Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/78—Controlling or regulating not limited to any particular process or apparatus
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C11/00—Component parts, details or accessories not specifically provided for in groups B05C1/00 - B05C9/00
  • B05C11/10—Storage, supply or control of liquid or other fluent material; Recovery of excess liquid or other fluent material
  • B05C11/1002—Means for controlling supply, i.e. flow or pressure, of liquid or other fluent material to the applying apparatus, e.g. valves
  • B05C11/1005—Means for controlling supply, i.e. flow or pressure, of liquid or other fluent material to the applying apparatus, e.g. valves responsive to condition of liquid or other fluent material already applied to the surface, e.g. coating thickness, weight or pattern
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C1/00—Apparatus in which liquid or other fluent material is applied to the surface of the work by contact with a member carrying the liquid or other fluent material, e.g. a porous member loaded with a liquid to be applied as a coating
  • B05C1/04—Apparatus in which liquid or other fluent material is applied to the surface of the work by contact with a member carrying the liquid or other fluent material, e.g. a porous member loaded with a liquid to be applied as a coating for applying liquid or other fluent material to work of indefinite length
  • B05C1/08—Apparatus in which liquid or other fluent material is applied to the surface of the work by contact with a member carrying the liquid or other fluent material, e.g. a porous member loaded with a liquid to be applied as a coating for applying liquid or other fluent material to work of indefinite length using a roller or other rotating member which contacts the work along a generating line
  • B05C1/0817—Apparatus in which liquid or other fluent material is applied to the surface of the work by contact with a member carrying the liquid or other fluent material, e.g. a porous member loaded with a liquid to be applied as a coating for applying liquid or other fluent material to work of indefinite length using a roller or other rotating member which contacts the work along a generating line characterised by means for removing partially liquid or other fluent material from the roller, e.g. scrapers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C9/00—Apparatus or plant for applying liquid or other fluent material to surfaces by means not covered by any preceding group, or in which the means of applying the liquid or other fluent material is not important
  • B05C9/04—Apparatus or plant for applying liquid or other fluent material to surfaces by means not covered by any preceding group, or in which the means of applying the liquid or other fluent material is not important for applying liquid or other fluent material to opposite sides of the work

