Method and device for detecting and controlling the printing pressure in flexographic machines

A method and a device for detecting and controlling the printing pressure for a flexographic machine having at least one printing group comprising first ink supply means, a second cylinder capable of removing a quantity of ink from said first supply means, a third cylinder rotatable and provided on its periphery with at least one covering element, the peripheral surface of which is defined by portions reproducing the motif to be printed, and a fourth cylinder defining, together with the third cylinder, a passage for the transit of a strip of material on which printing is to be performed; the cylindrical surface of the third cylinder has connected to it at least one sensor element for emitting signals proportional to the forces with which the sensor element itself is stressed during the course of contact of the portion of the third cylinder with the strip and with the second cylinder.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and device for detecting and 
controlling the printing pressure, in particular, in flexographic 
machines. 
As is known, flexographic printing machines are usually employed for 
performing printing, in one or more colours, on films of plastic material, 
paper or cardboard. For the application of each colour the flexographic 
machines are usually provided with a first rotating inking roll, or 
fountain roll, which is partially immersed inside a container with ink 
inside it, and a second rotating screened roll, or anilox cylinder, which 
is substantially tangential to the fountain roll and is able to remove at 
each rotation, during use, a given quantity of ink from the fountain roll 
itself, a third rotating plate-carrying roll, or plate cylinder, which is 
provided on its periphery with at least one printing plate, this plate 
consisting of lowered surface portions and raised surface portions 
reproducing the motif to be printed, being arranged substantially 
tangential to the anilox cylinder and capable, at each rotation, of 
retaining on the surface of the raised portions themselves a given 
quantity of ink supplied by the anilox cylinder, and a fourth printing 
roll, or impression cylinder, arranged substantially tangential to the 
plate cylinder and defining, together with the plate cylinder itself, a 
narrow passage intended for the transit of a strip of material on which 
printing is to be performed. 
According to another embodiment of the known type, the fountain roll, the 
ink container and the anilox cylinder may be replaced by a group 
consisting of an ink container closed by doctor blades and an anilox 
cylinder which removes the quantity of necessary ink directly from the 
container. 
The mutual distance existing between the plate cylinder and the impression 
cylinder is considered to be a particularly critical parameter since the 
result of the printing operation depends on this distance: if this 
distance differs appreciably from an optimum value, the printing operation 
performed is of mediocre quality. In practice, this distance may be 
modified, during operation of the flexographic machine, by temperature 
variations of the said cylinders, by wear of the cylinders themselves and 
by deformations in the structure which supports the cylinders themselves, 
and the optimum value of this distance varies with variation of the said 
strip of material and the subject to be printed. 
In order to verify the printing quality it is known to use telecamera 
equipment. This equipment, however, in addition to being very costly, in 
the event of defective printing is unable to eliminate the cause of the 
malfunction, but merely indicates the malfunction itself to an operator. 
In any case, the operator must modify manually the distance between the 
said cylinders by means of long and extremely critical operations which 
require a considerable amount of experience on the part of the operator 
him/herself, also because the procedures for carrying them out depend on 
the material on which the printing operations are performed and the design 
to be printed. 
These manual adjustments are also required when at the start of a printing 
operation, for example with a new plate, the distances between the plate 
cylinder and the impression cylinder must be set. 
A further cause of uncertainty able to prevent optimum operation-of the 
flexographic machines in question arises from the fact that the periphery 
of the said plate cylinder is defined by at least one printing plate which 
is fixed to the plate cylinder itself by means of thin bi-adhesive film. 
This film is subject to variations in thickness following temperature 
changes and also following the printing action of the cylinder, thereby 
influencing the printing quality. 
A further critical parameter of the said printing machines is the distance 
which the anilox cylinder and the plate cylinder have between them, since 
correct inking of the plate also depends on this distance. It is obvious 
that these distances between the plate cylinder and impression cylinder 
and between the plate cylinder and anilox cylinder are influenced by too 
many variables and parameters which in practice are difficult to control. 
Consequently, an idea which is proposed is that of changing the parameter 
on the basis of which detection and control are performed. 
In fact, since it is difficult, if not impossible, to constantly control 
the optimum distance between the cylinders, the parameter "distance 
between the cylinders" has been replaced by the pressure existing between 
the plate cylinder and impression cylinder, namely, instead of controlling 
the position of the cylinders it is proposed to control directly the 
printing pressure. 
Moreover, it is proposed to detect also the pressure existing between the 
plate cylinder and the anilox cylinder, namely the inking pressure. 
