Conveyor for cut sheets

Abstract

A conveyor (10) for conveying cut sheets (18) in a sheet processing apparatus, the conveyor comprising a perforated endless belt (12), a suction box (16) disposed underneath a sheet transport path (TP) that is formed by a run of the belt (12), and a cover plate (26, 28) extending over and being spaced apart from a section of the sheet transport path, the cover plate having an air-permeable portion (30) at or near an edge that extends across the sheet transport path, characterized in that the air permeability of said air-permeable portion (30) is larger over a central region of the sheet transport path than over lateral regions thereof.

Description

[0001] CONVEYOR FOR CUT SHEETS

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a conveyor for conveying cut sheets in a sheet processing apparatus, the conveyor comprising a perforated endless belt, a suction box disposed underneath a sheet transport path that is formed by a run of the belt, and a cover plate extending over and being spaced apart from a section of the sheet transport path, the cover plate having an air-permeable portion at or near an edge that extends across the sheet transport path.

[0005] 2. Description of the Related Art

[0006] In sheet treatment apparatus such as inkjet printers and the like, a perforated conveyor belt is frequently used for conveying print media sheets through a number of processing stations such as a printing station, a curing station, a treatment station for treating the sheets with superheated steam, and the like. The perforations in the conveyor belt and the suction box have the purpose to attract the sheets to the conveyor belt in order to assure that the sheets are conveyed in a perfectly flat state without forming any wrinkles or cockles.

[0007] A conveyor having the features indicated in the preamble of claim 1 has been disclosed in US 2016 129 712 A1. In this conveyor, the cover plate that is disposed above the sheet transport path has perforations that are uniformly distributed over the width of the cover plate so that part of the ambient air that is sucked into the suction box through the perforations of the conveyor belt may flow through the perforations of the cover plate, i.e. in a direction normal to the cover plate. When a leading edge of a sheet passes underneath a row of the perforations of the cover plate, the air flowing through the perforations will be deflected by the sheet into a horizontal direction in parallel with the transport direction of the sheet, i.e., towards the edge of the sheet, before it enters the suction box. The purpose of these air currents is to blow away dust that has settled on the edges of the sheets. In other sheet conveyors, a cover plate may be provided for different purposes. For example, the cover plate may serve as a guidance aid for safely guiding the sheets at an interface between the belt conveyor and an adjacent upstream or downstream sheet transport device. In other cases, the cover plate may have the purpose to prevent heat being radiated away from pre-heated sheets, or to control the humidity content of the atmosphere immediately above the sheet surface. In these cases, the cover plate has not been provided with any perforations.

[0008] It has been found however that in certain cases, especially when the sheets to be conveyed have a large format, the cover plate may induce an air flow pattern that increases the risk of creases or folds to be formed in the sheets. The reason is that the air that is sucked into the suction box underneath the cover plate has to flow around the edges of the cover plate and therefore form horizontal flow currents which tend to lift the leading or trailing ends of the sheets off the conveyor belt due to the Venturi-effect. Then, when the sheet moves on and is again attracted to the conveyor belt, folds or cockles may remain in the sheet.

[0009] SUMMARY OF THE INVENTION

[0010] It is an object of the invention to provide a sheet conveyer in which the cover plate is less likely to induce creases or wrinkles or cockles in the sheets.

[0011] In order to achieve this object, the conveyor according to the invention is characterized in that the air permeability of said air-permeable portion is larger over a central region of the sheet transport path than over lateral regions thereof.

[0012] The air-permeable portion permits part of the air to enter directly into the suction box in a direction normal to the plane of the cover plate. As a consequence, when, for example, the leading edge of a sheet approaches the edge of the cover plate, the horizontal air currents that tend to lift the sheet off the conveyor belt will at first occur only at the lateral edge portions of the sheet transport path where there is practically no air flow path allowing a direct entry of the air to the suction box. In contrast, in the central region of the transport path, the air current normal to the cover plate will pin the widthwise center portion of the sheet against the conveyor belt. Then, as the transport of sheet proceeds, the area underneath the cover plate where the air flow through the perforated conveyor belt is blocked by the sheet itself increases, and less air will be sucked into the suction box. As a consequence, the velocity of the horizontal air flows between the sheet and the cover plate will also decrease and the lateral edge portions of the sheet that had been lifted off the conveyor belt will be drawn back onto the conveyor belt. Since, due to the presence of the air flow paths in the central region, the central portion of the sheet had been lifted to a lesser extent, the process of straightening out the sheet on the surface of the conveyor belt will smoothly proceed from the center towards the lateral edges without forming any cockles or folds.

