METHOD OF AND COATING APPLICATION UNIT FOR APPLYING A COATING ON A WEB OF MATERIAL

Abstract

 The invention relates to a method and a coating application unit for applying a coating on a web (1) of material, such as a paper web or a board web or flexible packaging material. The method comprises at least the following steps:
- moving the web (1) of material in a machine direction (M) at a web speed;
- supporting the web of material with rotating cylinders (2, 3, 4, 5, 6, 7);
- rotating a coating applying cylinder (2), such as a gravure cylinder, whereby surface (8) of the coating applying cylinder travels at a coating applying speed;
- arranging a layer of liquid coating material on the surface (8) of the coating applying cylinder;
- leading the web (1) of material close to the surface (8) of the coating applying cylinder so that a first side (9) of the web of material makes contact with the layer of liquid coating material at an applying zone (10), whereby some of the liquid coating material sticks on the first side (9) of the web of material thereby creating a coating layer on the web of material;
- rotating the coating applying cylinder (2) in reverse direction (R), i.e against the machine direction (M), so that the surface (8) of the coating applying cylinder moves in reverse direction (R) at a coating applying speed, whereby at the applying zone (10) the web (1) of material and the surface (8) of the coating applying cylinder (2) are moving in opposite directions;
- smoothing the coating layer on the web (1) of material by arranging it in contact with a rotating blade assembly (11) after the applying zone (10) at a smoothing contact zone.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to method of and coating application unit for applying a coating on a web of material according to preambles of the appended independent claims. The invention especially relates to a new way of applying a coating, such as a dispersion coating, onto a web of e.g. cardboard, paper or flexible packaging material. The invention may be used with various printing methods, such as gravure printing.

TECHNICAL BACKGROUND

[0002] Coating and printing on a moving web of material using rotating cylinders is well known. E.g. in gravure printing the image to be printed is engraved onto a rotatable cylinder. The engraving creates on the cylinder surface cells or engravings that are filled with liquid coating material, e.g. ink. The cylinder is brought into contact with the web of material, in order to transfer the coating to the web. The amount of coating contained in a cell corresponds to different amounts of coating transferred, e.g. colour intensities produced on a cardboard web. Deeper or larger cells will transfer more coating, e.g. produce more intense colours whereas smaller cells will produce less intense ones. Normally, an engraved cylinder is partially immersed in a tray or a source filled with liquid coating material. The cylinder is rotated in the source, thereby drawing coating into the cells and excess coating onto its surface. Usually, a doctor blade is arranged to scrape the cylinder before it makes contact with the web, removing the excess coating from areas with no recesses and leaving in the cells the right amount of coating. The web is transferred near the engraved cylinder surface, usually both moving in the same direction at a so-called machine speed. The web is pressed between a support cylinder and the engraved cylinder, whereby the coating is transferred from the cells onto the web. Usually the coating is then dried by leading the web with the coating layer on through a dryer.

[0003] Controlling coating processes could be more effective. E.g. it is often difficult to the achieve and maintain wanted levels of evenness, thickness and smoothness of the coating layer. Unwanted small holes, so-called pinholes are sometimes left in the coating. Problems usually get more severe with higher machine speeds. E.g. when gravure printing a web such as cardboard with dispersion coatings, it is sometimes difficult to get an adequate amount of coating material applied on the moving web.

OBJECT OF THE INVENTION

[0004] It is an object of the present invention to reduce or even eliminate the above-mentioned problems appearing in prior art.

[0005] It is an object of the present invention to enhance the effectiveness of coating processes, especially in gravure printing of e.g. cardboard, paper or flexible packaging material.

[0006] The present invention has especially an object of providing an effective process for gravure printing a web with a dispersion coating, such as a dispersion lacquer.

[0007] The present invention has especially an object of providing an effective process for reducing or eliminating pinholes in a dispersion coating layer on a web of material.