Definitions

• the invention relates to a method for controlling machine-directional coat weight in coating a paper or board web, in which coating material is applied on a moving substrate and metered by means of a doctoring element and in which the load of the doctoring element is adjusted based on the coat weight measured from the web.
• the invention also relates to an arrangement for implementing such a method.
• direct or indirect application can be used.
• direct application coating material is spread on the web as it moves past an application area on the surface of a counter roll.
• 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 applied excessively and the amount of coating that remains on the moving substrate is adjusted by means of a doctoring element.
• a doctoring element As the doctoring element is frequently 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 is carried away from the application area along with the moving substrate.
• 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.
• the profile of the coating layer is most usually adjusted by changing the load of the doctoring element.
• 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.
• 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.
• the machine-directional profile of coat weight is adjusted 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 coat weight cross profile. A considerable problem is that the pressure of the loading hose very easily drifts away from the optimal operating range.
• the dry content of coating material would be maximal, which means also maximal doctoring pressure. Frequently because of the diluting or dilution of the coating material or for some other reason, the dry content of coating material and thus the doctoring pressure are lower than desired. It is difficult for the machine operator to identify a corrective measure, because the coat weight is affected by many different variables, such as run speed, the properties of base paper, the dry content of coating material and the loading pressure of the doctoring element.
• the object of the invention is a control method by means of which the loading pressure of the doctoring element can be controlled better than earlier to keep it within a desired operating range.
• a method according to the invention is characterised by what is presented in the characterising part of claim 1.
• the loading pressure is prevented from drifting away from an optimal operating range by monitoring the load of the doc- toring element and by adjusting the dry content of coating material based on it.
• the loading pressure is possible to keep the loading pressure in a usable window for the whole time.
• Quick changes of coat weight are adjusted by changing the loading pressure of the doctoring element in the same way as before, and the slow adjustment of the operating range of loading pressure takes place by changing the dry content of coating material.
• Coat weight is affected by many different factors. A great deal of them relates to the properties of base paper, whereby they cannot be affected any more in the coating stage.
• An important controllable variable is the dry content of coating ma- terial. Its control is, however, comparatively slow, because the volume of coating material is large and changing the consistency of the coating material in the whole machine circulation takes a long time.
• a control system comprises two controllers, the first of which controls the load of the doctoring element based on the coat weight measured of the web, and the second controls the dry content of coating material based on the load of the doctoring element for keeping said load within a desired operating range.
• the pressure of the loading hose is given a target level in the vicinity of which pressure is desired to remain.
• the controller changes the dry content of coating material by increasing or decreasing the feed of dilution water into the coating material in the machine cir- culation. Then, the pressure of the hose can always be kept within a range in which no runnability problems or excessive wear and tear of the doctoring element occur. It is also easier to model the response of the control of the coat weight CD-profile as the pressure of the loading hose remains within a range in which the response can be assumed to be constant.
• DiIu- tion water is fed via a control valve to a machine container, which is provided with level control.
• coating material is guided via an air remover and a sieve to an application device from which there is a return circulation back to the machine container. Only a small part of coating material in circulation becomes applied on the web.
• the dry content of coating material can change during the circulation, e.g. because of the fact that the fibre web being coated absorbs more water than solids.
• the doctoring element When the dry content of coating material is very high and one wishes to apply a thin coating layer on the web, the doctoring element has to be loaded by high pressure. When in this situation the dilution of coating material is increased, the coat weight being applied on the web tends to decrease. The feedback control of coat weight leads then to decrease of the loading pressure of the doctoring element.
• the load of the doctoring element is low e.g. when coating material is dilute and one wishes to apply a thick coating layer on the web.
• the dilution of coating material is decreased, the coat weight being applied on the web tends to increase.
• the feedback control of coat weight leads then to increase of the loading pressure of the doctoring element.
• the new control system is automatic and user-friendly and enables increasing the change interval of the doctoring element. It also conserves the loading apparatus and improves the quality of the end-product and the uniformity of quality.
• the dry content of coating colour prepared in a coating colour kitchen can still be adjusted in the machine circulation. In the long run, the runnability of the coating machine improves and breaks become less frequent.
• 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 the dry content of coating material is controlled for keeping the load of a doctoring element within a desired range.
• 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 to be coated is guided to run.
• 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 a doctoring element 15, 16 which is pressed against the surface of the roll 11, 12.
• the amount of coating material exiting from 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 G 1 and G 2 describing the dry basis weight are lead to a processor 21 that calculates the coat weight applied on the web.
• Signal CW describing the total coat weight, which is produced by the processor 21, is compared with a coat weight set value CW 0 , which has been entered via a work station 22. Based on the comparison, a coat weight controller 23 gives the actuators 25, 26 a command signal S 1 which determines the pressure to be fed to the hoses loading the doctoring elements 15, 16.
• the actuator 25, 26 can be e.g. a pressure control valve.
• Fig. 2 shows a control system according to the invention, which comprises dilution controllers 27, 28 for adjusting the dry content of coating material based on the load of the doctoring elements 15, 16.
• the control of the total coat weight takes place in a similar manner as in Fig. 1.
• the processor 21 Based on the dry basis weights G 1 , G 2 measured from the web W, the processor 21 calculates the total coat weight CW, which is compared with the set value CWo of coat weight. Based on this comparison, the coat weight controller 23 produces a control signal Si for changing the loading pressures of the doctoring elements 15, 16.
• the system measures the load of the doctoring elements 15, 16 and, based on the measurement, the dry content of coating material to be fed to the application devices 13, 14 is adjusted.
• the control takes place by means of the dilution controllers 27, 28 which receive signals L 1 , L 2 describing loading pressure from the actuators 25, 26 and compare them with target values L 10 , L 20 of load pressure entered via the work station 22.
• the dilution controller 27, 28 Based on the compari- son, the dilution controller 27, 28 produces a control signal S 2 , S 3 to feed valves 31, 32 which are arranged in a dilution water feed line 29, 30. Via the feed line 29, 30 and the feed valve 31, 32, dilution water is fed into the coating material in the machine circulation 17, 18 of coating material preceding the application device 13, 14.
• the dilution controller 27 recognises that the load of the doctoring element 15 is too high, it gives a command to the dilution valve 31 to increase the feed of the dilution water into the coating material.
• the dilution controller 27 calculates the position of the valve required for the change of dry content based on the entry flow of coating colour.
• the coat weight controller 23 gives the actuator 25 a command to decrease the load of the doctoring element 15 in order for the coat weight to remain at a desired value. Therefore, the control of the load of the doctoring element does not compromise the coat weight control and the load can be continuously kept within the optimal operating range.
• the method can be applied in both one- and two-sided coating.
• the application method can be direct or indirect.
• the application device can be any known device by means of which coating material is applied on a moving surface excessively.
• the doctoring element can be a doctor blade or a rotating rod. It can be loaded against a transfer surface or a web by means of compressed-air hoses or other loading means known as such.

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• Application Of Or Painting With Fluid Materials (AREA)
• Coating Apparatus (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (2)

Cited By (1)

Citations (8)

• 2006
  • 2006-03-24 FI FI20065194A patent/FI118305B/fi active IP Right Grant
• 2007
  • 2007-03-22 WO PCT/FI2007/050155 patent/WO2007110475A1/en active Application Filing

Patent Citations (8)

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