly moistened, which can improve the calendering effect.
The higher the temperature of the nip, the more conducive to the plastic deformation of the paper sheet, and therefore conducive to improving the calendering effect.
Practice has shown that at a higher temperature, a better calendering effect can be obtained at a lower line pressure and a smaller number of nip passes. This can greatly reduce the side effects such as reduced sheet strength caused by mechanical pressing.
Therefore, the development of high-temperature calendering technology, through various types of hot calendering rolls (steam heating, electromagnetic heating and hot water heating) up to 200 ℃ roll temperature, when the internal temperature of the paper sheet to maintain a low, and the internal paper sheet does not produce a large compression deformation, so the strength of the paper sheet loss is very little.
However, this high-temperature calendering technology requires that the rolls must act evenly on the paper sheet, otherwise it is difficult to make the thickness pressed evenly at low pressure, thus seriously affecting the uniformity of the paper sheet calendering, so the processing of the rolls and the design of the medium and high, the requirements are very strict, and the paper sheet itself requires very good uniformity, which limits its application on general paper machines.
This calendering technology can be used with controllable mid-height rolls, as well as with interleaved paper machines using a set flow box with step diffusers and flutter sheets.
Types of calender rolls
The calender rolls of ordinary paper machine calenders are harder chilled cast iron rolls. For paper sheets with an uneven surface, the roll surface will only touch the raised part of the surface, but not the lower part. The tightness of the contacted part increases, and stronger pressure creates light spots, making the surface characteristics uneven.
In contrast, soft calendering technology, using the elastic roller degeneration to absorb the unevenness of the paper surface, so that the paper surface is completely in contact with the iron roller surface, the tightness of the paper sheet after calendering changes gently and continuously, and the surface characteristics of the paper sheet are invalid; in addition, the nip is wide, the calendering time is long and the specific pressure is relatively lower, so that the thickness can be reduced less (small loss of strength), and the smoothness is increased.
The property of the paper sheet
Paper sheet is relatively soft and plastic, it is easy to produce a good transfer effect in the pressure area, thus helping to improve the calendering effect.
For example, newsprint, although the fiber is relatively stiff, but because of the soft porous, coupled with more microfine components, easy to move each other deformation, so its calendering effect is better than the more brittle and hard straw pulp; moderate pulping of paper, due to its enhanced plasticity, and into the paper uniformity is better, so the calendering effect is better than the unpulped paper.
Similarly, the plasticity of soft pulp is better than that of hard pulp, and thus the calendering effect is better. However, pulp with high pulping degree and softer pulp is more prone to the phenomenon of blackening; the paper sheet with filling has enhanced plasticity, thus improving the calendering effect.
So the calender is one of the most essential machines to ensure the final quality of the paper.
In fact, there are many classifications of calendering machines. I will list some classfications and their features.
- Hard calender;
- Soft roll calender:Easy to operate, less calendering breaks, good adaptability to paper types; is difficult to change the working width after it is determined
- Supercalender:The maximum allowable temperature cannot exceed 85 degrees; large area, large investment
Choose the right calender according to your needs and budget.