Note: Descriptions are shown in the official language in which they were submitted.
C~,Z i i 7665
DE LA RUE GIORI S.A. LAUSANNE/SWITZERLAND
Printino unit for a web-fed printing machine
Field of the invention
The invention relates to a printing unit for a
web-fed printing machine with two cooperating cylinders
which form the printing nip and which, as in a sheet-fed
printing unit, have a plurality of printing saddles
separated by cylinder pits, for printing a web running
through the printing nip with variable formats adjoining
one another, the web being transported at a variable
speed in the so-called pilgrim-step mode and, after each
printing operation, when it passes a cylinder pit, being
retracted and accelerated again relative to the circum-
ference of the two cylinders in such a way that
successive printing images are lined up virtually without
a gap.
Prior art
Printing unite of this type are described in
US Patent 5,062,360. These known printing unite are
either offset printing units with two cooperating blanket
cylinders, which are both inked with printing images and
which serve for producing a recto/verso print, or
intaglio printing units with a plate cylinder which
cooperates with an impression cylinder. The use of sheet-
fed printing units for printing a continuous paper web
has the advantage that there is no need for the difficult
and time-consuming production of plate and impression or
blanket cylinders with a continuous seamless
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circumferential surface; use is made, instead, of the
cylinders which are simpler to produce and have printing
saddles separated from one another and on which the
individual printing plates or printing coverings can be
individually mounted, adjusted and exchanged without
great difficulty. Moreover, the transport of the paper
web in the so-called pilgrim-step mode affords the
possibility that images adjoining one another can always
be produced without a loss of paper as a result of
unprinted gaps between the images, hence that no
unprinted stripe occur between successive printing images
on account of the cylinder pits. For this purpose, as
described particularly in US Patent 5,062,360, the
transport of the paper web is controlled by means of a
control system in such a way that, after each printing
operation, the no longer clamped paper web, when it
passes the mutually aligned cylinder pits of the two
cylinders, is first braked, then moved backward and
finally accelerated again to the operating speed, before
it is clamped once more between the following cooperating
printing saddles of the two cylinders for the subsequent
printing operation. The distance over which the paper web
is retracted relative to the circumference of the two
cylinders is selected so that successive printing images
are printed virtually without a gap. To this effect,
therefore, the pilgrim-step movement must be controlled
in such a way that, after the pilgrim step, the paper web
assumes at least approximately the same position relative
to the printing nip between the two cylinders, that is to
say within the machine, as before the pilgrim step.
As a result of an appropriate adaptation of the
pilgrim-step movement, it is possible, with one and the
same printing machine, even in the case of varied formats
of the printed image, that ie to say in the case of
shorter formats, to print images adjoining one another
without a gap. The printing format is changed by mounting
a printing plate of the same size with a shorter printing
image, the printing zone thus remaining the same and not
being shortened.
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In the hitherto known web-fed printing machines
of this type, the two cylinders are always set relative
to one another in such a way that the printing saddles,
when they cooperate in the printing nip, are located
exactly opposite one another, as shown in Figures 1 and
2 for the two cylinders 1 and 4 which each have three
printing saddles 2 and 5 with the circumferential length
S. At the same time, therefore, the length D of the
printing zone, along which the paper web 7 is clamped
between the two cylinders, is always exactly as large as
the circumferential length S of a printing saddle,
specifically irrespective of the circumferential length
B of the printing image, that is to say of the format.
The print-free zone between two printing zones
according to the length G of a cylinder pit and
consequently the timespan available for retracting the
paper web are therefore, of course, also always the same.
Figure 1 shows the commencement of a printing
operation and Figure 2 the end of this printing
operation, that is to say after the cylinders have
rotated in the direction of the arrows through an angle
corresponding to the length D of the printing zone. If
the maximum possible format is being printed, that is to
say if the circumferential length of the printing image
is virtually equal to the circumferential length of a
printing saddle, then, after each printing operation, the
paper web 7, when it passes the cylinder pits 3, 6 in a
freely movable manner, must be braked, retracted and
accelerated again in such a way that, depending on the
printing speed, the obtainable deceleration and
acceleration and the time taken to run through the pit,
when printing commences again it assumes approximately
the same position relative to the printing nip as that
which it had assumed before the pilgrim step.
However, if smaller formats are to be printed, in
which, as illustrated in Figures 1 and 2, the
circumferential length B of a printing image is smaller
than the circumferential length S of a printing saddle,
then, after each printing operation, the paper web 7 must
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be retracted and accelerated again over a greater
distance which ensures that, when printing commences
again, the position of the paper web is additionally set
back relative to the printing nip, opposite to the
running direction, by the amount of the difference
between the length of the printing zone D and the length
of the printing image B, that is to say by the amount of
the distance D-B.
In an example according to Figures 1 and 2, the
print start A coincides with the image start, while the
print end E of the.printing zone is located behind the
image end F by the amount of said differential distance
D-B, this difference being designated by Z. In this case,
therefore, whenever printing commences again, the paper
web 7 must assume a position relative to the printing nip
which is offset rearward opposite to the running
direction by the amount of the length Z, if printing
images succeeding one another virtually without a gap are
to be obtained on the paper web, as indicated in Figure
1.
