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When the paper passes through the calender, under the action of the line pressure, the polyurethane roller and the paper are deformed to form a deformed area. The pressure is distributed over this area.

 

The width of the deformed area determines the amount of deformation of the polyurethane roller and the amount of reduction in the thickness of the paper. (The ratio of the line pressure to the width of the deformed area is called the specific pressure, and the unit is kPa/m2).

 

The use of a polyurethane roll on a calender enables a wider deformation area to be formed, as with the use of a polyurethane roll in the press section.

 

In this case, although the line pressure is large, it is smaller than the pressure, so the paper will not be crushed, and the pressure time of the paper in the roller nip can be prolonged.

 

The specific pressure is the main factor that determines the tightness, smoothness and gloss of the paper.

 

In the calender, the paper tightness increases with the increase of the specific pressure, and the physical properties of the paper are related to the tightness obtained in the calender.

 

The smoothness and gloss of the paper are due to the sliding of the paper between the rollers and the plastic deformation of the surface under pressure.

 

Due to the uneven surface of the paper in the nip between the rolls, the paper is only in partial contact with the rolls rather than over the full width.

 

When the pressure is increased, the contact area is increased, and thus the gloss and smoothness of the paper are improved.

 

When the pressure increases beyond a certain limit range (250 ~ 300kN/m), the pressure to continue to increase the smoothness of the paper has little effect.

 

The smaller the elastic modulus of the polyurethane roller, the greater the radial deformation and the greater the sliding between the rollers. Therefore, the paper with high requirements for smoothness and gloss, such as writing paper, printing paper and coated paper, needs to use polyurethane rollers with good elasticity for calendering.

 

According to the production of different paper products, coated polyurethane roller Shore hardness is generally 880~950.

 

Vehicle speed and number of rollers

 

When the line pressure is the same, increasing the number of rollers within a certain range can improve the gloss and smoothness of the paper, and slightly increase the tightness of the paper.

 

With the increase of the number of rollers, the elongation of the paper also increases, and the smaller the hardness of the polyurethane roller, the higher the elongation of the paper.

 

When the speed of the calender is increased, the amount of improvement in the smoothness of the paper is significantly reduced due to the reduction in the time during which the paper web is pressed in the nip between the rolls.

 

When the vehicle speed increases, the line pressure does not change, and the smoothness of the paper decreases; but when the vehicle speed increases, the gloss and smoothness of the resulting paper decrease little.

 

Paper Moisture and Barrel Temperature

 

The greater the moisture of the paper, the greater the plasticity and the easier the calendering.

 

Therefore, the paper is subjected to a wetting treatment before passing through calendering.

 

For writing paper and printing paper, the moisture before the calender should not be less than 8% to 9%, but rarely reaches 10%.

 

The wetting of these two kinds of paper is carried out before the paper machine, and the most suitable method is to put the wet paper in a constant temperature and humidity environment for 2 to 3 days, so that the moisture in the paper is evenly distributed.

 

However, when the production capacity of the paper machine is large, it requires a large stacking area and complicates the transportation of paper in the finishing workshop. Therefore, this method is currently used only when producing high-grade paper.

 

Industrial paper (such as capacitor paper, imitation parchment paper, etc.) requires 15% ~ 30% moisture before super-compaction, and wetting is carried out on a special wetting machine to ensure uniform wetting along the full width of the paper.

 

When the humidity of the paper is not sufficient, the paper can be moistened with steam in front of the top of the super calender or at the middle roller.

 

When the paper passes through the super calender or ordinary calender, the temperature of the roller determines the change of the moisture of the paper, and the change of the moisture has a significant impact on the calendering process.

 

Because super calender and ordinary calender have little effect on over-dried paper, it is necessary to adjust the supply of steam and water so that the dryness of the paper will not reach the final dryness when it passes through the upper rollers.

 

In order to obtain good quality paper, the roll surface should be polished frequently. Roll surface polishing should be carried out regularly, and the time interval between the two polishing is determined by the speed of the calender, the line pressure, the content of sand in the paper and the use of the scraper.

 

When the speed exceeds 300 m/min, the top roller and the middle roller shall be polished once every 4~6 months; the bottom roller shall be polished once every 12~18 months; when the speed is lower than 300 m/min, the top roller and the middle roller shall be polished once every 8~12 months; the bottom roller shall be polished once every 18~24 months. Since the surface of the calender roll is processed, when the speed does not exceed 500 m/min, the balance is generally not required.