PRINTER WITH VACUUM DEVICE

Abstract

A printer includes a print station; a media transport mechanism arranged for conveying print media on a transport path past the print station, the transport mechanism having a support surface for supporting the media; and a vacuum device arranged for attracting the media against the support surface on a section of the transport path downstream of the print station. The vacuum device is divided, in the direction along the transport path, into at least two segments in which the media are attractable with different non-zero suction pressures.

Claims

1. A printer comprising: a print station; a media transport mechanism arranged for conveying print media on a transport path past the print station, the transport mechanism having a support surface for supporting the media; and a vacuum device arranged for attracting the media against the support surface on a section of the transport path downstream of the print station, wherein the vacuum device is divided, in a direction along the transport path, into at least two segments in which the media are attractable with different non-zero suction pressures, and wherein the suction device has an upstream segment and a separate downstream segment and wherein the suction pressure in the upstream segment is larger than the suction pressure in the downstream segment.

2. The printer according to claim 1, the printer being an ink jet printer.

3. The printer according to claim 1, wherein the media transport mechanism has an upstream transport section for conveying the media past the print station, and a separate downstream transport section in which the vacuum device is arranged.

4. The printer according to claim 3, wherein the downstream transport section includes a perforated endless conveyer belt.

5. The printer according to claim 3, wherein a drying station is provided in the downstream transport section.

6. The printer according to claim 5, wherein the ratio between the suction pressure in the upstream section and the downstream section is between 1.3:1 and 5:1.

7. An ink jet printing method comprising the steps of: moving a media sheet past a print station and printing an image onto the media sheet; conveying the media sheet with the printed image through a drying station while the media sheet is supported on a support surface; and attracting the media sheet against the support surface by means of a suction device; wherein the step of attracting comprises a first sub-step of attracting the sheet against the support surface with a high suction pressure in order to remove cockles from the sheet, and a second sub-step of holding the sheet in engagement with the support surface by applying a suction pressure that is lower than the suction pressure in the first sub-step.

8. The printer according to claim 5, wherein the ratio between the suction pressure in the upstream section and the downstream section is between 2.5:1 and 3.5:1.

9. The printer according to claim 2, wherein the media transport mechanism has an upstream transport section for conveying the media past the print station, and a separate downstream transport section in which the vacuum device is arranged.

10. The printer according to claim 4, wherein a drying station is provided in the downstream transport section.

Description

[0021] Embodiment examples will now be described in conjunction with the drawings, wherein:

[0022] FIG. 1 is a schematic view of essential parts of a printer according to the invention;

[0023] FIG. 2 is a plan view of a media sheet with an image printed thereon;

[0024] FIG. 3 illustrates the effect of a swelling of the media sheet shown in FIG. 2 in the area of the printed image; and

[0025] FIG. 4 is an enlarged view of a detail in FIG. 1, showing a cockled sheet, with the height of the cockles being exaggerated.

[0026] As is shown in FIG. 1, an ink jet printer comprises a print station 10 and a media transport mechanism 12, 14 which, in this example, comprises two separate transport sections 12 and 14. Each transport section comprises an endless conveyer belt a top surface of which constitutes a support surface 16 supporting a media sheet 18 that is conveyed past the print station 10 in the upstream transport section 12 and is then handed over to the downstream transport section 14 which moves the sheet past a drying station 20.

[0027] The print station 10 may comprise a print head assembly with a plurality of ink jet print heads arranged for jetting ink droplets in different colors onto the surface of the media sheet 18. For example, the ink may be a water-based ink and the media sheet 18 may be a sheet of paper which is wetted by the ink applied thereto.

[0028] The drying station 20 may for example comprise a radiator for irradiating the sheet 18 with infrared light, in order to raise the temperature of the sheet and to dry the ink by evaporating the volatile ink components.

[0029] The downstream transport section 14 is equipped with a vacuum device 22 comprising two adjacent segments 24, 26 constituted by separate plenum chambers each of which is connected to a blower 28 and 30, respectively.

[0030] The plenum chambers in the segments 24 and 26 have a perforated top wall, and the conveyer belt in the transport section 14 is also perforated, so that air is drawn-in through the perforations of the conveyer belt and the top wall of the plenum chambers. In this way, the sheet 18 is attracted against the support surface 16 as it passes over the segments 24 and 26. Consequently, the conveyer belt is pressed against the perforated top walls of the plenum chambers, which causes a certain amount of friction as the sheet 18 and the part of the conveyer belt supporting it move jointly through the drying station 20.

[0031] A main purpose of the suction device 22 is to prevent the sheet 18 from cockling, which is an undesired effect that will now be explained in conjunction with FIGS. 2 and 3.

[0032] FIG. 2 is a plan view of the media sheet 18 with an image 32 printed thereon. When the image 32 is being printed in the print station 10, the liquid water-based ink is applied onto the sheet in the area of the image 32 and the water penetrates into the paper of the sheet 18 and causes the same to swell.

[0033] This has been symbolized in FIG. 3, where the image 32 has been shown slightly enlarged, due to the swelling, and the original contour 32′ of the image has been shown in phantom lines. The image 32 is surrounded by a margin portion 34 where the paper of the sheet does not swell. This leads to internal strains in the paper and causes the paper to form wrinkles or cockles 36 in the area of the image 32.

[0034] FIG. 4 shows a part of the printer that has been shown in FIG. 1 on an enlarged scale. A media sheet 18 is just leaving the print station 10, and a leading edge of the sheet has already reached the downstream segment 26 of the vacuum device in the drying station 20. The part of the sheet 18 onto which ink has been applied in order to form the image 32 starts to cockle with a certain delay time which corresponds to the time in which the water penetrates into the paper. As the sheet 18 moves from right to left in FIG. 4, the cockles 36 start to form slightly downstream of the print station 10. For illustration purposes, the height of the cockles 36 has been exaggerated in FIG. 4.

[0035] The cockles 36 pass over a transition area from the upstream transport section 12 to the downstream transport section 14. In this transition area, the sheet 18 cannot be attracted against the support surface (actually there is no support surface in the gap between the two conveyer belts), so that the formation of cockles cannot be prevented. However, as soon as the cockles reach the area of the segment 24, they are firmly attracted against the support surface 16 because the blower 28 associated with the plenum chamber of this segment is controlled to create a high vacuum pressure in the order of magnitude of, for example, 3 kPa. Consequently, the height of the cockles 36 decreases from the upstream end to the downstream end of the segment 24, as has been shown in FIG. 4.

[0036] The length of the segment 24 in the transport direction, and the vacuum pressure in that segment are selected such that the cockles are eliminated completely at the transition between the segments 24 and 26. Then, since the sheet 18 mates the support surface 16 on its entire area, a smaller vacuum pressure of, e.g., 1 kPa in the plenum chamber of the segment 26 is sufficient for holding the sheet in the flat state and for preventing the cockles from forming again. Eventually, when the corresponding region of the sheet 18 leaves the drying station 20, the paper has been dried to such an extent that no cockles will form anymore.

[0037] Thus, the zone above the segment 24 can be considered as a repair zone where a high suction pressure is applied for removing the cockles 36. Since this repair zone is relatively short, the energy consumption of the associated blower 28 and the friction between the conveyer belt and the top wall of the plenum chamber can be kept small. Then, when the sheet passes over the longer segment 26, the energy consumption (of the blower 30) and the friction are kept small because of the reduced vacuum pressure in this segment.

[0038] Of course, the concept that has been described above can easily be extended to a design with three or more successive suction zones in which the suction pressures can be controlled independently of one another.