Conveyor belts are frequently used for conveying sheets of paper, plastic film or the like through a sheet handling apparatus such as a printer or a copier. In many applications e.g. an inkjet printer, it is important that the sheets are reliably held in a stable position on the belt so as to be conveyed through the apparatus with high accuracy, and/or the sheets are held flat on the belt, i.e. without forming any cockles. For that purpose, it is known to provide an array of suction nozzles below the belt, distributed along and across the transport path, so that a suction pressure is generated which draws the sheets against the belt and holds them safely in position.

When the sheets reach the downstream end of the transport path, they must reliably be separated from the belt so that they may either be discharged onto a tray or may be passed over to a subsequent conveying device, e.g. a set of conveyor rollers or another conveyor belt. Sometimes, the sheets tend to stick to the belt, because the sheet material is sticky or because a static charge is developed which attracts the sheet against the belt. When the leading edge of the sheet is not separated quickly enough from the belt, a jam is likely to occur, or the sheet may be damaged.

Typically, the belt is deflected at a deflection roller at the downstream end of the transport path and the tendency of the sheet to stick to the belt may be reduced by reducing the radius of the deflection roller or by deflecting the belt at a relatively sharp knife-edge, so that the flexibility of the sheet is no longer sufficient for the sheet to follow the sharp turn of the belt. However, such a sharp turn may also cause strains in the belt itself, so that the lifetime of the belt is reduced. In order to mitigate this problem, it would be possible to increase the flexibility of the belt, which however, would make it even more difficult to assure the required transport accuracy and reliability.

It is generally known that the separation of the sheets from the belt may be assisted by blowing a stream of air against the leading edge of the sheet when it leaves the transport path at the downstream end.

US 2008/001347 A1 describes a sheet transport apparatus, in which a sheet is placed on an area of a transport belt and is held to the circulating transport belt by a negative pressure applied through the belt. The transport belt has through openings in the shape of perforations. Downstream, the sheet is lifted off the transport belt at a first blowing unit and is then further moved by the transport belt until it reaches an area of a guide roller. The guide roller has a plurality of projections, which are aligned with a pattern of the through openings in the transport belt, when the transport belt is guided around the guide roller. The sheet is lifted of the transport belt by the projections extending through the corresponding openings and projecting above an outer surface of the transport belt. In one example, a second blowing unit may be provided by the guide roller being formed as a hollow shaft having through openings extending through the projections or being provided there between. It is an object of the invention to provide a sheet conveying device which assures a high transport accuracy and reliability and also assures that the sheet is reliably separated from the belt at the end of the transport path.

According to the invention, this object is achieved by the conveying device further comprising an array of anti-friction blow nozzles interleaved with the array of suction nozzles and connected to a blower for blowing-out a gas towards the bottom surface of the belt during the traverse of the sheet along the marking engine.

By blowing out air or any other gas through the perforations of the belt at the end of the transport path, it is possible to lift the leading edge of the sheet off the belt already before the leading edge actually reaches the end of the transport path where the belt makes a turn. As a result, the sheet can more reliably and quickly be separated from the belt. The same perforations in the belt that are used for drawing the sheet against the belt in the upstream part of the transport path may also be used for blowing out the air at the downstream end, so that the invention may be implemented without modifying the configuration of the belt.

For example, the additional anti-friction blow nozzles may be distributed along and across the entire transport path, forming an alternating pattern with the suction nozzles. This has the advantage that the part of the belt that spans the distance between an upstream and a downstream deflection roller may be supported on an air cushion that is created by the blow nozzles, so that frictional resistance and deflection of the conveyor belt are reduced. The air that is blown out by the blow nozzles, except the blow nozzles immediately at the end of the transport path, will readily be sucked-in again by the suction nozzles, so that a short-circuited air flow is created on the bottom side of the belt. Since the dynamic pressure in this air flow will be lower than the ambient pressure, the sheet is still drawn against the top surface of the belt. Of course, the force with which the sheets are attracted may be increased as desired by increasing the flow rate through the suction nozzles relative to the flow rate through the blow nozzles.