[0013] Similarly, when the trailing edge of the sheet approaches the edge of the cover plate, the lifting effect will again be stronger in the lateral edge portions than in the central portion and, again, when the sheet is pulled back against the conveyor belt after having left the region of the cover plate, the process of straightening out the sheet will proceed from the center outwardly without forming cockles.

[0014] Similar effects are achieved when the air-permeable portion of the cover plate is formed adjacent the downstream edge in the direction of sheet transport. Thus, the likelihood of the sheets to form wrinkles can be decreased significantly.

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

[0016] The air flow paths in the direction normal to the cover plate may be formed by circular or slot-like perforations of the cover plate, for example. In the case of slots, these slots may extend in parallel with the transport direction and may reach right to the edge of the cover plate which will thus have a comb-like configuration in its central part.

[0017] In another embodiment, the air permeable portion may be formed by a cut-out in thee leading or trailing end of the cover plate, the cut-out being delimited, for example, by a concave V-shaped or trough-shaped edge contour of the cover plate.

[0018] The air-permeable portion may be formed at or near the upstream edge or the downstream edge of the cover plate or at or near both edges.

[0019] The cover plate may serve as a guidance aid at an interface between the belt conveyor and an adjacent conveyor. In that case, the cover plate will extend only over an end section of the belt conveyor, and another part of the cover plate will extend over an end portion of the adjacent conveyor.

[0020] In other embodiments, the cover plate may serve other purposes and may be arranged above the perforated conveyor belt in its entirety.

[0021] DETAILED DESCRIPTION OF EMBODIMENTS

[0022] Embodiment examples will no be described in conjunction with the drawings, wherein:

[0023] Fig. 1 is a schematic side view of a conveyor according to the invention;

[0024] Figs. 2 and 3 are viewings similar to Fig. 1, but showing a conventional conveyor;

[0025] Fig. 4 is an enlarged side view of a part of a conveyor according to the invention;

[0026] Fig. 5 is a plan view of a cover plate of the conveyor shown in Fig. 4;

[0027] Fig. 6 is a cross-sectional view of the conveyor taken along the line

[0028] VI-VI in Fig. 5; and

[0029] Figs. 7-13 show patterns of air-permeable portions of the cover plate according to other embodiments.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] As it is shown in Fig. 1 , a belt-type conveyor 10 has an endless belt 12 trained over rollers 14. The belt 12 has fine perforations uniformly distributed on its entire surface, and a suction box 16 is disposed below an upper run of the belt 12, so that ambient air can be drawn into the suction box through the perforations of the belt 12 as is generally known in the art. Thus, a sheet 18 to be conveyed on the conveyor 10 will be attracted against the surface of the conveyor belt by suction pressure.

[0032] In the example shown, the sheet 18 is supplied from an adjacent conveyor 20 having a sheet support plate 22 and pairs of transport rollers 24 (only one pair is shown). Thus, the nip of the pair of transport rollers 24 and the surface of the upper run of the belt 12 of define a continuous sheet transport path TP. At the transition between the conveyors 20 and 10, a cover plate 26 is disposed above the sheet transport path such that an upstream edge of the cover plate extends across the conveyor 20 and a downstream edge extends across the conveyor 10. Only a little spacing exists between the cover plate 26 and the sheet transport path TP, so that the cover plate 16 serves as a guidance aid for smoothly guiding the leading edge of the sheet 18 from the conveyor 20 to the conveyor 10.

[0033] Another cover plate 28 is disposed in its entirety above the conveyor 10. In the space between the cover plates 26 and 28, a treatment device (not shown) may be arranged for treating the sheets 18 with steam. In that case, the cover plate 28 may have the purpose to control the humidity of the atmosphere in the immediate environment of the sheets by preventing the steam from diffusing too quickly.

