U.S. patent number 3,554,123 [Application Number 04/691,039] was granted by the patent office on 1971-01-12 for control of color densities and tones in multicolor printing.
This patent grant is currently assigned to Hurletron Incorporated. Invention is credited to Bernard E. Lewallen.
United States Patent |
3,554,123 |
Lewallen |
January 12, 1971 |
CONTROL OF COLOR DENSITIES AND TONES IN MULTICOLOR PRINTING
Abstract
In web-fed, multicolor printing operations wherein individual
colors which make up the final composite copy are separately
applied to the web, each at a separate printing unit or station,
the invention provides a new and advantageous method of controlling
the color densities and tones obtained in said composite copy, so
that they closely correspond to those of the original art work or
composition to be reproduced. The method provided employs
electrical energy of controlled relatively high voltage and low
amperage which is applied in the area of each printing unit or
station wherein said individual colors are transferred to the web,
whereby to effect or assist said transfer. It further provides for
separately regulating the potential of the electrical energy
supplied to each of the separate printing units or stations, to
govern the amount of pigment there transferred to the web and
thereby independently control the density of each individual color
in the composite copy. By controlling the density of each
individual color applied to the web, the tone values of combined
colors in the copy (such as, for example, a purple formed by
overprinting blue on red) may also be controlled.
Inventors: |
Lewallen; Bernard E. (Danville,
IL) |
Assignee: |
Hurletron Incorporated
(Danville, IL)
|
Family
ID: |
24774924 |
Appl.
No.: |
04/691,039 |
Filed: |
December 15, 1967 |
Current U.S.
Class: |
101/170; 101/211;
101/152; 101/489 |
Current CPC
Class: |
B41M
1/14 (20130101); B41M 1/18 (20130101); B41M
1/42 (20130101); B41F 9/001 (20130101) |
Current International
Class: |
B41M
1/14 (20060101); B41M 1/42 (20060101); B41M
1/18 (20060101); B41M 1/00 (20060101); B41F
9/00 (20060101); B41m 001/10 () |
Field of
Search: |
;101/151,152,170,426,211 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Burr; Edgar S.
Claims
I claim:
1. In the process of applying to electrically resistive material a
pattern of work comprised of at least one color which is
transferred to said material from a patterned, pigment-carrying
surface within a transfer zone, a current diversion path being
provided to bypass the transfer zone, the method of controlling the
color density of said pattern which comprises supplying a
relatively high direct current potential across the transfer zone
in a range of from about 1,000 to 1,500 volts and adjusting the
current drawn to a value which is in excess of a predetermined
current within a range of from about 0.5 to 1.0 milliamperes which
said material is expected to normally draw, diverting current
through the current diversion path by increasing the potential
across the transfer zone to a value sufficient to effect current
diversion, inhibiting current diversion by adjusting the potential
across the transfer zone to a value below that which effects
negation of current diversion, further adjusting the potential
across the transfer zone while observing a printed copy to a value
below the last named value whereat the printed copy possess a
suitable color density to establish a potential adjustment range
between the two last-named values, readjusting the current
delivered to the transfer zone to a value immediately below that
which effects current diversion through the current diversion path,
and adjusting the potential across the transfer zone within said
potential range while observing the printed copy to vary the
colored density during printing.
Description
It is common practice, in web-fed, multicolor printing operations,
to employ a limited number of differently colored inks and to apply
each of them to the running web of paper or other material printed
upon at a separate printing unit or station. At these separate
units or stations, the printing cylinder or plate employed carries
an inked pattern corresponding to only that portion of the finished
copy which either consists of or contains one of the individually
applied colors. Combinations of the individually applied colors are
obtained by overprinting all or a part of the pattern carried by
one or more plates or cylinders upon all or a part of the pattern
carried by another plate or cylinder. For example, an area in the
finished copy which appears green may be obtained by applying to
that area both yellow and blue inks from corresponding areas of the
separate cylinders which apply yellow and blue.
To properly prepare a set of color cylinders or plates is an art as
well as a science. It involves making color separations from the
original composition, one separation for each of the different
colors to be employed in the printing, the production of a screened
resist from each color separation and the careful etching, through
each resist, into a printing plate or cylinder, the pattern carried
by that resist. In addition to careful control of the processing
conditions, this requires good judgment, based upon experience, and
considerable skill. Therefore, it is not unusual, upon proofing a
set of cylinders or plates to find that one or more of them must be
reworked by hand or completely remade before acceptable results can
be obtained in the printing.
Using rotogravure work as an example, the pattern etched into the
gravure cylinder, in intaglio, consists of a multiplicity of minute
cells which carry the ink to be applied to the web. These cells are
of varying depth, cross-sectional area and spacing in different
areas of the pattern, to give the desired shading, tone and density
of color in the pattern printed by that cylinder. In general, what
may be termed the texture of the inked pattern on a given cylinder
or plate largely determines the quantity of ink applied to the web
by that cylinder or plate, whether the type of reproduction
employed be gravure, letterpress, offset, or some other variety of
printing.
