FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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2 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ACKNOWLEDGEMENTS

Author/Editor: Peter Basler, Bobst Group (Corrugated Postprint)

Ed Engledow, Fife Corporation (Web Inspection and Guiding)

Kurt Freye, Windmoeller & Hoelscher (Wide Web)

Peter Kershner, Mark Andy, Inc., (Narrow Web)

Contributors: George Cusdin, Flexographic Printing Services

Jim Mack, Langston

Gordon McGee, Webtron

Richard Harrison, Ward Machinery Co.

Special thanks to Langston for illustrations 67, 69-75, 78, 81-86, 88-89.

FPRESSES AND PRESS EQUIPMENT 3

here are so many kinds of press-

es, auxiliary equipment and in-

line operations that it’s virtually

impossible to review all the

available combinations. How-

ever, each of these subjects will

be reviewed sufficiently to provide an under-

standing of flexo printing presses and their

related equipment.

In recent years, more and more flexo

presses have been supplied with in-line oper-

ations, such as laminating and/or coating,

die cutting, etc. Therefore, an understanding

of how these operations are incorporated

into the press is beneficial. Do not confuse

in-line operations with in-line presses. In-line

presses are just that – a specific variety of

printing press as described elsewhere in this

chapter. In-line operations are applications

other than printing that are performed on

other equipment that has been combined

with the press. A separate section in this

chapter has been devoted to in-line process-

ing with the narrow web press. There is also

a complete chapter devoted to the corrugat-

ed post-print operation.

Historically, the flexographic web printing

industry has been broken down into two

basic web configurations: wide web and nar-

row web. Web widths range from 6" on a

small adhesive label press, to over 150" on a

newspaper press. The break point between

the definition of narrow web and wide web

has been in the range of 20" to 24". That is to

say, web widths up to 24" may be considered

narrow web and web widths over 20" may be

considered wide web, leaving an area for

dispute. The term “mid-web presses” has

come into usage for a class of presses that,

although not strictly defined, has a web

width that lies somewhere in the range

between 20" and 47". For this publication,

narrow web is defined as being web widths

up to 20". Anything above 20" is considered

wide web.

The flexo web press has four major com-

ponents: the unwind with in-feed, the print-

ing section, the dryer and the rewinder with

out-feed. The auxiliary equipment, such as

web guides, web viewers, powder spray

units, air shafts, etc., are also important in

the overall operation of the flexo press.

There are many models and designs of this

equipment and each component will be cov-

ered in detail in other chapters.

Introduction

here are three basic types of wide

web flexographic printing press-

es: stack, central impression

cylinder and the in-line press.

Generally, these presses are used

for printing flexible packaging

materials, but they also do narrow web, cor-

rugated and publication printing operations.

Regardless of the end product, the printing

principles remain fundamentally the same.

THE STACK PRESS

In the stack press (Figure

), individual

color stations (sometimes called sections or

decks) are stacked one over the other on one

or both sides of a main press frame. Each

color station is driven through gear trains

supported by the main press frame. Stack

presses are made with one to eight color sta-

tions, although the most common configura-

tion is a six-color press.

FPRESSES AND PRESS EQUIPMENT 5

Wide-Web Presses

A typical 6-color wide-

web stack press, where

individual color stations

are stacked one over

the other on one or

both sides of a main

press frame.

D E

A Infeed Tension Nip Rolls

B Metering Roll

C Anilox Roll

D Plate Cylinder

E Impression Roll

F Print Station

G Between Station Dryers

To Main Dryer

6 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Stack presses have three main advantages.

First, the operator can usually reverse the

web to allow both sides to be printed during

one pass through the press. Various web-

threading arrangements allow complete ink

drying before the reverse side is printed, pro-

vided enough drying capacity is designed into

the area where the web passes between the

two series of stations. The second advantage

is the color-station accessibility, which facili-

tates changeover, wash-up, etc. The third

advantage is the ability to print large repeats.