[0034] In Fig. 1, the cover plates 26 and 28 have been shown in a longitudinal section in a sectional plane located in the width-wise center of the sheet transport path TP. Each of the cover plates 26, 28 has an air-permeable portion 30 at both, its upstream and downstream edges. In the example shown the air-permeable portions are formed by perforations. These perforations allow an air flow 32 in a direction normal to the plane of the respective cover plate. The purpose of the air-permeable portions 30 and the air flows 32 created thereby will be explained by reference to Figs. 2 and 3 which show a conventional conveyor 10’ which differs from the conveyor 10 only that it has cover plates 26’ and 28’ which do not allow for air flows such as the air flows 32.

[0035] Fig. 2 shows the conveyor 10' in state in which no sheets are conveyed thereon. The suction box 16 of the conveyor 10’ is connected to a vacuum source so that ambient air is drawn in through the perforations of the conveyor belt 12. In the regions outside of the cover plates 26’ and 28’, the ambient air will flow vertically towards the perforated conveyor belt. However, where the belt is covered by the cover plates 26’, 28’, the vertical air flow is obstructed, so that the air has to flow around the edges of the cover plates. This creates horizontal air flows 34 in the gap between the conveyor belt 12 and the cover plate 28 and, to a lesser extent, also in the gap between the sheet support plate 22 of the conveyor 20 and the cover plate 26’. The suction from the suction box 16 and the rotation of the roller 14 further create an air flow 36 through a gap between the sheet support plate 22 and the roller 14. Due to the Venturi-effect, these horizontal air flows create a dynamic underpressure in the spaces between the cover plates 26’ and 28’ and the sheet transport path. This underpressure counteracts the suction from the suction box 6, as has been symbolized by contoured arrows in Fig. 2. If a sheet were present underneath the cover plate 26’ or 28’, the perforations of the conveyor belt 12 would initially be obstructed by the sheet, but the Venturi-effect could locally overwhelm the force created by the suction box, especially underneath the edge portions of the cover plates.

[0036] Fig. 3 shows a situation in which a sheet 18a and 18b, respectively, is present on underneath each of cover plates 26’ and 28’. Due to the Venturi-effect, the leading and trailing edges of both sheets 18a, 18b have been lifted off the conveyor belt and off the sheet support plate 22, respectively.

[0037] Since the sheet 18b obstructs the perforations of the conveyor belt, the horizontal air flow underneath the cover plate 28’ is low, at least in the width-wise central area, where the belt perforations are closed off by the sheet. However, since the cover plate 28’ and the suction box 16 will generally extend beyond the sheet 18’ in the width direction, there are unobstructed regions at the lateral edges of the sheet 18b, and horizontal air currents will tend to lift the lateral edge portions of the sheet. Once the lateral parts of the sheet have been lifted, more perforations of the conveyor belt will be exposed, more air will be drawn in and impinge on the sheet from below so that the leading and trailing edges of the sheet are lifted even further. This lifting process will proceed from the lateral edges towards the center until the entire leading and trailing edges of the sheet have been lifted. Then, when the leading edge of the sheet 18b has moved beyond the downstream edge of the cover plate 18’, a vertical air flow will be possible again and the edge of the sheet will be pressed downwards against the conveyor belt again. However, this downward movement is not guaranteed to be uniform over the entire width of the sheet, and those regions of the sheet which happen to hit the conveyer belt first will be firmly pressed against the belt due to the differential air pressure and will prevent any horizontal movement of the sheet. Consequently, any bulges in the sheet in those areas where the downward movement was slower cannot be smoothened out again. As a result, folds or cockles will remain which will interfere with subsequent treatment processes and lead to print artefacts. Essentially the same is true for the trailing edge of the sheet 18b because the horizontal air flow will weaken when the edge of the sheet moves further away from the upstream end of the cover plate 28’.

[0038] Essentially the same problems arise also for the sheet 18a underneath the cover plate 26’.

[0039] Fig. 4 shows an interface between the conveyor 20 and a conveyor 40 which differs from the conveyor 10 according to Fig. 1 only in that its cover plate 26" has the air- permeable portion 30 allowing the air flow 32 only at its downstream edge that is located above the suction box 16. There are no perforations near the upstream edge because the distance of this edge from the suction box 16 is so large that the air flow around this edge of the cover plate 26 is too small to cause problems.

