WO2006079678A1

WO2006079678A1 - Apparatus for regulating the application width of a slide-fed curtain coater

Info

Publication number: WO2006079678A1
Application number: PCT/FI2005/050442
Authority: WO
Inventors: Markku GRÅSTEN; Kai Aalto; Jukka Koskinen; Juha Lipponen; Timo Nurmiainen; Heikki Vatanen
Original assignee: Metso Paper, Inc.
Priority date: 2005-01-27
Filing date: 2005-12-02
Publication date: 2006-08-03
Also published as: CN101103155B; EP1841918A1; FI117176B; JP2008526483A; EP1841918B1; JP4681005B2; CN101103155A; FI20055041A0; EP1841918A4

Abstract

The invention relates to an apparatus for regulating the application width of a slide- fed curtain coater, said curtain coater having at least one nozzle slot (30), which opens (31) onto a flow plane, the leading edge of said flow plane providing a feeding lip (33). The apparatus comprises a laterally movable regulating mechanism (40), which is disposed in the vicinity of both edges of the flow plane and which has a blade element (42) defining the lateral extent of a coating color (1) flowing from the nozzle slot (31), and a return flow guide element (41, 50) for deflecting the flow of coating color emerging from a portion of the nozzle slot extending beyond the application width defined by the blade element (42) back to machine circulation or for disposal.

Description

Apparatus for regulating the application width of a slide-fed curtain coater

The present invention relates to an apparatus for regulating the application width of a slide-fed curtain coater, said curtain coater having at least one nozzle slot opening onto a flow plane, the leading edge of said flow plane providing a feeding lip.

As coated paper grades and coating become more and more common, the coating processes and equipment are subject to more and more requirements. In coating, more specifically in pigment coating, the surface of paper is formed with a layer of coating color in a coating station, followed by performing drying of excess water. The formation of a coating color layer can be distinguished into supplying the coating color to the surface of paper, i.e. application, as well as into adjusting the final amount of coating. The most important pigment coating method is so-called blade coating, in which the amount of coating is adjusted by means of a so-called doctor blade. Blade coating stations of the most common type include a blade coater equipped with an application roll and a blade coater of the nozzle application type. In addition, coating is performed by using a so-called film transfer coater, which have become quite common in recent times. Another new technique being introduced comprises the use of curtain coaters.

Curtain coaters can be distinguished as slot-fed and slide-fed coaters. In use, the slide-fed curtain coater delivers coating by means of a nozzle unit onto an inclined plane, along which the coating runs towards a plane edge providing a feeding lip, developing a curtain as coating trickles over the feeding lip. The resulting curtain is maintained at a desired width by means of an edge guide which, as suggested by its name, is positioned along the edge of the feeding lip.

In slot-fed application beams, the coating is pumped through a distribution chamber to a narrow vertical slot, a curtain forming along its lip and trickling onto a web under the guidance of edge guides. The coating can be applied in one or more layers.

A multiplicity of various paper and board grades exist and can be divided according to grammage for two categories: papers with a single ply and a grammage of 25- 300 g/m², and boards manufactured in multiply technique and having a grammage of 150-600 g/m². As noted, the borderline between paper and board is floating as boards of the lightest grammage are lighter than the heaviest papers. Generally, paper is used for printing and board for packaging. Papers and boards can be coated or uncoated.

The subsequent descriptions are examples of currently employed values for coatable fibrous webs and may include substantial fluctuations from the given values. The descriptions are principally based on the source publication Papermaking Science and Technology, Papermaking Part 3, Finishing, ed. Jokio, M., publ. Fapet Oy, Jyvaskyla 1999, 361 pages.

Coated magazine paper (LWC = light weight coated) contains mechanical pulp at 40-60%, bleached softwood pulp at 25-40% and fillers and coating agents at 20- 35%. General values for LWC paper can be considered as follows: grammage 40-70 g/m², Hunter gloss 50-65%, PPS SlO roughness 0.8-1.5 μm (offset) and 0.6-1.0 μm (roto), density 1100-1250 kg/m³, brightness 70-75% and opacity 89-94%.