Heretofore, with a given set of plates or cylinders and a given
choice of inks, the pressman has had little latitude in altering
the density of any individual color as it is applied to the web. By
the same token, he has had little control over the tone values of
composite colors produced by separately applying individual colors
in the same area of the copy. However, there are certain expedients
which can be and are used to obtain some slight variations.
In letterpress work, shims may be inserted between the plate and
the cylinder on which it is mounted to exert greater pressure
between the web and the plate in areas where the ink transfer is
deficient. This is a time consuming, cut-and-try procedure, usually
involving prolonged down time on the press and consequent loss of
production.
In gravure work, and to a lesser extent in offset printing, the
impression pressure employed at each unit may be adjusted to give
greater or lesser contact between the web and the gravure cylinder
or offset blanket which carries the ink. However, if there is much
inequality between the impression pressures carried at different
units, resulting web tension variations cause misregister between
the separate printings.
As a last resort, to avoid remaking plates or cylinders, one or
more of the inks may be doctored by adding more of the same
pigment, a little different pigment, more or less solvent,
extenders, etc. However, when expedients such as mentioned are
necessary, the resulting printed copy is quite apt to be a
compromise between what is really wanted and what is
unacceptable.
The present invention provides a convenient and effective method of
controlling color densities and tone values in multicolor work,
during the press run. This method does not prevent the use of
previously available expedients but greatly extends the latitude of
control available to the pressman. It is not intended to obviate
careful preparatory work nor the judicious choice of inks, but
provides, during the printing operation, correction for minor
deficiencies commonly encountered in preparing plates and
cylinders. In most instances, when reasonable care and skill are
exercised in the preparatory work, this new method will produce
results superior to those otherwise obtainable from a given set of
plates or cylinders and a given choice of inks.
The method herein provided of controlling color densities and tones
involves the use of electrical energy (sometimes referred to as
electrostatic energy because of the low amperage and relatively
high voltage employed) to at least assist the transfer of ink or
other pigment to the web from the patterned surface on which it is
carried. It also provides a procedure for determining the operating
conditions which may be safely employed, particularly with respect
to the amperage and the range of voltage supplied to each printing
unit. It further provides for separately regulating the potential
of the electrical energy supplied to each printing unit, whereby to
independently control the color density in the pattern of printing
applied to the web at each printing unit.
The use of electrical or electrostatic energy in the transfer of
pigmented materials is well known particularly as applied to spray
painting. Neither is its use unknown in printing, but until quite
recently it has not been applied successfully to commercial work
such as publication, catalog and package printing. Most of the
proposed electrostatic printing systems undertake to effect the
transfer of dry, powdered pigment from a patterned surface upon
which it is deposited, or through a patterned screen, to the
surface to be printed upon through an ionized air gap, with no
physical contact between the web and the surface carrying the
pigmented pattern.
Recently, a system for electrostatically assisting the transfer of
liquid ink in otherwise conventional rotogravure printing
operations has been offered to the industry by the firm to which
the present invention is assigned. It has enjoyed rapid and
widespread adoption and has proven beneficial with a wide variety
of printing stocks. While it utilizes a printing impression (i.e.,
physical contact between the web and the printing cylinder), it
permits the use of lower impression pressure than would give
acceptable ink transfer without the electrostatic assistance
provided.
This electrostatically assisted system has been promoted and
accepted entirely on the basis that it accomplishes more complete
emptying of the ink carrying cells to give better and more uniform
ink distribution on the web. This eliminates or greatly reduces the
phenomenon known as snowflaking or skipped dots in the printed
copy, which is a rather common fault occurring when some of the
cells give up little or no ink to the web. This fault is
particularly prevalent in delicate or lightly shaded areas of the
copy and also occurs in more heavily pigmented areas when employing
either relatively rough surfaced or relatively rigid and unyielding
printing stocks, such as, for example, paperboard and uncalendered
or lightly calendered stock made from ground wood pulp (e.g.,
newsprint). The electrostatically assisted system thus permits the
use of rougher and lower priced stocks than would otherwise be
suitable for the quality of printing generally demanded in
publication, catalog and packaging color work.
The apparatus used in the aforementioned electrostatically assisted
system is suitable for conducting the process of the present
invention. Because of its much greater successful use than other
proposed apparatus for electrostatically effecting or assisting
printing operations, it is the form of apparatus preferably
employed for practice of the method of operation herein provided
and is included as a part of the accompanying drawing. However, it
is not intended to limit the invention to use with this nor with
any other specific form or type of apparatus, since the invention
involves a method of operation, rather than a means. To the extent
that other means are successfully usable in or may be readily
adapted to the practice of the invention, such use falls within the
intended scope of the invention.
When employed as herein provided, for the purpose of controlling
color densities and tones, the way in which the apparatus is
initially set up to operate and its mode of operation differ from
the setup procedure and mode of operation previously contemplated.
These differences have to do, principally, with setting the upper
limits of voltage and amperage to be employed and with separate
regulation of the voltage actually employed during the run at each
of the printing units or stations. The recommended setup procedure
and mode of operation will be described later, with reference to
the accompanying drawing.