The fact that the length D of the printing zone
always remains ,constant, even when printing-3mages of
smaller format are produced, has various disadvantages:
Since, after each printing operation, the paper
web must additionally be retracted by the amount of a
distance Z, this signifies, for smaller formats, that ie
to say for larger Z, a considerably greater deceleration
and acceleration of the paper web in comparison with the
printing mode in which the maximum printing format is
produced. At the same time, the following has to be taken
into account:
When the paper web is released after the printing
zone, it first moves a little way further according to
the magnitude of the printing speed and of the obtainable
deceleration, until it stops. This distance which the
paper web has thereby covered is doubly critical, since
it has to be taken into account once again during
acceleration after the standstill of the paper web. The
paper web therefore has to be retracted by double this
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distance, so that, after subsequent acceleration,- it
arrives again with the printing speed at the same point
which it occupied before the pilgrim step.
If the printing-saddle length is larger than the
printing-image length, the paper web cannot be
decelerated immediately after the end of the .printing
image, but is transported further at the printing speed, .
before the pilgrim step can commence. As mentioned, this
distance must be covered additionally in the pilgrim
step.
However, since limits are placed on the
deceleration and acceleration of the paper web, a
reduction in the speed of the machine is necessary. This
loss of capacity thus has a twofold effect, since, during
each revolution, a shorter printing image is produced
and, in addition, the speed must be reduced
Furthermore, particularly in intaglio printing,
the paper web is compacted by the very high pressure,
which can amount to 80 t per meter of web width, and by
the high-gloss chromium-plated surface of the printing
plate,- in such a way that after retraction, during the
subsequent printing operation, the pressed, bu-t not ink-
printed portion Z of the paper web is printed With poorer
quality than the non-pressed web. Figures 1 and 2
illustrate the excessively pressed web portion Z and the
double-pressed portion P. The pressing of the portion Z
in Figure 1 originates from the preceding printing
operation.
Objects of the present invention are avoidance of
the disadvantages explained above, and to provide a
printing unit in which no losses of capacity during
the transition to a smaller format and also no double
pressings of the paper web occur.
The invention provides a printing unit, for a web-
fed printing machine with first and second cooperating
cylinders which form a printing nip and which have a
plurality of printing saddles separated by non-printing
grooves, for printing a web running through the
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printing nip with variable formats adjoining one
another, the web being transported at variable speed in
a so-called pilgrim-step mode and after each printing
operation, when it passes a groove, being retracted and
accelerated again relative to the circumference of the
two cylinders in such a way that successive printing
images are lined-up virtually without a gap. The two
cylinders are set angularly relative to one another, in
respect of the positions of their printing saddles co-
operating in the printing nip, depending on a format,
in a manner offset by an amount of a circumferential
distance which is at least approximately equal to a
difference between a circumferential length of a
printing saddle and a circumferential length of a
printing image; so that a length of the impression
zone, along which the two printing saddles clamp the
web between them when they pass the printing nip, is
only approximately as large as the circumferential
length of a printing image. As on cylinders of a
sheet-printing machine, the cylinders comprise cylinder
pits forming said grooves. A first release member. is
fastened on the circumference of the first cylinder on
which the beginning of a printing saddle, with respect
to a direction of rotation of the cylinder, corresponds
to a beginning of a printing image, and a first sensor
activatable by this release member is installed in
proximity to the circumference of this cylinder. A
second release member is fastened on the circumference
of the second cylinder, on which the end of a printing
saddle corresponds to an end of the printing image, and
a second sensor activatable by this release member is
installed in proximity to the circumference of this
cylinder. The release members and the sensors are so
arranged and designed that the first sensor generates a
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first signal at the moment at which the beginning of a
printing saddle passes a connecting line between the
axes of the two cylinders, and the second sensor
generates a second signal at the moment at which the
end of the printing saddle cooperating with said
printing saddle and belonging to the second cylinder
passes the connecting line. The signals and a time
span between the two signals represent measured values
for the angular position and the circumferential length
of the printing zone, and are used for controlling
movement of the paper web in the pilgrim-step mode
during passage of an impression-free zone.
The invention is explained in more detail by
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means of an exemplary embodiment with reference to the
drawings.
As already mentioned, Figures 1 and 2 illustrate
the printing operation of a known sheet-fed printing unit
during the printing of a paper web which is transported
in the pilgrim-step mode, and
Figures 3 and 4 illustrate a printing unit
according to the invention and its operation,
Figure 3 showing the commencement of a printing
operation and Figure 4 the end of this printing
operation, that is to say after the two cylinders have
rotated through an angle corresponding to the printing
zones D.
Description of the preferred embodiment
The two cylinders 1 and 4 illustrated in Figures
3 and 4 are, for example, the plate cylinder 4 and the
impression cylinder 1 of an intaglio printing unit. The
direction of rotation of the cylinders is indicated by
curved arrows. The two cylinders have three printing
saddles 2 and 5 of equal size and .with the
circumferential length S which are arranged equally
distantly along their circumference and -which are
separated from one another by cylinder pits 3 and 6 with
the circumferential length G. Printing coverings are
mounted on the printing saddles 2 of 'the impression
cylinder 1 and intaglio printing plates are mounted on
the printing saddles 5 of the plate cylinder 4. The
printing image on the printing plates has a
circumferential length B which is smaller than the length
S of the respective printing saddle 5.
