Coating device for applying coating color onto a fiber web and method for coating of a fiber web

Abstract

A coating device and method for coating a fiber web for applying coating color onto a fiber web has a nozzle unit with at least one nozzle part (10), a feeding chamber (11) and at least one equalizing chamber (13; 13), a feed hole (12) between the feeding chamber (11) and the first equalizing chamber (13), and a nozzle slot (14) from which the coating color is discharged through the nozzle slot (14) outlet opening (15). A mixing chamber (20) between the outlet area of the feed hole (12) and the equalizing chamber (13) functions as a means for distributing uniformly the inhomogeneous coating color flow entering the equalizing chamber (13) from the feed hole (12). The coating from the feed hole is directed to a wall (22) of mixing chambers (20) which distribute uniformly the coating entering the equalizing chamber (13).

Claims

1. A coating device for applying coating color onto a fiber web, comprising: a nozzle unit; wherein the nozzle unit has at least one nozzle part; wherein first portions of the nozzle part form a feeding chamber extending in a cross machine direction to form a manifold for coating color; wherein second portions of the nozzle part form at least in part at least one equalizing chamber extending in the cross machine direction; wherein third portions of the nozzle part form a plurality of feed holes spaced apart in the cross machine direction along the nozzle part, the plurality of feed holes defining a plurality of feed hole outlets; wherein fourth portions of the nozzle part form a plurality of mixing chambers arrayed in the cross machine direction and opening into the equalizing chamber, and wherein each of the plurality of feed hole outlets opens into a corresponding one of the plurality of mixing chambers; wherein each feed hole extends between the feeding chamber and the equalizing chamber via one of the mixing chambers; wherein each mixing chamber has a wall surface opposite a corresponding one of the plurality of feed holes, the mixing chamber and the wall surface being arranged to distribute uniformly an inhomogeneous coating color flow entering the equalizing chamber from the corresponding one of the plurality of feed holes; and a nozzle slot having an outlet opening, formed at least in part by a surface of the nozzle part, the nozzle slot being arranged to conduct coating color from the equalizing chamber to be discharged through the outlet opening of the nozzle slot onto an upper surface of the coating device; wherein each mixing chamber has a curved bottom part and two opposed side walls that extend upwardly from the curved bottom part towards the equalizing chamber; wherein each of the plurality of feed hole outlet openings is directed tangentially in respect to the corresponding curved bottom part of the mixing chamber.

2. The coating device of claim 1 wherein an opening angle B of less than 150 is defined between the two opposed side walls.

3. The coating device of claim 2 wherein the opening angle B is less than 90.

4. The coating device of claim 3 wherein the opening angle B is less than 60.

5. The coating device of claim 1 wherein the mixing chamber where it opens to the at least one equalizing chamber is round, or it is elliptic or oval so it extends in the cross machine direction.

6. The coating device of claim 1 wherein each of the plurality of feed holes defines a feed hole diameter which is between 1-5 mm.

7. The coating device of claim 6 wherein each of the plurality of feed holes defines a feed hole length which is at least 2-3 times the feed hole diameter.

8. The coating device of claim 6 wherein the curved bottom part has a radius greater than the feed hole diameter divided by 2.

9. The coating device of claim 8 wherein the radius is greater than 0.5 mm.

10. The coating device of claim 1 wherein the feed hole outlet opening is a distance of 0-5 mm from a bottom of the bottom part of the mixing chamber.

11. The coating device of claim 10 wherein the feed hole outlet opening is a distance of 0-3 mm from the bottom of the bottom part of the mixing chamber.

