Sheet surface treatment apparatus

Abstract

A sheet surface treatment apparatus comprising a number of treatment elements (24) and a sheet conveyer (10) arranged to convey the sheets (30, 32) along a transport path (12) that is arranged such that the sheet surface to be treated is spaced apart from the treatment elements (24) by a specified distance, the sheet conveyer (10) comprising a suction box (22) for attracting the sheets against an air-permeable support surface (20), characterized in that the suction box (22) is arranged on the same side of the sheet transport path (12) as the treatment elements (24) and the support surface (20) is formed by support elements (18) that are interleaved with the treatment elements (24).

Description

[0001] SHEET SURFACE TREATMENT APPARATUS

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a sheet surface treatment apparatus comprising a number of treatment elements and a sheet conveyor arranged to convey the sheets along a transport path that is arranged such that the sheet surface to be treated is spaced apart from the treatment elements by a specified distance, the sheet conveyor comprising a suction box for attracting the sheets against an air-permeable support surface.

[0005] More particularly, the invention relates to a corona treatment apparatus for media sheets.

[0006] In the printing industry, media sheets are frequently subjected to a corona treatment before printing, in order to modify the surface energy of the sheets so as to improve the wettability of the sheet surface with liquid ink. In this case, the treatment elements are corona electrodes disposed on the side of the sheet that is to be treated, and a ground electrode is disposed on the opposite side of the sheet, so that, when a high voltage is applied to the corona electrodes, a corona discharge occurs between the corona electrodes and the ground electrode.

[0007] 2. Description of the Related Art

[0008] The documents EP 2 802 455 B1 , JP 2024 008724 A, JP 2022 106645 A and WO 2023 / 074632 A1 have been cited in the European Search Report. The first three documents disclose treatment apparatus in which a suction box is disposed on the same side of the sheet transport path as the treatment elements. The purpose of the suction box is to remove dust or gases produced in the treatment process. The fourth document shows a similar apparatus, but without suction box.

[0009] US 10 2018 154 B2 discloses a corona treatment apparatus of the type described above wherein ground electrodes are formed by a sequence of metal rollers that are disposed opposite to each of a plurality of corona electrodes, and the roller-shaped ground electrodes form the support surface for the sheet. This has the advantage that the sheets can be conveyed over the support surface with low friction. However, a drawback is that the cylindrical shape of the rollers serving as ground electrodes decreases the efficiency of the corona discharges.

[0010] Another parameter that is essential for achieving a highly efficient corona treatment is the distance between the surface of the sheets and the corona electrodes. In this respect, the known apparatus has the drawback that the distance between the corona electrodes and the surface to be treated depends upon the thickness of the sheets.

[0011] SUMMARY OF THE INVENTION

[0012] It is an object of the invention to provide a sheet surface treatment apparatus wherein the distance between the treatment elements and the surface to be treated can be controlled with high accuracy.

[0013] According to the invention, in order to achieve this object, the treatment apparatus is characterized in that the suction box is arranged on the same side of the sheet transport path as the treatment elements, and the support surface is formed by support elements that are interleaved with the treatment elements.

[0014] In the apparatus according to the invention, the distance between the treatment elements and the surface to be treated is determined entirely by the position of the support elements and is independent of the sheet thickness.

[0015] More specific embodiments of the invention are indicated in the dependent claims.

[0016] If the treatment apparatus is a corona treatment apparatus, the treatment electrodes are formed by corona electrodes, and a ground electrode which may take the form of a continuous plate is arranged on the side opposite to the suction box.

[0017] In an embodiment, a single ground electrode having a flat surface facing the sheet transport path is provided for all treatment elements, e.g., treatment electrodes, e.g., corona electrodes. In an alternative embodiment, the surface of the ground electrode facing the sheet transport path does not necessarily have to be flat. Instead, in areas between the treatment electrodes the surface of the ground electrode may be further away from the sheet transport path.

