Method and arrangement for improving the usability of a slide-fed curtain coater and reducing the loss of coating material

The invention relates to a method for improving the usability of a slide-fed curtain coater, which is used for coating a fibrous web, and for reducing the loss of coating material, said curtain coater having a nozzle unit which comprises at least two adjacent nozzle parts and which nozzle unit includes a nozzle slot between said at least two adjacent nozzle parts, which extends in a longitudinal direction of the coating device and which has coating material supplied across its entire breadth and further discharged through an outlet opening of the nozzle slot onto a flow plane established by a top surface of the nozzle parts. The invention relates also to an arrangement for performing this method.

An objective of the present invention is to provide an improvement in a slide-fed curtain coating device intended for the application of a paper/board web coating paste, especially in connection with web breaks. Currently available curtain coaters of a coating material involve several problems, which all have an adverse effect on the usability and runnability of an entire fibrous web production line. The nozzle unit of a slide-fed curtain coater has generally its feeding chambers washed and flushed during the occurrence of web breaks and the like malfunctions leading to a momentary stoppage of the production line in order to prevent the coating material from drying and the slot from becoming blocked.

Essential problems in such a procedure include the duration of a washing sequence and the coating paste going to waste as the coating process is restarted for as long as the removal of all water from the beam is successfully completed. Operations performed in cleaning engage workforce unnecessarily when it would be rather needed for mending defects caused by the actual break and for preparations regarding a restart of the rest of the fibrous web production line. Another problem in a currently applied washing method is constituted by an increased risk of air bubbles developing inside the beam. These bubbles cause flaws in the coating curtain and end up in the coating with a degrading effect on its quality. Moreover, the effluent resulting from a washing process of the beam and containing coating agents also incurs unnecessary extra costs.

Another adverse influence is that changes occurring in the temperatures of a nozzle unit during a break may have a harmful impact on the quality of a curtain coating, especially right after a restart. In the event that the temperature of nozzle unit structures finds a way to decrease at any locations, this may exert an impact on the width of nozzle slots across the entire breadth of a web and thereby also on the cross-profile of the coating.

Accordingly, the present invention is aimed at providing an improved curtain coater, which first of all is quickly activable immediately after a break in the run and which also enables avoiding the other foregoing problems at the same time. The invention provides a solution in which the nozzle unit washing sequence has been upgraded in such a way that the beam's feeding chamber need not be emptied and washed in web breaks and other short-term interruptions. Also, with regard to all of its running parameters, the nozzle unit is maintainable all the time at values consistent with its normal operating mode, such that the quality of curtain coating is flawless immediately after a restart and thereby the amount of waste products is minimized.

In order to accomplish this aim, a method of the invention is characterized by what is presented in the characterizing clause of the first claim. An arrangement of the invention for implementing this method is in turn characterized by what is presented in the characterizing clause of the 9th claim.

Preferred embodiments and further evolutions of the invention are disclosed in the dependent claims.

The invention will now be described more closely with reference to the accompanying drawing, in which:

Hg. 1 shows in a schematic cross-sectional view a nozzle unit for one slide- fed curtain coater, in which the nozzle unit has its top edge provided, among others, with a washing jet according to one exemplary embodiment of the invention.

Referring to fig. 1, there is shown one nozzle unit 1 for a slide-fed curtain coater. The nozzle unit comprises nozzle parts 3, 3a and 32 mounted on top of a support

structure (not shown), which jointly constitute a nozzle unit including three nozzle slots 30, 30b. This design also enables multiple curtain coating. The nozzle unit has one of its ends provided with coating paste feeding pipes (not shown), which open into feeding chambers 12. The substance to be fed travels along a feeding chamber towards the opposite end, which is optionally provided with a by-pass route. When advancing in the feeding chamber, the substance to be discharged passes at each point of the feeding chamber's length into a equalizing chamber 13 and thence further into the nozzle slot 30 across the entire width thereof. Presented in the figure are also directions used in this application, i.e. a longitudinal direction W of the coating device, a height of the nozzle parts and at the same time a longitudinal direction L of the nozzle slot 30, and a lateral direction D of the nozzle slot. Thus, in the applicator beam 1 shown in fig. 1, a coating curtain 5 is made up from three individual layers of coating material. In order to create the coating curtain 5, the successive layers of coating material (not shown), discharging from the nozzle slots' 30 outlet openings 31 and trickling along a nozzle unit's flow plane 4 established by the nozzle parts' top surfaces, are guided over a feeding lip established by an edge 41 of the applicator beam.