SUMMARY OF THE INVENTION

[0008] In order to realize the objects mentioned above, the method and coating application unit and other objects according to the invention are characterized by what is presented in the enclosed independent claims. The dependent claims present some further possible features of the invention.

[0009] The embodiments, examples and advantages mentioned in this text relate, where applicable, as well to the method and the coating application unit as to uses according to the invention, even though it is not always specifically mentioned.

[0010] A typical method according to the invention a coating is applied on a web of material, such as a paper web or a board web or flexible packaging material. The method comprises at least the following steps:

• moving the web of material in a machine direction at a web speed;
• supporting the web of material with rotating cylinders;
• rotating a coating applying cylinder, such as a gravure cylinder. The surface of the coating applying cylinder travels at a coating applying speed;
• arranging a layer of liquid coating material on the surface of the coating applying cylinder;
• leading the web of material close to the surface of the coating applying cylinder. This way a first side of the web of material makes contact with the layer of liquid coating material at an applying zone. Some of the liquid coating material sticks on the first side of the web of material thereby creating a coating layer on the web of material;
• rotating the coating applying cylinder in reverse direction, i.e. against the machine direction. This way the surface of the coating applying cylinder moves in reverse direction at a coating applying speed. Thus at the applying zone the web of material and the surface of the coating applying cylinder are moving in opposite directions;
• smoothing the coating layer on the web of material by arranging it in contact with a rotating blade assembly after the applying zone at a smoothing contact zone.

[0011] A typical coating application unit according to the invention is meant for applying a coating on a web of material, such as a paper web or a board web or flexible packaging material. The unit comprises

• a coating applying cylinder, such as a gravure cylinder, arranged to be rotated. The surface of the coating applying cylinder is arranged to travel at a coating applying speed;
• a coating source arranged to apply a layer of liquid coating material on the surface of the coating applying cylinder;
• rotating cylinders arranged to support the web of material to be moved in the unit in a machine direction at a web speed. The surfaces of the rotating cylinders are arranged
  a) to define a path of travel through the coating application unit for the web of material and

  b) to lead a first side of the web of material to travel near the surface of the coating applying cylinder at an applying zone. This is done in order to allow some of the liquid coating material to stick on the first side of the web of material, thereby forming a coating layer on the web;
• a motor arranged to rotate the coating applying cylinder in reverse machine direction, i.e. against the machine direction. The surface of the coating applying cylinder is arranged to move at a coating applying speed. At the applying zone the web of material and the surface of the coating applying cylinder are arranged to move in opposite directions;
• a rotating blade assembly arranged along the path of travel after the applying zone. The rotating blade is arranged to contact the coating layer on the web of material at a smoothing contact zone in order to smoothen the coating layer.

[0012] Thus it has been found out that coating of a web of material may be done by moving the web and the coating applying cylinder in opposite directions. This provides a surplus of liquid coating material at the applying zone, i.e. in and near a nip between the web and the coating applying cylinder. This way an adequate amount of coating is transferred on the web. The so produced coating layer is then smoothed by contacting the coating layer with a rotating blade assembly at a smoothing contact zone. The rotating blade assembly ensures that at least most of the unwanted holes, tops and valleys left in the coating material after the applying zone are removed or levelled.

[0013] Although the examples in this text use predominantly gravure printing, the invention may be used with any printing method.

[0014] The liquid coating material may vary according to need. E.g. dispersion coatings, different types of lacquers and varnishes. In an embodiment the liquid coating material is a dispersion of polymer and mineral pigments. In an embodiment the liquid coating material is a barrier dispersion or a barrier varnish. The dry content of the liquid coating material may vary according to application, but some possible values are 30 to 50 % or 37 to 43 %. Viscosity of the liquid coating material may vary according to application, but some possible values are 10 to 20 s (DIN 4) or 12 to 15 s (DIN 4).

[0015] The invention relates to coating a web of material, such as a paper web or a board web or flexible packaging material web. The invention is suitable for producing different types of packaging materials, such as food, tobacco or washing powder packaging materials. Further examples of possible products are materials for hydrophobic or moisture barrier packaging or heat sealable materials. E.g. coated cardboard produced with the invention may be of weight 120-400 g/m².