The stack press has proved useful in many

applications and has been used to print on

almost every type of substrate. It does have

limitations that don’t make it completely

acceptable for some applications. When

printing substrates that are extensible or of

extremely thin gauges, the stack press is

generally restricted to color registrations

that do not require greater accuracy than

±0.0312". When heavier gauge materials are

being printed, such as papers, laminated film

structures and others that can tolerate fairly

high web tension values, the stack press can

profitably produce commercially acceptable

registered products.

The stack press lends itself well to appli-

cations such as printing in-line with other

types of machinery. These add-ons may

include extruders, bag machines, sheeters,

laminators or other equipment.

With some special color-station designs,

it’s possible to have 360° register on each

station and independent engaging and disen-

gaging to allow the remaining part of the

C A typical six-color

central impression

press supports all of its

color stations around a

single impression

cylinder.

A In Feed Guide

B Nip Roll

C Central Impression Cylinder

D Inter Station Dryer

E Hydraulic Vertical Lock

F Hydraulic Horizontal Lock

G Fine Impression Adjustment

H Impression Indicators

I Metering Roll

J Anilox Roll

K Plate Cylinder

machine to operate. The stack press also has

been used both as a coating and tinting (all-

over coloring) machine. Since each color

station is independent from the others, it is

easy to mechanically arrange various rota-

tions of the inking parts. It is also possible to

change the web lead to flood coat a sheet, or

print coatings in a standard fashion. These

techniques have been applied in the pre-

printing of corrugated liner material.

THE CENTRAL

IMPRESSION PRESS

The central impression press, sometimes

called a drum, common impression or CI

press, supports all of its color stations around

a single steel impression cylinder mounted in

the main press frame. (Figure

). The

impression cylinder supports the web, which

is thereby “locked” to the cylinder as it pass-

es all color stations. This configuration helps

prevent register shift from color to color.

Since the greatest advantage of the central

impression cylinder press is its ability to

hold excellent register, it has become the

mainstay of the converting industry. Also,

with graphic designs becoming more com-

plicated and the demand for process printing

remaining steady, the positive register ability

of the CI press makes it suitable for all types

of substrates. The most common press in

use today is still the six-color central impres-

sion press, although this is being superceded

by the eight-color CI press. Even ten-color CI

presses are being now built.

Impression cylinders of various diameters

have been used. At first, four-color presses

were the most common, and they generally

used 30- or 36-diameter impression cylin-

ders. To get better speed and allow room for

interstation drying, impression cylinders up to

60" were used for four-color presses. The first

six-color central impression cylinder presses

used 83" diameter cylinders. The latest eight-

color central impression presses have cylin-

ders up to 94" in diameter. As drying tech-

niques have improved and the distance

required for drying between colors has

decreased, smaller impression cylinders have

come back into use. The most common eight-

color single impression cylinder press today

has an 89" (2.26 meter) diameter cylinder.

Thanks to advances in between-color dry-

ing, the adage that “larger cylinder presses

usually offer higher speeds” no longer

applies. In general, however, it is still possible

to get longer printing repeat lengths on the

larger impression cylinder presses than those

of smaller design.

The central impression press has found lim-

ited use when it comes to printing both sides

of a web during one pass through the press,

most commonly in tubular film printing.

Development of the CI Press

Beginning in late 1989, major technolog-

ical advances were made regarding con-

struction of wide web CI presses.

The first significant change was making the

individual printing deck a single piece cast-

ing. The previous manufacturing technique

utilized a pair of machined side frames joined

together by two or more bolts on transverses.

The new development required major ad-

vances in machine tool design and construc-

tion. However, it was immediately apparent

that a single piece cast deck provided a more

stable platform from which to print.

Simultaneous with the development of sin-

gle piece cast decks was the elimination of

all the hydraulics on these presses, replaced

by digital electronic and pneumatic controls.

This change eliminated the messy hydraulic

operation and maintenance problems asso-

ciated in the past with hydraulically actuated

printing decks.

Further enhancement to these decks was

brought about with the use of prismatic linear

recirculating ball guides, which are perma-

nently lubricated and preloaded, allowing

movement of the printing deck only in the

PRESSES AND PRESS EQUIPMENT 7

8 FLEXOGRAPHY: PRINCIPLES & PRACTICES

direction of the x-axis. Previous hydraulic

deck designs required the printing deck to

move in all three directions, i.e., along the x, y

and z axes in order to move the printing decks

forward and back, on and off impression, and

in or out of gear engagement.

