FI82109B - Method for improvement of quality of a coating of a paper web - Google Patents

Description

82109

The present invention relates to a method for improving the quality of a paper web coating according to the preamble of claim 1.

It has been found that the drying method is very crucial for the quality of the coated paper and in particular the so-called mottling, i.e. the mottling of the coating. Uneven ink absorption occurs during the printing step. To understand the drying process, the drying operation should be divided into three different stages. Thus, in the drying process there is a so-called a critical step (solidification step of the coating paste) during which great care must be taken to obtain optimum coating quality. Outside this critical phase, high dryer evaporation powers can be used, i.e. before or after the critical zone for final humidity control. The beginning and end of the critical zone are thus very sensitive points, because in these the evaporation power changes very quickly.

The object of the present invention is to provide a new control method by means of which the above-mentioned critical step can be effectively controlled in order to obtain a good quality coating on a moving paper web. The invention is based on the fact that the drying is carried out in three successive stages, so that the first and second stages, the latter of which is the critical stage, are adjusted on the basis of the moisture reading of the web measured between the first and second and second and third stages. More specifically, the method according to the invention is characterized by what is set forth in the characterizing part of claim 1.

According to the invention, it has been found that the critical phase can be controlled and also controlled by means of humidity meters 2 82109. A good correlation between the moisture content of the mottling and the surface of the coated paper provided by the moisture meter has been observed when the moisture meters are installed at both ends of the critical stage and the drying profile is changed so that the evaporation efficiency of the meters is very much changed.

Quality control with respect to mottling is thus possible by means of a control system which adjusts the drying power of the drying section of the coating machine at different stages of the process to be optimal for the quality of the product. It is possible to obtain the control parameter by means of a device which reacts to moisture, in particular to the moisture of the coated surface of the product. The critical humidity limits are determined by a drying study and printing tests. Once the limit humidities at the beginning and end of the critical phase are obtained, the control system keeps them constant by adjusting the drying of the pre-critical phase and the critical phase.

The invention provides considerable advantages. Thus - fine tuning of the coating quality can be performed, - quality can be adjusted quickly after process changes, - the effect of temporary disturbances on drying can be compensated, shorter machine lay-out can be obtained, - the method gives a better understanding of the drying process, easier to maintain uniform and good quality, - species-specific drying parameters can be used, and - a primer level is obtained more quickly after line breaks.

The invention will now be examined in more detail with the aid of the accompanying drawing. The drawing schematically shows one embodiment of the method according to the invention.

3 82109

In the drawing, the paper web 4 coming from the coating station 5 in the direction of arrows C passes through rollers 10 and 11 to cylinders 12-14. In the area between rollers 10 and 11, the web 4 passes through three successive drying zones 1-3. The drying powers of these zones 1-3 are individually adjustable. Zone 1 is the zone preceding the critical phase, zone 2 itself covers the critical phase and in zone 3 the final moisture of the web 4 is adjusted. Zones 1-3 are preferably operated on an infrared or fluid bed drying principle.

A first moisture meter 6 is arranged between zones 1 and 2, which measures the moisture of the web 4 at A by means of its sensor 8. A second moisture meter 7 is arranged between zones 2 and 3, which measures the moisture of the web 4 at B by means of its sensor 9, respectively. Based on the reading of 6, the drying power of the first zone 1 is adjusted to keep the moisture of the web 4 at the first set value kA at the beginning of the critical zone 2. The reading of the second humidity meter 7 B. The setpoints kA and kB have been determined either empirically or computationally so that the quality of the coating in the final product is optimized.

As mentioned, caution should be exercised at the critical stage (zone 2). For example, when the reading of the first humidity meter 6 is less than kA, the control system (not shown) reduces the drying power of the first zone 1 (i.e. the blowing speed and temperature in the case of fluidized bed drying and the electrical power in the case of infrared drying) until the set value kft is reached again. When kA is exceeded, the drying power in zone 1 is in turn increased. The humidity meter 7 monitors the humidity value of the end B of the critical zone 2, giving the control system the necessary stimuli to adjust the drying powers 4 82109 of both the first zone 1 and the second zone 2 so that the moisture of the web 4 remains at the set value kg. Simultaneous control of both the first zone 1 and the critical zone 2 is important because the drying zone cannot be allowed to exceed a certain maximum value Pmax in the critical zone. The critical area must also not be moved to zone 3, where effective drying is again allowed. At the beginning of the critical zone A, the coating paste on the web 4 is partially solidified and at the end B is completely solidified.

EXAMPLE

When using LWC-type coating paste and measuring the moisture of the web with surface moisture meters calibrated with air-conditioned paper samples, typical setting values are e.g.

kA = 13 ... 15% (humidity with air-conditioned paper kg = 10 ... 12% calibrated with the sample)

In the case of a fluidized bed dryer, the temperature of the blowing air in the critical zone must be <200 ° C and the blowing speed <30 m / s.

Within the scope of the invention, solutions that deviate from the above can be considered. Thus, it is also possible to measure the moisture of the web already leaving the coating station and to take the obtained reading into account when adjusting the drying power of the first zone.

Il

Claims (4)

A method for improving the quality of the coating on a paper web (4) coming from a coating station (5) and passing through a drying process (1-3), by controlling the drying effect of the drying process (1-3) , wherein - the drying is carried out in three successive zones (1-3), of which at least the first two (1-2) are individually adjustable, - the moisture content of the web (4) is fed with a first moisture meter (6, 8). ) at a site (A) between the first (1) and the second zone (2), - the moisture content of the web (4) is measured by a second moisture meter (7, 9) at a site (B) between the second ( 2) and the third zone (3), - starting from the ridge of the first moisture meter (6, 8), the drying effect of the first zone (1) is controlled to keep the moisture content of the web (1) at a first set value at the location (A) between the first (1) and the second zone (2), characterized in that the drying effect in both the first (1) and the second (2) the zone is controlled externally from the ridge of the second humidity meter (7, 9) to keep the moisture content of the web (4) at a second set value at the site (B) between the second (2) and the third (3) zone. Method according to claim 1, characterized in that the first and second intended values are determined empirically. Method according to claim 1, characterized in that the first and second intended values are calculated. β 82109 Method according to Claim 1, characterized in that the moisture content is also measured in the web coming from the coating station (5) and the impact is taken into account in determining the drying effect in the first zone (1). II