In the accompanying diagrammatic drawing, FIG. 1 illustrates four
printing units of an otherwise conventional rotogravure printing
press embodying apparatus by means of which the method of operation
provided by the invention may be practiced.
FIG. 2 of the drawing is an enlarged longitudinal section of one
end of the impression cylinders of FIG. 1, showing the two layer
resilient covering preferably employed thereon.
FIG. 3 is an enlarged view of the end portion of one of the
conductive rollers of FIG. 1, showing one way in which they may be
electrically insulated from the frame of the press and one way in
which electrical energy may be supplied to these rollers.
FIG. 4 is essentially a circuit diagram of a power supply usable in
the practice of the invention and incorporating desirable safety
features.
FIG. 5 illustrates the general form of curves obtained by plotting
the voltage of the electrical energy applied to one of the printing
units of FIG. 1 against (in curve A) the ink density in the printed
copy produced by that unit and (in curve B) the percentage of
skipped dots in the printed copy produced by that unit.
Referring now to FIG. 1, that portion of the rotogravure press here
represented comprises the separate printing units A, B, C and D,
which are substantially identical in construction. Corresponding
parts of these separate units are designated by the same reference
numbers, suffixed by the letters a, b, c and d, respectively.
The web 1, which may be paper, paperboard, plastic film, laminates
of such materials, or any other substantially nonconductive
material, is drawn from rolls thereof on a suitable reel or unwind
stand, not shown, and passes over idler rollers 2a and 3a to the
first impression formed between printing cylinder 4a and impression
cylinder 5a. Carrying the pattern of color applied at unit A, the
web then passes over idler roller 6a to a suitable dryer
represented at 7a, where heat is applied to dry or set the ink. The
web is then directed over idler roller 8a, compensator roller 9a,
and thence, in succession, through printing units B, C and D, its
path of travel through each unit being as illustrated and described
with respect to unit A.
The compensator rollers 9a, 9b, and 9c are bidirectionally movable,
as indicated by the arrows, to controllably lengthen and shorten
the path of web travel between impressions and thus control
circumferential register between patterns separately applied to the
web at the several impressions.
At printing unit A, an ink fountain 10a is provided, into which the
lower portion of gravure cylinder 4a extends, and a suitable level
of ink, indicated at 11a, is maintained in the fountain. Cylinder
4a is rotated in the direction indicated by the arrow thereon, by
well known drive means, not shown, and the intaglio pattern which
it carries picks up ink as the cylinder rotates through the pool in
the fountain. Some ink is also picked up on the smooth, unetched
areas of the cylinder and is doctored (scraped) from these areas
before reaching the impression, by means of a suitable doctor blade
indicated at 12a. Similar fountain and doctor blade arrangements,
designated in the drawing by corresponding reference numerals,
serve the same purposes at units B, C and D.
For present purposes it may be considered that the portions of the
printed copy consisting of and containing yellow are applied to the
web at printing unit A, those consisting of and containing red,
blue and black are applied to the web, respectively, at printing
units B, C and D. Thus, the web leaving printing unit D carries, on
one side, a complete four-color reproduction which may include
areas printed in only one of any of the four colors and areas in
which one or more of the four colors is overprinted on a previously
applied color.
If the web is to be printed on both sides, it is turned over upon
leaving unit D, by well know means not shown, and directed to one
or more additional printing units, similar or the same as A, B, C
and D, wherein its reverse side is printed upon either in monotone
or in two or more colors, as desired. One or more subsequent
operations such as slitting, rewinding, folding, cutting and
creasing, etc., are also, normally, performed on the web after it
leaves the last printing unit, but these are not pertinent to the
present invention and are not illustrated.
Electrically conductive, metal rollers 13a, 13b, 13c and 13d apply
the electrical energy employed in electrostatically assisting each
of the separate printing operations to the respective impression
cylinders 5a, 5b, 5c and 5d, these conductive rollers each being
connected, as will be described later, to an individual power
supply.
Conventionally, the impression cylinders are loaded by means of
pneumatic or hydraulic cylinders or by means of screws, not shown,
This permits adjustment of the impression pressure to regulate the
area of the indentation or nip, formed in a resilient covering
provided on the impression cylinders, by their pressurized contact,
through the web, with the printing cylinders. It also permits
retraction of the impression cylinders from contact with the web
and with the printing cylinders, whenever desired.
During the electrostatically assisted printing operation, the
conductive roller serving each printing unit being used is in firm,
rolling contact with the surface of an outer, resilient, conductive
covering provided, and later described, on the impression
cylinders. However, the conductive rollers are movable with
vertical movement of the impression cylinders and preferably, they
may also be completely retracted, whenever desired, from contact
with the impression cylinders. One of the numerous suitable means
of accomplishing both purposes is shown in FIG. 1 and in more
detail in FIG. 3. As illustrated in FIG. 1, the end shafts of the
conductive rollers are carried in one end of pivoted arms such as
shown in 14a, 14b, 14c and 14d, the opposite ends of which are
pivoted, in the case illustrated, about the end shafts of the
respective idler rollers 3a, 3b, 3c and 3d. Suitable hand-operated
levers or, when desired, remotely controlled air cylinders, not
shown, may be employed to lift the conductive rollers from contact
with the impression cylinders.
