COATER

Abstract: An appliance for a double-sided coating of a moving web by means of two coating substance applicators (2,4) and a smoothing nip (11) comprising a back-up roll (9) on one side of the web and a adjustable levelling blade (10) or the like on the other side of the web. The coating substance is supplied on the web (1) directly on the side facing the levelling blade (10) and on the surface of the back-up roll (9) or the web (1) on the other side of the web (1). The smoothing nip (11) and the both applicators (2, 4) are located within a 30 degrees sector of the back-up roll (9), preferably within a 20 degrees sector.

Description: The invention relates to an appliance of the kind presented in the preamble of claim 1 and as it is known from GB-A-2 096 025. In a coater, where a moving web is coated on both sides, the supply of coating substance on the web generally takes place with two coating applicators, the first one being located between a back-up roll and the web and the second one being located at the web side facing a doctor blade or the like. The first applicator may be arranged to apply coating substance at first on the surface of the back-up roll from where it is transferred to the web surface, according to GB-A-2 096 025. This kind of appliance is useful when applying different coating substances on both web sides. In another embodiment according to DE-A-34 07 255 (priority date March 1, 1983, filing date Febr. 28, 1984 publication date Sept. 6, 1989) the first applicator is arranged to apply coating substance directly on the web and generally at the opposite position of the second applicator. The coating substances on both web sides are brought into a substantially non-pressurized contact with the running web for adhering thereto and travelling therewith upwards to the smoothing nip. This kind of appliance is useful when applying equal coating substances on both web sides. The object of the invention is to improve the state of function of a coater. By this is meant that the coater of the invention may be adapted to different coating conditions according to the respective coating substances and web materials. The characteristic features of the invention are disclosed in claim I. In the coater of the invention the nozzles of both applicators are located close to the smoothing nip, i.e. within a 30° sector of the back-up roll, preferably within a 20° sector. By this means the first applicator located at the web side facing the back-up roll is mountable info different positions, in a first one to apply coating substance on the surface of the back-up roll and in a second one to apply coating substance directly on the web. In a preferred embodiment of the invention both applicators are of the extrusion type, which through a supply slit by means of a very light excess pressure supply coating substance on the moving surfaces. It is important, that the applicator supplying coating substance on the roll so is adjusted so. that the amount of coating substance supplied does not excess the amount of required coating substance so much, that an excessive pool would be formed below the smoothing nip by influence of the web moving upwards at high speed. When the coating substance is almost pressure-less in the nozzles of the applicators, the air following the coating substance will thus not dilate as the pressure remains constant and will so not cause uncoated areas on the web, which happens in known coaters, where the moving direction of the web is opposite and the smoothing nip forms the bottom slit of the coating substance damming. In known coaters of this type a further drawback is the hydrostatic pressure of the damming, which rises to a remarkable degree, because the height of the damming surface has to be big enough to let the excess of coating substance flow away from the ends of the basin. Because of these leakages it is not possible to leave the edge zones of the web uncoated nor to arrange the control devices of the blades, nozzles etc. at the gables. which is possible and preferable in a coater according to the invention. Leaving the edge zones uncoated is a necessity, when for example one side is coated with surface sizing and the other side with pigment coating, when it is wanted to have different basis weights for the coatings of the different sides of the web, which succeeds in the best manner by changing the content of dry substance of the coating substance, or when it is wanted to leave one side wholly uncoated. The applicator located at the web side facing the levelling blade of the like operates together with a rigid support located at the opposite side of the web. In a favourable embodiment this support is a rigid level blade, the surface of which is made of a plastic material having a low friction coeffcient. Instead of a level blade there can be used a back-up roll or the like. The space that the support demands in the direction from the web to the back-up roll is small, and due to this the applicator in question can be placed close to the smoothing nip. The moving direction of the web is from below upwards, but it does not have to be fully perpendicular, instead it may prior to the smoothing nip differ somewhat from the perpendicular direction. After the smoothing nip the moving direction of the web preferably differs clearly from the direction of a tangent line of the back-up roll drawn through the smoothing nip. This being the case there is generated a refracting point at the levelling blade or the like, which improves the quality of the coating. The invention will in the following be explained more accurately with reference to the figures 1 and 2. In the drawing, numeral 1 indicates a moving web, 10 a doctor blade and 9 a back-up roll comprising a back-up roll shaft 15. The doctor blade 10 can be replaced by a bar 10a. The smoothing nip 11 is the section between the back-up roll 9 and the doctor blade 10 or the like mounted on a blade support 18. On the roll side of the web there is a wedge-shaped coating substance applicator 2, which spreads coating substance 16 on the surface of the back-up roll 9, from which it is supplied to the web 1 at the smoothing nip 11. On the opposite side of the web 1 there is a corresponding applicator 4, which spreads coating substance 16 directly on the web. The coating substance 16, paste or sizing, is pumped through the connections 7 and 8 to the applicators 2 and 4, from which it is extruded from the nozzles of both applicators 2 and 4 through the slits 3 and 5 to the surface of the web 1 or the roll 9. The applicators 2 and 4 are independently mountable and adjustable and they are on a distance of a few millimetres from the point to be coated without touching the moving surface, which reduces the risk of web rupture. The distance from the slit of the nozzle to the web 1 or the roll 9 is then suitable, when only a small amount of coating substance 16 is falling to the collection tanks 13 below the applicators 2 and 4. The coating substance 16 spread from the slit 3 on the roll 9 is moving with the rotating roll 9 towards the smoothing nip 11, in front of which it is fastened to the web 1 and in which the amount of coating substance 16 is regulated by the pressure of the doctor blade 10 or bar 10a. The amount of coating substance 16 remaining on both sides of the web 1 is determinated by the same regulation of blade pressure. The coating substance 16 coming from the slit 5 of the nozzle of applicator 4 presses the web 1 against the rigid support plate 6 on the opposite side of the web, which plate prevents the web from bending and the surface of which preferably is coated with plastic in order to get a very smooth surface. The support plate 6 prevents at the same time the excess of coating substance 16 between the roll 9 and the web 1 taken off at the smoothing nip 11 from falling down on the web 1, leading it instead to the collection tank 13. Numeral 17 indicates schematically a vertically movable support for the support plate 6. In a coater, where the smoothing nip 11 and the center point of the back-up roll 9 are in the same horizontal plane, the tangent line 14 of the back-up roll drawn through the smoothing nip 11 is perpendicular. The direction of the web 1 below the smoothing nip diverges at most 5° from the direction of the tangent line, preferably away from roll 9. After the smoothing nip 11 the web 1 turns off in a direction away from the roll 9 preferably 3°...5°, at most 7°-10°. This change of the direction of the web 1 effects advantageously upon the distribution of the surface layer into the surface of the roll 9 and into the part remaining on the surface of web 1 so, that the surface of the part remaining on the web 1 becomes smoother than if the web 1 should move in the direction of the tangent line 14 of the back-up roll 9. Due to their wedge-shaped construction the nozzles of the applicators 2 and 4 can be brought very close to the smoothing nip 11. The distance from the smoothing nip 11 to the slits 3 and 5 of the said nozzles can be 100 mm or smaller. In practice the distance is dependent on the diameter of the back-up roll 9. In a coater according to the invention the attempt is, that the angle a of the sector of the back-up roll, the lines of demarcation 12a and 12b of which sector go through the smoothing nip 11 and the spreading point 3 most far from it, is at most 30°, preferably at most 20°. In figure 2 is described an alternative way of using a coater according to the invention. The nozzle 2 is mounted in a turned position so, that the slits 3 and 5 of the nozzles 2 and 4 are opposite each other on different sides of the web. Hereby the support plate 6 has to be lowered to the position 6a. The state of function of the coater is also in this mode very good.

