Sheet handling device for wide format sheets

A sheet handling device for wide format sheets including a sheet support plate having a top surface containing suction holes which are connected to at least one suction chamber, the at least one suction chamber being divided into compartments that are connected, though an opening, to a suction device adapted to create a subatmospheric pressure in the compartments, wherein at least one internal wall between adjacent ones of the compartments defines a flow restriction orifice, and at least one of the compartments is directly connected to the suction device, and at least another one of the compartments is indirectly connected to the suction device through the orifice.

BACKGROUND OF THE INVENTION

The present invention relates to a sheet handling device for wide format sheets including a sheet support plate, said plate having a top surface containing suction holes which are connected to at least one suction chamber, said at least one suction chamber being divided into compartments that are connected to a suction device adapted to create a subatmospheric pressure in the compartments.

In the copying and printing industry, a sheet support element with suction holes is frequently used for supporting an image receiving sheet and at the same time ensuring that the sheet lies perfectly flat on the support element. For example, in an ink jet printer, a sheet, e.g. a sheet of paper, is advanced over a sheet support plate while the image is being printed. The sheet is held on the sheet support plate due to the subatmospheric pressure in the suction chamber which is connected to suction holes facing the bottom side of the sheet. A certain subatmospheric pressure is required to hold the sheet sufficiently flat on the support plate.

When sheets of different width are to be printed, the smaller sheets do not cover the sheet support element completely. Therefore, some of the suction holes are not covered. When the suction chamber is made of one large compartment extending over the whole area of the sheet support element, an air flow through the uncovered suction holes leads to an increase of the pressure in the suction chamber. This results in the sheet not being held firmly on the support element. If, on the other hand, a suction device of higher power is used to compensate for the increased air flow into the suction chamber, the suction will be too strong when all suction holes are covered by a large sheet, and the advance of the sheet over the support element is impeded.

From the European patent application EP 0 997 308 A2 a media hold down unit is known that comprises two or more vacuum chambers, a first vacuum chamber being directly connected to a vacuum source, while the other vacuum chambers are each connected to the vacuum source via separate bypass channels. The sizes of the chambers correspond to different widths of sheets that are to be handled. If one of the chambers is not covered by a sheet, an air flow is generated from this chamber through the bypass channel. The air flow affects the uniformity of the pressure inside the first vacuum chamber. Since the bypass channel and the first vacuum chamber are connected at an entrance into a conduit leading to the vacuum source, the uniformity of the pressure provided at the first chamber is affected only to a reduced extent. However, only a limited number of different sheet widths can be handled. Furthermore, for each partition of the support plate an extra bypass channel is needed and requires extra space.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a sheet handling device in which sheets of different sizes can be held down on a sheet support element with appropriate suction pressure, the sheet handling device being of simple and space saving construction.

According to the present invention, this object is achieved by a sheet handling device of the type indicated above, wherein at least one internal wall between adjacent ones of said compartments defines a flow restriction orifice, and at least one of said compartments is directly connected to the suction device, and at least another one of the compartments is indirectly connected to the suction device through said orifice.

The flow restricting orifices restrict the air flow from a compartment that is not covered by a sheet. Thus, the necessary subatmospheric pressure can be maintained in those compartments that are covered by the sheet. When, however, a sheet of larger size covers also the suction holes for which the flow path is restricted by the orifices, the flow rate will be reduced substantially. Thus, the effect of the flow resistance of the orifices will also be reduced, and the suction effect will be essentially uniform over the whole area of the sheet. Furthermore, no extra space is needed for bypass channels and the like. Another advantage is that the size of the compartments can be chosen by arranging the internal walls at appropriate positions in the suction chamber. Therefore, when manufacturing the sheet handling device, the sheet support place is readily adaptable to the required sizes of the compartments. Furthermore, since the internal walls do not require extra space outside of the suction chamber, the number of compartments may easily be increased. Therefore, the sheet handling device can readily be adapted to handle sheets of more than two sizes.