Each of the printing units has a power supply individual to that
unit and comprising a source of low amperage, high voltage current.
The power supplies for units A, B, C and D of FIG. 1 are shown in
block form at 15a, 15b, 15c and 15d, respectively. Their components
and circuitry preferably are identical and the diagram of a
suitable circuit is illustrated in FIG. 4. The functions of this
circuit will be described later with reference to FIG. 4.
As shown in FIG. 1, a two phase line 16, of any conveniently
available voltage and frequency, is connected to the power supplies
through individual switches 16a, 16b, 16c and 16d. Thus, any
printing unit not being used may be disconnected from its primary
source of electrical energy. The positive terminals of the DC
output of the power supplies are connected by the conductors 17a,
17b, 17c and 17d to the respective conductive rollers 13a, 13b, 13c
and 13d, which feed the current supplied thereto into the
resilient, conductive covering on the impression cylinders 5a, 5b,
5c and 5d. The purpose of conductors 19a, 19b, 19c and 19d, leading
from the power supplied to ground, will be explained in conjunction
with the description of FIG. 4.
In the impression or nip area between each impression cylinder and
the corresponding gravure printing cylinder, electrical energy
passes from the conductive covering on the impression cylinder,
through the web to the gravure cylinder. The gravure cylinders are
at ground potential, being connected to ground in the case
illustrated by conductors 18a, 18b, 18c and 18d. However, no
special grounding wire is normally required, since the end shafts
of the printing cylinders are journaled in bearings on the frame of
the press and this usually establishes a good path to ground
potential.
The web 1, being of relatively nonconductive material, offers
resistance to the flow of electrical energy from the impression
cylinder to the printing cylinder and, with the latter grounded, a
substantial difference in potential normally exists through the web
at the impression. This causes ionization of the ink particles, in
the cells of the intaglio pattern on the gravure cylinder, at a
polarity opposite to that prevailing in the conductive covering of
the impression cylinder. Consequently, the normal meniscus of the
ink in the cells is deformed, attracting it to the interposed web.
Thus the cells of the intaglio pattern on the printing cylinder are
more completely emptied, as they pass through the impression, to
deposit on the web a greater quantity of ink than otherwise would
be transferred.
The increased quantity of ink transferred to the web and, hence,
its color density in the printed pattern, is directly related to
the difference in potential between the impression and printing
cylinders in the nip area. This, in turn, is a direct function of
the voltage applied to the conductive covering on the impression
cylinder. Therefore, by regulating the voltage output of the
connected power supply, the color density of the pattern printed at
each unit may be controlled.
In accordance with conventional practice, each of the impression
cylinders is made vertically movable by journaling its end shafts
in a frame which slides vertically on suitable ways provided on the
frame of the press. To avoid unnecessary detail in the drawing, the
well-known structure comprising the press frame and the slidable
mounted frames in which the impression roller shafts are journaled
are not shown. However, it should be noted that this structure
normally establishes an electrical path to ground potential from
the impression cylinder. One way in which this may be avoided is
illustrated in FIG. 2.
Referring to FIG. 2, the metal shell of the impression cylinder is
indicated at 21 and is covered by two layers, 22i and 22s, of
resilient material. The layer 21i is of high dielectric strength
and preferably is a compound of rubber or similar elastomer,
suitably bonded to the metal shell 21. The layer 22s is conductive
to the extent required to transmit electrical energy from the
conductive roller to the impression formed between the impression
and printing cylinders. It may also be a compound of rubber or
other elastomer containing sufficient carbon black or other
conductive material to render it semiconductive.
Usually the width of the web is less than the length of shell 21 of
the impression cylinder. The printing cylinder may also be of
greater length than the width of the web. The location of one edge
of the web is indicated at W in FIG. 2 and, to prevent the
electrical energy employed from short circuiting from the
impression cylinder to the printing cylinder at their end portions
which extend beyond the web width, the insulating material 22i
preferably is extended at these ends to a level which is flush with
or slightly below the outer surface of the insulating layer 22s.
This construction is indicated at 23 in FIG. 2.
FIG. 3 illustrates one type of construction whereby the conductive
rollers are insulated from the frame of the press. The conductive
roller 13a and its supporting pivotal arm 14a of printing unit A
are shown in FIG. 2, the corresponding assemblies for the other
printing units being of the same construction.
Both end shafts of the metal conductive roller are mounted in
suitable bearings, one of which is shown at 31 in FIG. 3. Each of
these bearings is mounted in a housing, indicated at 32, which is
constructed of suitable high dielectric material, such as, for
example, glass or cotton fabric impregnated with phenolic resin or,
as an alternative, wood fibre bonded under pressure with
urea-lignin resin (hardboard). The housing 32 is encased in one end
of the metal arm 14a, the opposite end of which is pivoted as
indicated in FIG. 1. The conductor 17a from the power supply
serving this press unit terminates within housing 32 in a spring
33, preferably of beryllium-copper, having a suitable contact
button 34 which bears upon and makes contact with a hard metal
contact 35 attached to the metal end shaft 36 of the conductive
roller.