[0016] Flexible packaging is material used in package or part of a package whose shape can be readily changed. It includes packaging made with flexible packaging materials, such as paper, plastic, film, foil, metalized or coated paper, and film, or a combination of any of these materials. Flexible packaging comprises e.g. rollstock, bags, pouches, liners, shrink sleeves, shrink wrap, stretch wrap, bulk shipping sacks, flexible lidding, film wrap around labels.

[0017] In an embodiment the coating application unit comprises a rotatable support bar or cylinder arranged along the path of travel between the applying zone and the rotating blade assembly. The rotatable support bar is arranged to support the second side of the web of material.

[0018] In an embodiment the position of the rotatable support bar is arranged adjustable towards or away from the web of material. This way it is possible to adjust the position of the web of material compared to the surface of the coating applying cylinder and/or compared to the rotating blade assembly. Said adjusting the position of the the web of material with the rotatable support bar makes it possible to easily and accurately adjust tension and position of the web at the applying zone and/or at the smoothing contact zone. Said adjusting also makes it possible to easily and accurately adjust tension of the web between the coating applying cylinder and the rotating blade assembly with movement of the support bar. It also makes it possible to adjust the amount of coating material transferred from the coating applying cylinder to the web with movement of the support bar. It also makes it possible to adjust the smoothing of the coating layer on the web at the smoothing contact zone with movement of the support bar.

[0019] In an embodiment the rotatable support bar is freely rotating, i.e. not driven. In another embodiment the rotatable support bar is machine driven, possible with adjustable speed and direction.

[0020] In an embodiment the web of material is pressed towards and against the surface of the coating applying cylinder at least mainly with rotating cylinders that are situated along the path of travel, but a distance from the coating applying cylinder. This means that there is no need for a support cylinder used in the prior art to press the web between itself and the coating applying cylinder.

[0021] In an embodiment the rotating blade assembly comprises a frame and supported to the frame a rotatable smoothing cylinder. The smoothing of the coating layer is then performed by arranging it in contact with the rotatable smoothing cylinder at the smoothing contact zone. Smoothing cylinders as known as such.

[0022] In an embodiment the position of the rotatable smoothing cylinder compared to the frame of the rotating blade assembly is adjustable with position adjusting means. This makes it possible to e.g. remove the smoothing cylinder from contact with the web, when needed.

[0023] In an embodiment the rotatable smoothing cylinder is freely rotating, i.e. not driven. In another embodiment the rotatable smoothing cylinder is machine driven, possible with adjustable speed and direction.

[0024] In an embodiment the smoothing of the coating layer is adjusted by adjusting position of the web of material compared to the rotating blade assembly.

[0025] In an embodiment the thickness of the coating layer on the web of material after the applying zone is adjusted by adjusting position of the the web of material compared to the surface of the coating applying cylinder.

[0026] In an embodiment speed ratio between the coating applying speed and the web speed is set between 0.1 and 10, or between 1 and 2, or between 1.2 and 1.4. In an embodiment the speed ratio may be adjusted and chosen according to need. The speed ratio can be adjusted e.g. by adjusting the rotating speed of the motor arranged to rotate the coating applying cylinder. The speed ratio may be selected and/or optimized according to need.

[0027] In an embodiment the adjustable speed ratio between the coating applying speed the web speed is used to adjust thickness of the coating layer on the web of material after the applying zone. In most cases, in normal running conditions of the coating application unit, increasing the speed ratio causes more coating material to be brought and kept at the application zone, which normally leads to a thicker coating layer on the first side of the web of material. Also, normally, decreasing the speed ratio makes the opposite to happen. E.g. if it measured or noticed that the amount of coating material should be higher, the speed ratio may be increased and vice versa.

[0028] The coating applying cylinder may e.g. have a diameter of 400 to 800 mm.

