It is an object of the present invention to provide an improvement in a cur- tain coater intended for the application of a paper/board web coating paste.

Curtain coaters can be categorized as slot-fed or slide-fed coaters. In the slide-fed coater, the coating is fed by means of a nozzle unit onto an inclined plane, the coating flowing therealong towards the edge of the plane to form a curtain as the coating drips from the edge of the plane. In slot-fed applica- tor beams, the coating is pumped via a manifold chamber into a narrow ver- tical slot, the curtain forming at its lip and drips onto a web. The coating can be applied in one or several layers. The curtain thus formed is guided by means of an edge guide, which, as suggested by its name, is located at the edge of a feed slot/feed lip.

The current curtain coaters for a coating paste are hampered by several problems, all having an adverse effect on the manageability of a cross-profile in the coating paste. In curtain coating, the nozzle slot must have its width strictly within certain size limits for feeding a proper amount of coating paste onto the top surface of a nozzle unit and further onto a web to be coated. In practice, this has called for highly precise machining, as well as a careful management of thermal loads applied to the nozzle unit.

The currently available nozzle units constitute a part of a support structure.

Being clamped together, the nozzle units provide a single solid supporting unit by means of external bracing. Such a construction causes certain prob- lems in manufacturing. The supply ducts for a coating paste are all machined in a nozzle part, which comprises a single, integral, solid bar-shaped block and, hence, in the process of machining supply ducts it is also necessary to consider structural strength aspects. In this respect, a coexisting problem is also the limited size of supply ducts, and the problem of managing the size of a flow slot resulting partly therefrom. Thus, the system of supply ducts is subjected to certain limits regarding both size and shape, thus curtailing op- tions for sizing the ducts to provide optimal flow conditions for a coating paste.

Due to the structure of the prior art nozzle beam, there will also be high pre- cision requirements in manufacturing technique. All inaccuracies in manufac- turing, regarding both the supply duct systems and particularly the nozzle slot for a coating paste, have an irreversible effect on profile. This problem becomes more pronounced as the size of pieces and the feeding width of a coating paste increase. The nozzle blocks/beams of current manufacturing technology have a maximum width in the order of 4,5 m and, consequently, are not suitable for wide machines.

In addition, the management of cross-profile is also problematic in varying applications when using a curtain coater of the prior art. This applies particu-

larly to a slide-fed coater. The shape of supply ducts, which can have an ef- fect on overall profile, is determined in the process of designing an applicator block/beam. As the coating paste varies in terms of its properties and/or supply rate, the latter have a distinct effect on cross-profile, which can no longer be corrected.

Accordingly, one important objective of the present invention is to provide an improved curtain coater, which firstly enables a consistent supply of coating paste across the entire width of a nozzle unit and thereby an improved man- agement over the cross-profile of the amount of coating, and secondly pro- vides substantially less demanding requirements in terms of manufacturing technique and is thereby more attractive in terms of its manufacturing costs, as well.

In order to accomplish this objective, a coating apparatus of the invention is characterized in that the feeding chamber is defined entirely within one of the nozzle parts in a position substantially offset relative to the nozzle slot and the equalizing chamber, such that the delivery of a coating paste from the feeding chamber into the equalizing chamber is effected by way of a flow passage provided inside the nozzle part.

An applicator according to one preferred embodiment of the invention is characterized in that the feeding chamber is designed to be convergent in its lengthwise direction from the inlet end of supplied substance towards the opposite end, and that at least one of the feeding chamber's walls is adjust- able for setting the feeding chamber's convergence as desired.

The invention relates also to a method for coating a fibrous web, said method employing an apparatus as set forth above.

The invention will now be described in more detail with reference to the ac- companying drawings, in which:

Fig. 1 shows in a schematic perspective view one nozzle unit for a slide- fed curtain coater, which can be implemented according to the in- vention, fig. 2 shows in a schematic cross-sectional view one nozzle unit of the prior art for a multiple curtain coater, fig. 3 shows in a schematic cross-sectional view one embodiment for nozzle parts containing a feeding chamber and a nozzle slot in the nozzle unit of a coater as shown in fig. 2, fig. 4 shows in a schematic cross-sectional view another embodiment for a nozzle part in the nozzle unit of a curtain coater as shown in fig.

2.

