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Description  |
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The present invention relates to improvements in a printing apparatus for
printing a running web.
FIG. 1 shows a conventional color printing apparatus in which as the web 4
to be printed passes between an impression cylinder 1 and plate cylinders
2, it is printed by plates 3 mounted on the plate cylinders. The
impression cylinder and the plate cylinders are driven from a single motor
through a geared transmission for synchronized printing. Since the
diameter of the plate cylinders is determined by the printing length which
is the length of the plate, each time the printing length changes, new
plate cylinders having a circumference corresponding to the new printing
length become necessary. This increases the printing cost. Further, each
time the printing length changes, heavy plate cylinders have to be
replaced with new ones. New plate cylinders with new plates mounted
thereon have to be mounted on the impression cylinder with a suitable
contact pressure. This is very troublesome work. Further, fine adjustments
of the transmission were necessary for phase tuning between the plate
cylinders for accurate register. Another disadvantage is that a lot of
plate cylinders have to be maintained.
An object of the present invention is to provide an improved printing
apparatus which obviates the necessity of preparing different plate
cylinders for different printing lengths.
In accordance with the present invention, there is provided a printing
apparatus for printing a running web, comprising at least one printing
unit having a plate cylinder having a circumference (B) longer than double
the printing length (L) and adapted to carry two plates, a pair of
impression cylinders mounted on the plate cylinder to press the web
against the plates on the plate cylinder in diametrically opposite
positions, and means for guiding the web so that the length of the web
existing between one contact point (between one of the impression
cylinders and the plate cylinder) and the other contact point (between the
other of the impression cylinders and the plate cylinder) will be three or
more odd number of times as long as the printing length, first drive means
for driving the web, second drive means for driving the plate cylinders,
and control means for controlling the first or second drive means so that
during the period from the print start to the print end, the former will
be equal to the latter and, from the print end to the next print start,
the plate cylinder will move for a distance equal to (B/2-L) while the web
moves for a distance equal to the printing length (L).
Other objects and features of the present invention will become apparent
from the following description taken with reference to the accompanying
drawings, in which:
FIG. 1 is a schematic view of a prior art color printing machine;
FIG. 2 is a block diagram showing a preferred embodiment;
FIG. 3 is a graph showing how the speed of the plate cylinder changes; and
FIGS. 4a-4f are views sequentially showing how the web is printed
continuously.
Referring to FIG. 2, a web 10 passes between a pair of feed rolls 11,
around a first paper roll 12, through printing units 13.sub.1, 13.sub.2,
13.sub.3 and 13.sub.4, around a second paper roll 14 and between a pair of
pull rolls 15, and is fed to the next station.
Between the adjacent printing units there are web length adjusting units
16.sub.1, 16.sub.2 and 16.sub.3 for adjusting the length of web existing
between the adjacent printing units to the printing length multiplied by
an integer. Each web length adjusting unit comprises a vertically movable
adjuster roll 17. By adjusting the height or vertical position of the
adjuster roll, the length of web existing between the adjacent printing
units is adjusted to the printing length multiplied by an integer.
Each printing unit 13.sub.1 -13.sub.4 comprises a plate cylinder 19 having
a circumference longer than double the printing length and adapted to
carry two plates 18 in diametrically opposite positions, a pair of
impression cylinders 20a and 20b adapted to press the web 10 against the
plates 18 on the plate cylinder 19, a pair of adjuster rolls 21, driers 22
disposed downstream of the impression cylinders 20a and 20b for drying the
ink applied by the plates 18 to the surface of the web, and ink rolls 24
for applying ink in ink reservoirs 23 to the plates 18 on the plate
cylinder 19. After having been printed by the plates 18, the web 10 is
dried by the driers 22 and discharged from the printing station. The
adjuster rolls 21 are in such a position that the length of web 10
existing between one contact point (between one of the impression
cylinders and the plate cylinder) and the other contact point (between the
other of the impression cylinders and the plate cylinder) will be three or
more odd number of times as long as the printing length. (Actually, the
plate cylinder and the impression cylinders are not in direct contact but
through the web and the plate.)
