The present invention concerns a procedure for controlling the doctor coating of a moving paper web, on the surface of the web being applied coating materi¬ al in at least two steps, the coating being levelled and the excess being removed by scraping.

The invention furthermore concerns a means for coating a paper web in at least two steps, comprising a preceding coating means which has been provided with a preceding applicator and a preceding doctor blade, and a following coating means which has been provided with a following applicator and a following doctor blade, and a moving base against which said coating means are disposed to coat the web. The means is particularly meant for controlling the doctor coating process.

Paper is here understood to mean paper proper, such as printing paper and cardboard, or other fibre- based web-like products manufactured on paper and card¬ board machines and equivalent.

Multi-step coating taking place without inter¬ mediate drying has heretofore mainly been applied in manufacturing functional special coatings. The coating material is mostly a solution or a polymer melt, occa¬ sionally also a dispersion (e.g. the Patents DE 3 235 151, GB 1,158,298, GB 1,159,598, GB 1,207,133 and US 4,537,801). The purpose with coating is to in¬ fluence, for instance, the permeability to gases, fla- vour sealing, separation properties, light sensitivity, etc. It is important in all these instances that the coating should constitute a layer of uniform thickness and totally covering the surface of the paper. For this reason means of the nozzle type are frequently employ- ed, with the aid of which a quantity of coating materi¬ al, regulated in a preceding step, is conveyed onto the surface of the paper.

In the art is further known, through the Fin¬ nish patent application FI 885678, a multi-step coating procedure and means in which the following coating step is implemented immediately after the preceding step so that the surface of the coating spread, and levelled by scraping, on the paper in the preceding step is wet when the coating of the following step is being ap¬ plied, that is, the following coating step is imple¬ mented before the surface of the coating of the preced- ing step has time to dry, before the water has time to be drained from the surface of the coating. Thanks to this procedure, the coating can be implemented as an economic tailor-made job, taking the customer's special requirements into account. The characteristics of the paper can be regulated within wider limits than before. Coating material of high-grade quality is economically usable in the surface coating, e.g. special pigments, such as plastic pigments which improve the gloss, pre¬ cipitated pigments improving printer's ink absorption and gloss, and optic lighteners and titanium dioxide which boost the light colour. The procedure may also be applied for surface sizing with which thin pigmentation is combined. Thanks to the low ^' quantity, if so desired, of the coating spread in the following step, i.e., to its low base mass, the procedure affords a chance to use expensive special pigments, and other raw materi¬ als, in an economic way.

The object of the present invention is to develop further the above-described procedure and means disclosed in the Finnish patent application No. 885678 so that the coating can be controlled more accurately, and in wider limits, than before.

It is a further object of the invention to provide a procedure and a means by which the coating of paper can be regulated even during a run.

It is a further object of the invention to provide a procedure and a means for controlling the

coating of a paper web so that one and the same coating machine can be used in manufacturing products of vari¬ ous kinds and qualities with short preparation time, i.e., the coating machine can be rapidly and accurately adjusted to conform to any desired quality criteria of the particular paper brand going to be coated.

Regarding the features which are characterist¬ ic of the invention, reference is made to the claims. The invention is based on the fundamental idea that the locational implementation of two coating steps employed in coating a paper web is regulated by moving the respective coating means relative to each other, in the direction of travel of the web. The system can be implemented e.g. in the way that the doctor blades be- longing to two coating means are moved relative to each other in the web travel direction, in which connection it is not absolutely necessary to move the applicators of said coating means relative to each other. The sys¬ tem may further be implemented in the way that applica- tors belonging to at least two coating means are moved relative to each other. The doctor blades, or the coat¬ ing means together with their applicators, may then for instance be moved tangentially to the backing roll of these coating means. The displacement may for instance be axially centred with reference to the axis of the backing roll. On the other hand, the displacement may equally be on a straight, curved or otherwise configu¬ rated path mainly in the direction of web travel, e.g. carried by a guide or guides, such as rails. As taught by the invention, one of the coating means, e.g. the preceding coating means, may be fixed and the following coating means is movable relative to the preceding one. Similarly, one of the coating means, e.g. the following coating means, is fixed and the pre- ceding coating means is movable relative to the follow¬ ing one. The number of coating means is in no way con¬ fined. The system may comprise two or more, e.g. three,

four or five coating means. The system may comprise at least one additional coating means provided with an additional applicator and an additional doctor blade, said additional coating means being movable relative to said preceding and following coating means. It is thus understood that in the system of the invention one, i.e., any one, of the coating means comprised in the system may be movable relative to the others. Further¬ more, two or more coating means may be movable relative to the others. Equally, all coating means belonging to the system may be movable relative to each other.