[0029] The length of the application zone may be arranged e.g. as 30 to 100 mm or 40 to 80 mm or 45 to 55 mm.

[0030] In an embodiment the distance between the applying zone and the smoothing contact zone is between 150 and 500 mm, or 200 and 350 mm or 280 and 320 mm.

[0031] In an embodiment the coating layer is cured or dried after the smoothing contact zone with a curing or drying apparatus e.g. by hot air curing, UV curing or electrical IR curing.

[0032] In an embodiment the surface of the rotatable smoothing cylinder is made of polished metal, such as stainless steel. A low roughness of the surface of the rotatable smoothing cylinder seems to be advantageous. In an embodiment roughness of the surface of the rotatable smoothing cylinder is arranged between Ra 0.004 and Ra 0.05 or between Ra 0.006 and Ra 0.025 or less than Ra 0.025 or less than Ra 0.01. Polishing of steel cylinders is known as such, but using a surface with low roughness on a rotatable smoothing cylinder in a method and application unit of the invention gives excellent smoothing of the coated web.

[0033] In an embodiment the surface of the coating applying cylinder is engraved, in an embodiment with spiral engraving. Depth and form of the engraving defines how much lacquer or other coating material is applied on the web of material.

[0034] In an embodiment the coating applying cylinder is a metal gravure cylinder, i.e. an engraved image carrier used in gravure printing. In gravure printing cells or depressions or engravings in a gravure cylinder are filled with coating material or ink and transferred to the substrate, i.e. the web of material.

BRIEF DESCRIPTION OF THE FIGURES

[0035] The invention is described in more detail below with reference to the enclosed schematic drawing, in which

Figure 1

shows a coating application unit according to the invention in perspective view,

Figure 2

shows a coating application unit according to the invention in side view,

Figures 3 and 4

show a rotating blade assembly according to the invention in two different perspective views.

DETAILED DESCRIPTION OF THE EXAMPLES OF THE FIGURES

[0036] For the sake of clarity, the same reference numbers are used for some corresponding parts in different embodiments.

[0037] Figures 1 and 2 each show a coating application unit 14 for applying a coating on a web 1 of material, such as a paper web or a board web or flexible packaging material. The web 1 is supported by rotating cylinders 2, 3, 4 (in Figs 1&2) and 5, 6, 7 (in Fig. 2) and moved in the unit in a machine direction M at a web speed. The unit comprises a coating applying cylinder 2, such as a gravure cylinder. The cylinder 2 is arranged to be rotated by a motor 13 (Fig. 1). The coating applying cylinder 2 transfers liquid coating material from a coating source (not shown) on its surface 8.

[0038] Surfaces of the rotating cylinders 2, 3, 4, 5, 6, 7 define a path 15 of travel through the coating application unit 14 for the web 1 of material. The surfaces of the rotating cylinders lead the web so, that a first side 9 of the web travels near the surface 8 of the coating applying cylinder at an applying zone 10. This allows at least some of the liquid coating material on surface 8 to stick on the first side 9 of the web of material, thus forming a coating layer on the first side 9 of the web.

[0039] The motor 13 is arranged to rotate the coating applying cylinder 8 in reverse machine direction R, i.e. against the machine direction (M). Thus, the surface 8 of the coating applying cylinder is arranged to move at a coating applying speed in reverse machine direction R. This way at the applying zone 10, or where the web and the surface 8 align, the web and the surface 8 of the coating applying cylinder 2 are arranged to move in opposite directions. This causes a surplus of coating material to reach the applying zone 10. The speed of the motor 13 and thus the coating applying speed can be adjusted as needed. With a suitable speed ratio between the coating applying speed and the web speed the liquid coating material starts to pile up at the applying zone 10. This ensures a sufficient amount of coating material to stick on the web 1 as a coating layer.

