Method and apparatus for decurling web material

Abstract

 In a steam decurling method and apparatus in which a web is passed through a steam treatment station and one surface of the web is subjected to a steam shower, improved decurling is achieved by maintaining a substantially constant distance between the web and the source of the steam shower by directing a gas flow obliquely towards the web from a gas source disposed adjacent the opposite surface of the web from the steam shower, in a manner such that the gas flow is substantially parallel to the web after the gas flow has impinged on the web. The gas source is preferably an air foil assembly. The web may be of coated or uncoated paper.

Description

[0001] This invention relates to a method and apparatus for decurling web material, particularly a web of coated or uncoated paper.

[0002] When a web is unrolled, the unrolled web frequently has a tendency to curl. The extent of this depends ou factors such as the length of time the roll has been stored in stock and the previous position in the roll of the part of the unrolled web in question (it will be appreciated that the curvature of the different convolutions making up the roll differs). In order to compensate for these factors, it has been found desirable to treat the web to reduce its tendency to curl when unrolled, particularly if it is ultimately to be cut into sheets. It will be appreciated that the handling, appearance and convenience of use of such sheets will be facilitated if the undesirable curling effect is eliminated. Proposals for decurling treatment may be found, for example, in US Patents 3185616 and 3649447. While such proposals have been found useful, they have not been altogether satisfactory in the sense that they are expensive and/or require complicated equipment.

[0003] In the case of a paper web, it is known that increasing the moisture content of the paper will lead to less trouble from curling than paper having a low moisture content. Thus it is well-known in the paper industry to seek to counteract curling by the application of moisture to raw paper stock and/or to coated paper stock. In either case the moisture is normally applied to the surface opposite to that to which the web will otherwise tend to curl. With respect to a coated web it is known that the web will tend to curl towards the coated surface, or, in the case of a web coated on both surfaces, to the surface last coated, and so the moisture is normally applied to the uncoated surface, or to the first coated surface.

[0004] Several types of apparatus have been proposed for applying moisture to a moving web. Examples include roll applicators such as so-called Dahlgren-type systems, electrostatic systems such as disclosed in US Patent 3467541 and steam applicator systems. The last-mentioned include firstly systems incorporating a heat conducting backing body to facilitate condensation of the steam, whereby an increased moisture content may be obtained, as disclosed in US Patent 2370811 and secondly, systems in which a humid atmosphere is directed towards the web, as disclosed in US Patent 3238635.

[0005] While these and other techniques have been found useful in applying moisture to a paper web so as to minimise the undesirable curling properties.of the paper, they have not proved entirely satisfactory. Electrostatic systems tend to suffer from nozzle clogging, arcing and misting, and to apply moisture unevenly. Moreover, the need to take special ; safety precautions is a disadvantage. So far as non-electrostatic systems are concerned, mere direction of a humid atmosphere towards the web has been found to be inefficient so far as decurling is concerned. If direct steam showers are employed, the decurling effect is governed by the steam pressure employed at the shower source. In order to obtain effective decurling, excessive steam pressures and/or reduced machine speeds may be necessary. It should be appreciated in this context that steam pressure affects other important variables such as the web temperature, web to shower distance and steam billowing. In addition, decurling regulated by steam pressure alone is uneconomical and inefficient. If the web is free floating, the distance between the web and the shower source tends to increase and steam tends to billow. These effects result in an increased total amount of steam being used, in order to ensure that the amount of steam required for bringing abour decurling actually reaches the web. The additional steam is wasted since it is merely drawn away by exhaust. The provision of extra exhaust capacity is also expensive.

[0006] It is an object of the present invention to provide a method and apparatus for steam decurling a web, such as a web of coated or uncoated paper, which will overcome or at least mitigate the above-mentioned disadvantages.

[0007] Accordingly, the present invention provides in a first aspect a method for decurling a web by substantially uniform application of moisture thereto, comprising the steps of subjecting one surface of the web to a steam shower as the web passes through a steam treatment station, characterized in that a substantially constant distance is maintained between the web and the source of the steam shower by directing a gas flow obliquely towards the web from a gas source disposed adjacent the opposite surface of the web path from the steam shower, in a manner such that the gas flow is substantially parallel to the web after the gas flow has impinged on the web.

[0008] In a second aspect, the present invention provides apparatus for decurling a web by substantially uniform application of moisture thereto, comprising means for passing the web through a steam treatment station and a steam source for providing a steam shower to one surface of the web, characterized in that gas flow directing means are provided on the opposite surface of the web path from the steam source, whereby a substantially constant distance is maintained in use between the web and the steam source as a result of impingement of the gas flow on the web and subsequent flow of gas parallel to the web.

