EP1838456A1

EP1838456A1 - Method for removing an edge bank formed at the edge of a coated web in curtain coating effected as a dry edge coating process

Info

Publication number: EP1838456A1
Application number: EP05818866A
Authority: EP
Inventors: Heikki Vatanen; Jukka Koskinen; Timo Nurmiainen
Original assignee: Metso Paper Oy
Current assignee: Metso Paper Oy
Priority date: 2005-01-18
Filing date: 2005-12-22
Publication date: 2007-10-03
Also published as: EP1838456A4; CN101080279A; WO2006077277A1; FI20055024A; FI20055024A0; JP2008526484A

Abstract

The invention relates to a method and apparatus for removing an edge bank formed at the edge of a coated web (30) in curtain coating effected as a dry edge coating process. The method comprises subjecting the edge bank to fluid blasting by means of elements (31-32) disposed downstream of a coating curtain application point for removing and/or spreading the edge bank for a thinner formation, said blasting being delivered from a center line direction of the web (30) towards the web edge.

Description

Method for removing an edge bank formed at the edge of a coated web in curtain coating effected as a dry edge coating process

The invention relates to a method and apparatus for removing an edge bank formed at the edge of a coated web in curtain coating effected as a dry edge coating process.

Coated paper grades and coating becoming more and more popular, the coating processes and equipment have increasing demands imposed thereon. In coating, more specifically in pigment coating, the surface of paper is formed with a layer of coating color at a coating station, followed by effecting the draining of excess water. The formation of a coating color layer can be divided in supplying the coating color onto a paper surface, i.e. application, as well as in the adjustment of a final amount of coating. The most important pigment coating technique is so-called blade coating, wherein the amount of coating is adjusted by means of a so-called doctor blade. The most common types of pigment coating stations comprise a blade coater equipped with an application roll and a blade coater with a nozzle application feature. In addition, coating is performed by using a so-called film transfer coater, which have lately become quite popular. Another emerging new technique involves the use of curtain coaters.

Curtain coaters can be categorized in slot-fed and slide-fed coaters. In the slide-fed curtain coater, a coating is fed by means of a nozzle assembly onto an inclined plane, the coating flowing down towards an edge of the plane constituting a feeding lip, the curtain forming as the coating falls off the feeding lip. In slot-fed application bars, a coating is pumped by way of a distribution chamber into a narrow vertical slot, the curtain forming at its lip and falling onto a web. A coating can be spread in one or more deposits. The resulting curtain is maintained at full width by means of an edge guide which, as suggested by its name, is located along the edge of a feeding slot/feeding lip. A multitude of various grades of paper and board are in existence and can be sorted by basis weight in two classes: papers of a single ply and with a basis weight of 25-300 g/m² and boards made in multiply technique and with a basis weight of 150-600 g/m². As noted, there is a floating line between paper and board, since boards of the lightest basis weight 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 following descriptions are examples of presently used values for fibrous webs to be coated and substantial deviations from the given values may occur. The descriptions are mainly based on the source publication Papermaking Science and Technology, section Papermaking Part 3, Finishing, edit. 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 coatings at 20-35%. General values for LWC-paper can considered as follows: basis weight 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: basis weight 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: basis weight 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: basis weight 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 basis weight of 100-135 g/m² and can be coated even more than twice.

In coated pulp-based woodfree printing papers or fine papers (WFC), the amounts of coating vary to a great extent according to demands and intended use. The following are typical values for once and twice coated pulp-based printing paper: once coated basis weight 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, basis weight 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 basis weight grades with a basis weight as high as 500 g/m² and low basis weight grades with a basis weight of about 120 g/m², the grades ranging from those based on primary fiber to those based by 100% on recycled fiber and from uncoated ones to those with multiple coating. Coated boards include the following:

- primary-fiber based (folding board (FBB = folding boxboard), bleached pulp board (SBS = solid bleached board), liquid packaging 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 suitable for making basically all the above-mentioned coatable grades. Curtain coating does not provide a smoothness equal to what is achieved by blade coating, but it is better than blade coating in terms of coverage.

One practical problem in curtain coating is that, when curtain coating is implemented as so-called dry edge coating, in which the edge region of a web is left with an uncoated zone, this boundary region of coating develops a thicker formation of coating material, i.e. an edge bank. This edge bank causes trouble in keeping the coating machine clean and in the winding of a paper/board web unless this dry edge is cut off upstream of the winder. There are basically two types of prior known solutions for removing an edge bank and/or inhibiting its formation.

The first type of solution involves inhibiting the formation of an edge bank by sucking off the edge zone of a falling coating curtain at the very surface of a web by means of vacuum. One solution like this is described in US patent 4,830,887, relating to the manufacture of photographic paper. In this solution, the bottom end of edge guides is provided with a suction feature. Fig. 1 shows schematically the principle of such prior known solution. Reference numeral 4 represents an edge guide having its bottom end provided with a suction feature 5 for vacuuming the edge zone of a curtain, such that the web develops a dry edge 2. Reference numeral 1 represents a coated portion of the web and reference numeral 3 indicates a backing surface, e.g. a roll.