General values for MFC paper (machine finished coated) can be considered as follows: grammage 50-70 g/m², Hunter gloss 25-70%, PPS SlO roughness 2.2-2.8 μm, density 900-950 kg/m³, brightness 70-75% and opacity 91-95%.

General values for FCO paper (film coated offset) can be considered as follows: grammage 40-70 g/m², Hunter gloss 45-55%, PPS SlO roughness 1.5-2.0 μm, density 1000-1050 kg/m³, brightness 70-75% and opacity 91-95%.

General values for MWC paper (medium weight coated) can be considered as follows: grammage 70-90 g/m², Hunter gloss 65-75%, PPS SlO roughness 0.6-1.0 μm, density 1150-1250 kg/m³, brightness 70-75% and opacity 89-94%.

HWC (heavy weight coated) has a grammage of 100-135 g/m² and it can be coated even more than twice.

In coated pulp-based woodfree printing papers or fine papers (WFC), the amounts of coating fluctuate a great deal according to requirements and intended use. The following are typical values for once- and twice-coated pulp-based printing paper: for once-coated, grammage 90 g/m², Hunter gloss 65-80%, PPS slO roughness 0.75-2.2 μm, brightness 80-88% and opacity 91-94%, and for twice-coated, grammage 130 g/m², Hunter gloss 70-80%, PPS SlO roughness 0.65-0.95 μm, brightness 83-90% and opacity 95-97%.

Boards make up quite a heterogeneous group, including high grammage grades with a possible grammage as high as 500 g/m², and low grammage grades with a grammage of about 120 g/m², the grades possibly ranging from those based on primary fiber to those based by 100% on recycled fiber and from uncoated grades to multiple coated ones. Coated boards include the following:

- virgin fiber based folding board (FBB = folding boxboard), bleached pulp board (SBS = solid bleached board), LPB = liquid packaging board, coated white top liner, carrier board)

- recycled-fiber based white lined chipboard (WLC = white lined chipboard), coated recycled board).

Curtain coating is useful for manufacturing basically all the above grades to be coated. Curtain coating is not able to produce a smoothness equal to that obtained by blade coating, but it provides a better coverage than blade coating.

Adjustment of the application width is generally effected in curtain coating by limiting the width of a curtain falling onto a web either just before the web surface or at the nozzle beam's feeding lip, the curtain width being maintained in the latter case by means of edge guides. One problem in this prior known technique is that the feeding width remains constant (wider than web), whereby adjustment of the width of a coating color layer arriving on the web, i.e. adjustment of the application width, has the effect, especially in multiple coating, of amalgamating various feeding layers of a portion beyond the application width into a single composition, which is not reusable or the reuse of which is a clearly cost-increasing factor.

Another approach is to limit the flow of coating color onto a flow plane by closing the edge zone of feeding chambers over a desired section, e.g. by using dampers, the coating color flowing onto the flow plane and further over the feeding lip, forming a curtain of a desired width maintained by means of edge guides. One problem in this solution is the occurrence of defects in the crosswise feeding profile as a result of blocking the edge zone of the chambers and at the same time the chambers develop regions which offer the coating color a possibility of drying. When adjusting the width, these dried coating color particles result in coating defects and even in a localized obstruction of the nozzle beam.

Accordingly, it is an objective of the present invention to provide a solution capable of eliminating the problems relating to prior art adjustments of the application width.

In order to accomplish this objective, an apparatus of the invention is characterized in that the apparatus comprises a laterally movable regulating mechanism, which is disposed in the vicinity of at least one edge of the flow plane and which has a blade element defining the lateral extent of a coating color flowing from the nozzle slot, and a return flow guide element for deflecting the flow of coating color emerging from a portion of the nozzle slot extending beyond the application width defined by the blade element back to machine circulation or for disposal.

The dependent claims 2-8 disclose a few preferred embodiments of the invention.

The inventive solution is capable of maintaining the beam's feeding profile constant in lateral direction regardless of the application width and the coating color collected from a portion exceeding the coating width is totally reusable for the present coating layer without any extra treatments, except for possible deaeration of the coating color. By remaining open, the feeding chambers shall not experience any harmful drying of coating color or other obstructions.