Referring now to FIG. 4, which is essentially a circuit diagram, it
also shows in diagrammatic form the printing cylinder 4a,
impression cylinder 5a, web 1, and conductive roller 13a of FIG. 1,
to indicate how the circuit between these elements and the source
of power for the electrostatically assisted system is connected and
completed. Certain elements and subassemblies in the circuit of the
power supply and its associated parts are of well-known
construction and they function, individually, in a well-known
manner. To avoid unnecessary complexity in the diagram, these
well-known subassemblies are shown in block form. It should be
understood, of course, that although FIG. 4 shows the power supply
connected to the first printing unit (A of FIG. 1), it is typical
of the power supplies 15a, 15b, 15c and 15d of FIG. 1, one such
power supply serving each printing unit employed.
Readily available AC line current (such as 115 V., 60 cycle, for
example, although other voltages and frequencies may be used) is
supplied as shown on the upper left hand corner of FIG. 4, to
low-voltage, full-wave rectifier 40 having, for example, an output
of +50 volts DC This low-voltage DC is supplied to the voltage
regulator 41, which is essentially a three-stage transistor
amplifier with an internal source of reference voltage, such as a
battery, and a regulating feedback which will be later described.
The regulated low-voltage DC output from 41 is fed to the DC-to-DC
transistor inverter-rectifier 42, where, in accordance with
well-known practice, the low voltage DC input is first converted
into high-frequency, high voltage square waves, in the inverter
section. The inverter-rectifier assembly also contains a
bridge-type rectifier section wherein the high-voltage square waves
from the inverter section are converted to high-voltage direct
current.
The positive output side of the inverter-rectifier is connectable,
through relay contacts K5A and, as described in more detail with
reference to FIG. 3, through line 17a to the conductive roller 13a.
The negative side of the inverter-rectifier output is connected
through potentiometer P2, meter M2, and line 19a to ground. The
printing cylinder 4a is also grounded to complete the circuit and
provide energy for electrostatically assisting the transfer of ink
as previously described.
The circuitry provided and illustrated in FIG. 4 makes provision
for the following automatically-controlled safety features:
1. Temporarily interrupting operation of the electrical system
whenever the load imposed on the power supply reaches a preselected
safe limit.
2. Discontinuing operation of the electrical system until it is
intentionally started after clearing the difficulty, when a
persistent or frequently recurring load corresponding to the
preselected safe limit is imposed on the power supply.
3. Preventing operation of the electrical system until the press
reaches a predetermined operating speed (i.e., cylinder r.p.m. or
linear web speed).
4. Preventing operation of the electrical system until the
impression is made (i.e., the impression cylinder has been brought
into firm contact with the printing cylinder).
The aforementioned safety provisions and their mode of operation
are explained in conjunction with the following description of the
sequence and mode of operation of the circuit components.
The aforementioned AC supply line has a main disconnect switch 16a
and, with this switch closed, the line is connected with the
low-voltage AC-to-DC rectifier 40. The supply line is also
connected through this switch with another rectifier 43 of the same
type to provide -20 volts DC, for example, which is used as will be
later described. Also, the AC power line is connected with a
transformer T3 which provides a suitable filament voltage to the
Thyratron tube V1.
As current flows through the Thyratron filament circuit from
transformer T3, the current sensitive relay coil K1 is energized.
This closes the corresponding contacts K1A of the relay to connect
the negative side of the output from rectifier 43 with the time
delay unit 44, which functions in the following manner to prevent
energization of the rest of the system until the filament of the
Thyratron tube warms up and renders this tube operative.
With contacts K1A closed, the capacitor C1 in the time delay unit
starts charging from voltage drawn through relay coil K2 and the
resistors R1 and R2. As soon as the voltage of capacitor C1 has
risen to a sufficient value to cause the unijunction transistor Q1
to conduct, the resulting positive pulse of voltage generated at
its unijunction base is applied to the gate of SCR 1, turning it
on. Conduction through SCR 1 permits sufficient voltage to be
applied to relay coil K2 to cause operation of this relay, closing
its contacts K2A and K2B in a branch of the AC supply line. Until
this happens, the pushbutton PB1, the relay contacts K6A and the
relay contacts K7A in the aforesaid branch line are rendered
ineffective to close the circuit of this branch line by virtue of
the open contacts at K2A and K2B.
The function of the momentary contact pushbutton PB1 is to initiate
the electrically assisted operation, but until relay contacts K2A,
and K2B, K6A and K7A are all closed, no energy can be transmitted
through the branch line in which these contacts are located, and
the pushbutton PB1 remains ineffective. The manner in which
contacts K2A and K2B are closed has been described above. The
manner in which contacts K6A and K7A may also be closed, so that
the operation can be initiated by pushbutton PB1, will now be
described.