[0009] The gas flow is conveniently a flow of air, and the gas flow directing means is preferably an air foil assembly.

[0010] The present method and apparatus are particularly suited to the decurling of paper webs, either coated or uncoated. In the case of a coated web, the steam shower is normally directed at the uncoated surface of the web, or, if both web surfaces are coated, at the surface opposite the last-coated surface.

[0011] Normally, the web path is horizontal and the steam shower is directed at the web from beneath the web.

[0012] The air foil assembly is preferably offset with respect to the source of the steam shower so as to be just prior thereto in the direction of web movement. This minimises the possibility of damage to the web material at high or excessive steam pressures.

[0013] The leading surface of the air foil assembly is preferable approximately 0.0125m to 0.0190m (¹/₂ to ³/₄ inch) prior to the steam source, as viewed in the direction of web travel. The vertical displacement of the web material from the steam source is preferably approximately half that from the air foil assembly to the steam source. This may be achieved by suitable balancing of the air pressure and the steam shower pressure. The preferred air pressure in the air foil assembly is 178-229 kPa (7-9 inches of water). The preferred steam shower pressure is approximately 34.5kPa (5 psi), although this is not critical. Steam pressure above about 206.8kPa (30 psi) should however preferably be avoided.

[0014] Preferably, a plurality of steam shower sources are provided with a corresponding number of gas flow directing means.

[0015] The or each steam shower source may be a steam pipe with at least one steam emitting orifice, which is or are normally located along the centre line of the pipe. Such a steam shower source permits the pressure of steam emitted to be varied, so as to facilitate uniform application of moisture to the web being treated. The or each steam shower is preferably located below the web, adjacent to the web surface.

[0016] The invention stems from the finding that the factors most significant in controlling the curling effect, particularly in relation to paper webs and more especially to coated paper webs, are the distance between the web and the source of the steam shower, the web temperature, the steam pressure and the exhaust level. The present invention permits an efficient regulation of these factors and provides for reliable control of the distance between the web and the source of the steam shower.

[0017] This minimises any web fluttering effect which might otherwise result from excessive steam pressures and results in more uniform steam application and penetration of the web. By controlling the distance between the web and the steam source, billowing is minimised and the necessity for extreme exhaust conditions is eliminated. The invention also permits the use of steam pressures less than would be needed with a comparable conventional method and apparatus.

[0018] It will be appreciated that in the present method and apparatus, there is no need to make use of an electrostatic field for guiding the moisture to be applied to the web, and that this is a significant benefit.

[0019] It will also be appreciated that the use of one or more air foil assemblies has the advantage of permitting a constant distance to be maintained between the web and the steam source without the need for contact with the web surface. This reduces the possibility of the surface of the web being impaired or blemished, which is particularly significant where a coated surface is involved, especially if it is a relatively recently coated surface. Use of an air foil assembly also provides a cushioning effect on the web surface in the area immediately above the steam pipe orifice(s). There is also a cooling effect on the web surface since the air film promotes temperature reduction, thus causing more efficient steam condensation. Also, when an air foil assembly is used, it is not usually necessary, as it is with conventional steam decurling apparatus, to increase the steam pressure when the speed of the web has been increased in order to achieve the desired decurling effect.

[0020] In order to enable the present invention to be more readily understood, reference will now be made to the drawings, which illustrate diagrammatically and by way of example some embodiments thereof and of a conventional apparatus, and of which :-

Fig. 1 is a schematic side view of a conventional steam shower decurling apparatus; and

Fig. 2 is a schematic side view of an embodiment of steam shower decurling apparatus according to the present invention.

[0021] Referring first to Fig. 1, a steam shower decurling apparatus generally indicated 1 comprises a steam shower housing 2 containing two steam pipes 3,4 having respective orifices 5,6. The steam shower housing 2 includes exhaust chambers 10.

[0022] When in use, a web 7 is unwound from a feed roll 8, is passed over the steam pipes 3,4 and is taken up on an idle roll 8. Steam showers issue from the orifices 5,6 of the steam pipes 3,4 and impinge on the web 7 as it passes over them. Any excess steam is removed via the exhaust chambers 10.

[0023] In this conventional steam decurling apparatus, the decurling effect is regulated by steam pressure only and there are no means for maintaining a substantially constant distance between the web and the steam sources. The steam pressure employed affects this distance and if it becomes too large as a result of a high steam pressure, excessive steam billowing may result. To counter this, exhaust levels have to be increased, but this leads to excessive removal of steam and thus to an increase in steam usage.