Useful details and optional features of the invention are indicated in the dependent claims.

In one embodiment, blow nozzles may be arranged immediately upstream of a deflection roller which defines the end of the transport path. For example, the blow nozzles may be incorporated in a nozzle body that also forms the suction nozzles.

As an alternative or in addition, blow nozzles may also be provided in the peripheral surface of the deflection roller.

Preferred embodiments of the invention will now be explained in conjunction with the drawings, wherein:

Fig. 1 shows a top plan view of a sheet conveying device of a printing system

according to the invention, with a part of a conveyor belt being broken away;

Fig. 2 is a schematic side-elevational view of the device shown in Fig. 1 ;

Fig. 3 shows a top plan view of a sheet conveying device according to a modified

example;

Fig. 4 is a side-elevation of the device shown in Fig. 3, with a deflection roller for the belt being shown in cross-section; and Fig. 5 illustrates a printing system in accordance with the present invention.

As is shown in Fig. 1 , a belt 10 is passed around an upstream deflection roller 12 and a downstream deflection roller 14, so that a top portion of the belt forms a transport path for conveying sheets 16, e. g. print substrates or the like. At least one of the deflection rollers 12, 14 is actively driven, so that the top portion of the belt 10 moves in the direction indicated by an arrow A and conveys the sheets 16 that have been placed on the belt in that direction. The belt 10 has an array of perforations 18 that are evenly distributed over the entire surface area of the belt and are aligned in rows that extend in the conveying direction A.

A plate-like nozzle body 20 is disposed between the deflection rollers 12, 14 and extends over the entire width and almost the entire length of the transport path. A series of slot-like suction nozzles 22 are formed in the flat top surface of the nozzle body 20 and extend in the row direction of the perforations 18, each suction nozzle 22 be aligned with one of the rows of the perforations 18.

As is shown in Fig. 2, the top surface of the nozzle body 20 is slightly elevated above the top apex of the deflection rollers 12, 14, so that the top portion of the belt 10 does not freely span the distance between the deflection rollers 12, 14 but is supported by the nozzle body 20.

As is further shown in Fig. 2, the suction nozzles 22 communicate with a suction port of a blower 24, so that air is drawn-in through the perforations 18 of the belt 10, as has been symbolised by arrows B in Fig. 2. As a result, the sheet 16 will be firmly drawn against the surface of the belt 10 and will be held flat on the belt as long as it moves along the transport path together with the belt. This reliably prevents the sheets 16 from being distorted or shifted relative to the belt, so that the sheets can be conveyed with high transport accuracy and high reliability.

Fig. 2 illustrates a condition in which the leading edge of the sheet 16 is about to reach the downstream end of the transport path as defined by the deflection roller 14. Here, the nozzle body 20 forms a slot-like blow nozzle 26 that extends across the entire width of the transport path immediately upstream of the deflection roller 14. This blow nozzle 26 communicates with the discharge port of the blower 24, so that at least a part of the air that has been sucked-in by the blower 24 is blown out through the blow nozzle 26 and through the perforations 18 when they successively move across the blow nozzle. As has been symbolised by arrows C in Fig. 2, this creates an air flow that is upwardly directed against the leading edge of the sheet 16 and lifts the same of from the surface of the belt 10 even before it reaches the apex of the deflection roller 14 where the belt makes a sharp turn. As a result, the leading edge of the sheet 16 is safely separated from the belt 10 so that it may reliably be caught by a subsequent conveying device (not shown) which may be for example be formed by a nip between two transport rollers, or the like. As the belt 10 and the sheet 16 continue to move beyond the position shown in Fig. 2, the entire length of the sheet 16 will gradually be separated from the belt 10 as it passes over the blow nozzle 26.

