FIELD OF THE INVENTION
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The invention relates to the field of stacking processed
sheets, in particular stacking processed printing plates.
BACKGROUND OF THE INVENTION
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Before a printing plate can be used on an offset press, it is
first imaged and then processed. When large numbers of printing
plates are processed, it is customary to stack the processed
printing plates automatically in a so-called plate stacker, at the
output side of the processor. An operator can then carry away the
stack of plates on a transport trolley.
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Figs. 1 and 2 show the operation of a Lithostar™ plate stacker
LS82 Ultra (Lithostar is a trade mark of Agfa-Gevaert N.V.). A
plate 25 is processed by a processor 10 (only partially shown in
Figs. 1-2) and transferred by the output roller pair 11 of processor
10 to a plate stacker 30, in the direction of arrow A. In the plate
stacker 30, plate 25 is supported and transported by a number of
strings 31 in the direction of arrow B. Fig. 1 only shows one
string 31, guided by pulleys 32, 33. In reality, plate 25 is
supported by several strings 31 at regular distances in the
direction perpendicular to the plane of Fig. 1. Plate 25 is
transported up to hinge 22 of transport trolley 20. Then, as shown
in Fig. 2, plate 25 is put onto transport trolley 20 by rotating the
tilting bars 21 in the direction of arrow C. Only one tilting bar
21 is shown in Fig. 2; in reality, transport trolley 20 has several
tilting bars 21, at regular distances in the direction perpendicular
to the plane of Fig. 2 and alternating with the strings 31. One
plate 25 after the other is put onto trolley 20, so that a stack of
plates 25 is formed on bottom 23 of transport trolley 20. When
there are enough plates 25 on the transport trolley 20, it is
wheeled away by an operator.
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Patent application EP-A-1 155 835 discloses a plate stacker
that is adapted for large format plates; the stacker contains a
device that avoids buckling of the plates.
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Existing plate stackers require much floor space and are rather
complex.
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There is thus a need for an improved plate stacker.
SUMMARY OF THE INVENTION
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The present invention is a device for stacking sheets as
claimed in independent claim 1, and a method for stacking sheets as
claimed in independent claim 9. Preferred embodiments of the
invention are set out in the dependent claims. The device in
accordance with the invention can be incorporated in a processor, as
claimed in claim 7, or in a transport trolley, as claimed in
claim 8.
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In this document, a "sheet" is preferably a printing plate,
more preferably an aluminum printing plate. A "sheet" may however
also be a, generally rectangular, piece of poly(ethylene
terephtalate), of paper, cardboard, plastic, sticker material, etc.
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A device in accordance with the invention comprises supporting
means to support a sheet, so that the supporting means can protrude
from the device over a protruding length. The device further
comprises means to increase the protruding length; advantageously
the protruding length is increased while the sheet is supported by
the supporting means.
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Before a sheet is supported, the protruding length may be zero.
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In a preferred embodiment of the invention, the supporting
means is rollable or collapsible.
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An important advantage of a device according to the invention
is that it is space-saving. Another advantage is that it is simple
and inexpensive.
-
Since a device in accordance with the invention is compact and
cheap, it may be incorporated into a transport trolley or into a
processor.
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Preferred embodiments of a device in accordance with the
invention may include features of a method - as claimed or as
described below - in accordance with the invention, and vice versa.
-
Further advantages and embodiments of the present invention
will become apparent from the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
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The invention is described with reference to the following
drawings without the intention to limit the invention thereto, and
in which:
- Figs. 1 and 2 show a prior art device, in diagram form;
- Fig. 3A shows, in diagram form, a device in accordance with the
invention;
- Fig. 3B shows a first embodiment of a supporting means in
accordance with the invention;
- Figs. 4A and 4B show a second embodiment of a supporting means
in accordance with the invention;
- Fig. 5 shows a third embodiment of a supporting means in
accordance with the invention.
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DETAILED DESCRIPTION OF THE INVENTION
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Fig. 3A shows a first embodiment of a stacking device 30 in
accordance with the invention. A sheet 25 is output by a processor;
for simplicity, only the output roller pair 11 of the processor is
shown in Fig. 3A. The output roller pair 11 moves sheet 25 to
stacking device 30, where sheet 25 is supported by a supporting
means 40.
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In the embodiment shown in Fig. 3A, the supporting means 40
includes a flexible strip 40, that is shown in more detail in
Fig. 3B. The flexible strip 40 may be a steel tape of a type that
is customarily used for measuring purposes. The stiffness of such
steel measuring tapes is low enough so that they are rollable, and
that a measuring tape can be stored coiled up in its housing; yet a
steel measuring tape has a sufficiently high stiffness so that an
unrolled, straightened tape can support a load. Advantageously,
instead of a steel measuring tape, a flexible strip 40 as shown in
Fig. 3B is used. Moreover, it is preferred to use a plurality of
steel tapes or flexible strips 40. E.g. an aluminum printing plate
having a width of 67 cm (in the direction perpendicular to the plane
of Fig. 3A) and a length of 70 cm may be supported by four such
flexible strips 40, spaced at regular distances in the direction
perpendicular to the plane of Fig. 3A.