[0024] Referring now to Fig. 2, a steam shower decurling apparatus generally indicated 20 comprises a steam shower housing 21 containing two steam pipes 22,23 having respective orifices 24,25. These orifices are disposed along the centre line of their respective pipes 22,23. The steam shower housing 21 includes exhaust chambers 29 for controlled venting of billowing steam from the steam pipes 22,23.

[0025] Two air foil assemblies generally indicated 31,32 are positioned above the respective steam pipes 22,23 so as to be on the opposite surface of the web path from the steam pipes. Each air foil assembly comprises an air foil housing 33,34 and an air supply duct35,36. The ducts 35,36 are attached to a remote air blower (not. shown). Each air foil housing has an outlet 37 arranged so as to direct air obliquely towards the web, with the result that the air flow is parallel to the web.

[0026] The use of air foil assemblies of this kind gives rise to a Venturi effect (as witnessed at the surface of an aeroplane wing). The air emitted through the outlet 37 establishes a uniform air flow or film pattern directed parallel to the web surface and this prevents contact of the web with the air foil assemblies whilst at the same time maintaining the web at a constant distance from the air foil assemblies, and thus also from the steam shower assemblies.

[0027] Each air foil assembly 33,34 has its leading edge positioned or offset so as to be just before the orifice 24,25 or centre line of its respective steam pipe 22,23 as viewed in the direction of web movement. The extent of the offset is indicated as ℓ and is approximately 0.0125m to 0.0190m (¹_/2 to ³/₄ inch). The vertical displacement between each air foil assembly and its respective steam pipe is indicated asot.

[0028] When in use, a web 26 is unwound from a feed roll 27, is passed between the air foil assemblies 33,34 and the steam pipes 22,23 and is taken up on an idle roll 28. The air foil pressure is maintained such that the web floats uniformly between the air foil assemblies 33,34 and the steam pipes 22,23, with the result that the distance between the steam pipe orifices 24,25 and the web material 26 is maintained substantially constant at a value approximately half the vertical displacement α between each air foil assembly and its respective steam pipe.

[0029] The offset of the air foil assemblies 33,34 with respect to the steam pipes 22,23 minimises the possibility of damage to the web due to high steam pressures. The use of air foils provides a cushioning effect on the web in the area immediately above the steam pipes. The upper surface of the web does not actually contact the air foil assemblies, which is especially significant when the upper surface of the web carries a coating, since the possibility of impairing the web surface is reduced.

[0030] The steam pressure employed in the apparatus of Fig. 2 is generally about 34.5kPa (5 psi) and is preferably not greater than 206.8kPa (30 psi).

[0031] It will be appreciated that the apparatus of Fig. 2 does not utilise electrostatic means for guiding moisture to be applied to the web.

Claims

1. A method for decurling a web by substantially uniform application of moisture thereto, comprising the step of subjecting one surface of the web to a steam shower as the web passes through a steam treatment station, characterized in that a substantially constant distance is maintained between the web and the source of the steam shower by directing a gas flow obliquely towards the web from a gas source disposed adjacent the opposite surface of the web from the steam shower, in a manner such that the gas flow is substantially parallel to the web after the gas flow has impinged on the web.

2. A method as claimed in Claim 1, characterized in that the source of the flow of gas is offset with respect to the source of the steam shower, so as to be just prior thereto as viewed in the direction of web movement.

3. A method as claimed in Claim 2, characterized in that the gas flow pressure is approximately 178 to 229kPa (7-9 inches of water) and the steam shower pressure is approximately 34.5kPa (5 psi).

4. A method as claimed in any preceding claim, characterized in that a plurality of steam shower sources and a corresponding number of gas flows are provided.

5. A method as claimed in any preceding claim, characterized in that the web comprises an optionally coated paper sheet.

6. Apparatus for decurling a web by substantially uniform application of moisture thereto comprising means for passing the web through a steam treatment station and a steam source for providing a steam shower to one surface of the web, characterized in that gas flow directing means are provided adjacent the opposite surface of the web path from the steam source, whereby a substantially constant distance is maintained in use between the web and the steam source, as a result of impingement of the gas flow on the web and subsequent flow of gas parallel to the web.

7. Apparatus as claimed in Claim 6 characterized in that the gas flow directing means comprises an air foil assembly.

8. Apparatus as claimed in Claim 7, characterized in that the air foil assembly is offset with respect to the steam source so as to be just prior thereto, as viewed in the direction of web movement.

9. Apparatus as claimed in Claim 8, characterized in that the extent of the offset is approximately 0.0125m to 0.0190m (¹/₂ to ³/₄ inch).

10. Apparatus as claimed in any of Claims 6 to 9, characterized in that a plurality of steam shower sources and a corresponding number of gas flow directing means are provided.

Drawing