The plate cylinders 19 in the printing units 13.sub.1 -13.sub.4 are coupled
to a motor 25 through a transmission (not shown) having gears, sprockets
and/or chains for synchronized driving. A tachometer generator 26 and a
first pulse generator 27 for detecting the amount of revolutions of the
plate cylinder 19 are connected to the motor 25 for the plate cylinders
(hereinafter referred to as the cylinder motor).
The feed rolls 11 and the pull rolls 15 are connected to a web driving
motor 28 through a transmission (not shown) having gears, sprockets and/or
chains so that they will turn at the same peripheral speed. A second pulse
generator 29 is connected to one of these rolls (to the pull roll 15 in
the embodiment of FIG. 2) to detect the length for which the web 10 has
run. A print start detector 30 is provided adjacent to the plate cylinder
19 of one of the printing units 13.sub.1 -13.sub.4 in order to detect the
start of printing.
The web length adjusting unit 16.sub.n (n is 1, 2 and 3) serves to adjust
the length existing between the contact point (between the plate cylinder
19 of the printing unit 13.sub.n and its impression cylinder 20b) and the
contact point (between the plate cylinder 19 of the printing unit
13.sub.n+1 and its impression cylinder 20a) to the printing length
multiplied by an integer.
On a setter 31, the circumference B of the plate cylinder 19 (with the
thickness of a plate taken into consideration) and the printing length L
(that is usually the length of the plate 18) can be set. Since the value B
is fixed once the size of the plate cylinder 19 has been determined, only
the printing length L may be settable.
In response to the print start signal from the print start detector 30, a
preset counter 32 reads the printing length L and simultaneously starts to
count a first pulse signal .phi..sub.1 from the first pulse generator 27.
When its count becomes equal to the printing length L, the preset counter
will give a print end signal to show that the printing with the plate 18
is complete.
A computer 33 reads a compensation value 2L-B/2 in response to the print
end signal, adds the first pulse signal .phi..sub.1 from the first pulse
generator 27 and subtracts a second pulse signal .phi..sub.2 from the
second pulse generator 29. Namely, the computer 33 performs computation
2L-B/2-.phi..sub.2 +.phi..sub.1. The result M of computation is converted
by a D/A converter 34 to an analog error voltage V.sub.c.
An F/V converter 35 receives the pulse signal .phi..sub.2 from the second
pulse generator 29 and outputs a line speed voltage V.sub.A proportional
to the frequency of the pulse signal .phi..sub.2. An analog computer 36
subtracts the error voltage V.sub.C from the line speed voltage V.sub.A to
obtain a speed command voltage V.sub.o (=V.sub.A -V.sub.C). A motor
controller 37 compares the speed command voltage V.sub.O with a speed
voltage V.sub.B from the tachometer generator 26 and gives a motor voltage
V.sub.D to the motor 25 for the plate cylinders so that it will be driven
at just the speed command voltage V.sub.O.
Next, operation will be described below in detail.
Firstly, the circumference B of the plate cylinder 19 and the printing
length L are set in the setter 31. The adjuster rolls 17 in the web length
adjusting units 16.sub.1 -16.sub.3 are moved to adjust the length of the
web existing between the adjacent ones of the printing units 13.sub.1
-13.sub.4 to the printing length multiplied by an integer. Two plates 18
are then set on the plate cylinder 19 of each printing unit in
diametrically opposite directions, and ink of a required color is filled
in the ink reservoirs 23 in each printing unit.
The speed for the web driving motor 28 is set on a speed setter 38. A
machine switch is turned on to start printing. The motor 28 will be driven
at the speed preset on the speed setter 38, so that the web 10 will run at
the preset constant speed. The F/V converter 35 gives a line speed voltage
V.sub.A proportional to the web speed.