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The supporting means 40 of this embodiment of the invention is
thus rollable; however this does not imply that the supporting means
is coiled up when it is not in use. In Fig. 3A, the supporting
means 40, when not in use, is oriented vertically with respect to
the floor 50, so that it only requires minimal floor space. When a
sheet 25 arrives from the processor, the supporting means 40 is
moved in the direction of arrow B and bent to a horizontal
orientation by guiding means 36. The portion of the supporting
means 40 that is guided by guiding means 36 is in a curved state;
when it leaves guiding means 36, this portion is straightened by its
own elasticity and may then support a sheet 25.
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In the embodiment shown in Fig. 3A, a sheet 25 is put onto a
stack as follows. Sheet 25 exits from the processor 10 (not shown
in Fig. 3A) and is still transported by the output roller pair 11 of
the processor. Then, sheet 25 as well as supporting means 40 are
moved in the direction of arrow B by rollers 37 and 38, i.e. the
protruding length L of supporting means 40 is increased while the
supporting means 40 supports sheet 25. As shown in Fig. 3A, the
axes of rollers 37 and 38 are not located in the same vertical
plane: the leading edge 26 of sheet 25 first reaches roller 37 and
afterwards roller 38. The supporting means 40 passes over roller 37
and under roller 38. Sheet 25 is supported by supporting means 40,
and also, initially, by roller 37; roller 38 presses slightly on
sheet 25. Supporting means 40 and the leading edge 26 of sheet 25
bend somewhat downwards as they are moved further in the direction
of arrow B, but this does not affect the operation of the stacking
device 30. When the trailing edge 27 of sheet 25 is moved past
roller 38, rollers 37 and 38 are stopped and their direction of
rotation is reversed. Supporting means 40 and sheet 25 are now
moved in the direction opposite to that of arrow B. Sheet 25, which
is supported only by supporting means 40, is stopped by stop means
35, as explained further below, so that supporting means 40 is
withdrawn but sheet 25 is not. When supporting means 40 is
completely withdrawn, sheet 25 drops by gravity into receiving means
such as tray 45 shown in Fig. 3A. Now, the next sheet 25 exiting
from the processor may be handled, so that a stack of sheets 25 is
formed in tray 25. Advantageously, the withdrawal speed of
supporting means 40 is larger than the transport speed in the
direction of arrow B (e.g. twice as large); this allows short delays
between successive sheets.
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In a preferred embodiment, the stop means includes two
identical cams 35 and the supporting means comprises four flexible
strips 40; the two cams 35 are positioned respectively between the
first and the second and between the third and the fourth flexible
strips 40, when viewed in the direction perpendicular to the plane
of Fig. 3A. The position and the shape of cams 35 are such that the
leading edge 26 of sheet 25 pushes against the cams 35 so that they
rotate around point P in the sense of arrow D; sheet 25 can now
freely pass the cams. After the trailing edge 27 of sheet 25 passed
cams 35, the cams rotate back to their rest position by gravity, so
that their sides 34 are approximately vertical. When withdrawing
supporting means 40, in the direction opposite to that of arrow B,
the trailing edge 27 of sheet 25 is caught by edges 39 of cams 35.
Cams 35 are blocked so that they cannot rotate beyond their rest
position in the sense opposite to arrow D.
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As is clear from the explanation above, a single drive means
suffices for rollers 37 and 38, and cams 35 do not need a drive
means. The single drive means is used for increasing the protruding
length L of supporting means 40 and for withdrawing the supporting
means 40. Using only one drive means is an advantage with respect
to prior art devices as shown in Figs. 1 and 2, which use two
motors: a first motor to move the strings 31 to transport a sheet 25
in the direction of arrow B (see Fig. 1) and a second motor to
rotate the tilting bars 21 and sheet 25 in the direction of arrow C
(see Fig. 2) .
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Another advantage of a stacking device 30 in accordance with
the invention is that it is much smaller than a prior art device.
When the stacking device is not in use, tray 45 in Fig. 3A may
manually be rotated around hinge 46 over an angle of 90° in the
direction of arrow D, so that stacking device 30 requires much less
floor space than the prior art device 30 shown in Figs. 1 and 2.
Such a rotation may also be used to transfer a stack of sheets 25
from stacking device 30 to a transport trolley 20. Alternatively,
stacking device 30 may be incorporated in a transport trolley 20;
this transport trolley is then wheeled away by an operator and
replaced by an empty transport trolley 20 (also including a stacking
device 30) when a certain number of sheets 25 have accumulated in
tray 45. In yet another embodiment, a portion of the stacking
device 30 is incorporated in the transport trolley 20, while another
portion is either a separate apparatus or is incorporated in the
processor 10. For example, of the embodiment of stacking device 30
as shown in Fig. 3A, the receiving means 45 may be incorporated in
the transport trolley 20, while the supporting means 40, the means
36, 37, 38 for increasing the protruding length L of the stacking
device 30 and the stop means 35, are either part of a separate
apparatus or are incorporated in the processor 10.