In fact, it has been discovered that detection of the aforementioned 
pressures and controlling thereof within suitable values enable 
optimum-quality prints to be obtained. 
In this way all the variables which modified in an unpredictable manner the 
distance between the plate cylinders and impression cylinder and anilox 
cylinder are overcome, since the printing pressure and the inking pressure 
are controlled directly, these being the parameters which influence most 
of all the printing quality. 
The main problem to be solved with regard to the printing methods in 
flexographic machines of the known type consists in avoiding variations in 
the printing quality due to variations in the temperature conditions, the 
bi-adhesive film for fixing the plate, wear of the plate and wear of the 
flexographic machine itself and any other parameter which modifies the 
optimum printing conditions. 
SUMMARY OF THE INVENTION 
The solution to the problem is obtained by the features of the 
characterizing part of the first claim. 
The solution to the problem provides a printing method for flexographic 
machines such that the operator is able to use easily at least one 
parameter for maintaining the optimum printing conditions. The parameter 
used is at least the printing pressure. 
The solution to the problem provides, moreover, a device for detecting the 
printing pressure in flexographic machines. 
The parameter of the printing pressure is obtained by providing the 
cylindrical surface of the said plate cylinder with at least one sensor 
element designed to emit electrical signals proportional to the force with 
which the sensor element itself is stressed during contact of the portion 
of the plate cylinder with which it is associated, with the strip adjacent 
to the impression cylinder. 
According to a preferred embodiment, the method and the device also 
envisage detecting the force with which the sensor element itself is 
stressed during contact with the said anilox cylinder. 
According to a further embodiment, the method and the device envisage using 
the signals from the sensor element in order to modify automatically the 
mutual position of the cylinders so as to keep the forces within the 
optimum values.

DESCRIPTION OF THE PREFERRED EMBODIMENTS. 
In accordance with the accompanying figures, and in particular FIG. 1, i1 
denotes in its entirety a flexographic machine, only some of the 
components of which intended for the monochromatic printing of a strip 2 
of material are shown in a schematic manner. 
In the description which follows reference will be made to only the 
components necessary for carrying out monochromatic printing, it being 
obvious to a person skilled in the art that these components, in the case 
of multiple-colour printing, must be present for each colour present in 
the graphic motifs imprinted on the strip 2. 
According to the embodiment shown in FIG. 1, the flexographic machine 1 
comprises a fountain roll 3 partially immersed inside a container 4 with 
ink inside it and rotatable clockwise about a horizontal axis, an anilox 
cylinder 5 rotatable in an anti-clockwise direction, substantially 
tangential to the roll 3 and capable of removing, at each rotation, a 
quantity of ink from the roll 3 itself. 
Alternatively, according to the embodiment shown in FIG. 2, the 
flexographic machine 1 comprises an ink container 13 provided with doctor 
blades from which an anilox cylinder 5 rotatable in an anti-clockwise 
direction removes, at each rotation, a quantity of ink directly from the 
container 13 itself. 
The flexographic machine 1 comprises moreover a plate cylinder 6 rotatable 
in a clockwise direction, substantially tangential to the cylinder 5 and 
provided on its periphery with a plurality (three in FIG. 1) of covering 
or plate elements 7, the peripheral surfaces of which are defined by 
lowered surface portions and by raised surface portions reproducing the 
motif to be printed. The cylinder 6 is capable during use, at each 
rotation, of retaining on the said raised surface portions a given quanity 
of ink supplied by the cylinder 5. The covering elements 7 are fixed to 
the cylinder 6, in a known manner, by means of a strip of bi-adhesive 
material not shown. 
The flexographic machine 1 comprises moreover an impression cylinder 8 
rotatable in an anti-clockwise direction, arranged substantially 
tangential to the cylinder 6 and defining, together with the cylinder 6 
itself, a narrow passage intended for the transit, in a direction 
indicated by an arrow F, of the said strip 2 of sheet material on which 
printing is to be performed. 
In accordance with that shown in FIGS. 2 and 3, the cylinder 6 is keyed 
onto a horizontal shaft 9 and has formed on its cylindrical surface a 
recess 10 which extends in a direction parallel to the axis of the 
cylinder 6 itself and which preferably has a cross-section in the form of 
a "dovetail" with its vertex arranged on the outside of the cylinder 6. 
This recess 10 stably houses inside it two sensors 11 and 12 which are 
arranged respectively on the left-hand side and right-hand side in FIG. 2 
and which are fixed inside the recess 10 for example by a resin filling 
material which surrounds it in a substantially complete manner. These 
sensors 11, 12 may be of any type able to provide a signal following 
application of a force to the sensors themselves and in particular may be 
of the piezo-resistive or piezo-electric type or may consist of strain 
gauges. The signal output by the sensors 11, 12 may be of any type and, in 
particular, of the electric or optical type. 