Fig. 1 shows in general view one applicator beam 1 for a slide-fed curtain coater. The beam can be constructed also according to the present inven- tion. The applicator beam comprises nozzle parts 3,3a, and 32, which are mounted on top of bearers 2 providing a support structure and which to- gether establish a nozzle unit with three nozzle slots 30 to enable multiple coating.

At one end of the nozzle unit are arranged feeder pipes 4 for a coating paste to be fed, which open into feeding chambers 12 (fig. 2). The substance be- ing fed passes along the feeding chamber towards the opposite end, which is optionally provided with a by-pass. While advancing in the feeding chamber, the substance to be fed moves at each point over the length of the feeding chamber to the equalizing chamber 13 and thence further to a nozzle slot 30 over the entire width of the nozzle slot 30 (fig. 2).

In this apparatus, the nozzle parts 3 and 32 are arranged in a movable man- ner on top of the bearers 2. In view of shifting the movable nozzle parts 3,

32, the applicator block includes a fixed support 5 resting on the bearers 2, on one side of which, between the rearward nozzle part 32 and the fixed support 5, is a closing tube 6 and on the opposite side of the fixed support is an opening tube 7, whereby the closing tube 6 can be pressurized for shift- ing the nozzle parts 3,32 towards the immobile nozzle part 3a to close the nozzle unit and pressure can be released from the closing tube 6 and the opening tube 7 can be pressurized for shifting the movable nozzle parts 3, 32 away from the fixed nozzle part 3a to open the nozzle unit, e. g. for removing dried pasta from the nozzle slot and/or various chambers of the nozzle. The figure also indicates the directions mentioned in this application, i. e. a longi- tudinal direction W of the coating apparatus, 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.

Fig. 2 depicts schematically a general construction for the nozzle unit of one prior known slide-fed multiple curtain coater. The nozzle unit comprises noz- zle parts 3,3a, and 32, each of which, except for the last-mentioned one, has a feeding chamber 12 and a equalizing chamber 13, as well as a nozzle slot 30. The outermost nozzle part 3a has its edge 33 forming a feed lip, over which is delivered a coating paste emerging from outlet openings 31 of the nozzle slots 30 and flowing along a top surface of the nozzle unit for produc- ing a coating curtain and guiding the same by means of edge guides (not shown) onto the surface of a paper/board web to be coated, which pro- gresses beneath the coater.

The feeding chamber 12, the equalizing chamber 13, as well as the nozzle slot 30 are all machined in a thick beam of steel constituting the nozzle unit, and even arranged one after the other in the longitudinal direction L of the nozzle slot and all being entirely defined by a boundary surface between two successive nozzle parts. Such a construction is hampered by a variety of drawbacks regarding both functional and manufacturing aspects of the appa- ratus.

Fig. 2 indicates also a so-called unsupported height H for the nozzle. It has an essential significance regarding the consistency of a feeding rate of the nozzle. In a nozzle unit with this type of construction, the unsupported height H extends from a bottom edge of the feeding chamber 12 all the way to an outer edge 31 of the nozzle slot 30. Thus, the feeding chamber's bot- tom edge constitutes the highest possible point of support, i. e. the height H represents a distance over which the adjacent nozzle parts do not provide support for each other. In a typical nozzle unit, the height H has a value of about 75 mm.

Within this range, the width of the nozzle slot 30 depends both on the shape of surfaces in nozzle parts and on thermal expansions occurring in nozzle parts, caused by temperature differences due to thermal loads carried by the coating paste. Such a construction sets high precision requirements in terms of manufacturing technique, particularly as the beam size increases.

The feeding rate of a coating paste varies quite linearly as a function of the slot width as far as typically employed slot widths are concerned. For exam- ple, if the objective is to allow a coating profile to have a maximum fluctua- tion of one percent in the amount of coating, the accuracy of a 0,5 mm slot in the direction D must have an accuracy of 0,005 mm. Thus, in the con- struction of fig. 2, which in this case has a total height of 120 mm, the sup- port surface must have a flatness of 0,002* (120-75) /75 mm = 0,003 mm, which thus represents here a maximum surface deviation from an imaginary flawless flat surface.

On the other hand, the growing size of pieces, as the feeding amount of coating paste and the width of a coating profile increase, also makes the management of e. g. thermal loads more difficult. The pieces also become very heavy and impose requirements on bracing. The prior art nozzle beams have a maximum width in the order of 4,5 m.