Inherent in hydraulic deck construction is

the need for additional gaps between the vari-

ous metal pieces in order to allow for this

movement. Each additional gap between the

metal pieces, and movement along all three

axes allows for more movement of the print-

ing deck, and in turn more potential for regis-

tration errors and inaccurate impression set-

ting. The electronic decks allow movement

along only one axis, and therefore have a

more stable platform and a lower centerline of

the plate and anilox rolls relative to the print-

ing decks. These modifications result in better

print quality at higher speeds, which is a dis-

tinct advantage on jobs that previously were

not able to run as fast on hydraulic decks.

The new electronic decks were further

enhanced by using true closed-loop digital

stepping motors with built in encoders, feed-

ing back actual deck positions to the host

computer now controlling the printing

decks. This digital control enables the oper-

ator to set or reproduce previous impression

and register settings on the decks with a

high degree of accuracy and virtually 100%

reliability, something that is not possible

with hydraulic decks and their inherently

looser construction.

Modern CI presses also have fully auto-

matic plate cylinder-to-bull-gear-engagement

cally rotate each plate cylinder and its repeat

gear into initial register and engagement

position relative to each other, and then into

engagement with the bull gear to within

±0.0002" initial register accuracy, without

pulling any material through the press.

First developed during the late 1980s and

incorporated by virtually all wide-web press

manufacturers today, was the addition of

robotic cylinder loading/unloading systems

for both the plate and anilox rolls (Figure

The addition of these robotic loading systems

significantly reduces the potential for damage

to the press, anilox rolls, plate cylinders and

personnel, while speeding up overall change-

over times. Today, most wide-web presses are

ordered with a robotic cylinder system.

These robotic cylinder loading systems

have gone through a number of iterations by

various manufacturers; however, as of this

writing most robotic cylinder systems are

virtually 100% reliable.

Chambered doctor blades with automatic

wash-up systems, coupled with the advances

of electronic printing decks and robotic cylin-

der handling systems, have led to the devel-

opment of modern CI flexo presses controlled

via a central operator console. The various

press components – including unwind, in-feed

draw, printing deck movement, register cor-

rection, drying system temperature settings,

air flows, out-feed chill-roll draw settings, slit-

ter settings and winder settings – are all inte-

grated into the main operator console with

direct digital connections. The central con-

sole further reduces makeready times by

allowing operators to automatically preset all

these settings quickly and easily for later

recall of similar or same jobs, or to use as a

template for a family of jobs.

With the development of electronic print-

ing decks, robotic cylinder loading systems,

on press wash-up systems and automatic

gear engagement, it is now also possible for

press manufacturers to provide a safe system

for allowing operators to make ready unused

printing decks while the press is running at

full press speed. These free deck safe sys-

tems have proven to be economically viable

for those customers running three-, four- or

five-color jobs. However, with six colors or

more, the above-mentioned features allow

jobs to be changed fast enough while the

press is stopped, so as not to require the free

deck makeready system.

In recent years flexo has achieved print

quality comparable to gravure, and as a

result, more and more work from the

gravure sector is moving to flexo. This

increase in print quality has driven the devel-

opment of the 10-color CI press, since most

gravure jobs are up to 10 colors.

Central Impression Drum

Drum construction in a central impression

press can be of double-wall steel or cast iron.

In either case the drum will be temperature-

controlled by a heating/cooling device. As

markets have advanced in flexography, and

the product expected from a CI press has

improved to a high degree of excellence,

press manufacturers have been compelled to

hold more demanding tolerances in regard to

the CI drum. Very commonly employed today

is the use of digitally controlled heating/cool-

ing elements, which hold drum temperature

within a range of ±1° F. This close tolerance

is a necessary element in the printing excel-

lence being demanded of press manufactur-

ers and converters today. If the press experi-

ences variation in drum temperature, which

causes CI drum size variation, the operator

will be continually attempting to compen-

sate, causing missed impressions or over-

impression.