The above-described nozzle unit design, as well as its operation, is essentially known in the art and explained in this context just for background information and easier understanding. The invention is by no means limited to just this particular type of slide-fed curtain coating device.

Hence, when e.g. in the event of a web break or some other malfunction leading to a stoppage of the fibrous web production line, the supply of coating paste to the applicator beam is also discontinued, the result will be a hazard of the coating material drying or becoming deposited in nozzle slots, along a flow plane, as well as in flow paths and chambers. In order to avoid this, the currently employed washing sequence has involved a process of cleaning and washing the feeding chambers clear of a coating material, the same being done on the flow plane itself. All this requires plenty of labor and thereby engages manpower and most importantly takes time during a break, which perhaps otherwise would prove to be very short indeed. In general, the duration of web breaks is typically within the range of 7 minutes to 1 hour, in on-line machines typically less than 10 minutes. When using a current washing sequence, based on the washing and flushing of feeding chambers, this

break duration may easily double. Another drawback in the above type of washing process is the resulting large quantity of wash water containing coating material, the proper treatment of which water incurs more costs.

As shown in fig. 1, one exemplary embodiment of the invention is provided with means for flushing the flow plane with a flushing liquid. In this embodiment of the invention, the nozzle unit has its upper edge 35 provided with a washing jet 36 covering the entire breadth of the beam, wherein flushing liquid is adapted to be delivered from a small gap 33 whenever the supply of a coating paste is stopped.

This flushing liquid cleans the beam's flow plane of coating material while preventing the coating material from drying in the nozzle slot's 30 top section 31. The flushing liquid is preferably dispensed in such an abundant amount that said flushing liquid establishes a substantially unbroken layer as it trickles along the flow plane 4. After the coating materials have flushed away from the flow plane, the amount of flushing liquid is basically reducible, yet making sure that there is no coating material drying in the nozzle slots' outlet openings 31. In another possible arrangement, the flushing liquid remaining in passages after the flushing is concluded is allowed to escape at least partially from the system of passages, thus making sure that no more flushing liquid is allowed onto the flow plane during the course of an actual working cycle.

Thus, the flushing is not brought to a total closure until just before a restart. The flow plane can be easily dried simply in a manual fashion, for example by wiping with an appropriate piece of cloth. The commencement of a coating process can be performed very quickly, the beam's feeding chambers being full of ready-to-apply coating paste. Likewise, by having the flushing liquid at an appropriate temperature in a flushing process, the beam temperature can be maintained consistent and in line with a normal operating temperature even during a break. Thus, among others, the sizing of nozzle slots remains constant and the coating can be given a flawless cross-profile right from the start of a run. Depending on coating materials, the operating temperature of a coating material and a nozzle unit is typically in the order of +20 ⁰ C to +45°C. Regulating the temperature as well as the amount of flushing liquid makes it possible to control also the temperature of a beam and the

consistent temperature distribution thereof during a break and to retain it within the above-mentioned range.

Hence, an essential benefit offered by a method of the invention is a possibility of maintaining a slide-fed curtain coating device in a simple and cost-cutting manner in a very quickly activable readiness, i.e. in a sort of standby status, in relation to restarts in the event of malfunctions as well as also restarts e.g. after shutdowns, A method of the invention can be applied to set the curtain coating device in a ready- to-operate condition and to retain it in that condition while the rest of a fibrous web production line is being prepared for activation. As soon as the rest of the production line is started, the curtain coating device can be put to service immediately.

A useful flushing liquid may even be water alone, or water can be supplied at least at some point of a flushing process with additives or detergents or surface finishes suitable for reducing a surface tension and enhancing a washing process with regard to the qualities of each coating material. At the same time, these additives can be used to resist the formation of films on a flow plane. Such films impede the sliding of a coating curtain along the flow plane and thereby cause disturbances in the coating curtain. A consequence of flushing with water alone is that the flow plane is liable to develop a film which hinders a uniform spreading of the coating material throughout the slide in connection with a restart.

The washing process can be accompanied by a mechanical cleaning as well, e.g. simply by wiping with a sponge or by means of an appropriate, especially automatically controlled appliance. Another improvement in terms of flushing and washing can be provided by using particularly dissolving flushing liquids in the early part of a flushing cycle. After the coating materials have been substantially flushed off, it is possible to use additives for removing or impeding the formation of the above-mentioned possible films which have developed on the surface of a flow plane.