[0040] A rotating blade assembly 11 is arranged along the path 15 of travel after the applying zone 10. The rotating blade assembly 11 comprises a frame 20 and a rotatable smoothing cylinder 3 arranged to contact the coating layer on the web at a smoothing contact zone 21. The smoothing of the coating layer is performed by the smoothing cylinder 3, which has its steel surface polished. A handle bar 22 is attached to the bar 3 to move the position of the bar 3 in relation to the frame 20, when needed.

[0041] A rotatable support bar 4 is arranged along the path 15 of travel between the applying zone 10 and the rotating blade assembly 11. It arranged to support the second side 12 of the web 1 of material. Position of the support bar 4 can be adjusted towards or away from the web 1. This way the tension of the web may be adjusted. Also, this allows to adjust the position of the web 1 compared to the surface 8 of the coating applying cylinder 2 and compared to the smoothing cylinder 3. Position of the the support bar 4 and the cylinder 6 together define which length of the web 1 is in contact with the surface 8 of the coating applying cylinder 2. There is no need for a special support cylinder used in the prior art to press the web between itself and the coating applying cylinder.

[0042] The adjustable functions, such as the position of the support bar 4, the speed ratio between the coating applying speed and the web speed, the position of the smoothing cylinder 3, can be used to control and adjust the coating process, e.g. the thickness or smoothness of the of the coating layer. And, this can in most cases be done without the need to adjust the machine speed or functioning of the coating source.

[0043] The figures show only a few preferred embodiments according to the invention. Facts of secondary importance with regards to the main idea of the invention, facts known as such or evident for a person skilled in the art, such as power sources or support structures possibly required by the invention, are not necessarily separately shown in the figures. It is apparent to a person skilled in the art that the invention is not limited exclusively to the examples described above, but that the invention can vary within the scope of the claims presented below. The dependent claims present some possible embodiments of the invention, and they are not to be considered to restrict the scope of protection of the invention as such.

Claims

1. Method of applying a coating on a web (1) of material, such as a paper web or a board web or flexible packaging material, the method comprising at least the following steps:

- moving the web (1) of material in a machine direction (M) at a web speed;

- supporting the web of material with rotating cylinders (2, 3, 4, 5, 6, 7);

- rotating a coating applying cylinder (2), such as a gravure cylinder, whereby surface (8) of the coating applying cylinder travels at a coating applying speed;

- arranging a layer of liquid coating material on the surface (8) of the coating applying cylinder;

- leading the web (1) of material close to the surface (8) of the coating applying cylinder so that a first side (9) of the web of material makes contact with the layer of liquid coating material at an applying zone (10), whereby some of the liquid coating material sticks on the first side (9) of the web of material thereby creating a coating layer on the web of material;

characterised by

- rotating the coating applying cylinder (2) in reverse direction (R), i.e against the machine direction (M), so that the surface (8) of the coating applying cylinder moves in reverse direction (R) at a coating applying speed, whereby at the applying zone (10) the web (1) of material and the surface (8) of the coating applying cylinder (2) are moving in opposite directions;

- smoothing the coating layer on the web (1) of material by arranging it in contact with a rotating blade assembly (11) after the applying zone (10) at a smoothing contact zone.

2. A method according to claim 1, characterised by

- supporting a second side (12) of the web of material with a rotatable support bar (4) between the applying zone (10) and the rotating blade assembly (11).

3. A method according to claim 2, characterised by

- adjusting position of the rotatable support bar (4) towards or away from the web (1) of material and thereby adjusting position of the web (1) of material compared to the surface (8) of the coating applying cylinder (2) and/or compared to the rotating blade assembly (11).

4. A method according to any previous claim 1 to 3, characterised by

- adjusting the smoothing of the coating layer by adjusting position of the web (1) of material compared to the rotating blade assembly (11).

5. A method according to any previous claim 1 to 4, characterised by

- adjusting thickness of the coating layer on the web (1) of material after the applying zone (10) by adjusting position of the the web (1) of material compared to the surface (8) of the coating applying cylinder (2).