Preferably, the compartments are arranged symmetrically. Thus, the maximum distance of a compartment from the suction means is reduced as compared to, e.g., a configuration in which the suction device is connected to a first compartment in a chain of interconnected compartments. With this preferred arrangement, smaller sheets may be passed over the first compartment in a middle section of the sheet support plate instead of being fed at a lateral edge of the sheet support plate.

In a preferred embodiment, at least two groups of compartments are arranged in the direction of advance of the sheet, and only one group has a compartment that is directly connected to the suction device, whereas the compartments of the other group or groups are connected indirectly through flow restricting orifices. This helps to reduce the air flow into the suction chamber when a sheet covers only part of the support plate in the transport direction of the sheet. For example, when a new sheet is printed and advanced onto the sheet support plate, the sheet may already cover a compartment of the first group while the leading edge of the sheet has not yet reached a neighboring compartment of the second group.

One or more hollow spaces may be provided in the sheet support plate in order to reduce the weight of the plate and also its heat capacity, in the case that the temperature of the plate must be controlled, and yet to obtain a sufficient thickness and stability of the support plate. Then, the suction holes are preferably drilled in such positions that they lead into the hollow space which itself is connected to the compartments of the suction chamber by a number of apertures. As a result, an additional air flow would take place from one compartment to the other through the hollow space. It is therefore preferable that the internal wall separating these compartments comprises, for each of said hollow spaces, a protrusion extending through the aperture into said hollow space, so as to restrict the air flow through the hollow space.

DETAILED DESCRIPTION OF THE INVENTION

As is shown inFIG. 1, a hot melt ink jet printer includes a platen10which is intermittently driven to rotate in order to advance a sheet12, e. g. a sheet of paper, in a direction indicated by an arrow A over the top surface of a sheet support plate14. A number of transport rollers16are rotatably supported in a cover plate18and form a transport nip with the platen10, so that the sheet12, which is supplied from a reel (not shown) via a guide plate20, is paid out through a gap formed between an edge of the cover plate18and the surface of the sheet support plate14.

A carriage22which includes a number of ink jet printheads (not shown) is mounted above the sheet support plate14so as to reciprocate in the direction of arrows B across the sheet12. In each pass of the carriage22, a number of pixel lines are printed on the sheet12by means of the printheads which eject droplets of hot melt ink onto the sheet in accordance with image information supplied to the printheads. For the sake of simplicity, guide and drive means for the carriage22, ink supply lines and data supply lines for the printheads, and the like, have not been shown in the drawing.

The top surface of the sheet support plate14has a regular pattern of suction holes24which pass through the plate and open into a suction chamber26that is formed in the lower part of the plate14. The suction chamber is connected via a tube27to a blower28which creates a subatmospheric pressure in the suction chamber, so that air is drawn-in through the suction holes24. As a result, the sheet12is drawn against the flat surface of the support plate14and is thereby held in a flat condition, especially in the area which is scanned by the carriage22. Thereby, a uniform distance between the nozzles of the printheads and the surface of the sheet12is established over the whole width of the sheet, and a high print quality can be achieved.

The droplets of molten ink that are jetted out from the nozzles of the printheads have a temperature of 100° C. or more and cool down and solidify after they have been deposited on the sheet12. Thus, while the image is being printed, the heat of the ink must be dissipated with a sufficient rate. On the other hand, in the initial phase of the image forming process, the temperature of the sheet12should not be too low, because otherwise the ink droplets on the sheet12would be cooled too rapidly and would not have time enough to spread-out. For this reason, the temperature of the sheet12is controlled via the sheet support plate14by means of a temperature control system30which circulates a temperature control fluid, preferably a liquid, through the plate14. The temperature control system includes a circulating system with tubes32that are connected to opposite ends of the plate14. One of the tubes passes through an expansion vessel33containing a gas buffer for absorbing temperature-dependent changes in the volume of the liquid. As will be readily understood, the temperature control system30includes heaters, temperature sensors, heat sinks, and the like for controlling the temperature of the fluid, as well as a pump or other displacement means for circulating the fluid through the interior of the sheet support plate14.