The flushing liquid is of course also suppliable onto a flow plane by other useful techniques, for example by mist blowing, delicate spraying and sprinkling or in other such fashion or by a combination of the above. In fig. 1, means for spraying

water are indicated by reference numeral 66. Likewise, it is possible to provide a supply pipe extending crosswise of the flow plane and represented by reference numeral 68, from which the flushing liquid can be delivered onto the flow plane across the full width thereof. The above dispensing elements for flushing liquid can also be designed to be movable both laterally and longitudinally of the nozzle unit. In addition to the design shown in the embodiment of fig. 1, the washing jet 36 can be equally well provided e.g. directly between the nozzle parts 32 and 34 and the distributing chamber 37 for flushing liquid can be provided in the nozzle part 34.

In any event, in the process of supplying the flushing liquid onto a flow plane, it is advisable to avoid splashes and to make sure that the ambient gas volume, for example moist air alone, no longer during a restart procedure needlessly contains droplets which could have an adverse impact on the film of coating material on top of the flow plane and further on the curtain of coating material. Likewise, in a preferred arrangement, it is advisable to disable the development of e.g. condensation-inflicted droplets on surfaces defining and included within a gas space or generally on surfaces above the flow plane or to prevent the dripping thereof onto the flow plane, especially during normal running.

During the course of a run, it is preferable to establish optimum conditions in the ambience of a flow plane for the behavior of coating material layers, i.e. a gas volume which is stable in terms of its properties, substantially moist and in the same category as the nozzle unit and coating material in terms of its temperature. A gas space 62 surrounding the flow plane can be given for example a substantially closed design, e.g. by means of an appropriate hood 60 which can further be designed movable. It is preferred that this gas space 62 be provided with air moisture and temperature regulation. It is preferred that the control of said gas space be adapted to a flow plane flushing and washing arrangement of the invention such that, in the event of breaks, the gas space surrounding the flow plane can be quickly established after a washing cycle in a condition as favorable as possible from the viewpoint of coating material layers. In a method of the invention, the washing operation of a nozzle unit and the regulation of a gas space surrounding the flow plane are easily arranged to proceed automatically without actions performed by the staff.

Drying of a flow plane can be performed not only by simple manual drying but also by means of automated scrapers, bars or the like 64 for removing water from the flow plane prior to a restart. Drying can also be performed, for example, by means of appropriate air blowing.

According to a further aspect of the invention, the delivery of a flushing liquid onto a flow plane can also be effected by way of one or several nozzle slots 30, 30b, 33. Thus, in the event that one or several of the nozzle slots are not needed for producing a coating, such slots can instead be used during a break for the feeding of flushing liquid onto a flow plane. Most preferably, this purpose is served by the topmost nozzle slot or, in any case, a nozzle slot located higher up than the topmost nozzle slot currently in coating material service. In the nozzle unit shown in fig. 1, for example, the flushing liquid can be arranged to be supplied onto the flow plane 4 by way of the topmost nozzle slot 30b.

The feeding chamber and other passages can be preferably adapted to be drained of flushing liquid as soon as the washing cycle is finished and normal running is restored. Naturally, this applies also to other elements mentioned in optional embodiments for passing flushing liquid onto a flow plane. This way, a possible passage of flushing liquid onto a flow plane during normal operation is avoided.

In one evolution of the invention, the flushing is extended, in addition to the foregoing, also into the actual nozzle slots, as well as into flow paths and feeding chambers, on a counterflow principle. In other words, the flushing liquid is arranged to flow from a flow plane into nozzle slots and further to internal members of the nozzle unit in a direction reverse with respect to the flow of coating materials. At the same time, this provides a beneficial effect on the efficiency of a cleaning process. Separate pressurization of flushing liquid is not necessary as it is possible to carry out washing and flushing by means of gravity alone. The washing of an applicator beam effected as described above can be performed either as a continuous flow or the chambers and other flow paths can be filled with flushing liquid, its dissolving effect can be allowed an appropriate time and flushing can be continued with flowing flushing liquid in the next cycle. Likewise, the draining of feeding and distributing chambers filled with coating material prior to the application of flushing liquid can be arranged to proceed as appropriate or by

means of the flushing liquid itself. The flow of flushing liquid and thereby the effective extent of flushing and washing can be limited to desired members of an applicator beam, e.g. by means of existing valve elements or the like.

Especially in the event of a lengthy break, if it is nevertheless necessary to ultimately drain an applicator beam of coating materials, the washing of internal elements of the applicator beam can be performed as described by using a counterflow washing process through nozzle slots. Similarly, in the process of restoring an applicator beam to a ready-to-operate condition, it is possible to proceed as described above, i.e. the beam is supplied with full load of coating materials, the flushing and washing of the flow plane is initiated and readiness for a commencement of the coating process is maintained.