On the other hand, the cylinder motor 25 is controlled by the computer 33,
etc. so that the peripheral speed of the plate cylinders 19 (which is,
strictly speaking, the peripheral speed of the plate 18, but hereinafter
referred to as the peripheral speed of the plate cylinder for simplicity)
will be as shown on the graph of FIG. 3. Namely, it will be controlled to
be equal to the web speed during the period from the print start point
T.sub.11 to the print end point T.sub.12. During the period, the plate
cylinder turns for a distance equal to the printing length L, that is, the
length of the plate 18. During the period from the print end point
T.sub.12 to the next print start point T.sub.21, the web 10 runs for a
distance equal to the printing length L whereas each plate cylinder 19 is
decelerated and accelerated until its peripheral speed again becomes equal
to the web speed at the next print start point T.sub.21 at latest, so that
it will turn for a distance equal to (B/2-L) which is the peripheral gap
between the two plates 18.
The arrangement is such that the web is printed for the printing length L
while it passes between one of the two plates 18 and one impression
cylinder 20a of each printing unit, and runs unprinted for a distance
equal to the printing length L, and is printed for the printing length L
while it passes between the other of the two plates 18 and the impression
cylinder 20a. Thus, on the web passed under the first impression cylinder
20a, the printed portions and the non-printed portions will appear
alternately. However, the portions that were not printed at the side of
the impression cylinder 20a are printed without fail at the side of the
other impression cylinder 20b, since the length of the web existing
between one contact point (between the plate cylinder 19 and one
impression cylinder 20a) and the other contact point (between the plate
cylinder 19 and the other impression cylinder 20b) is three or more odd
number of times as long as the printing length L. This results that the
web is printed continuously without any gaps, as will be best understood
from FIG. 4, in which A and B designate two identical plates and A.sub.1,
A.sub.2, A.sub.3 and B.sub.1, B.sub.2, B.sub.3 are the portions of web
printed with the plate A and the plate B, respectively. Some unprinted
portions appear at the beginning, but some time after, the web is printed
continuously without leaving any portions unprinted.
Also, since the length of the web existing between the adjacent printing
units is the printing length L multiplied by an integer, the web is
printed with desired colors overlapped with accurate register. The
abovesaid control assures that the web has been multicolor printed
continuously when it leaves the pull rolls 15.
It will be described in more detail how the peripheral speed of the plate
cylinder 19 is controlled.
When the printing with the plate 18 ends, a print end signal is given to
the computer 33 by the preset counter 32. In response to it, the computer
33 will read the printing length L and the circumference B of each plate
cylinder 19 and starts the computation (2L-B/2-.phi..sub.2 +.phi..sub.1).
At the start, the result M of computation should be sufficiently large.
Thus, the speed command voltage V.sub.O =V.sub.A -V.sub.C <O. Because the
cylinder motor 25 is controlled by the voltage V.sub.O, the plate
cylinders 19 will be decelerated. Accordingly, the first pulse signal
.phi..sub.1 decreases whereas the second pulse signal .phi..sub.2 is
constant because the web 10 is fed by the motor 28 at a predetermined
constant speed. Therefore, the result M of computation and thus the error
voltage V.sub.C gradually decreases. Accordingly, the speed command
voltage V.sub.O will become from negative to positive, so that the
cylinder motor 25 will be accelerated until the peripheral speed of the
plate cylinder becomes again equal to the web speed at the next print
start point (where the print start detector 30 operates) at latest. While
such an equi-speed state is maintained, the result M of computation and
thus the error voltage V.sub.C will remain zero.
If the peripheral speed of the plate cylinder 19 becomes higher than the
web speed, the first pulse signal .phi..sub.1 will increase, so that the
result M of computation and thus the error voltage V.sub.C will become
larger than zero and the speed command voltage V.sub.O (=V.sub.A -V.sub.C)
will decrease by the error voltage V.sub.C. As a result, the cylinder
motor 25 will be decelerated until the peripheral speed of the plate
cylinder becomes again equal to the web speed. Thus, the motor 25 is
controlled so that M and V.sub.C will be kept at zero.