When the impression cylinder 5a is lowered into contact with the
printing cylinder, as shown on the right hand side of the diagram,
the contacts of limit switch S1 will close. This causes power from
the AC supply line to energize relay coil K6, closing the
corresponding contacts K6A.
After the press is started in the usual manner, not illustrated,
the printing cylinder is accelerated to the desired running speed
through the drive shaft 46 and the conventional gearing indicated
diagrammatically at 47. A suitable speed sensitive switch which,
for the sake of simple illustration, is here shown as a device 48
similar to a flyball governor attached to drive shaft 46 and having
contacts 49, closes these contacts when cylinder 4a reaches a
predetermined speed. Closing contacts 49 completes the circuit
through relay coil K7, energizing the same to close the
corresponding relay contacts K7A.
With relay contacts K2A, K2B, K6A and K7A all closed, the
impression between cylinders 4a and 5a will have been made, the
cylinders will be rotating at the desired speed and the time delay
unit 44 will have functioned to insure that the Thyratron V1 is
ready to operate. The system is thus ready to start supplying power
to the conductive roller 13a. By then manually operating the
momentary contact pushbutton PB1, to close its contacts, this will
complete the closing of the entire series of contacts in the branch
AC supply line and relay coil K4 will energize to close the
contacts K4A and K4B. Contacts K4A remain closed after pushbutton
PB1 is released, thus keeping the aforesaid branch line conductive
until the stop button PB2 is pushed or one of the series of other
contacts in the branch line drops out.
When contacts K4B close, the DC output from rectifier 40 enters the
voltage regulator 41 and the latter supplies its regulated output
to the inverter-rectifier 42. Meanwhile, since contacts K2A, K6A
and K7A have closed, the relay coil K5 has become energized to
close its contacts at K5A, which connects the positive output
terminal of the inverter-rectifier with the conductive roller 13a.
Its corresponding negative terminal is connected to ground, as will
be later described, to complete the circuit from the
inverter-rectifier through the conductive roller 13a, web 1, and
printing cylinder 4a to ground, and return to the
inverter-rectifier.
A portion of the positive output from the inverter-rectifier is fed
to ground through resistor R4, microammeter M1, which measures
voltage, and potentiometer P1. A sample voltage, the value of which
depends upon the setting of the wiper arm of P1 and which is
positive with respect to ground, is picked from the potentiometer
by the wiper arm and fed back to the voltage regulator 41. Here the
sample voltage is compared in a bridge circuit with the
aforementioned reference voltage in the regulator and this
comparison initiates an impedence change in the input to the
inverter-amplifier from the regulator to bring the sample and
reference voltages to equilibrium. Thus, changing the setting of
the wiper arm of potentiometer P1 changes the proportion of the
voltage fed back to the regulator and causes the latter to alter
its high voltage output to the conductive roller 13a. Meter M1
indicates the voltage level selected.
The negative output terminal of the inverter-rectifier 42 is
connected to ground through potentiometer P2 and meter M2. Since
all load current flows through this circuit, meter M2 monitors the
actual current drawn by the electrostatic assist applied to the
printing operation. To prevent this current from reaching a
dangerously high level, which can happen, for example when web 1
breaks or when holes in the web pass through the impression,
provision is made for limiting the current drawn by the conductive
roller 13a and shorting the inverter-rectifier output to ground if
and when the current level becomes excessive. To accomplish this,
part of the voltage drop across potentiometer P2 is picked off by
its wiper arm and applied to the current sensor 45. The current
sensor functions as follows.
If and when the current flowing through potentiometer P2 reaches a
level at which the voltage at the wiper arm setting exceeds the
threshold voltage of the zener diode D1, to which this pickoff
voltage is applied, the transistor Q2, to which the diode is
connected as shown, conducts and grounds the grid of Thyratron V1.
This causes the Thyratron to conduct heavily (crowbar) and
effectively short circuit the high voltage output of the
inverter-rectifier to ground through the Thyratron.
In addition, when transistor Q2 conducts to trigger the Thyratron,
the coil of relay K3 will be energized to open the relay contacts
K3A and interrupt the rectifier low voltage supply to the voltage
regulator. This, of course, turns off the inverter-rectifier and,
when its high voltage output decays to zero, Thyratron V1 ceased to
conduct, no current passes through the current sensor 45 and, with
relay coil K3 thus deenergized, relay contacts K3A drop back to
closed position. If the cause of the amperage surge has meanwhile
been corrected or cleared, the system is then back in operation. If
the fault persists or is repeated, the above described cycle is
repeated.
When relay coil K3 is energized, relay contacts K3B are closed to
connect a counter 50 with the AC supply line. Counter 50 is of the
type which closes an internal contact when a selected count is
reached and counts one digit each time relay contacts K3B close.
When the count reaches the preselected number, say three or five,
for example, the internal contacts complete the circuit from the AC
supply line through relay coil K8, energizing the same to open the
normally closed contacts K8A in the branch AC supply line. This
deenergizes relay coil K4, causing contacts K4A and K4B to open and
thereby discontinue the operation of the system until pushbutton
PB1 is manually operated. Thus, if there is a persistent heavy
drain of current or this condition is repeated for a number of
times corresponding to the preselected number on the counter, the
current supply to the conductive roller 13a is discontinued until
operation is intentionally renewed, giving the operator an
opportunity to find and correct the difficulty. When desired, a
warning, such as a light or audible signal, may be turned on
automatically when the counter reaches the preset count, and/or
operation of the press may be stopped automatically.