[0040] Fig. 5 shows a plan view of the cover plate 26” shown in Fig. 4. Here, it can be seen, that, in addition to the perforations in the width-wise center of the sheet transport path, there are additional perforations 42 which, however, are located closer to the downstream edge of cover. In other words, the air-permeable portion 30 has the shape of a triangle that has its base at the downstream edge of the cover plate and its tip pointing towards the center of the cover plate. Since there are more perforations in the central region than in the lateral regions, the air-permeability will be largest in the center.

[0041] The effect of these perforations 42 and the air flows 32 created thereby has been illustrated in Fig. 6 which shows a sectional view along the line VI-VI in Fig. 5. This sectional line passes through two of the perforations 42. When the leading edge of the sheet 18 approaches the downstream edge portion of the cover plate 26”, its lateral edges are lifted by horizontal air flow 32. However, in the central portion of the sheet, the leading edge hits the vertical air flow 32 through the perforations 42 and is held down on the surface of the conveyor belt 12. Then, as the sheet transport proceeds, the leading edge of the sheets hits more and more air flows 32 through the perforations 42 that are located closer to the lateral edges of the cover plate. In this way, the sheet is straightened out from the center towards the lateral edges without any cockles or folds being tracked on the service of the conveyor belt. A corresponding effect occurs also when the trailing edge of the sheet passes underneath the perforations 42.

[0042] The air-permeable portion 30 with the pattern illustrated in Fig. 5 may be defined by other structures than the perforations 42. Fig. 7 shows an outline 43 of a cover plate that has air-permeable portions 30 at both ends and may replace the cover plate 28 and / or 26 in Fig. 1. The air-permeable portion at the downstream edge is formed by slots 44 that are open towards the edge of the plate and form the same triangular pattern as the perforations 42 in Fig. 5. On the other transversal edge, the air- permeable portion is formed by a concave edge contour 46 of the cover plat, which allows for a vertical air flow predominantly in the width-wise center and defines a flow pattern that is capable of smoothing out the sheets in the same way as illustrated in Fig. 6.

[0043] Figs. 8 to 13 show alternative patterns for arranging perforations in the cover plate. In Fig. 8, the perforations are circular holes 48 arranged in two rows in a V-shape. In Fig. 9, the perforations are elliptical holes 50 slanting towards the corners of the cover plates. In Fig. 10, the perforations are circular holes 52 arranged in regularly spaced rows and columns, with the innermost column being shorter than the other two columns. In Fig. 11 , the perforations are circular holes 54 arranged in a V-shaped pattern similarly as in Fig. 8. In Fig. 12, the perforations are circular holes 56 arranged in rows and columns similarly as in Fig. 10. In Fig. 13 circular holes 60, 62 are arranged in columns that extend in parallel with the edge of the cover plate, but the holes 62 closer to the center have a larger diameter than the holes 60 on the lateral sides so that the airpermeability is higher in the central region.

Claims

9CLAIMS1. A conveyor (10) for conveying cut sheets (18) in a sheet processing apparatus, the conveyor comprising a perforated endless belt (12), a suction box (16) disposed underneath a sheet transport path (TP) that is formed by a run of the belt (12), and a cover plate (26, 28; 26") extending over and being spaced apart from a section of the sheet transport path, the cover plate having an air-permeable portion (30) at or near an edge that extends across the sheet transport path, characterized in that the air permeability of said air-permeable portion (30) is larger over a central region of the sheet transport path than over lateral regions thereof.

2. The conveyor according to claim 1, wherein the length of said air permeable portion (30) in the sheet transport direction is larger over the central region of the sheet transport path than over the lateral regions.

3. The conveyor according to claim 1, wherein said air permeable portion (30) is formed by a cut-out of the cover plate that is delimited by a concave edge contour (46) of the cover plate.

4. The conveyor according to claim 1 or 2, wherein said air permeable portion (30) is formed by perforations (42; 48; 50; 52; 54; 56; 58; 60, 62) in the cover plate.

5. The conveyor according to claim 1 or 2 wherein said air permeable portion (30) is formed by slots (44) that are open towards the edge of the cover plate.

6. The conveyor according to any of the preceding claims, wherein the cover plate has an air permeable portion (30) on both edges that extend across the sheet transport path (TP).

7. The conveyor according to any of the preceding claims, wherein the cover plate (26) extends over an interface between two subsequent conveyors (10, 20).

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