If the peripheral speed of the plate cylinder 19 becomes lower than the web
speed, the first pulse signal .phi..sub.1 will decrease, so that the
result M of computation and thus the error voltage V.sub.C will become
smaller than zero and the speed command signal V.sub.O will increase by
the error voltage V.sub.C. Therefore, the cylinder motor 25 will be
accelerated until the cylinder speed becomes again equal to the web speed.
The motor 25 is controlled so that M and V.sub.C will be kept at zero.
Thus, as shown in FIG. 3, during the period from the print start T.sub.11
to the print end T.sub.12, the web 10 moves for a distance equal to the
printing length L whereas the plate cylinders 19 are controlled on the
basis of the line speed voltage V.sub.A to be driven at the same speed as
the web speed. The peripheral distance covered by the plate cylinder
during the period, too, will be equal to the printing length L, which
corresponds to the area of the rectangle T.sub.11 T.sub.12 PO. During the
period from the print end T.sub.12 to the next print start T.sub.21, the
web moves for a distance equal to the printing length whereas the plate
cylinders 19 are controlled to be firstly decelerated as shown at P-Q and
then accelerated as shown at Q-R so that they will move for a peripheral
distance equal to (B/2-L) which is the length of the gap between the two
plates 18 and corresponds to the area of a pentagon T.sub.12 T.sub.21 RQP.
In other words, during the period from the print start T.sub.11 to the next
print start T.sub.21, the web 10 moves for a distance equal to 2L
(corresponds to the area of a square T.sub.11 T.sub.21 RO) whereas the
plate cylinders 19 are controlled to turn for a peripheral length equal to
B/2 (corresponds to the area of a hexagon T.sub.11 T.sub.21 RQPO)
FIG. 3 shows a mere example of change of the peripheral speed of the plate
cylinders 19. How it changes depends on the printing length L (that is,
the length of the plates) relative to the circumference B of the plate
cylinder 19. It may be controlled otherwise so long as the distance
covered by the plate cylinders during the period from the print end
T.sub.12 to the next print start T.sub.21 will be exactly equal to
(B/2-L). For example, the plate cylinders may be maintained at the web
speed for some time even after the print end T.sub.12. Or, they may be
kept at a low speed or kept stopped for some time after deceleration. Or,
they may attain again the web speed some time before the next print start
T.sub.21.
Further, if B/2-L<L (namely, 2L-B/2<O), the plate cylinders 19 are firstly
accelerated and then decelerated after the print end until their speed
becomes again equal to the web speed, so that the distance covered by the
plate cylinders 19 during the period from the print end T.sub.12 to the
next print start T.sub.21 will be (B/2-L).
In the embodiment of FIG. 2, the peripheral speed of the plate cylinders 19
are controlled with the web speed kept constant. However, the web speed
may be controlled with the speed of the plate cylinders kept constant.
Such a control will be described below.
This manner of control differs from the abovementioned embodiment in that
the F/V converter 35 receives the pulse signal .phi..sub.1 proportional to
the number of revolutions of the plate cylinders, not the pulse signal
.phi..sub.2 proportional to the distance covered by the web, and gives to
the analog computer 36 the line speed voltage corresponding to the
frequency of the pulse signal .phi..sub.1. Also, the tachometer generator
26 and the pulse generater 27 are connected not to the cylinder motor 25
but to the web driving motor 28. In this embodiment, the web driving motor
28 is controlled with the speed command signal V.sub.O in the same manner
as in the first embodiment whereas the cylinder motor 25 is driven at a
constant speed.
Although in the preferred embodiment the print start detector 30 and the
preset counter 32 are used, they may be replaced with a print end detector
which detects the end of printing with the plate 18.
The abovesaid printing length may not be a single printing length in a
continuous printing with no blank between the adjacent printed portions,
but include any allowance for subsequent cutting, glueing or other
processing as well as the actually printed portion.
Although in the preferred embodiment a plurality of printing units are used
for multicolor printing, a single printing unit will do for monochrome
printing. The apparatus according to the present invention is applicable
to monochrome printing, too.
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