The CI cylinder, independent of its method

of construction, must be manufactured to

meet high Total Indicated Runout (TIR) char-

acteristics. It is very common today that

specifications on a new press dictate that the

drum not exceed 0.0003" TIR. Ideally, the

PRESSES AND PRESS EQUIPMENT 9

Modern wide-web

presses are almost

always ordered with

a robotic cylinder

loading/unloading

system, which helps

in reducing potential

damage to the press,

anilox rolls and plate

cylinders, as well as

personnel.

10 FLEXOGRAPHY: PRINCIPLES & PRACTICES

drum will be held to a lower actual number if

possible. Realizing that the TIR of the drum

will only be as good as the supporting journal

and bearings, manufacturers of presses are

demanding the use of printer-quality roller

bearings with a TIR of around 0.0002". The

use of custom hand-fit bronze bushings,

which were very prevalent in the past, is los-

ing favor with manufacturers today. The

bronze bushings must be constantly lubricat-

ed, usually with a lube pump. The advantage

of printer-quality roller bearings manufac-

tured to acceptable tolerances is that they

can be lubricated in a similar manner to a

gearbox (enclosed oil bath or grease pack).

IN-LINE PRESS

The in-line press is the third commonly

used type of wide-web press. Its color sta-

tions are separate, complete units horizontal-

ly mounted one to the other, and may be dri-

ven by a common line-shaft. In-line presses

can be manufactured with any number of col-

ors. This type of press can be easily designed

to handle various web widths, from narrow to

extremely wide, since a single frame need not

support all colors. The in-line press is com-

monly used in folding carton, corrugated post

print and multi-wall bag operations, as well as

in other special applications.

In-line presses are also common in narrow

web widths for printing pressure-sensitive

and standard label stocks, where they offer

the advantages of quick setup and accessi-

bility. This design is also used in those spe-

cialized areas where a specific product line

may need a press designed for short runs.

The in-line press has the versatility to print

on both sides of a given web by either turn-

ing the web over with the turning bar system

or using alternate threading. They can be

used to flood-coat where all-over coloring of

absorbent materials is required.

FOLDING CARTON PRESS

Folding cartons have been printed by flexo

for many years, but it has only been in the

past 10 years that the industry has started

using state-of-the-art central impression

presses for this application. Pizza cartons

illustrate the type of work that was originally

relegated to the flexo process. The ability to

print multi-colors in close register on wide

webs, however, has made the process more

attractive to other point-of-purchase folding

carton applications.

Normally the folding carton press is tied

in to an in-line process that would include

cutter-crease or platen die-cutting opera-

tions. In the past, most cutter-creasing oper-

ations were mated only with roto-gravure or

sheet fed offset presses. With the improved

quality of flexographic printing, the use of

water base or UV inks, and the lower cost of

equipment, converters can no longer ignore

the option of printing high quality folding

cartons flexographically.

Folding carton equipment differs as a

result of the substrate. Heavy board roll

stock is used, which requires higher tension

levels, larger idle rolls and more sophisticat-

ed splicing and control units. The unwind

section must handle 72" diameter rolls and

automatically splice them without an overlap

(butt splicing). Idle rolls must be of a large

enough diameter to eliminate the risk of

weakening the board fiber bond or creasing

the board.

From the unwind through the cooling

drum and to the out-feed unit, the press oper-

ator is dealing with the usual tension zones

and a constantly moving web. From this

point on to the cutter creaser or platen die-

cut section, the motion is intermittent (stop

and go). Web control is critical to both print

and die-cut register, as the printing must be

on a smooth flowing, tensioned web, and the

die-cutting on a stationary, relaxed web.

Electronic print sensors register the image

on the web to the die cutter.

PLATE CYLINDERS

The plate cylinder shaft and cylinder wall

may be of integral construction similar to the

roll body of the fountain roll and anilox roll, or

of a de-mountable type. Whether integral or

demountable, it must be very accurate. Its total

TIR should not exceed 0.001" for line work,

and 0.0005" for fine screen or process work

(some feel this should be reduced to 0.00025"

for process printing), when turned on its bear-

ing journals. Further, it should be made with a

taper not to exceed 0.0002" for every 12" of

face length, and should be dynamically bal-

anced to operate smoothly at high press

speeds.