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Fig. 3B shows a preferred embodiment of a flexible strip 40.
It has the shape of a very flattened U. It may be made of a metal,
preferably steel. Its thickness, e.g. 0.25 mm, is chosen to obtain
an appropriate stiffness: large enough to support a sheet 25, and
small enough to be easily deformed by guiding means 36. In Fig. 3B,
only a short portion of a flexible strip 40 is shown; in practice, a
flexible strip 40 has a larger length.
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Instead of a flexible strip 40, other embodiments of supporting
means can be used. Figs. 4A and 4B show in diagram form a
supporting means 41 comprising successive segments, interlinked by
hinges, and further comprising means to allow the supporting means
to span a long self-supporting length. Such a type of supporting
means 41 is called a "multi-segment supporting means" in this
document; it is customarily used in cable carrier systems, wherein
it guides one or more cables from a stationary connection to a
movable device. More information on cable carrier systems and on
multi-segment supporting means can be found e.g. in patent
US-A-4 625 507. Figs. 4A and 4B illustrate two variants of an
embodiment with multi-segment supporting means 41; for simplicity,
only a sheet 25, a multi-segment supporting means 41 and their
orientation with respect to the floor 50 are shown; the rest of the
stacking device 30 can easily be designed by a person skilled in the
art, starting from the stacking device illustrated in Fig. 3A. The
multi-segment supporting means 41 is moved at a speed v2, which is
preferably twice the speed v1 of the sheet 25 since both the portion
43 of the supporting means 41 that supports the sheet 25 and the
portion 44 that does not support the sheet 25 have to be advanced,
so that sheet 25 may be supported adequately, over its entire
length.
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Fig. 5 shows in diagram form yet another embodiment of a
supporting means; here also, only a sheet 25, the supporting means
42 and their orientation with respect to the floor 50 are shown.
The supporting means 42 is telescopic (as a telescopic car antenna).
It includes several segments that may be moved e.g. by a rope
transfer mechanism: the telescopic supporting means 42 is retracted
by pulling the rope 48; it is extended by pushing the rope 48. Rope
48 may be made of a synthetic resin having moderate hardness and
elasticity. The portion of rope 48 outside the supporting means 42
may be coiled on a spool.
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As in the case of flexible strips 40, it is preferred to use
more than one, e.g. four, multi-segment supporting means 41 or
telescopic supporting means 42.
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The supporting means shown in Figs. 3A to 5 are rollable or
collapsible, so that they may assume a compact shape when not in
use. This allows a space-saving construction of the stacking device
30. In this document, a "rollable" supporting means does not mean
that the supporting means has to be coiled upon a spool; it suffices
that the supporting means can be deformed to a curved state (as e.g.
by the guiding means 36 in Fig. 3A).
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The invention is not limited to the types of supporting means
discussed above; any other means as known in the art and falling
within the scope of the claims may be used.
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The invention may be applied to any kind of sheets, but is
especially useful for printing plates, because of their larger
stiffness. Film sheets and other flexible sheets can easily be bent
when output from the processor, and stacked vertically in a tray,
thus requiring only little floor space. Printing plates, on the
other hand, have a larger stiffness and often have large dimensions,
so that prior art stacking devices are bulky, as illustrated by
Figs. 1 and 2.
EXAMPLE
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In the embodiment shown in Figs. 3A and 3B:
- number of flexible strips 40 : 4
- spacing between two strips : 20 cm
- dimensions of a flexible strip :
- thickness : 0.25 mm
- width : 15 mm
- length : 120 cm
- sheet 25 :
- aluminum printing plate
- thickness : 0.15 mm
- length : 70 cm
- width : 67 cm
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Those skilled in the art will appreciate that numerous
modifications and variations may be made to the embodiments
disclosed above without departing from the scope of the present
invention.
List of reference signs
-
- 10 :
- processor
- 11 :
- roller pair
- 20 :
- transport trolley
- 21 :
- tilting bar
- 22 :
- hinge
- 23 :
- trolley bottom
- 25 :
- sheet
- 26 :
- leading edge
- 27 :
- trailing edge
- 30 :
- stacker
- 31 :
- string
- 32 :
- pulley
- 33 :
- pulley
- 34 :
- side
- 35 :
- cam
- 36 :
- guide
- 37 :
- roller
- 38 :
- roller
- 39 :
- edge
- 40 :
- flexible strip
- 41 :
- multi-segment support
- 42 :
- telescopic support
- 43 :
- portion
- 44 :
- portion
- 45 :
- tray
- 46 :
- hinge
- 48 :
- rope
- 50 :
- floor
- A :
- arrow
- B :
- arrow
- C :
- arrow
- D :
- arrow
- L :
- length
- P :
- point