In accordance with that shown in FIG. 4, the sensors 11, 12 are connected 
to respective signal transmission cables 14 (only one of which is shown) 
which are housed inside a cavity 15 formed in an end wall of the recess 10 
and are able to send the respective signals towards the outside of the 
cylinder 6 by means of a commutator 16 of the known type keyed onto the 
shaft 9. 
According to two variants (not illustrated) of the present invention, the 
recess 10 may house inside it a number of sensors greater than two, or 
only one sensor. 
According to a first embodiment, the sensors 11, 12 (see also FIG. 6) are 
covered by one of the covering elements 7 connected to the cylinder 6, 
while according to a further embodiment of the present invention, which 
will be considered below, the sensors 11, 12 are connected (FIG. 7) to a 
portion of the cylinder 6 itself adjacent to the zone situated between two 
contiguous covering elements 7. With reference, in particular, to FIGS. 5 
and 6, each of the edge zones of the cylindrical surface of the cylinder 6 
has arranged next to it a device 17 for activating the emission of a 
reference signal, functioning of which will be explained below. 
Each device 17 comprises an idle roller 18 rotatable about an axis parallel 
to the axis of rotation of the cylinder 6 and having its cylindrical 
peripheral surface arranged in contact with a portion of the cylindrical 
peripheral surface of the cylinder 6 itself. 
This roller 18 is supported by one end of a substantially horizontal bar 
19, a middle portion of which is hinged on a fixed pivot 20 parallel to 
the axis of the cylinder 6 and the other end of which has a bottom portion 
adjacent to a top portion of a resilient element consisting of a helical 
spring 21 operating by means of compression, the other end of which rests 
against a fixed opposition element 22. 
In accordance with that shown in FIG. 6 and as will become clear below, 
each of the sensors 11, 12 is connected to a respective memory device 23, 
of a type known per se, which is able to store, at each rotation of the 
cylinder 6, a reference signal emitted by the sensor 11, 12 itself when 
this sensor 11, 12 reaches a condition substantially in contact with a 
roller 18; an output of each memory device 23 is connected to an input of 
an associated comparator device 24, a second input of which is connected 
directly to the sensor 11, 12 and an output of which, in a simplified 
embodiment of the present invention, is connected, via elements, 
preferably of the electronic type, which are obvious to a person skilled 
in the art in the light of that stated in the present description, to a 
device 25 for displaying messages indicating characteristics of the 
signals from the comparator device 24 itself. 
The signals which the sensor 11, 12 sends, at each rotation of the cylinder 
6, to the associated memory device 23, and, respectively, to the 
associated comparator device 24 are proportional, as will emerge clearly 
below, to the force to which the sensor 11, 12 itself is subjected at 
different moments during operation of the flexographic machine 1. 
During use, in accordance with the embodiment shown in FIG. 2, the 
container 13 constantly transfers ink to the peripheral surface of the 
cylinder 5; this cylinder 5 in turn transfers the ink to the covering 
elements 7 of the cylinder 6, which imprint in succession the motif to be 
printed on the strip 2, with cooperation of the cylinder 8 which presses 
the strip 2 itself against the cylinder 6. 
During the course of each rotation of the cylinder 6, the sensors 11, 12 
associated therewith make contact in succession, substantially, with the 
cylinder 5, the rollers 18 arranged next to the cylinder 6 itself, and the 
strip 2 adjacent to the cylinder 8. During the course of contact of the 
sensors 11, 12 with the rollers 18, the sensors 11, 12 themselves, which 
are subjected to compression with a known force depending on the 
characteristics of the helical springs 21, act as signal emitting elements 
and thus send a reference signal to the associated memory devices 23, 
which is retained by the memory devices 23 themselves and permanently sent 
to the associated comparator devices 24 until a following signal is 
received. Moreover, when the sensors 11, 12 come into the vicinity of the 
cylinder 5, they are compressed by means of the covering element 7 located 
next to them and send to the comparator device 24 respectively associated 
with them a corresponding signal which is a finction of the compression to 
which the sensors 11, 12 themselves have been subjected. Similarly, when 
the sensors 11, 12 come into the vicinity of the cylinder 8, they are 
compressed by means of the covering element 7 located next to them, and 
send to the associated comparator device 24 an associated signal which is 
a function of the compression to which the sensors 11, 12 have been 
subjected. It should be noted that the sensors 11, 12 subjected to 
compression emit respective signals porportional to the mean of the 
compressive forces applied to the various zones of the sensors 11, 12 
themselves. 
