A method for controlling the coating colour build-up on a blade at coating stations in a paper coating machine

Description

[0001] The invention relates to a method for controlling the coating colour build-up on a blade at the coating stations in a paper coating machine. In the method, energy is supplied to the coating colour build-up (stalagmite) by means of a dryer and/or the temperature of the coating mixture is adjusted so that the coating colour build-up will not cause problems in the coating. The energy supplied to the coating colour build-up by the dryer is converted into heat, thus further evaporating liquid from the build-up. The rheology of the coating mixture can be altered by adjusting its temperature so that the coating colour build-up will not cause problems in the coating.

[0002] Paper coating is a very important process stage in the production of coated printing paper. In the coating step, a uniform coating mixture containing pigments, binding agents, additives and water is applied to the surface of paper at the coating stations, and then dried. The coating mixture is called coating colour.

[0003] The purpose of paper coating is to improve the paper properties, such as brightness and print quality. The gloss of calendered coated paper is also considerably higher as compared with uncoated paper.

[0004] The amount of coating material to be applied to the surface of paper is adjusted by varying the dry solids content of the coating colour, or by varying the scraping pressure that the doctor blade exerts on the paper at the coating station of the coating machine.

[0005] The scraping pressure is also called blade pressure. The amount of applied paper coating material decreases with increasing blade pressure or with decreasing dry solids content of the colour. Correspondingly, the amount of paper coating material increases with decreasing blade pressure or with increasing dry solids content of the coating colour.

[0006] The blade pressure depends not only on the desired amount of coating material but also on the properties of the pigments and the binding agent, the roughness and porosity of the base paper to be coated, and the speed of the coating machine.

[0007] The coating colour build-up on the blade becomes problematic when the coating colour build-up splashes, and the splashes stick to the coated paper, where they cause stripes and round coating colour accumulations. The stripes or coating colour accumulations cause breaks and printability problems in further processing. As the build-up of coating colour on the blade of the coating machine increases with the speed of the coating machine, the build-up of coating colour restricts the running speed of the coating machine.

[0008] It has now been discovered that by drying the coating colour build-up on the blade at the coating station and/or by adjusting the temperature of the coating mixture, the speed of the coating machine can be substantially increased without that the coating colour build-up causes stripes, splashes or coating colour accumulations in paper.

[0009] The object of the present invention is to dry up the coating colour build-up and/or to alter the rheology of the coating colour by adjusting the temperature of the coating colour so that the speed of the coating machine need not be decreased. This object is achieved by means of a method according to the invention, which is characterized in that a dryer is mounted on the blade of the coating machine or in the vicinity of the blade so as to dry the coating colour build-up so that it remains dry at different running speeds of the coating machine, and/or a heat exchanger is mounted in the machine or storage circulation of the coating colour for cooling and/or warming the coating colour so that the rheological properties of the coating colour can be affected by adjusting the temperature of the coating colour in such a way that the coating colour build-up remains dry at different running speeds of the coating machine.

[0010] The new method can be applied to all coating machines in which a dryer for the coating colour build-up can be mounted in the vicinity of the blade or on the blade and/or a heat-exchanger can be provided in the machine or storage circulation of the coating colour.

Claims

1. A method for controlling the coating colour build-up on a blade used at coating stations in a paper coating machine, characterized in that a dryer is mounted on the blade of the coating machine or in the vicinity of the blade so as to dry the coating colour build-up so that it remains dry at different running speeds of the coating machine, and/or a heat exchanger is mounted in the machine or storage circulation of the coating colour, the temperature of the coating colour being adjustable by means of the heat-exchanger so that the coating colour build-up remains dry at different running speeds of the coating machine.

2. A method according to claim 1, characterized in that an electric resistor or electric resistors are mounted in the blade of the coating machine over the entire width of the blade, and the total drying capacity of the resistor or resistors is from 0.1 to 5.0 kW per one metre of the width of the blade.

3. A method according to claim 1, characterized in that an electric resistor or electric resistors are mounted at a distance of 0.01 to 0.7 m from the blade of the coating machine over the entire width of the blade, and the total drying capacity of the resistor or resistors is from 0.1 to 5.0 kW per one metre of the width of the blade.

4. A method according to claim 1, characterized in that a microwave dryer or microwave dryers are mounted at a distance of 0.01 to 0.7 m from the blade of the coating machine over the entire width of the blade, and the total drying capacity of the microwave dryer or dryers is from 0.1 to 5.0 kW per one metre of the width of the blade.

5. A method according to claim 1, characterized in that an IR dryer or IR dryers are mounted at a distance of 0.01 to 0.7 m from the blade of the coating machine over the entire width of the blade, and the total drying capacity of the IR dryer or dryers is from 0.1 to 5.0 kW per one metre of the width of the blade.

6. A method according to claim 1, characterized in that hot-air nozzles are mounted at a distance of 0.01 to 0.7 m from the blade of the coating machine over the entire width of the blade, and the total drying capacity of the hot-air nozzles is from 0.1 to 5.0 kW per one metre of the width of the blade.

7. A method according to claim 1, characterized in that a pipe is mounted at a distance of 0.01 to 0.7 m from the blade of the coating machine, drying air being blown through the pipe over the entire width of the blade, and the total drying capacity of the drying air flowing through the pipe is from 0.1 to 5.0 kW per one metre of the width of the blade.

8. A method according to claim 1, characterized in that a heat-exchanger is mounted in the machine circulation of the coating colour, the temperature of the coating colour being adjustable between +5°C and +90°C by means of the heat exchanger.

9. A method according to claim 1, characterized in that a heat-exchanger is mounted in the storage circulation of the coating colour, the temperature of the coating colour being adjustable between +5°C and +90°C by means of the heat exchanger.