In the example under consideration, the printing
plates are mounted in such a way that the end E of the
printing image coincides with that end of the printing
saddle 5 at the rear in the direction of rotation, while
the start A of the printing image B is offset corres-
pondingly relative to the start of the printing saddle 5.
As illustrated, the impression cylinder 1 ie set
angularly relative to the plate cylinder 4 so that the
printing saddles 2 and 5 cooperating in the printing nip
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and belonging to the two cylinders are offset relative to
one another in such a way that the circumferential length
D of the printing zone, in which the cooperating printing
saddles clamp the paper web 7, exactly includes only the
format to be printed, that is to say the circumferential
length B of the printing image. In practice, of course,
the printing zone must be slightly longer than the
printing image so as to allow for the free edges of the
printing images applied to the paper web.
According to Figures 3 and 4, that end of the
printing saddle 2 of the impression cylinder 1 at the
front in the direction of rotation is offset relative to
the front end of the printing saddle 5 by the amount of
a portion which is equal to the difference S - B. As
mentioned, in practice, the size of this offset is very
slightly smaller.
Since, during the printing operation, as a result
of this arrangement of these two cylinders 1 and 4, the
clamping region of the web is shortened essentially to
the circumferential length B of the format to be printed
and consequently the print-free zone is enlarged to the
length G + (S - B), more time is available for-retracting
the paper web, that is to say for deceleration and
acceleration. Moreover, the paper web does not need to be
additionally retracted, since it is freed immediately
after the end of the printing image and can be decel-
erated. It is therefore essential that the paper web be
clamped only directly in front of the start A of the
printing image and be freed again immediately after the
end E of the printing image. The advantage of this is
that, in the case of constant deceleration and accel-
eration moments which are drive-related, the machine
speed can be increased by means of the lengthened print-
free zone. The shorter repeat length in the case of a
small printing image can therefore be compensated by a
higher speed. The smaller the length B of the printing
image, the larger the print-free zone which is available
for retracting the correspondingly large paper-web
portion.
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A further advantage is that, during a printing
operation, no additional paper-web portion is pressed
outside the printing image and therefore double pressing
according to the doubly pressed portion P of the paper
web according to Figure 2 is absent, thereby preventing
the quality from being impaired.
In the example according to Figures 3 and 4, the
cylinder 1 could also be the plate cyclinder and the
cylinder 4 the impression cylinder. In this case, the
start A of the printing image coincides with that end of
the printing saddle of the plate cylinder at the front in
the direction of rotation, and it is this plate cylinder
which is offset opposite to the direction of rotation in
relation to the impression cylinder.
In order to utilize fully the above-described
advantageous effects of an offset of the two cooperating
cylinders, therefore, the printing image on a printing
plate must either with its start A coincide with the
start of the respective printing saddle or else with its
end E coincide with the end of the respective printing
saddle.
The embodiment illustrated in Figures 3 and 4 can
also be an offset printing unit with two cooperating
blanket cylinders, by means of which either a one-sided
offset print can be produced With an inking of only one
blanket cylinder or a recto/verso print can be produced
with an inking of both blanket cylinders. All the
advantages mentioned above also apply to an offset
printing unit of this type.
The length D and the angular position of the
printing zone can be determined automatically. For this
purpose, in the example according to Figures 3 and 4, a
release member R is fastened to the circumference of the
impression cylinder 1 in such a way that it moves past a
fixedly installed initiator or sensor I, responding to
this member, exactly when the start of the printing zone,
that is to say the start A of the printing image, passes
the connecting line between the axes of the two cylinders
1 and 4; this position is shown in Figure 3. The response
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of the sensor I results in the generation of a signal
which, in a counter controlled by a rotary transmitter,
fastened to the plate cylinder 5, in the form of a pulse
generator, fixes a count and stores this. A release
member L is likewise fastened to the plate cylinder 5 in
such a way that it passes a fixedly installed initiator
or sensor M exactly at the moment when the end of the
printing zone of the plate cylinder, that is to say the
end E of the printing image, passes said connecting line;
this position is shown in Figure 4. The signal generated
by the sensor M causes the storage of a second count.
Said two counts serve as reference values for the control
of the drawing rollers which move the paper web 7 in the
pilgrim-step mode. The difference between the two counts
ie a measure of the printing zone actually effective and
the print-free zone, in which the paper web is free. This
measure is used to calculate the optimum deceleration and
acceleration during the pilgrim step, by means of which
the paper web is retracted and accelerated again, in
order to guarantee a gap-free printing of the web. The
sensors I and M can, for example, be inductive or optical
sensors, in the case of ari inductive sensor, the release
member FC or L being, for example, a steel block.
In general, therefore, the two signals and the
time span between the two signals represent measured
quantities which, at a given rotational speed of the
cylinders 1 and 4, determine respectively the angular
position and the length of the printing zone.
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