The reference signals arising from the described contact of the rollers 18 
with the cylinder 6, at each rotation of the cylinder 6, are compared with 
those emitted when the condition of substantial contact of the sensors 11, 
12 with the cylinders 5 and 8 is reached. As a consequence of that stated 
above, each comparator device 24, at each rotation of the cylinder 6, 
sends to the display device 25 four signals indicating the force with 
which the sensors 11, 12 have. been stressed by the portions of the 
cylinders 5 and 8 momentarily adjacent to them during the course of each 
operating cycle of the flexographic machine 1. 
These signals are displayed, by means of the display device 25, for example 
in numerical form, and this information provides a useful aid for an 
operator in charge of adjusting the flexographic machine 1. On the basis 
of these signals, in fact, the operator is able to operate each of the two 
support elements 26 (FIG. 8) of each of the cylinders 5 and 6, varying in 
a manner known per se the positions of the cylinders 5 and 6 themselves 
and modifying the force with which the cylinders 5, 6 and 8 interact with 
one another. 
In the case where the compression to which the sensors 11, 12 are subjected 
should differ excessively from a compression considered to be optimum, the 
comparator devices 24 could cause automatic stoppage of the flexographic 
machine 1. 
In the said further embodiment of the present invention, in which the 
sensors 11, 12 are connected (FIG. 7) to a portion of the cylinder 6 
adjacent to the zone located between two contiguous covering elements 7, 
the sensors 11, 12 themselves are covered by respective small plates 27 
arranged in the vicinity of respective edge zones of the cylindrical 
surface of the cylinder 6; these small plates 27 are connected to the 
periphery of the cylinder 6 and have a thickness and a consistency 
substantially the same as those of the covering elements 7. 
These small plates 27 are affected by the action of the said rollers 18 in 
the manner described above and act on the sensors 11, 12 in a manner 
similar to that considered above so as to subject the sensors 11, 12 
themselves to compression during the course of contact of the small plates 
with the peripheral cylindrical surface of the cylinder 5 and with the 
strip 2 adjacent to the cylinder 8. The signals from the sensors 11, 12 
are used in the manner described above. 
FIG. 8 shows a further embodiment of the flexographic machine I considered 
above. 
In this embodiment of the flexographic machine 1, the support elements 26 
of the cylinders 5 and 6 are supported in a manner not shown by the base 
1' of the flexographic machine 1 and are able to translate horizontally 
either way, in a direction perpendicular to the axis of the cylinder 6, 
under the action of the respective actuating elements 28 subject to the 
control of the said comparator devices 24. 
During use, in the case where a comparator device 24, following the 
comparison between a reference signal from a memory device 23 and the 
signal from one of the sensors 11, 12 associated with the memory device 23 
itself, should encounter an excessive divergency between an optimum 
compression value and the compression to which the sensor 11, 12 itself is 
subjected, for example, during contact between the cylinder 7 and the 
cylinder 8 (with the strip 2 arranged in between), the actuating element 
28 associated with the support element 26 of the cylinder 6 situated on 
the same side as the sensor 11, 12 causes horizontal translation of the 
support element 26 itself so as to regulate the compression in question. 
The same applies to the other sensor 11, 12 and to the other support 
element 26 of the cylinder 6, and with similar procedures the comparator 
devices 24 respectively associated with the sensors 11, 12 ensure that the 
support elements 26 of the cylinder 5 are kept in optimum positions. 
It should be noted that, according to variants, not shown, of the 
flexographic machine 1, the sensors 11 and 12 could have any form and 
extension and could be arranged in positions not coinciding with a 
generatrix of the cylinder 6. 
It should be noted, moreover, that the signals from the sensors 11 and 12 
could be sent to the comparator devices 24 without the use of the said 
commutator 16; these signals, for example, could be sent to the comparator 
devices 24 via radio or in the form of signals of the optical, acoustic or 
any other type suitable for the purpose, by means of a transmitter device 
schematically shown in the form of a block 14' associated with the wires 
14 (FIG. 4). 
It should be noted, finally, that the said devices 17 could be omitted, 
with the said reference signals being able to be emitted by signal emitter 
devices of the known type, not shown. 
The invention thus conceived may be subject to numerous modifications and 
variations, all of which falling within the scope of the inventive idea. 
Moreover, all the details may be replaced by technically equivalent 
elements.