6. A method according to any previous claim 1 to 5, characterised by that the rotating blade assembly (11) comprises a frame and a rotatable smoothing cylinder (3), whereby in the method:

- the smoothing of the coating layer is performed by arranging it in contact with the rotatable smoothing cylinder (3) at the smoothing contact zone.

7. A method according to claim 6, characterised by

- adjusting position of the rotatable smoothing cylinder (3) compared to the frame of the rotating blade assembly (11).

8. A method according to any previous claim 1 to 7, characterised by

- arranging a speed ratio between the coating applying speed and the web speed between 0.1 and 10, or between 1 and 2, or between 1.2 and 1.4.

9. A method according to any previous claim 1 to 8, characterised by

- adjusting thickness of the coating layer on the web (1) of material after the applying zone (10) by adjusting the speed ratio between the coating applying speed and the web speed.

10. A method according to any previous claim 1 to 9, characterised by

- curing the coating layer after the smoothing contact zone with e.g. hot air curing, UV curing or electrical IR curing.

11. Coating application unit (14) for applying a coating on a web (1) of material, such as a paper web or a board web or flexible packaging material, the unit comprising

- a coating applying cylinder (2), such as a gravure cylinder, arranged to be rotated, whereby surface (8) of the coating applying cylinder is arranged to travel at a coating applying speed;

- a coating source (16) arranged to apply a layer of liquid coating material on the surface (8) of the coating applying cylinder;

- rotating cylinders (2, 3, 4, 5, 6, 7) arranged to support the web (1) of material to be moved in the unit (14) in a machine direction (M) at a web speed, whereby surfaces of the rotating cylinders (2, 3, 4, 5, 6, 7) are arranged

a) to define a path (15) of travel through the coating application unit (14) for the web (1) of material and

b) to lead a first side (9) of the web of material to travel near the surface (8) of the coating applying cylinder at an applying zone (10) in order to allow some of the liquid coating material to stick on the first side (9) of the web of material forming a coating layer;

- a motor (13) arranged to rotate the coating applying cylinder (8);

characterised by that

- the motor (13) is arranged to rotate the coating applying cylinder (8) in reverse machine direction (R), i.e. against the machine direction (M), so that the surface (8) of the coating applying cylinder is arranged to move at a coating applying speed, whereby at the applying zone (10) the web (1) of material and the surface (8) of the coating applying cylinder (2) are arranged to move in opposite directions;
and by the coating application unit (14) further comprising

- a rotating blade assembly (11) arranged along the path (15) of travel after the applying zone (10) and arranged to contact the coating layer on the web (1) of material at a smoothing contact zone in order to smoothen the coating layer.

12. A coating application unit (14) according to claim 11, characterised by that the rotating blade assembly (11) comprises a frame and supported to the frame a rotatable smoothing cylinder (3) arranged to be in contact with the coating layer at the smoothing contact zone.

13. A coating application unit (14) according to claim 11 or 12, characterised by a rotatable support bar (4) arranged along the path (15) of travel between the applying zone (10) and the rotating blade assembly (11) and arranged to support the second side (12) of the web (1) of material.

14. A coating application unit (14) according to any previous claim 11 to 13, characterised by that distance between the applying zone (10) and the smoothing contact zone is between 150 and 500 mm, or 200 and 350 mm or 280 and 320 mm.

15. A coating application unit (14) according to any previous claim 11 to 14, characterised by that the web (1) of material is pressed towards and against the surface (8) of the coating applying cylinder (2) at least mainly with rotating cylinders (4, 6) that are situated along the path (15) of travel, but a distance from the coating applying cylinder (2).

16. A coating application unit (14) according to any previous claim 12 to 15, characterised by that the surface of the rotatable smoothing cylinder (3) is made of polished stainless steel.

17. A coating application unit (14) according to any previous claim 12 to 16, characterised by that roughness of the surface of rotatable smoothing cylinder (3) is between Ra 0.004 and Ra 0.05 or between Ra 0.006 and Ra 0.025 or less than Ra 0.025 or less than Ra 0.01.

Drawing