In this embodiment, as is shown in Fig. 1 , the top surface of the nozzle body 20 is also formed with a series of anti-friction blow nozzles 28 which extend in parallel with the suction nozzles 22 and are arranged alternatingly therewith. For illustration purposes and for facilitating the distinction between the blow nozzles 26, 28 and the suction nozzles 22, the blow nozzles have been hatched in Fig. 1. A distribution manifold 30 which has only schematically been shown in Fig. 2 connects the array of anti-friction blow nozzles 28 to the discharge port of the blower 24, so that a part of the air that has been drawn in by the blower is discharged upwardly against the belt 10 via the blow nozzles 28. As can be seen in Fig. 1 , the blow nozzles 28 are aligned with the gaps between the rows of perforations 18, so that the air ejected by the blow nozzles 28 will impinge on non-perforated parts of the belt 10. As a result, an air cushion is formed between the bottom surface of the belt 10 and the top surface of the nozzle body 20, so that the belt can pass over the nozzle body with reduced friction. This reduces not only a wear and distortion of the belt 10 carrying the sheets 16 but also facilitates the separation of the sheets from the belt by means of the blow nozzle 26 at the downstream end.

Since the air current discharged by the blower 24 is divided between the blow nozzle 26 and the distribution manifold 30, the flow rate of air discharged through the anti-friction blow nozzles 28 is smaller than the flow rate of air sucked-in by the suction nozzles 22, resulting in a net force that attracts the sheet 16 towards the belt. Of course, it would also be possible to provide separate blowers for the blow nozzle 26 and the distribution manifold 30 and/or to provide a distribution valve for controlling the flow rate of air through the anti-friction blow nozzles 28. In yet another embodiment, the anti-friction blow nozzles 28 may be connected directly to the blow nozzle 26.

Figs. 3 and 4 show a modified example, in which the nozzle body 20 has neither the blow nozzle 26 at the downstream end nor the longitudinally extending anti-friction blow nozzles 28. Instead, an array of blow nozzles 26' is formed in the peripheral surface of the deflection roller 14. These blow nozzles 26 take the form of circumferential grooves in the surface of the deflection roller 14.

As is shown in Fig. 4, the deflection roller 14 has an international distribution manifold connecting the blow nozzles 26' via a rotary connector (not shown) to the blower 24 or to a separate blower. As a result, an air flow, symbolised by arrows D in Fig. 4, is ejected radially from the surface of deflection roller 14 and through the perforations 18 of the belt 10, so that the leading edge of the sheet 16 is readily separated from the belt. As can be seen in Fig. 3, the blow nozzles 26 are aligned with the suction nozzles 22 and, consequently, with the rows of perforations 18, so that the air discharged by the blow nozzles can readily pass through the perforations 18. Additional baffle plates 34 may be provided around the peripheral surface of the deflection roller 14 so as to concentrate the air flow to the peripheral region that faces the sheet 16.

In this example the position where the sheet 16 is separated from the belt 10 is shifted towards the downstream end of the transport path as compared to the embodiment shown in Figs. 1 and 2.

Fig. 5 illustrates a printing system in accordance with the present invention. The printing system comprises a media supply station 100 where the sheets of printing substates are supplied towards the marking engine 102. This marking engine 102 is a piezo-based inkjet page wide array, but may alternatively comprise any typing of marking process, such as e.g. a thermal or piezo-based scanning or page wide inkjet process, an electro(photo)graphic process, magnetographic process, or the like. The marking engine applies an image of marking substance onto the sheet of printing substrate such as e.g. a sheet of paper, cloth, plastics. After applying the image of marking substance onto the printing substrate, the printing substrate is fed towards the delivery station, where a sheet may be collected by an operator, or fed towards one or more post-processing units, such as e.g. a stacking unit, a folding device and/or any binding device. The printing system utilizes the sheet conveying device as described here above to transport the sheet of printing substrate during the complete path from media supply station towards the media delivery station 101 , or during a portion thereof.

Figure 5 illustrates the use of such a sheet conveying device during the traverse of the sheet along the marking enging, as this conveying requires a large precision in positioning.

Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. In particular, features presented and described in separate dependent claims may be applied in combination and any advantageous combination of such claims are herewith disclosed.

Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.