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

Hg. 1 shows one embodiment for a solution of the invention in a diagrammatic sectional view over one edge zone of a nozzle beam,

Hg. 2 shows the embodiment of fig. 1 in a perspective illustration in a working position, Hg. 3 shows the embodiment of fig. 1 in a perspective illustration in an opened position, and

Hg. 4 shows in a schematic view one prior art nozzle element for a curtain coater's nozzle unit.

In reference to fig. 4, there is shown in a schematic view a nozzle element for a curtain coater's nozzle unit, as described in the application PCT/FI2004/050122. The nozzle element is only depicted as an example of one nozzle design useful in the inventive solution. The nozzle unit may consist of a plurality of nozzle elements, each including a feeding chamber 12 and an equalizing chamber 13, as well as an nozzle slot 30. Hg. 4 illustrates the nozzle unit's outermost nozzle element 39, whose edge 33 provides a feeding lip for passing thereover a coating color emerging from the nozzle slots 30 outlet openings 31 and running along the nozzle unit's top surface, i.e. a flow plane, for producing a coating curtain and guiding the same by means of edge guides (not shown) onto the surface of the paper/board web to be coated which is traveling below the coater. The resulting coating curtain extends across the web to be coated.

Between the feeding chamber 12 and the equalizing chamber 13 is a feed hole 18, which in the position shown in fig. 4 opens horizontally into the feeding chamber 12 and vertically into the equalizing chamber 13. Each feed hole 8 is further provided with a bore 19a opening out of the nozzle element and merging with the vertical section of the feed hole 18, and said bore being provided with a lengthwise movable adjusting pin 19, having its inner end 23, extending into the feed hole 18, preferably beveled. The adjusting pin 19 has its outer end 25 provided with an adjustment nut 21, which can be screwed for displacing the adjusting pin 19 in its lengthwise direction for varying an effective cross-sectional area of the feed hole 18 in view of supplying a desired amount of coating color into the equalizing chamber 13 at various points of its lengthwise direction. In association with the adjusting pin is further provided a adjustment plate 20, whereby the adjustment plate can be disengaged for varying a basic setting of the feed hole's effective cross-sectional area for reducing the feeding rate. The adjusting pin 19 is sealed in the bore 19a by means of packings 22. The feed holes 18 have a distance from each other in the nozzle element's longitudinal direction of e.g. 100-600 mm, preferably 150-300 mm, for providing a desired cross-profiling by varying the effective cross-sectional area of the feed holes. Operation of the adjusting pins is preferably automated to enable profiling also during running in the crosswise direction of an object to be coated. Reference numeral 24 is used to represent the attachment plane of nozzle element assembly screws. The assembly screws extend through between the successive bores 19a across a water chamber 17 and are fitted with necessary packing means. This type of solution can be implemented as a single-ply or multiple coater, in which the number of nozzle slots 30 can be e.g. 1-24.

Hg. 1 shows diagrammatically one embodiment of the invention, in which at least one edge region of the nozzle unit is provided with an application width regulating mechanism 40, comprising in the illustrated embodiment a lid member 41, a blade element 42 and a base member 50. The base member 50 is attached by means of a connecting piece 51 to the nozzle unit's lateral end. Reference numeral 43 represents an edge guide for guiding the developing coating curtain as it falls onto a web. The lid member 41 is attached to the blade element 42, e.g. by screws. The blade element and the lid member are movable in the nozzle unit's lateral direction in view of using the blade element's 42 inner edge for limiting, as desired, the width of a coating color 1 presently on the flow plane. The coating color flowing out of the nozzle slot opening 31 left outside the application width limited by means of the blade element 42 flows into a guiding element constituted by the base member 50 and the lid member 41 and having chute sections 44, 45 in alignment with each nozzle slot to enable the flowing of this coating color remaining beyond the application width into conduit elements 46 included in the base member 50, by way of which the flow of coating color is deflected back to machine circulation or, if necessary, out of the machine circulation. This solution allows for treating in a desired manner a coating color emerging from an individual nozzle slot, whereby e.g. some of the color layers can be recycled to machine circulation and some can be guided away. Reference numeral 47 represents a conduit by-passing the nozzle unit's feeding chambers 12. The regulating mechanism 40 is preferably provided in the vicinity of each edge of the nozzle unit.