12. A method for coating of a fiber web, in a coating device comprising: a nozzle unit having at least one nozzle part; wherein first portions of the nozzle part form a feeding chamber extending in a cross machine direction to form a manifold for coating color; wherein second portions of the nozzle part form at least in part at least one equalizing chamber extending in the cross machine direction; wherein third portions of the nozzle part form a plurality of feed holes spaced apart in the cross machine direction along the nozzle part, the plurality of feed holes defining a plurality of feed hole outlets; wherein fourth portions of the nozzle part form a plurality of mixing chambers arrayed in the cross machine direction and opening into the equalizing chamber, and wherein each of the plurality of feed hole outlets opens into a corresponding one of the plurality of mixing chambers; wherein each feed hole extends between the feeding chamber and the equalizing chamber via one of the mixing chambers; wherein each mixing chamber has a wall surface opposite a corresponding one of the plurality of feed holes, the mixing chamber and the wall surface being arranged to distribute uniformly an inhomogeneous coating color flow entering the equalizing chamber from the corresponding one of the plurality of feed holes; a nozzle slot having an outlet opening, formed at least in part by a surface of the nozzle part, the nozzle slot being arranged to conduct coating color from the equalizing chamber to be discharged through the outlet opening of the nozzle slot onto an upper surface of the coating device for applying coating color; wherein each mixing chamber has a curved bottom part and two opposed side walls that extend upwardly from the curved bottom part towards the equalizing chamber; wherein each of the plurality of feed hole outlet openings is directed tangentially in respect to the corresponding curved bottom part of the mixing chamber; the method comprising the steps of: feeding a coating color to the feeding chamber which extends in a cross machine direction and therefrom to the plurality of feed holes which extend in a machine direction, wherein the coating color develops an inhomogeneous viscosity as it flows; discharging the coating color flow which forms an inhomogeneous viscosity flow from each of the plurality of feed holes into the plurality of mixing chambers each with an impact surface so that the coating color impinges on the impact surfaces; subjecting the flow of coating color to a kinetic energy change by impacting the flow of coating color onto the impact surface with sufficient velocity to make friction unimportant and thereby mixing the coating color flow independent of viscosity; uniformly feeding the flow of coating color from the mixing chambers to the equalizing chamber and then to the nozzle slot; and discharging coating color after mixing, through the outlet opening of the nozzle slot.

13. The method of claim 12 wherein each of the mixing chambers forming the opening area of the feed hole into the equalizing chamber distributes the coating color flow uniformly in the equalizing chamber indifferent of any inhomogeneity of the viscosity of the coating color flow.

14. The method of claim 12 wherein the direction of the coating color flow is changed immediately at the outlet from the feed hole into the mixing chamber.

15. The method of claim 12 wherein the plurality of feed holes have a diameter which defines a maximum shearing rate of the coating color flow in the feed holes wherein the diameter of the plurality of feed holes is 1-5 mm and flow velocity of the coating color flow is 2-7 m/s at the feed holes.

16. A coating device for applying coating color onto a fiber web, comprising: a nozzle unit; wherein the nozzle unit has at least one nozzle part; wherein first portions of the nozzle part form a feeding chamber extending in a cross machine direction to form a manifold for coating color; wherein second portions of the nozzle part form at least in part at least one equalizing chamber extending in the cross machine direction; wherein third portions of the nozzle part form a plurality of feed holes spaced apart in the cross machine direction along the nozzle part, the plurality of feed holes defining a plurality of feed hole outlets; wherein fourth portions of the nozzle part form a plurality of mixing chambers arrayed in the cross machine direction and opening into the equalizing chamber, and wherein each of the plurality of feed hole outlets opens into a corresponding one of the plurality of mixing chambers; wherein each feed hole extends between the feeding chamber and the equalizing chamber via one of the mixing chambers; wherein each mixing chamber has a wall surface opposite a corresponding one of the plurality of feed holes, the mixing chamber and the wall surface being arranged to distribute uniformly an inhomogeneous coating color flow entering the equalizing chamber from the corresponding one of the plurality of feed holes; and a nozzle slot having an outlet opening, formed at least in part by a surface of the nozzle part, the nozzle slot being arranged to conduct coating color from the equalizing chamber to be discharged through the outlet opening of the nozzle slot onto an upper surface of the coating device; wherein each mixing chamber has a curved bottom part and two opposed side walls that extend upwardly from the curved bottom part towards the equalizing chamber; wherein each of the plurality of feed hole outlet openings is at an angle A defined between a direction centered along the feed hole and a direction defined by a middle line of the mixing chamber, the angle A being less than 90.