[0018] The sheet conveyor may comprise one or more pairs of rollers each forming a nip through which the sheets pass through. The axes of the rollers on the side of the ground electrode may be held in fixed positions relative to the ground electrode, and the entire unit comprising the rollers and the ground electrode may be elastically biased against the sheets and against the peripheries of the rollers on the side of the suction box, respectively. Upon accommodating a sheet, said entire unit is pushed down a distance equivalent to the thickness of the sheet, such that the top surface of the ground electrode and the (virtual) support surface that is formed by the support elements remain parallel planes. In this way, the distance between the ground electrode and the sheets passing through the nips of the roller pairs will automatically be adapted to the thickness of the sheets and may be arranged such that the spacing between the sheets and the ground electrode is made as small as possible, without the sheets contacting the ground electrode. This assures a high efficiency of the corona discharges and at the same time minimizes the friction between the ground electrode and the sheets as well as a heat transfer from the ground electrode to the sheets.

[0019] In order to minimize friction on the side of the suction box, the support elements may be configured as rollers. Since these rollers do not serve as electrodes, the cylindrical shape of the support elements will not compromise the discharge process.

[0020] The treatment apparatus may be arranged such that, in operation, the suction box is arranged above the sheet transport path, so that the sheets are lifted off the ground electrode by the suction pressure.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS

[0022] An embodiment example will now be described in conjunction with the drawings, wherein: Fig. 1 is a sketch of a corona treatment apparatus according to the invention;

[0023] Fig. 2 and 3 show a detail of Fig. 1 under a condition where the sheets to be treated have different thicknesses; and.

[0024] Fig. 4 shows a sketch of a corona treatment apparatus according to an embodiment of the present invention.

[0025] DETAILED DESCRIPTION OF EMBODIMENTS

[0026] As shown in Fig. 1 , a sheet surface treatment apparatus comprises a conveyor 10 for conveying media sheets to be treated along a sheet transport path 12. In the example shown, the conveyor is formed by two pairs of rollers 14,16, each pair forming a nip through which the media sheets pass through. In the space between the two roller pairs, a number of support elements 18 in the form of rollers are arranged above the sheet transport path 12 which is thus defined by the lower vertices of the upper rollers 14 of the conveyer and the support elements 18. Since the support elements 18 are separated by certain spacings, their lower vertices form an air-permeable support surface 20 for the media sheets moving along the sheet transport path 12. This support surface 20 forms the bottom of a suction box 22 to which a suction pressure is applied for attracting the media sheets upwards against the air-permeable support surface 20.

[0027] A number of rod-like or wire-like corona treatment elements 24 are arranged to extend across the sheet transport path 12 and are interleaved with some of the support elements 18. Since the apparatus described here is a corona treatment apparatus, the treatment elements 24 are corona electrodes that cooperate with a ground electrode 26 disposed on the opposite side of the sheet transport path 12. In operation, a high voltage is applied to the treatment elements 24, so that corona discharges occur between each of the treatment elements 24 and the ground electrode 26. Thus, when media sheets pass through the apparatus, the top surface of the media sheets is subjected to a corona treatment that results in an increase of the surface energy of the (paper) sheets. The efficiency of the corona treatment depends on the number of treatment elements 24 and also depends critically upon the distance between the corona electrodes and the sheet surface. The treatment elements 24 have been arranged such that the treatment elements 24 (the lower vertices thereof) are spaced apart from the support surface 20 by a specified distance that assures an optimal efficiency of the corona discharges. This distance is independent from the sheet thickness because the sheets are sucked against the support surface 20 and are lifted off from the ground electrode 26.

[0028] The ground electrode 26 is rigidly mounted on a frame 28 in which the lower rollers 16 of the conveyor 10 are rotatably supported. The ground electrode 26 has a flat top surface which is approximately level with the top vertices of the rollers 16. The entire unit comprising the frame 28, the lower rollers 16 of the conveyor and the ground electrode 26 is elastically biased upwards so that, when no media sheets are present, the lower rollers 16 engage the upper rollers 14 while the top surface of the ground electrode 26 does not quite contact the support elements 18.