Thanks to the invention, the coating process can be controlled in a more versatile manner than be¬ fore. When the coating takes place onto a wet precoat, which is the operation for development of which the invention is particularly meant, the dry matter content of the surface paste may, for instance, be increased and that of the thickener reduced because no immediate water draining occurs from the paste. The changes will improve both gloss and roughness. Moreover, greater freedom of choice than before can be exercised as to the composition of the paste.

Furthermore, when the present invention is applied in double blade coating upon a wet precoat, the invention enables machine renovation and regulation to be carried out with lower investments than before. The quality of the paper or cardboard that is turned out can be elevated by adding one or several coating courses, without changing the external dimensions of the paper or cardboard machine and without adding any drying units.

When the system of the invention is applied in double blade coating upon a wet precoat, it becomes possible to take into account the influence of various pastes, and of pastes used in different coating steps, and of the running conditions, on the solidifying ca¬ pacity of the pastes, without changing the running con-

ditions. When the running speed is varied on the coat¬ ing machine, e.g. within 100 to 350 m/min in a card¬ board machine, the application of paste upon the pre¬ ceding coating layer can be regulated in accordance with running conditions, pastes employed, web to be produced, and desired product criteria. Thanks to the invention, the location where a coating layer is ap¬ plied and/or the scraping location can be adjusted to be such that the quality of the surface to be coated is such as is desired, considering the running conditions, the base web and/or the changes taking place in the pastes that are being used.

The invention is described in detail in the following, with the aid of embodiment examples and re- ferring to the attached drawings, wherein:-

Fig. 1 presents, in elevational view and schematically, a coating procedure and means according to the inven¬ tion, and

Fig. 2 presents, in elevational view, another coating procedure and means according to the invention.

In Fig. 1 is seen a means according to the invention for coating a paper web 1 in three steps. The means comprises a preceding coating means 2, a follow¬ ing coating means 5 and an additional coating means 9. The coating means have been disposed in conjunction with a backing roll 12, which constitutes the base 8 for the web. The coating means 2, 5 and 9 further com¬ prise a preceding applicator 4, a following applicator 6 and, respectively, an additional applicator 10. The three coating means further comprise a preceding doctor blade 4, a following doctor blade 7 and an additional doctor blade 11. The web 1 is conducted to run, in the direction of travel indicated by arrows 5, around the backing roll 12. As taught by the invention, the preceding coating means 2 can be moved relative to the following coating means 5 and to the additional coating means 9,

in the web travel direction. Furthermore, the addition¬ al coating means 9 can be moved relative to the preced¬ ing coating means 2 and to the following coating means 5, in the web travel direction. The structural design of the coating means 2,

5 and 9 is known in itself in the art, and they are not more closely described in this connection.

The coating means 2, 5 and 9 are meant to coat the web 1 upon a wet coating, i.e., so that the coating of the preceding step is wet when the coating of the following coating step is spread on the web, and the coating of the following step is wet when the coating of the additional step is applied on the web.

In the embodiment depicted in Fig. 1, the coat- ing means 2 and 5 are carried to be turnable around the axis 13 of the backing roll 12. Hereby e.g. the posi¬ tion of coating means 2 and 5 relative to each other, and similarly that of coating means 2 and 5 relative to the coating means 9, can be adjusted to be as desired, in the web travel direction. The coating means 9 can for instance be moved about the axis 13 of the backing roll, or e.g. parallel to the surface of the backing roll, carried by a guide. Hereby the characteristics dependent on the drying stage of the coating material that is being applied in either coating step can be regulated by regulating the position relative to each other of the coating means, and thereby of the coating steps.

The structural design of the coating means 2, 5 and 9 depicted in Fig. 1 is known in itself in the art, and they are not more closely described in this connection. The coating means may be of any kind what¬ soever, e.g. devices based on roll application or on spray application, or other types of coating means. In each coating step a doctor blade 4, 7 and 9, respec¬ tively, is used for levelling the coating and for scraping off the excess of coating material.

In Fig. 2 is seen, in schematic presentation, another embodiment of the invention. The web 1 has been conducted to run over the rotating backing roll 12, which constitutes the base 8 for the coating process. The directions of travel, or rotation, of web and back¬ ing roll are indicated by arrows 15. In conjunction with the backing roll has been mounted a conventional, preceding coating means 2, with applicator roll 3 and doctor blade 4, and an SDPA coater 5, with applicator means 6 and doctor blade 7. In the embodiment here de¬ picted, the following coating means 5 is fixed and the preceding coating means 2 has been mounted movably, carried on guides 13 so that the coating means can be moved parallel to the surface of the backing roll 12 and to the web, relative to the following coating means 5, either towards it or away from it. In this embodi¬ ment, too, the structural design of both coating means is conventional in itself, and they are not more close¬ ly described in this connection. The embodiment examples are meant to illus¬ trate the invention without confining it in any way whatsoever, and embodiments of the invention may vary within the scope of the claims following below.