WO2007110474A1

WO2007110474A1 - Method and arrangement for controlling coat weight in coating a fibre web

Info

Publication number: WO2007110474A1
Application number: PCT/FI2007/050154
Authority: WO
Inventors: Tapio Anttila; Olli Niemi; Tarmo Sulopuisto; Aleksi Sipilä
Original assignee: Metso Paper, Inc.
Priority date: 2006-03-24
Filing date: 2007-03-22
Publication date: 2007-10-04
Also published as: FI118304B; FI20065193A0

Abstract

In coating a paper or board web, coating material is spread by means of two application devices (13, 14) on two surfaces of the web by means of two film-transfer rolls (11, 12), after which the total coat weight of the coated web (W) is measured. The amount of coating material consumed by each application device (13, 14) is measured and, based on the data, the ratio of coat weights applied onto the different surfaces of the web is calculated. The metering of coating material on the different surfaces of the web is adjusted in order to keep the ratio of coat weights and the total coat weight at a desired value. A first controller (23) controls the load of doctoring elements (15, 16) to keep the total coat weight at a desired value. A processor (24) receives information (M1, M2) on the coating material consumption of each application device (13, 14) and calculates the ratio of coat weights applied onto the different surfaces of the web. A second controller (24) changes, if required, the load of each doctoring element (15, 16) so that the ratio of coat weights being applied is at a desired value, the total coat weight remaining on the level defined b the first controller (23).

Description

Method and arrangement for controlling coat weight in coating a fibre web

The invention relates to a method for controlling machine-directional coat weight in two-sided or double coating of a paper or board web according to the preamble of claim 1. The invention also relates to an arrangement for implementing such a method.

In coating paper or board, either direct or indirect application can be used. In di- rect application, coating material is spread on the web as it moves past an application area on the surface of a counter roll. In indirect application, coating material is first spread on a film-transfer roll or other transfer surface from which it is transferred onto the web in a coating nip. Coating material is often applied excessively and the amount of coating that remains on the moving substrate is adjusted, i.e. coating is metered by means of a doctoring element. As the doctoring element can be used a doctor blade or a rotating doctor rod, which is pressed against the moving substrate. The load of the doctoring element is provided by means of loading hoses operating with pressure medium or mechanical force driving the doctoring element. It depends on the load of the doctoring element how much coating material as carried away from the application area along with the moving substrate. There are also contact-free coating methods that require no doctoring element. In these methods, all of the applied coating material remains on the web, whereby the metering of coating takes place by means of a pump or a valve that feeds coating into the application device.

Coat weight, which means the dry content of coating colour on paper as grams per square metre, is an important measure when evaluating the coating result. One tries to keep the profile of the coating layer in both machine direction and cross- direction of the web as even as possible by means of feedback controls. In the machine direction, the profile of the coating layer is most usually adjusted by changing the load of the doctoring element, hi the cross-direction of the web, the profile of the coating layer is adjusted by changing the load of the doctoring element locally at those points in which there is too much or too little of coating. A prerequisite of the profile control in both machine- and cross-direction is the accurate measurement of coat weight. In measuring coat weight is usually used a measuring frame which has a carriage moving over the whole width of the paper web and being provided with sensors.

In known methods, the machine-directional profile of coat weight is controlled by changing the pressure in the pressure-medium hoses which are arranged to press the doctoring element against the moving substrate. The usable adjustment area of the hose pressure is fairly narrow, typically 0.8-2.5 bar. Outside this area, coat weight cannot nowadays be controlled by automatic control. Too low a loading pressure causes runnability problems and too high a pressure speeds up the wear and tear of the doctoring element. The loading pressure of the doctoring element also affects the response of the control of the cross profile of coat weight. A considerable problem is that the pressure of the loading hose very easily drifts away from the optimal operating range.

The measurement of coat weight is based on measuring the dry weight of the web before the coating head and after the coating head. In two-sided coating, this way the total coat weight is discovered but not the coat weights applied on different sides of the web. Also in multi-layer coating, there can occur situations in which the coat weight cannot be measured between two successive coating occasions.