The sheet support plate14, which has been shown in cross-section inFIG. 2, is made of a material, such as a metal, having a relatively high heat conductivity and also a relatively high heat capacity. A number of elongated cavities34are formed in the interior of the plate14so as to extend in parallel with one another and in parallel with the direction (B) of travel of the carriage22between opposite ends of the plate14, where they are connected to the tubes32through suitable manifolds. Each cavity34is delimited by a top wall36, a bottom wall38and two separating walls40. The top walls36, together, define the top surface42of the plate14which is machined to be perfectly flat.

Between each pair of two separating walls40, which delimit to adjacent cavities34, a hollow space44is formed. The hollow spaces44extend parallel to each other and in parallel with the direction (B) of travel of the carriage22between opposite ends of the plate14. The suction holes24pass through the top wall36into the hollow spaces44. The hollow spaces44are connected to the suction chamber26via apertures60having the form of slits extending in the direction of the hollow spaces.

FIG. 3schematically shows the interior of the suction chamber26in a horizontal cross-section. The suction chamber26comprises internal walls62extending in the sheet advance direction (A) and internal walls64extending in the reciprocating direction (B) of the printheads.

The internal walls62and64divide the suction chamber26into two groups of compartments66A,66B,66C and68A,68B,68C, respectively. The blower28is directly connected to the compartment66C in the middle of the first group of compartments through an opening69and the tube27(FIG. 1). The compartment68C is connected to its neighboring compartment66C via three flow restricting orifices70of circular cross section in the wall64. The compartments66B and68B are connected to the compartments66C and68C, respectively, through orifices70in the walls62. The outermost compartments66A and68A are only connected to their neighboring compartments66B and68B within the same group of compartments via orifices70in the walls62. However, the neighboring compartments66A and68A are not directly connected to each other. Also, the neighboring compartments66B and68B are not directly connected to each other, the compartments66A,66B,68A,68B and68C are indirectly connected to the suction device through the orifices70and the compartment66C.

When the sheet12is in the position ofFIG. 1, it covers approximately the rectangular area of the first group of compartments66A,66B and66C. The suction holes24which are connected to the second group of compartments68A,68B,68C are not covered by sheet12, so that air flows through the corresponding suction holes. However, the air flow to the compartment66C is restricted by the flow restriction orifices70. Thereby, the necessary subatmospheric pressure can be maintained in the compartments66A,66B and66C.

FIG. 4shows a cross sectional view of the sheet support plate14along the line IV-IV inFIG. 3. Unlike inFIG. 2, the cross section is through one of the walls62. The orifice70connecting the compartments66A and66B is shown.

The walls62comprise a plurality of protrusions72extending upwards through the apertures60(FIG. 2) in the bottom wall38into the hollow spaces44up to the bottom of the top wall36. Accordingly, air flow through the hollow spaces44is reduced by the protrusions72of the flow restricting members62.

When a sheet74(FIG. 1) of small width is advanced over the sheet support plate14, the sheet74covers, for example, only the area of the compartments66C and68C. The non-covered suction holes24which lie in the area of the compartments66A,68A,66B, and68B, permit an air flow into the hollow spaces44. Then, the protrusions72restrict the air flow through the hollow spaces44from the area of the compartment66A to the area of the compartment66B, for example, and from the area of the compartment66B to the area of the compartment66C. At the same time, the orifices70restrict the air flow from the compartment66A to the compartment66B and from the compartment66B to the compartment66C. Thereby, the required subatmospheric pressure can be maintained in the compartment66C.

FIG. 5shows a modified embodiment in a cross sectional view along a line that is parallel to the direction B. In this embodiment, the width of the suction chamber26is reduced as compared to the width of the sheet support plate14. The outermost compartments66A,68A have a reduced size but nevertheless are connected to a similar surface area of the sheet support plate14as described in connection withFIGS. 1 to 4. This is achieved by the hollow spaces44connecting the respective suction holes24via the apertures60to the compartment66A, for example. Thereby, the top surface42with the suction holes24can have a width that is larger than the width of the underlying suction chamber26.