Since the Thyratron tube V1 is depended upon to ground the output
of the inverter-rectifier in the event of an excessive current
drain, provision is made for deenergizing the system in case the
Thyratron filament fails. In this event the current through relay
coil K1 collapses causing contacts K1A to open and disconnect the
time delay unit 44 from the rectifier 43. This deenergizes relay
coil K2, causing its contacts K2A and K2B to open, thus
deenergizing relay coil K4 and opening its contacts K4A and K4B,
which shuts down the system and returns all circuits to their
initial inoperative condition.
It should be noted that, in FIG. 4, all relay contacts are shown
with the corresponding coils in deenergized condition.
When electrical energy is applied at the impression, as described,
a web of given composition, structure, density and thickness will
draw an amount of current which is characteristic of that
particular web. But upon the occurrence of a web break, a hole in
the web or extreme porosity, so that current can flow from the
conductive covering on the impression cylinder directly to the
metal printing cylinder, the amperage drawn at the impression will
greatly increase unless the output of the power supply is limited.
A large and sudden increase in current at the impression, even
though momentary, is hazardous on a gravure press, since the ink
contains volatile hydrocarbon solvents and the ignition point of
the hydrocarbon-air mixture in the vicinity of the impression may
be low. This danger is avoided, as has been explained in greater
detail with reference to FIG. 4, by setting at potentiometer P2
(see FIG. 4) an adjustable upper limit to the amperage supplied to
the printing unit. Preferably this limit is set quite close to but
slightly above the amperage normally drawn through the particular
printing stock being run.
There is also a potential or voltage level at which the web passing
through the impression will be punctured or at which excessive
electrical energy will otherwise be caused to prevail in the
impression area. This also varies with the composition, structure,
density, and thickness of the particular printing stock employed.
However, it has been assumed, heretofore, that a voltage closely
approaching the maximum safely permissible is desirable.
Accordingly, it previously has been recommended and has been the
practice, in employing apparatus of the type illustrated and
described, to set the voltage output of the power supply at only
slightly less than the safe maximum. In general, this voltage
setting has been selected, by trial, to suit the particular
printing stock being run. For example, a relatively thin or porous
stock may tolerate say 800 volts, while heavy paperboard may
tolerate up to 5,000 volts, with intermediate stocks, including
coated papers and lighter paperboards, falling between these
extremes.
I have found that, with a given set of cylinders and inks,
utilizing the highest or nearly the highest safely permissible
voltage at the impression may not and, frequently, does not give
the desired results with respect to color densities and tones in
the printed copy. Apparently, this may be attributed to lack of
exact predictability and lack of precise control in preparation of
the color separations, resists, and in etching the cylinders.
I have further found that, without materially endangering the
quality of the printed copy with respect to good ink distribution
and a low percentage of skipped dots, there usually is a
considerable range over which the voltage applied at the printing
units may be adjusted and that such adjustments produce very
noticeable changes in the color densities and tone values obtained
in the printed copy. This is shown graphically in FIG. 5 of the
drawing, which will now be explained.
In FIG. 5, the horizontal axis of the graph represents the voltage
of the electrical output from the power supply, increasing from
zero on the left to the maximum safe voltage for a given stock in a
specific press unit on the right. In curve A, the voltage values
are plotted against color density in the copy printed at that unit.
In curve B, the voltage values are plotted against the percentage
of skipped dots in the same copy. In general, the color density
increases with an increase in voltage and the percentage of skipped
dots decreases with an increase in voltage. Both curves follow a
generally S-shaped pattern but they are not identical.
While the curves shown in this graph are believed to be fairly
representative, it should be understood that variables in the
operation, such as a change in printing stock or in the moisture
content, caliper (thickness) or density of a given stock, will
cause them to change. Therefore, no specific quantitive values are
assigned on the graph. However, on the basis of tests conducted to
date, it appears that there is little increase in color density and
little decrease in the percentage of skipped dots until the applied
voltage reaches approximately 40 percent of its predetermined,
maximum safe value. From this point up to about 70 to 75 percent of
the maximum safe voltage, the percentage of skipped dots decreases
at a greatly accelerated rate and then starts to flatten out rather
abruptly. The color density follows a similar pattern but does not
flatten out as rapidly in its upper region.
By way of example, let us assume that, with a given stock and on a
given press unit, the highest potential which may be employed,
leaving some margin of safety, is 1,500 volts. In the
aforementioned electrostatically assisted printing operation, not
utilizing the present invention, it has been the practice to
determine this maximum safe voltage for the specific printing stock
being run and to operate the system at a constant voltage only
slightly below this predetermined value. However, in the practice
of the present invention, the following procedure is
recommended:
a. At the start of the operation, the potentiometer of the current
sensor (P2 in current sensor 45 of FIG. 4) is set for an amperage
output to the printing unit substantially in excess of the current
which the web is expected to normally draw. Let us assume, for
example, that we expect the web to draw somewhere between 0.5 and 1
milliampere and that the potentiometer is set to permit an output
from the power supply of 3 milliamperes.
b. Starting with a low voltage output from the power supply, it is
gradually increased (by means of potentiometer P1 in FIG. 4) until
the Thyratron tube (V1 of FIG. 4) crowbars rather consistently.