Fig. 3 illustrates one nozzle unit configured in accordance with the invention, wherein the feeding chambers 12 are provided completely inside the nozzle units. The coating paste is now fed from the feeding chamber 12 in a sub- stantially lateral direction D directly into the equalizing chamber 13 and thence further to the nozzle slot 30 in its extending direction L. This way, the previously mentioned unsupported height H of a nozzle part is substantially reduced, thus relaxing substantially the precision requirements imposed on surfaces of the nozzle part with regard to manufacturing technique. In the construction of fig. 3, the unsupported height is only 37 mm or 50% shorter than in a typical state-of-the art solution depicted in fig. 2. Hence, if the ob- jective is still to achieve a coating profile with one percent maximum fluctua- tion in the amount of coating, the support surface must have a flatness of 0,005* (120-37)/37 mm = 0,011 mm. Consequently, the precision required in the prior art solution is almost quadruple relative to the inventive solution.

What is also essential in the inventive construction of a nozzle part is that the entire nozzle unit is able to retain its integrity without external bracing. All elements constituting a nozzle unit can be preferably welded to each other comprehensively prior to installation in the actual coating apparatus, pro- vided that the feeding chamber 12 is made entirely immobile. If, for exam- ple, the feeding chamber has its lower wall made movable, it is possible to provide packings (not shown) between sections 42 constituting the bottom part of a nozzle part. With regard to the nozzle unit of fig. 3, for example, the nozzle part may have its upper sections 40 preferably secured to each other by welding. The nozzle parts can be preferably clamped against each other by means of bolts 41 to provide a nozzle unit. The fastening of a bot- tom section 43 is also preferably obtainable by a screw attachment (not shown).

A principal advantage gained by the novel construction is that the unsup- ported height remains unchanged regardless of the chamber size. The inven- tive solution also enables the construction of a considerably larger chamber

with the same external dimensions and coating paste flow rates, and at the same time facilitates the management of a coating paste flow profile. For example, the feeding chamber of fig. 2 has a cross-sectional area of 1330 mm2. On the other hand, if the nozzle unit of fig. 3 is provided with an equally high construction (in this case 120 mm), the feeding chamber will have a cross-sectional area of 2325 mm2 or 75% larger than the construction of fig. 2.

It is also particularly simple to provide the construction of fig. 3 with a space 17 for a cooling medium above and below the feeding chamber 12, in case the temperature fluctuations in nozzle structures become excessive. A pre- ferred cooling medium is for example water. Fig. 4, in turn, depicts one solu- tion of the invention for a nozzle part, wherein a space 17 extending longitu- dinal of the nozzle part encloses the feeding chamber from above and below, as well as from the side opposite relative to the equalizing chamber 13, The space 17 can be used for equalizing temperature differences resulting from thermal loads created by a coating paste (and other factors). In any case, the construction has material thicknesses which are highly consistent and the coating paste provides its heating or cooling effect over a very large area, whereby the deformations or strains due to temperature differences remain insignificant as compared with the prior art solution.

According to one further aspect of the invention, the feeding chamber 12 is designed to be convergent in its longitudinal direction from the inlet end of supplied material towards the opposite end, and that at least one of the feeding chamber's walls is adjustable for setting the feeding chamber's con- vergence as desired. In a particularly preferred case, it is the feeding cham- ber's 12 bottom wall which is made adjustable. The solution enables further facilitating the optimization of coating paste flow rates and thereby provides an even more substance material profile for the coating paste flowing out of an outlet opening 31 of the nozzle slot 30. This provides benefits, especially

in situations in which the feeding amounts are highly fluctuating. This is the case, particularly when it is desired to apply coating by using both multiple technique and single-layer technique.

In a further preferred embodiment, the coating paste discharging from a nozzle slot has the adjustment of its profile automated, whereby a crosswise profiling can be effected even during a run, which is not possible in currently known solutions.

The inventive solution can be further applied even in a more general sense in applicators intended for spreading various fibrous or other liquids or pastes in a paper/board or pulp machine environment, such as, for example, in the headbox of a paper machine. The inventive construction is also conceivable for use in the blade coating of a fibrous web, in connection with so-called jet application feed.