The blade element 42 present at either edge of the nozzle unit, along with the lid member 41, is preferably adjustable over the range of 0-500 mm, e.g. about 0- 250 mm. The guide element is conceivable not only as the above-described closed design constituted by a lid structure and a base member but also as an open design by fitting the blade element with rod-shaped or plate-like elements, which extend outward from the blade element towards the nozzle unit's edge and are disposed on either side of each nozzle slot in such a way that in alignment with the nozzle slot develops an upward open guide channel, along which the flow of coating color emerging from a portion of the nozzle slot extending beyond the application width is deflectable back to machine circulation or for disposal. In this case, the base member 50 can be e.g. similar to that shown in conjunction with the lid 41 or also provided e.g. with a flat surface without chute sections 44.

The inventive regulating mechanism is preferably adapted to be movable in a stepless manner for defining a desired application width. The movement can be automated and can also be performed in such a way that the adjustment of width can be effected, if necessary, also during running, the regulating mechanism having its control possibly based e.g. on the continuous tracking of the web edge.

The nozzle unit can be further provided with elements for supplying certain one or more nozzle slots not only with a coating color but, in addition or instead, also with some other material, such as water or a surface tension releasing agent, for preventing the occurrence of various edge defects in a coating curtain, such as e.g. edge necking or curtain splitting at the web edge, which may cause the deflection of a coating curtain underneath the web edge. The elements may comprise valve means arranged either across the entire width of the nozzle unit or only in the vicinity of its edges for guiding the supplemental material to a desired nozzle slot at desired locations. This further enables profiling the coating as desired.

Claims

1. An apparatus for regulating the application width of a slide-fed curtain coater, said curtain coater having at least one nozzle slot (30), which opens (31) onto a flow plane, the leading edge of said flow plane providing a feeding lip (33), characterized in that the apparatus comprises a laterally movable regulating mechanism (40), which is disposed in the vicinity of at least one edge of the flow plane and which has a blade element (42) defining the lateral extent of a coating color (1) flowing from the nozzle slot (31), and a return flow guide element (41, 50) for deflecting the flow of coating color emerging from a portion of the nozzle slot extending beyond the application width defined by the blade element (42) back to machine circulation or for disposal.

2. An apparatus as set forth in claim 1, characterized in that the regulating mechanism (40) is adjustable within the range of 0-500 mm.

3. An apparatus as set forth in claim 2, characterized in that the regulating mechanism (40) is adjustable within the range of 0-250 mm.

4. An apparatus as set forth in any of claims 1-3, characterized in that the curtain coater's nozzle unit comprises at least two nozzle slots, and that the regulating mechanism's (40) guide element comprises an openable lid member (41), which are formed with return chutes (45) aligning with the nozzle slots and isolated from each other, and a base member (50), which is fixedly attached to the nozzle unit and includes complementary chutes (44) coinciding with the lid's (41) return chutes (45), whereby, with the lid in a closed position, the guide element presents closed return channels in alignment with each nozzle slot.

5. An apparatus as set forth in claim 4, characterized in that the lid member (41) is attached to the blade element (42) to be movable therewith.

6. An apparatus as set forth in any of claims 1-3, characterized in that the guide element comprises rod-shaped or plate-like members attached to the blade element and extending outward from the blade element on either side of each nozzle slot for providing, in alignment with the nozzle slots, an upward open guide channel, along which the flow of coating color emerging from a portion of the nozzle slot extending beyond the application width can be routed back to machine circulation or for disposal.

7. An apparatus as set forth in any of the preceding claims, characterized in that the regulating mechanism is movable in a stepless manner for defining a desired application width.

8. An apparatus as set forth in claim 7, characterized in that the regulating mechanism has its movement automated.

9. An apparatus as set forth in any of the preceding claims, characterized in that the regulating mechanism is adapted to be automatically controlled on the basis of tracking the web edge.