Printing unit of a sheet-fed printing press

Details Printing unit of a sheet-fed printing press Printing unit of a sheet-fed printing press

2003-04-16

2004-08-31

Chau, Minh (Department: 2854)

Printing

Print plate feeding or delivering

C101S451000, C101S418000

Reexamination Certificate

active

06782825

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application claims priority from Japanese Patent Application Nos. 2002-115028 and 2002-317441, which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing unit of a sheet-fed printing press.
2. Related Art
As illustrated in
FIG. 6
, a conventional printing unit of the printing press includes plate cylinder
2
, which carries printing film
1
therearound and rotates, ink feeding part
3
for feeding ink on the printing film
1
on the plate cylinder
2
, rubber cylinder (not shown) onto which ink is transferred from the printing film
1
, and an impression cylinder (not shown) for pressing and holding a sheet with the rubber cylinder so as to enable the ink to be transferred from the rubber cylinder to the sheet.
The printing press of
FIG. 6
is a digital printing press, in which the printing film
1
having such a length as to enable several images to be formed thereon is rolled up and set to the plate cylinder
2
for easy replacement of the printing film
1
. Specifically, the plate cylinder
2
forms on its circumference recess
4
, in which a roll of the printing film
1
is mounted, and includes feed reel
5
for feeding the printing film
1
onto the circumference of the plate cylinder
2
, and rewind reel
6
for rewinding the printing film
1
by holding the leading edge
1
a
thereof so as to draw the film from the circumference of the plate cylinder
2
.
Once the printing film
1
is set on the plate cylinder
2
, image forming device
7
, which is secured in place with a predetermined clearance to the circumference of the plate cylinder
2
, forms an image on the printing film
1
placed on the circumference of the plate cylinder
2
so as to use the printing film
1
as a press plate. The ink feeding part
3
feeds ink on the printing film
1
so that the image is printed on the sheet via the rubber cylinder. Subsequent to completing series of printing operations, the rewind reel
6
rewinds the printing film
1
so as to draw the same from the circumference of the plate cylinder
2
, while the feed reel
5
successively feeds the printing film
1
onto the circumference of the plate cylinder
2
. That is, in the printing press having the printing unit as illustrated in
FIG. 6
, another printing image can be formed on the printing film
1
fed from the feed reel
5
by the image forming device
7
, thereby omitting the necessity to replace the plate every time the printing operation is performed, and hence achieving repeated printing operations by a single roll of the printing film
1
.
According to the printing unit as described above, various sizes of sheets can be printed. In each operation, the image forming device
7
forms an image on an image forming region of the printing film
1
. The image forming region corresponds in size to each of the sheets and has a first edge and a second edge respectively corresponding to a leading edge and a trailing edge of each of the sheets. The image is formed on the image forming region with the first edge thereof positioned at an edge of the recess closer to the rewind reel. In the conventional printing unit, the printing film
1
is rewound to present a new region onto the circumference of the plate cylinder
2
, thereby replacing the entire region of the printing film
1
extending between the feed reel
5
and the rewind reel
6
for another printing image, even if each sheet has a length shorter than this circumferential length. That is, regardless of the length of sheets to be printed, the printing film
1
is fed from the feed reel
5
so as to have the entire region on the circumference replaced by a newly fed region, which has substantially the same length as the entire circumferential length of the plate cylinder
2
between the feed reel
5
and the rewind reel
6
.
In consideration of the above, it is an object of the present invention to provide a printing unit of the sheet-fed printing press that is capable of reducing the amount of the printing film to be consumed than a conventional printing unit does.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a printing unit of a sheet-fed printing press for printing on sheets that includes a plate cylinder, a feed reel, an image forming device, a rewind reel and a control unit. The plate cylinder has a circumference, on which a recess is formed. The feed reel is mounted within the recess for feeding a printing film onto the circumference of the plate cylinder so as to introduce an image forming region for each printing operation. The image forming region corresponds in size to each of the sheets and having a first edge and a second edge respectively corresponding to a leading edge and a trailing edge of each of the sheets. The image forming device is designed to form an image on the image forming region of the printing film fed on the circumference of the plate cylinder with the first edge of the image forming region positioned at an edge of the recess closer to the rewind reel. The rewind reel is mounted within the recess for rewinding the printing film from the circumference of the plate cylinder. The control unit is designed to control the length of the printing film to be fed from the feed reel onto the circumference of the plate cylinder, based upon the length of sheets to be printed for each printing operation.
In the printing unit having the above arrangement, the length of the printing film to be fed from the feed reel is controlled based upon the length of sheets to be printed. This feeding arrangement achieves the reduction of the printing film to be consumed as compared with the conventional printing unit, in which the printing film is fed from the feed reel by a constant length regardless of the length of sheets to be printed so as to replace a region on the circumference of the plate cylinder entirely by a new region. As used throughout the description, the length of sheets or a sheet is meant to be a length thereof along the circumferential direction of the plate cylinder.
Preferably, the control unit controls the length of the printing film to be fed from the feed reel so that a region of the printing film positioned at a boundary between the circumference of the plate cylinder and the recess in a previous printing operation is prevented from being positioned within a film forming region to be used in a current printing operation when the printing film has been fed from the feed reel.
Since the printing film is sharply bent at the boundary between the circumference of the plate cylinder and the recess, the printing film is likely to be creased at the boundary. This creased portion may cause uneven printing if an image is formed on that portion. Therefore, in the above arrangement, the length of the printing film to be fed from the feed reel is controlled so as not to have the creased portion positioned in an image forming region of the printing film for each printing operation. Thus, uneven printing can be prevented.
Preferably, the control unit controls the length of the printing film to be fed from the feed reel so that, when the total of the length of sheets to be printed in a current printing operation and the length of sheets which have been printed in a previous printing operation is longer than the circumferential length of the plate cylinder between the feed reel and the rewind reel, the printing film is fed from the feed reel by a length substantially equal to the circumferential length so as to replace a region of the printing film on the circumference of the plate cylinder entirely by a new region, and when the total of the length of sheets to be printed in a current printing operation and the length of sheets which have been printed in a previous printing operation is shorter than the circumferential length of the plate cylinder between the feed reel and the rewind reel, the printing film is fed from the feed reel by a length not less than the length of sheets which ha

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