Incremental printing of symbolic information – Ink jet – Medium and processing means
Reexamination Certificate
2001-11-01
2004-09-14
Liang, Leonard S (Department: 2853)
Incremental printing of symbolic information
Ink jet
Medium and processing means
C347S101000, C400S642000, C400S645000, C400S023000
Reexamination Certificate
active
06789889
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to media handling systems for inkjet printing devices, and more particularly to a media handling system which is capable of achieving small bottom margins.
Known inkjet printing devices which use single sheet or cut sheet media have a limited bottom margin capability. One of the smallest bottom margins achievable is approximately 11.7 mm by the Hewlett Packard 800 series DeskJet™ printers. Many applications could take advantage of a smaller bottom margin, if available on single sheet and cut sheet inkjet printers. Continuous form inkjet products are able to achieve smaller bottom margins because a current page is attached to a subsequent page during printing. The pages are detached after printing.
Other inkjet printing concerns which impact the bottom margin limitation are the need for accurate dot placement and the need to account for the effects of wet ink printing. Both of these concerns impact a larger portion of the media sheet than simply the immediate area being printed at any given time. Media handling is one function controlled to achieve accurate printing and wet ink control. In the series 800 DeskJet™ printers, for example, pinch rollers keep the media sheet in contact with a drive roller as the media sheet is fed through a print zone adjacent to a printhead. The pinch rollers prevent media slippage and allow for accurate dot placement. Cockle control devices such as ribbed devices place a known bend pattern in the paper downstream from the print zone which limits cockle growth in the print zone. The pinch rollers isolate the cockled area from a flat media sheet area in the print zone.
FIG. 1
shows a conventional inkjet printing apparatus
10
including an inkjet pen
12
having a printhead
14
. The printhead
14
includes a plurality of inkjet nozzles which eject ink onto a media sheet
16
during printing. The media sheet is moved along a media path in a direction
17
by one or more rollers, including a drive roller
18
. A pinch roller
20
presses the media sheet to the drive roller
18
. A platen
22
supports the media sheet as the media sheet
16
is moved through a print zone
24
. Typically, the print zone is located close to the pinch roller's line of contact with the media sheet, but further along the media sheet path than the pinch roller's line of contact. More specifically, the print zone
24
is located adjacent to the printhead nozzles between the printhead
14
and the platen
22
. The relative location of the pinch rollers relative to the print zone determines how small the bottom margin can be. Once a trailing edge
26
of the media sheet
16
passes beyond the pinch roller
20
, there is nothing securing the media sheet as the trailing edge
26
advances through the print zone
24
. Accordingly, printing on the media sheet after contact is lost with the pinch roller
20
is subject to inaccuracies. The manufacturer avoids these inaccuracies by making the minimum bottom margin large enough that the media sheet is still in contact with the pinch roller at the bottom margin. Typically the distance from the nearest edge of the print zone to the pinch roller line of contact equals the minimum bottom page margin achievable for an inkjet print apparatus. Referring to
FIG. 1
, the minimum bottom page margin for the media sheet
16
is limited by the distance d
p
from the pinch roller
20
line of contact to the nozzle area of the printhead
14
.
In a page wide array inkjet printhead the nozzle rows are oriented 90 degrees about the typical scanning printhead nozzle row orientation. In the
FIG. 1
canning inkjet pen
12
the nozzle rows are oriented in a direction across the drawing sheet from let to right in the areas marked for the printhead
14
.
FIG. 2
shows a page wide array configuration. Inkjet pen
12
′ includes a pagewide array printhead
14
′ having a pair of rows
15
,
19
for each color (e.g., YMCK-yellow, magenta, cyan and black). The rows
15
,
19
are oriented to extend into the page of the drawing sheet.
Referring to
FIG. 2
, the media sheet
16
is moved along a media path in the direction
17
by one or more rollers, including a drive roller
18
. A pinch roller
20
presses the media sheet to the drive roller
18
. A platen
22
supports the media sheet as the media sheet
16
is moved through a print zone
24
. Once a trailing edge
26
of the media sheet
16
passes beyond the pinch roller
20
, there is nothing securing the media sheet as the trailing edge
26
advances through the print zone
24
. Accordingly, the minimum bottom margin is made large enough that the media sheet is still in contact with the pinch roller. With the nozzles oriented in the direction parallel to the length of the pinch rollers and drive rollers, the minimum bottom margin is limited by the first row of nozzles located farthest from the pinch roller
20
(row
19
of color K in FIG.
2
). Referring to
FIG. 2
, the minimum bottom page margin for the media sheet
16
is limited by the distance d
p
′ from the pinch roller
20
line of contact to the nozzle area of the printhead
14
′. For a YMCK printhead, the minimum bottom margin would be greater than 1 inch.
One way of reducing the minimum bottom margin is to place the pinch roller
20
closer to the print zone
24
. There is a limit, however, to how close the pinch roller line of media sheet contact can be to the print zone. Another scheme is to make the pinch roller diameter smaller, so that the distance between the print zone and pinch roller can be shorter. However, media advance accuracy suffers as the pinch roller becomes too small.
The pinch roller also serves to provide a reverse bowing which reduces cockle growth from the wet ink printing. Cockle growth refers to the buckling or ridges in a media sheet due to the presence of wet ink soaking into the media sheet. As the pinch roller becomes too small the reverse bow desired for limiting cockle growth becomes difficult to maintain. Accordingly, there is a need for a method and apparatus for allows for smaller bottom margins than the distance between pinch roller and print zone.
SUMMARY OF THE INVENTION
According to the invention, a media handling system provides accurate positioning of a media sheet through a print zone to allow for smaller bottom margins than known minimum bottom margins for single sheet or cut sheet inkjet printing devices. Minimum bottom margins less than 5 mm, and as low as 1 mm or 2 mm are achieved. This is a substantial improvement over the 11.7 mm minimum bottom margin achieved by current inkjet printing devices.
According to one aspect of the invention the media sheet is positioned on a support while traveling through the print zone. In addition to a pinch roller located along the media path prior to the print zone (i.e., an upstream pinch roller), there may be an another optional pinch roller located after the print zone (i.e., a downstream pinch roller). The support and pinch rollers stabilize the media sheet while the media sheet moves through the print zone. The downstream pinch roller may be of a star wheel configuration to minimize contact with the media sheet and avoid smudging the wet ink on the media sheet. A function of the downstream pinch roller is to hold the media sheet down and away from the inkjet printhead. Another function is to assist in advancing the media, especially once the media sheet trailing edge has passed beyond the upstream pinch roller.
According to another aspect of the invention, a guide shim is operatively positioned with the upstream pinch roller. The guide shim extends along the media path beyond the upstream pinch roller toward the print zone. The guide shim abuts or comes close to the print zone. The location of a lead edge of the guide shim relative to the print zone determines the minimum bottom margin for the inkjet printing device. One function of the guide shim is to provide media advance accuracy as the media sheet trailing edge departs contact with the upstream pinch roller an
Elgee Steven B
Rasmussen Steve O
LandOfFree
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