Claims: An appliance for double-coating a moving web, for instance a paper or a cardboard web (1), in which the web (1) moves from below upwards and the coating substance applied to the web is levelled in a smoothing nip (11), which comprises a back-up roll (9) on one side of the web and an adjustable doctor blade (10) or -bar (10a) on the other side of the web, the appliance comprising two coating applicators (2,4) for applying the coating substance on the moving surfaces of which applicators the first one (2) is located between the back-up roll (9) and the web (1) to apply coating substance via the surface of the back-up roll and the second one (4) is located at the other side of the web to apply coating substance directly on the web (1), characterized in that the nozzle of the first applicator (2) is located close to the smoothing nip (11), i.e. within a 30 degrees sector of the back-up roll, and in that the first applicator (2) is mountable into a turned position between the web (1) and the back-up roll (9) so that coating substance can be supplied by the applicator (2) directly on the web (1). Appliance according to claim 1, characterized in that the nozzle of the first applicator (2) is located within a 20 degrees sector of the back-up roll. Appliance according to claim 1 or 2, characterized in that both applicators (2,4) are of the extrusion type. Appliance according to any of the preceding claims, characterized in that the second applicator (4) can operate together with a rigid support blade (6) or a support roll, located at the opposite side of the web, when the first applicator (2) supplies coating substance on the surface of the back-up roll (9). Appliance according to claim 4, characterized in that the said support blade (6) or roll is mountable into a position (6a) which enables it to apply coating substance directly on both sides of the web (1), in which position (6a) the nozzles of the applicators (2,4) being located mainly opposite each other. Appliance according to any of the preceding claims, characterized in that prior to the smoothing nip (11), the moving direction of the web (1) differs at most 5° from the direction of the tangent line (14) of the back-up roll (9) passing through the smoothing nip (11), said angle being formed away from the back-up roll (9), and, after the smoothing nip (11), differs at least 3° and at most 10° from the direction of said tangent line (14) in a direction pointing away from the back-up roll (9). Appliance according to any of the preceding claims, characterized in that the applicators (2,4) without touching are independently mountable and adjustable. Appliance according to claim 7, characterized in that, the distance between the applicator (2,4) and the moving surface (1,9) is arranged to allow a small amount of coating substance applied to drop down beneath the applicators.

Inventors: GRASTEN, MARKKU, TUHKANEN, RAIMO, WESTERGARD, SIVERT
Applicants: VALMET PAPER MACHINERY INC.
Application year: 1984
Main classification: D21H23
IPC codes: B05C9, B05C5, D21H27, B05D1, B05C1