17. A method for coating of a fiber web, in a coating device comprising: a nozzle unit having at least one nozzle part; wherein first portions of the nozzle part form a feeding chamber extending in a cross machine direction to form a manifold for coating color; wherein second portions of the nozzle part form at least in part at least one equalizing chamber extending in the cross machine direction; wherein third portions of the nozzle part form a plurality of feed holes spaced apart in the cross machine direction along the nozzle part, the plurality of feed holes defining a plurality of feed hole outlets; wherein fourth portions of the nozzle part form a plurality of mixing chambers arrayed in the cross machine direction and opening into the equalizing chamber, and wherein each of the plurality of feed hole outlets opens into a corresponding one of the plurality of mixing chambers; wherein each feed hole extends between the feeding chamber and the equalizing chamber via one of the mixing chambers; wherein each mixing chamber has a wall surface opposite a corresponding one of the plurality of feed holes, the mixing chamber and the wall surface being arranged to distribute uniformly an inhomogeneous coating color flow entering the equalizing chamber from the corresponding one of the plurality of feed holes; a nozzle slot having an outlet opening, formed at least in part by a surface of the nozzle part, the nozzle slot being arranged to conduct coating color from the equalizing chamber to be discharged through the outlet opening of the nozzle slot; wherein each mixing chamber has a curved bottom part and two opposed side walls that extend upwardly from the curved bottom part towards the equalizing chamber; wherein each of the plurality of feed hole outlet openings is at an angle A defined between a direction centered along the feed hole and a direction defined by a middle line of the mixing chamber, the angle A being less than 90, the method comprising the steps of: feeding a coating color to the feeding chamber which extends in a cross machine direction and therefrom to the plurality of feed holes which extend in a machine direction, wherein the coating color develops inhomogeneous viscosity and forms a coating color flow of inhomogeneous viscosity; discharging the coating color flow of inhomogeneous viscosity from each of the plurality of feed holes into the plurality of mixing chambers each with an impact surface so that the coating color impinges on the impact surfaces; subjecting the coating color of inhomogeneous viscosity to a kinetic energy change by impacting the flow of coating color of inhomogeneous viscosity onto the impact surface with sufficient velocity to make friction unimportant and thereby mixing the coating color of inhomogeneous viscosity independent of viscosity; uniformly feeding the flow of coating color of inhomogeneous viscosity from the mixing chambers to the equalizing chamber and then to the nozzle slot; and discharging coating color of inhomogeneous viscosity after mixing, through the outlet opening of the nozzle slot.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following the invention and its advantages are explained in greater detail below in the sense of examples and with reference to accompanying drawings, wherein:

(2) FIG. 1 is a schematic drawing showing an example of a nozzle unit of a multi-layer curtain coating device according to the prior art.

(3) FIG. 2 is schematic drawing showing an example of an advantageous embodiment of the invention.

(4) FIGS. 3A-3C are schematic drawing showing some detail examples of advantageous features of the invention.

(5) FIG. 4 is an isometric view of a section of a nozzle part of the nozzle unit of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) In the figures the corresponding elements, parts and part components of the arrangement are denoted by the same reference signs in the figures unless otherwise mentioned. For clarity reasons the reference signs are typically marked in the figure in respect of one component/part/part component.

(7) FIG. 1 shows schematically the general structure of the nozzle unit of a known multi-layer curtain coater according to the prior art. The nozzle unit is comprised of nozzle parts 10, each of which has a feeding chamber 11 and an equalizing chamber 13, a feed slot 14 between the feeding chamber and the equalizing chamber and a nozzle slot 14, which are machined in a thick steel plate. The edge 17 of the outermost nozzle part 10 forms a feeding lip, over which the coating color discharged from the outlet openings 15 of the nozzle slots 14 and flowing along the upper surface of the nozzle unit 10 is conveyed to form a coating color curtain and to guide it onto the surface of the fiber web to be coated which is traveling below the coater. The coating color curtain formed extends across the fiber web to be coated.