[0029] When a media sheet 30 or 32 passes through the apparatus, as has been shown on an enlarged scale in Figs. 2 and 3, the lower rollers 16 are urged downwards against the elastic bias so that they are separated from the upper rollers 14 by the thickness of the media sheet 30 or 32. The ground electrode 26 is configured such that its top surface is offset downwardly from the plane that is defined by the top vertices of the lower rollers 16 by a very small amount that is just large enough to prevent direct contact between the ground electrode and the media sheets. Since the height of the ground electrode 26 and the lower rollers 16 is controlled by the thickness of the media sheets, the spacing between the top surface of the ground electrode 26 and the bottom surface of the media sheet 30 in Fig. 2 is the same as the spacing between the ground electrode 26 and the bottom surface of the thicker media sheet 32 in Fig. 3. In this way, the height of the ground electrode 26 can be optimized, regardless of the sheet thickness, for assuring a highly efficient corona discharge while at the same time assuring the practically frictionless sheet transport. Since the ground electrode 26 may be heated by the current of the corona discharges, the gap between the ground electrode and the media sheets also reduces an undesired heat transfer to the media sheets.

[0030] Figs. 2 and 3 also show that the spacing between the treatment elements 24 and the top surface of the media sheets 30, 32 is independent of the sheet thickness because the suction pressure of the suction box acts to lift the media sheets off from the ground electrode 26 and to attract them to the support surface 20 which thus defines the position of the top surface of the media sheets. Fig. 4 shows an embodiment wherein the ground electrode 26, the lower rollers 16 are rigidly connected to a frame 28 and hence forming a unit and the entire unit may, in operation, be elastically biased against the sheets and against the peripheries of the rollers on the side of the suction box (not shown in Fig. 4, see Figs 1-3), respectively.

[0031] The frame 28 comprises a guiding slit 41 and two guiding elements 42 which cooperate with guiding slit 41. Alternatively, a single guiding element may be implemented, as long as rotation of the frame around the axis of the guiding element is prevented, i.e., a guiding element with a non-circular cross-section which engages with the guiding slit to accommodate a movement of the entire unit along the double arrow 43.

[0032] Upon accommodating a sheet (not shown in Fig. 4), said entire unit is pushed down a distance equivalent to the thickness of the sheet in a direction indicated with double arrow 4, such that the top surface of the ground electrode and the (virtual) support surface that is formed by the support elements remain parallel planes.

Claims

7CLAIMS1 . A sheet surface treatment apparatus comprising a number of treatment elements (24) and a sheet conveyer (10) arranged to convey the sheets (30, 32) along a transport path (12) that is arranged such that the sheet surface to be treated is spaced apart from the treatment elements (24) by a specified distance, the sheet conveyer (10) comprising a suction box (22) for attracting the sheets against an air-permeable support surface (20), characterized in that the suction box (22) is arranged on the same side of the sheet transport path (12) as the treatment elements (24) and the support surface (20) is formed by support elements (18) that are interleaved with the treatment elements (24).

2. The apparatus according to claim 1 , wherein the support elements (18) are rollers.

3. The apparatus according to claim 1 or 2, wherein the treatment elements (24) are corona electrodes and at least one ground electrode (26) is provided on the opposite side of the sheet transport path (12).

4. The apparatus according to claim 3, wherein a single ground electrode (26) having a flat surface facing the sheet transport path (12) is provided for all treatment elements (24).

5. The apparatus according to claim 3 or 4, wherein the conveyor (10) comprises a number of pairs of rollers (14, 16) each forming a nip for the sheets (30, 32).

6. The apparatus according any of the claims 3 to 5, wherein the ground electrode (26) is mounted on a frame (28) in which one roller (16) of each pair of rollers of the conveyor (10) is rotatably supported, and the unit comprising the frame (28), the ground electrode (26) and said one roller (16) is elastically biased against the other rollers (14) of the roller pairs.

7. The apparatus according to claim 5 and 6, wherein the conveyor (10) has two pairs of rollers (14, 16) disposed upstream and downstream of the ground electrode (26), and the surface of the ground electrode (26) is offset from the plane defined by the8 rollers (16) of the conveyor (10) by an amount just enough to prevent the ground electrode (26) from contacting the sheets (30, 32).

8. The apparatus according to claim 6, wherein the frame (28) comprises a guiding slit (41) and at least one guiding element (42) cooperating with guiding slit (41).

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