Usually, one wishes to apply different coat weights on different sides of the web. It is possible to measure the coat weights of each side separately by direct coat- weight measurement which is based on measuring different components of the coating material by means of infrared radiation. This measurement requires expensive infrared sensors and the coat weight obtained from it is strongly depend- ent on the coating colour recipe and the penetration of infrared radiation in the base paper. Compensating the fillers in the base paper is difficult. Typically by infrared measurement, a good idea of the cross profile of coat weight is obtained, but the information on the level of coat weight remains inaccurate.

Controlling the unequal sidedness of coat weight is important also in those ma- chines which have no two-sided measurement of coat weight. Such machines are a great majority of coating machines. Nowadays in these cases, one has to content with solely the control of the total coat weight. Then, coat weight is adjusted by changing the loading pressure of both doctoring elements for the same amount without the knowledge of on which side of the coated web the coat weight devi- ates from the target value.

The object of the invention is a control method that gives an opportunity to control the unequal sidedness of coat weight better than before.

To achieve the above-mentioned object and those which come out later, a method according to the invention is characterised by what is presented in the characterising part of claim 1.

Correspondingly, an arrangement according to the invention is characterised by what is presented in the characterising part of claim 9.

The idea of the invention is that the total coat weight is still adjusted by changing the loading pressure of both doctoring elements, but simultaneously the unequal sidedness of coating weights is corrected by a separate controller which is ar- ranged to transfer loading pressure from one doctoring element to the other, as required. The controller calculates changes, which are different of their sign but equal of their absolute value, for the loading pressure of each doctoring element. Consequently, this coat weight ratio adjuster will not affect the total coat weight but only how the coat weight is divided onto different sides of the web or, in multi-layer coating, onto layers being applied one upon the other. The consumption of coating material in each application device is measured by means of a measuring device in the machine circulation. Based on mass or volume flow, the consumption of coating material can be calculated on a long period of time extremely accurately. The coat weight can be calculated of the consumption of coating material by means of equation (1):

CW = ^■ SC (1) d - v

in which CW_m is calculated coat weight, Q is consumption of coating material, d is web width, v is web speed, SC is dry content of coating material.

Based on the coating material consumptions and the total coat weight measured from the web, the ratio of coat weights applied on different sides of the web is calculated. The coat weights on the bottom and top surface of the web can be calculated by means of equations (2) and (3):

CW CW_b = — & CW₁₀, (2)

* CW + CW

CW

CW₁ = ^-M CW₁₀. (3)

CW + CW

in which CW_mb is calculated coat weight on the bottom side of the web, CW_mt is calculated coat weight on the top side of the web,

CWtot is actual coat weight measured from the web, CWb is coat weight on the bottom side of the web proportioned to the total coat weight, CW_t is coat weight on the top side of the web proportioned to the total coat weight.

When knowing the coat weight on each side of the web, measures that change the load of doctoring elements can be applied appropriately to provide and maintain the desired coat weight ratio. For each paper or board grade, target values are given for the coat weights of the different sides of the web. The coat weight ratio adjuster aims at retaining a pre-determined coat weight ratio in the long run. The controller must be tuned slow, because calculation on the basis of the coating ma- terial consumption is not accurate on a very short period of time. The controller must also be locked so that it will not switch on immediately after a production break, because reliable measurement is obtained only when the process is stable enough.

The method according to the invention can also be applied in contact-free coating in which no doctoring elements are used. Then, an actuator for metering the coating can be, e.g., a pump or a valve through which the coating material is fed into the application device.

The new control method enables optimising and maximising the quality of the product (technical properties of paper) and better uniformity. The new adjusting manner also simplifies the control of the process and decreases the possibility of the machine operator's operating errors.

The coating material consumption data has not been, as far as is known, utilised in feedback control of coat weight, because the measurement is slow and the control thus functions with a delay. In the invention, these problems have been solved by combining the utilisation of the coating material consumption data with the traditional coat weight control which is based on measuring the total coat weight. The invention will now be described with reference to the figures of the accompanying drawings, to which the invention is, however, by no means intended to be solely restricted.