(8) In the example of FIGS. 2 and 4 the nozzle part 10 of the coating device comprises a feeding chamber 11 from which feed holes 12 located spaced apart in the cross direction of the fiber web i.e., in the longitudinal direction of the nozzle part 10, feed coating color to an equalizing chamber 13, from which the coating color is fed through a feed slot 14 extending in the longitudinal direction of the nozzle part 10 to another optional equalizing chamber 13 and to the nozzle slot 14 extending in the cross machine direction of the fiber web, i.e. in the longitudinal direction of the nozzle part 10, to be discharged from the outlet opening 15. According to the invention at a location between the outlet area of the feed hole 12 and the equalizing chamber 13, a mixing chamber 20 is arranged, which functions as a means for distributing uniformly the inhomogeneous coating color flow entering the equalizing chamber 13 from the feed hole 12. The mixing chamber 20 forming the opening area of the feed hole 12 going into the equalizing chamber 13 distributes the coating color flow uniformly in the equalizing chamber 13 indifferent of any inhomogeneity of the viscosity of the coating color and the direction of the coating color flow is changed immediately at the outlet from the feed hole 12 to the equalizing chamber 13, i.e. it is still high-speed and has high kinetic energy and at this point its flow direction is changed with the aid of one of the walls 22 forming a surface against which the coating color, from the feed hole 12 outlet, impacts. The mixing chambers 20 openings 24 into the equalizing chamber 13 are spaced apart as shown in FIG. 4 or may be substantially abutting or even overlapping.

(9) In FIGS. 3A-3C are schematically shown some detail examples of advantageous features of the invention and as shown in FIG. 3A the mixing chamber 20 has a cup-like form or shape and the opening 24 to the equalizing chamber 13 of the cup-like form of the mixing chamber 20 is round, elliptic or oval in longitudinal direction. As can be seen from FIGS. 3A-3C the mixing chamber 20 has a curved bottom part 21 and straight or curved upward extending wall structures 22, 23 comprising side walls 23 and end walls 22. The radius R of the curved bottom part 21 is greater than the diameter DR of the feed hole 12 divided by 2. Advantageously the radius R is greater than 0.5 mm. The mixing chamber 20 side walls 23 are positioned between the end walls 22 and are inclined towards the equalizing chamber 13 at an opening angle B of less than 150 measured between the diverging side walls 23 of less than 150, advantageously less than 90, more advantageously less than 60. In these cases H (height of the wall 23) >1.5DR (diameter of the feed hole), advantageously H>3DR, and distance W (minimum distance between the end walls 22 at the location of the opening of the feed hole 12) is at least 0.5DR.

(10) The diameter DR of the feed hole 12 is defined by the maximum shearing rate of the coating color flow in the feed hole 12 so that the coating color flow is not in the shear thickening area. Advantageously the diameter DR of the feed hole 12 is 1-5 mm and the flow velocity of the coating color is 2-7 m/s. The length of the feed hole is at least 2-3the diameter DR of the feed hole 12.

(11) FIG. 3A shows alternative embodiments of the position of the feed hole 12 which is located at 90 from a middle line 26 of the of the mixing chamber, showing a feed hole 28 at about 105 and a feed line 30 at about 45. The outlet opening of the feed hole 12 is in close vicinity of the bottom part 21 of the mixing chamber 20. Advantageously the distance D from the bottom of the bottom part 21 to the lowest point of the outlet opening of the feed hole 12 is 0-5 mm, advantageously 0-3 mm and the outlet opening of the feed hole 12 is tangentially directed in respect of the curved bottom part 21 of the mixing chamber 20, advantageously the angle A between the feed hole 12 direction and the direction of the middle line 26 of the cup-like form of the mixing chamber 20 is less than 150, advantageously less than 90.