Again, by way of example, let us assume that this will happen when
the reading on meter M1 of FIG. 4 reaches 1,600 volts.
c. The voltage output of the power supply is then slowly decreased
until crowbarring ceases. In the example being used, this may
happen when the reading of meter M1 decreases to 1,540 volts and,
to leave a margin of safety, we select, say 1,500 volts as the
maximum to be used.
d. The voltage output of the power supply is further decreased,
below the point where crowbarring ceases, until skipped dots begin
to be noticeable in the printed copy. Say this occurs, in the
example being used, at an output from the power supply of 1,000
volts, as indicated by the voltmeter (M1 of FIG. 4). The
potentiometer setting is then increased slightly, to produce copy
which does not have an objectionable percentage of skipped dots.
Say this setting gives an output of 1,050 volts, we may then select
1,050 volts as the minimum to be used. Thus a range of voltage
regulation of 1,050 to 1,500 volts has been established in this
specific example.
e. Returning to the amperage adjustment, after noting on the
milliammeter (M2 of FIG. 4) the current being drawn by the web, the
setting of the amperage output adjusting potentiometer (P2 of FIG.
4) is decreased until the output falls sufficiently that
crowbarring is encountered. Say the web has been drawing
approximately 0,65 milliamperes and crowbarring occurs at 0.58
milliamperes. To accommodate normal variations in the current drawn
by the web, with changes in its structure, moisture content,
thickness, etc., the potentiometer setting is increased to give a
maximum output from the power supply slightly above that previously
noted on the milliammeter. In this example, the maximum output may
be set at say 0.75 milliamperes. It is left at the setting selected
during the run, unless and until the characteristics of the web or
some other factor changes sufficiently to require some
readjustment. During the run the printed copy is examined
periodically to compare the density of each individually applied
color with an acceptable standard and the voltage employed at any
unit which is not producing the desired results is adjusted, within
the limits predetermined as above described, to give optimum
results.
It will be obvious that the sequence of the recommended setup
procedure (steps (a) through (d) above) may be altered in some
respects without violating their purpose. Also, the specific
figures given as examples are intended only to be indicative of
their order of magnitude. It should also be pointed out that the
recommended procedure applies to each individual press unit and the
power supply which serves it since there may be a slight difference
in their behavior and this should be determined on any new
installation.
As a general rule, when proper inks have been selected and the
cylinders have been prepared with reasonable care and expertize,
matching the densities of the individually applied colors (normally
yellow, red, blue and black) with those of the original art work to
be reproduced or with acceptable color swatches will also give the
desired tone values in the areas of the copy where individually
applied colors are combined. However, it should be noted that, with
respect to maintaining the desired tone values of combined colors,
the invention affords a choice of three procedures. This may be
clarified by a specific example, such as follows:
Say a certain shade or tone value of green in the copy begins to
deteriorate, taking on a cast which is too yellow. The
possibilities for correcting this are (1) decrease the voltage at
the unit which applies the yellow ink, (2) increase the voltage at
the unit which applies the blue ink, or (3) do both. The choice
will be dictated, in part, by the existing settings of the
potentiometers which control the voltage to these two units. If the
one controlling yellow is somewhere in mid range or near its upper
usable limit, it may, of course, be reduced, but if it is near its
lower limit, reducing it further is usually undesirable.
Conversely, if the potentiometer controlling the blue is near its
upper limit, its effectiveness is limited, but if it is somewhere
in mid range or near a low limit, it can be increased with good
effect. If the blue is near its upper limit and the yellow near its
lower limit, a small change in both, each in the proper direction,
is indicated. If neither potentiometer setting has a narrow
remaining range, in the proper direction, a knowledgeable operator
will also take into account what will happen to an area of the copy
printed only in yellow or only in blue if he changes the
potentiometer controlling that color in order to bring the green to
the desired tone value.
Devices are now available for continuously monitoring color
densities and tones in multicolor printed copy, as it is produced.
Such a device has special utility when used in conjunction with the
method of control herein provided and such use is definitely
desirable and is contemplated. However, the specific means of
continuously monitoring, indicating, and/or recording changes in
color tones and densities, while pertinent, is not a part of the
present invention.
While the foregoing specification refers particularly to gravure
printing, utilizing some impression pressure, it is not intended to
limit the scope of the invention to this nor to any other specific
means of reproduction. To the extent that other types of
reproduction, including offset or planographic, letterpress or
stereotype, flexographic (formerly called aniline), screen or
stencil printing, and the like, are susceptible to improvement by
the method herein provided, such use of the method does not depart
from the intended spirit and scope of the invention.
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