Fig. 1 shows a system according to the prior art for controlling coat weight in two- sided film-transfer coating.

Fig. 2 shows a system according to the invention in which, besides the total coat weight, the ratio of coat weights being applied on different sides of the web is controlled.

Fig. 1 schematically shows a coating head which utilises film-transfer technique and is provided with a coat weight control system according to the prior art. The coating head comprises two film-transfer rolls 11, 12, which form a nip through which the web W being coated is carried. Application devices 13, 14 and doctoring elements 15, 16 are shown only as reference. Coating material is carried from a machine circulation 17, 18 via a feed line 17', 18' to the application device 13, 14 which spreads the coating material on the surface of the film-transfer roll 11, 12. The coating film is smoothed and the excess is doctored away by means of the doctoring element 15, 16 which is pressed against the surface of the roll 11, 12. The amount of coating material leaving the application area along with the roll surface is dependent on the load of the doctoring element 15, 16, which is controlled by an actuator 25, 26.

The dry basis weight of the web W is measured before the coating head by means of a first measuring frame 19 and after the coating head by means of a second measuring frame 20. Signals G₁ and G₂ describing the dry basis weight are lead to a processor 21 which calculates the coat weight applied on the web. Signal CW describing the total coat weight, produced by the processor 21 , is compared with a coat weight set value CW₀ that has been entered via a workstation 22. Based on this comparison, a coat weight controller 23 gives the actuators 25, 26 a command signal S₁ which determines the pressure to be fed in the hoses loading the doctoring elements 15, 16. The actuator 25, 26 can be e.g. a pressure control valve.

Coat weight measurement as described above gives information on the total coat weight but it does not reveal how much of coating has been applied on each side of the web. Frequently, one wishes to have different coat weights on the top and bottom surface of the web. Coating materials to be applied on different sides of the web can also be different. The properties of base paper, such as water absorption ability, are frequently different on different sides of the web. As there is no possibility for separate measuring of coat weights, a separate adjustment of the load of the doctoring elements 15, 16 cannot be implemented. The coat weight controller 23 adjusts the total coat weight, whereby a similar control signal Si is given to the actuator 25, 26 controlling the load of each doctoring element 15, 16 irrespective of on which side the possible deviation from the set value of coat weight occurs.

Fig. 2 shows an improved system according to the invention, which comprises a coat weight ratio adjuster 24 which monitors and adjusts the coat weights being applied on different sides of the web. The control of the total coat weight takes place in a similar manner as in Fig. 1. The processor 21 calculates, based on the dry basis weights G₁, G₂ measured of the web W, the total coat weight CW which is compared with the set value CW₀ of coat weight. Based on the comparison, the coat weight controller 23 produces a control signal Si for adjusting the loading pressures of the doctoring elements 15, 16. The change of loading pressure will not, however, be divided equally for both doctoring elements 15, 16, but it is divided in a manner defined by the coat weight ratio adjuster 24.

The coat weight ratio adjuster 24 receives information on the total coat weight from the processor 21 and signals Mi, M₂ describing the coating material con- sumption from the machine circulation 17, 18 of each application device 13, 14.

Based on these, the coat weight ratio adjuster 24 calculates the ratio of coat weights applied on the bottom and top surface of the web. This ratio is compared with the set value RWo given to it via a workstation 22. If a deviation occurs, the coat weight ratio adjuster 24 gives the actuators 25, 26 that control the load of the doctoring elements 15, 16 control signals +S₂ and -S₂, which are equal but with different signs, for correcting the difference. The outcome is that the load of the doctoring elements 15, 16 is changed for an equal amount but in different directions, whereby as the ratio of coat weights changes, the total coat weight remains constant.