Also, the roll diameter should be of such a

size that when the stickyback and rubber or

photopolymer plate is applied, the printing

plate will run at the proper gear pitch line.

Most wide-web flexographic presses are

equipped with either 0.25 circular pitch or 10

diametrical pitch gearing. European presses

have metric pitch gearing. For more details

on gears see pages 132-141.

Demountable Plate Cylinders

There are several different types of de-

mountable plate cylinders. They are com-

monly used where a converter may have sev-

eral different flexographic presses of about

the same width, but which require different

types or lengths of journal. So it’s possible to

make demountable plate cylinder shafts for

each press. Otherwise, it would be necessary

to have complete sets of cylinders for each

different press because the cylinder shaft and

gearing would not necessarily fit each press.

What plate system to use sometimes

becomes a question of cost. The tolerances

for demountable plate cylinders and shafts

must necessarily be the same as those for

integral-constructed cylinders. Cylinders

and shafts must be well maintained. Each

time a new cylinder is taken off or put on a

plate-cylinder shaft, its seating arrangement

should be checked for damage. When oper-

ating a demountable system, it is essential to

check the concentricity of the plate cylinder

each time it is reattached to a cylinder shaft

to make sure it is within the proper operat-

ing tolerances.

There are a few common ways of attaching

demountable cylinders to shafts. One type of

cylinder lends itself to having its end wall

heated so it will expand and slide over the

shaft. Upon cooling, the cylinder wall will

shrink into position. Another type of demoun-

table cylinder has a threaded opening on the

end wall (Figure

). The shaft is also thread-

ed and, when tightened with a spanner

wrench, the cylinder and shaft lock together.

Another type is locked into position with a

pressure system using a grease gun that fits in

the shaft. Still another type is fastened with

screws, activating a “hydraulic” system.

Whatever system you use, the require-

ments for consistency, taper, balance and

concentricity must be maintained for proper

printing. Demountable plate cylinders take

up less storage space, but they do require

extra time to reshaft for each new job.

SIDE AND CIRCUMFERENTIAL

A number of devices are available to the

operator to adjust side and circumferential

registration. Most common is a mechanical

apparatus, such as a hand-wheel, which when

connected to the plate cylinder, will cause the

plate cylinder to be laterally moved.

Circumferential register can also be accom-

plished with a simple hand-wheel by allowing

the plate cylinder and associated gearing to be

connected to a helical gear. The male spline

clamped to the plate cylinder journal in con-

junction with a female spline will allow the

plate cylinder gear to slide forward and back

on the male spline to affect the circumferen-

tial register without affecting the side register

of the plate cylinder.

A number of hydraulic or electrical

PRESSES AND PRESS EQUIPMENT 11

12 FLEXOGRAPHY: PRINCIPLES & PRACTICES

devices can be designed and installed to

allow the operator not only convenient

access to the register controls but a degree

of automation. Normal use of automatic

deck positioning systems allows for the pos-

sibility of automatically centering both side

and circumferential register devices. With

properly positioned plates, the automatic

deck positioning system provides the opera-

tor, upon rack-in and gear mesh, a properly

registered print job very close to acceptable

quality without further adjustment.

The motor-operated type of registration

system, whether hydraulic or electric, can be

furnished with the capability to be pulsed –

that is, upon activation of a pushbutton or

switch, the units will be programmed to

move a certain increment. Incremental

movement is a great asset, especially for a

high-quality process print where fine register

capability is paramount. By providing the

capabilities for pulse registration, the opera-

tor has the ability to adjust and fine-tune reg-

istration from a remote area, such as a web

viewer or video monitor. The above capabil-

ity saves lost motion, which equates directly

to higher quality print with less waste.

Demountable plate

cylinders help provide

consistency, taper,

concentricity and

balance during printing.

This type, a robotic

cylinder, has a threaded

opening on the end wall.

The shaft is also thread-

ed and, when tightened

with a spanner wrench,

it locks into the cylinder.