The second prior known basic solution is to suck the formed edge bank off the surface of a coated web. Fig. 2 shows schematically a solution designed by Trailer Schweizer Engineering (TSE), Mugenthal, Switzerland, which is based on sucking off the edge bank. This solution is effected by using nozzles 11 mounted on the edge zones of a web 10, whereby an edge bank forming on the web surface is sucked off along a conduit 12. The nozzles are fitted to an adjustable mounting bracket 13. The arrangement works with compressed air 17 which is passed by way of a pressure regulator 15 to a venturi nozzle 16, which generates a required suction in the line 12. The coating color and/or flushing water sucked out of the venturi nozzle is passed into a drain pipe 18 for recycling or disposal. Reference numeral 19 represents a supply of flushing water, said flushing water being delivered by way of flow indicators 14 to the nozzles 11. A challenge in this type of suction-based solutions is maintaining the functionality of suction features in paper machine environment. A problem is e.g. the high precision required by structural aspects. Suction slots generally measure no more than a few tenths of a millimeter and a distance from the web is just a few hundredths of a millimeter.

It is an objective of the present invention to provide an improved solution capable of overcoming the problems associated with the prior art. In order to accomplish this objective, a method of the invention is characterized in that the method comprises subjecting the edge bank to fluid blasting by means of elements disposed downstream of a coating curtain application point for removing and/or spreading the edge bank for a thinner formation, said blasting being delivered from a center line direction of the web towards the web edge. An apparatus of the invention is in turn characterized in that the apparatus comprises blasting elements disposed downstream of a coating curtain application point for delivering a fluid flow to the edge bank for removing and/or spreading the same for a thinner formation, said blasting elements being arranged in view of delivering the fluid flow from a center line direction of the web towards the web edge.

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

Fig. 1 depicts one prior art principle for inhibiting the formation of an edge bank,

Fig. 2 depicts one prior art principle for removing a formed edge bank from the surface of a web, and

Fig. 3 depicts in a schematic view of principle one solution of the invention for eliminating an edge bank. In reference to fig. 3, the inventive arrangement comprises first blasting elements 31, 32 disposed above a web 30 for applying a fluid flow 40 for removing an edge bank developing in the coating deposit of the web 30. The fluid may comprise e.g. a liquid or a gas, e.g. water or air, or a mixture thereof. The fluid flow is directed from a center line of the web 30 towards the web edge at an angle as small as possible for making the blasting fluid bounce off and for keeping its penetration into the web as insignificant as possible. The nozzle is preferably set at an angle, whereat the blasting fluid, discharging e.g. in a fan-shaped pattern, has its center line preferably at an angle of less than 45° with respect to the web, preferably within the range of less than about 20°. The coating color disengaged by blasting is collected in a reclaiming funnel 35 and a discharge gutter 36. In association with the funnel can be provided a suction feature and/or dilution water. In the illustrated embodiment, the fluid is delivered from a supply 37 to various blasting elements under the control of valves. Below the web are provided second blasting elements 33, 34 capable of producing an under-web fluid flow for supporting the web. The fluid used for a support fluid is preferably air or some other gas. Instead of an underside support flow, the support can be provided by using a separate backing element, e.g. a roll. The underside support is preferably organized in such a way that the web has its edge remaining in an essentially horizontal position or possibly deflected slightly upward in order to expose the edge bank to a blast as effective as possible.

In addition to above-the-web blasting elements intended for removing an edge bank , it is possible to provide separate drying elements, comprising e.g. air blasting elements (not shown) disposed in succession with the edge bank blasting elements. This is feasible, especially when the blasting fluid comprises a liquid.

The inventive solution provides a solution improved in terms of its functionality and operating reliability, in which the distance of a jet-blasting element from a web can be several dozen times more than what is found in suction-based solutions. The spraying elements can be placed as far away as dozens of millimeters from the web, e.g. within the range of 0-100 mm, preferably about 15-50 mm. Neither is the precision of adjustment as critical as in the case of suction applications.

Claims

1. A method for removing an edge bank formed at the edge of a coated web (30) in curtain coating effected as a dry edge coating process, characterized in that the method comprises subjecting the edge bank to fluid blasting by means of elements (31-32) disposed downstream of a coating curtain application point for removing and/or spreading the edge bank for a thinner formation, said blasting being delivered from a center line direction of the web (30) towards the web edge.

2. A method as set forth in claim 1, characterized in that the fluid is selected from the group consisting of: water, air or a mixture thereof.

3. A method as set forth in claim 1 or 2, characterized in that the method comprises delivering not only a removal and/or spreading blast but also a separate drying blast with air flow.

4. A method as set forth in any of the preceding claims, characterized in that the fluid flow is delivered to the web at such a small angle that the fluid bounces off the web surface.

5. A method as set forth in any of the preceding claims, characterized in that the web is also subjected to a fluid flow below the web for supporting the web.

6. An apparatus for removing an edge bank formed at the edge of a coated web (30) in curtain coating effected as a dry edge coating process, characterized in that the apparatus comprises blasting elements (31-32) disposed downstream of a coating curtain application point for delivering a fluid flow to the edge bank for removing and/or spreading the same for a thinner formation, said blasting elements being arranged in view of delivering the fluid flow from a center line direction of the web (30) towards the web edge.

7. An apparatus as set forth in claim 6, characterized in that the blasting elements (31-32) are arranged to deliver a fluid flow at such a small angle that the fluid bounces off the web surface.

8. An apparatus as set forth in claim 6 or 7, characterized in that below the web are mounted extra blasting elements (33-34) for delivering a web- supporting fluid flow to the bottom side of the web.