Many variations of the invention are possible within the scope defined by enclosed claims. The method can be applied in both two-sided and double coating in which two coating layers are applied successively on the same side of the web without measuring the coat weight in between. The application method can be direct or indirect. The actuator for metering the coating material can be a doctor blade or a rotating rod which is pressed against a transfer surface or a web by means of compressed-air hoses or other loading devices known as such. When the method is applied for contact-free coating (spray coating, curtain coating, etc.), the actuator for metering the coating material can be e.g. a pump or a valve.

Claims

1. A method for controlling coat weight in two-sided or double coating of a paper or board web, in which method: - coating material is applied by means of a first application device (13) on a first moving substrate (11), coating material is applied by means of a second application device (14) on a second moving substrate (12), the total coat weight of the coated web (W) is measured and actuators (15, 16), which meter coating material on the first and the second moving substrate (11, 12), are controlled based on the measurement, characterised by measuring the amounts of coating material consumed by the first and the second application device (13, 14) in a time unit and calculating, based on the measured amounts, a ratio of coat weights applied on the first and the second moving substrate (11, 12), and controlling the actuators (15, 16) which meter the coating material to keep the ratio of coat weights at a desired value simultaneously as the total coat weight remains constant.

2. A method according to claim 1, characterised by comprising stages in which

- based on the total coat weight, a first command signal (Sj) is produced for controlling the actuators (15, 16) which meter coating material on the first and the second moving substrate (11, 12), and - if the ratio of coat weights differs from a target value, a second command signal (S₂) is produced, based on which the first command signal (Si) intended for the actuators (15, 16) is modified so that two new command signals (Si+S₂, S]-S₂) are created, the combined effect of which is equal to the effect that the first command signal (Si) alone would have had.

3. A method according to claim 1 or 2, characterised in that said first and second moving substrate are a first and a second film-transfer roll (11, 12) from the surfaces of which coating material is transferred onto the web (W) in a nip formed by the rolls (11, 12).

4. A method according to claim 1 or 2, characterised in that the moving substrate is a web onto the first surface of which is applied a first coating layer by means of the first application device and onto the second, opposite surface of which is applied a second coating layer by means of the second application device.

5. A method according to claim 1 or 2, characterised in that the moving substrate is a web onto the first surface of which is applied a first coating layer by means of the first application device, and onto the same, first surface of which is applied a second coating layer by means of the second application device.

6. A method according to any one of preceding claims, characterised in that said actuators (15, 16) for metering coating material onto the moving substrate (11, 12) are doctoring elements (15, 16), which are pressed against the moving substrate (11, 12) after application the coating material.

7. A method according to claim 6, characterised in that as the doctoring elements are used doctor blades (15, 16) or rotating doctor rods which are pressed against the moving substrate (11, 12) by means of loading hoses operating with pressure medium or other loading devices known as such.

8. A method according to any one of claims 1-5, characterised in that said actuators for metering coating material onto the moving substrate are pumps or valves through which the coating material is fed into the application devices.

9. An arrangement for controlling coat weight in two-sided or double coating of a paper or board web, which arrangement comprises: - a first application device (13) for applying coating material on a first moving substrate (11),

- a first actuator (15) for metering coating material, which actuator (15) is in co-operation with the first application device (13), - a second application device (14) for applying coating material on a second moving substrate (12),

- a second actuator (16) for metering coating material, which actuator (16) is in co-operation with the second application device (14), means (19, 20, 21) for determining the total coat weight of the coated web (W), and a first controller (23) for controlling the actuators (15, 16) metering coating material so that the total coat weight remains at a desired value, characterised in that it further comprises a processor (24) which receives information (M₁, M₂) on the coating mate- rial consumption of each application device (13, 14) and calculates the ratio of coat weights applied onto the first and the second moving substrate . (11, 12), and

- a second controller (24) which controls the actuators (15, 16) metering coating material so that the ratio of coat weights remains at a desired value simultaneously as the total coat weight remains in the level defined by the first controller (23).

10. An arrangement according to claim 9, characterised in that said actuators for metering coating material are doctoring elements (15, 16) which are pressed against the moving substrate (11, 12) after the application of coating material.

11. A method according to claim 9, characterised in that said actuators for metering coating material are pumps or valves which control the feed of coating material into the application device.