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

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PRESSES AND PRESS EQUIPMENT 103

Close coupled

printing units of the

printer/rotary die-cutter

and flexo folder-gluer

machine have roll-away

press units, which allow

access to the various

printing elements for

setup and wash-up.

Adhesive Section

Die Cutter/

Creaser

Section

Creaser-

Slotter

Section

Printing Section

Feed Section

smearing or rubbing by folding or die-cutting

components. For example, a jam-up on a

rotary or platen die-cutting line with in-line

printing may cause production stops requir-

ing printing plate washing and causing sub-

sequent start-up waste.

SHEET FEEDERS

On corrugated sheet-fed presses, the first

element of importance is the feeding unit for

the sheets, which must ensure very good

Registration tolerances have been in the

area of ±0.0625" for many years and have

steadily decreased for tighter tolerances.

Today, some manufacturers talk about toler-

ances of ±0.010", however, the reality is more

in the area of ±0.020" during production.

The following are short descriptions of dif-

ferent feeding principles, all of which can be

found on machines of different models.

Kicker Feeder

The kicker feeder is the oldest of all feeder

designs and works best with narrow sheets

in the throughput direction of a machine. It is

still found on so-called “mini-flexo” setups

because it is capable of introducing sheets at

speeds of 30,000 per hour. A stack of sheets

is placed on the vacuum bed and located

against a lead-edge stop plate (Figure

Die-cut

Floor Track

RailMotorized

Casters

Fixed

Out Feed

Conveyor

Print #2 Print #1 Feed Section

The vacuum draws the bottom sheet flat

against the feeder bed and the trail edge of

the sheet locates against the kicker fingers.

The kicker fingers, timed to the press rota-

tion, then push the bottom sheet into the feed

roll nip. Feeding accuracy depends on manu-

al adjustments, board warp, consistency of

the pre-trimmed sheets and whether or not

the feeder is equipped with a suction-assist.

Kicker feeders can only feed sheets with

their corrugation oriented in the machine

direction. If the direction of the corrugations

is across the machine, feeding against the

soft edge may result in very inaccurate regis-

ter or frequent jamming. E-flute is generally

the lowest flute profile that can be fed with

such feeders. F- and N-flute are too fine for

this type of feeder. It is safe to say that kick-

er feeders are not found on high graphics

machines.

Lead-edge Feeders

The most significant advantage of suction-

plate feeders is that the corrugated sheets

are accurately registered against their lead-

edge. Therefore, trimming variations from

the corrugator have less influence on feeding

accuracy. Vacuum distribution may be

achieved through rotary or piston-type

valves. On suction plate feeders (Figure

powerful vacuum pumps – which can main-

tain a vacuum of 18" to 22" Hg under produc-

tion conditions – are needed, otherwise, slip-

page between the suction plate and the sheet

will occur.

Vacuum timing at start and release is im-

portant. Most suction plate lead-edge feed-

ers are timed as follows:

• The stroke of the plate must correspond

to the distance from the in-feed gate to

the center of the feed rolls, at a minimum.

• Suction must begin during a standstill

cycle (hopefully prolonged), when the

plate is in its most backward position.

• The suction plate must release the sheet

when the sheet has arrived at the center

of the in-feed roll nip.

• The speed of the suction plate and the

surface speed of the in-feed rolls must

be 1:1 to eliminate the vacuum at the

time when the sheet has to be released.

Venting to the atmosphere or injection

of compressed air are two methods

commonly utilized.

Belt Type. Belt type lead-edge feeding by fric-

tion goes back several decades and is found

on folder-gluer machines, both in the folding

carton and the corrugated industries (Figure

). Continuously running belts pull one

blank after another, from underneath a stack

of blanks or sheets, in an “untimed” manner.

Obviously, to apply a similar technique on

a printing machine, the process needs to be

104 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Trail-edge kicker units

are still found on so-

called “mini-flexo”

setups because they are

capable of introducing

sheets at speeds of

30,000 per hour and

work best with narrow

sheets in the throughput

direction of the machine.

During production,the

suction-plate feeder

allows corrugated

sheets to be accurately

registered against their

lead-edge by using

powerful vacuums to

help prevent slippage

between the suction

plate and sheet.

Feed

Rolls

Board

Stack

Vacuum Chamber

Kicker

Feeder

Back

Plate

Lead

Edge

Plate

Feed

Rolls

Pull

Rolls

Board

Stack

Side

Plate

Vacuum

Chamber

Actuator Arm

Back

Plate

Lead

Edge

Plate

timed. A vacuum fan is utilized to ensure a

slippage-free transport of each sheet. In its

simplest form, such a feeder has continuous-

ly running belts over a suction box or cham-

ber which has lifters that, in a timed fashion,

lower the stack of sheets onto the belts,

allowing the lowermost blank to be intro-

duced into the feed roller nip. The vacuum is

continuous and needs only to be interrupted

when feeding needs to be stopped.

Reciprocating Belt Feeder. Another type of

lead-edge belt feeder has reciprocating belts

with forward and backward motion. With this

design, the belts are lifted through cam rollers

to meet the lowermost sheet of a stack lying

on top of stationary slats. The feeding cycle

consists of four steps:

• The stationary belts are lifted above the

slats.

• The belts move forward in lifted posi-

tion to introduce the sheet into the feed

rollers.

• The belts are lowered away from the

sheet at the end of the forward stroke.

• Backward movement of the belts in

lowered (non-contact) position below

the slats completes the cycle.

Roller-type Feed Wheels. This category of

lead-edge feeder is also based on a high vol-

ume vacuum, which transports the sheet on

continuously rotating feed wheels that are

covered with a high friction material. Timing

is controlled through a vertically-arranged

lifting grid (slats) that has a low friction sur-

face. The up-and-down cycling, generally

done through a cam arrangement, deter-

mines when a sheet starts its travel forward

and when it is to be taken over by the feed

rolls in pace with the machine.

With feed-wheel or feed roller-type lead-

edge feeders, many variables in design exist.

Generally speaking, such feeders are very

reliable and good feed accuracy is obtained.

Such feeders can feed stock with excessive

warp to the point that a jam-up somewhere in

the printing units may occur, which is poten-

tially a bigger evil compared to a jam-up in

the feeder. Flat sheets are, on all types of

feeders, the best insurance for trouble-free

operation of a machine.

Cam Roller Feeder. With this type of feeder,

small quantities of blanks are placed in the

feed table hopper by the operator or an auto-

matic pre-feeding device. The leading edge of

the sheet locates against vertical fingers that

are set above the feed table to allow only one

sheet to pass at a time (Figure

). A set of

cam-shaped feed wheels contacts the bottom

blank in the pile and pulls it into the nip of

the feed rolls. The feed rolls grip the blank

and delivers it to the printing section.

PRINT STATION

Having examined the different substrate-

feeding methods, it is now time to look at the

next step in the process: the print station.

There is more to printing than simply apply-

ing ink. Proper mounting, gear and roll posi-

tion and technique are all important factors.

Top Printing vs. Bottom Printing

Flexo printing on corrugated evolved in a

round-about fashion. It was developed or

integrated into various existing manufactur-

ing processes and has only existed as a self-

PRESSES AND PRESS EQUIPMENT 105

In a belt-type feeder, the

lowermost blank is

pulled from a stack of

blanks or sheets, and

pushed through the feed

rolls to the folder-gluer.

Feed

Rolls

Board

Stack

Vacuum Chamber

Back

Plate

Lead

Edge

Plate

106 FLEXOGRAPHY: PRINCIPLES & PRACTICES

contained off-line process since the late ’70s.

For this reason, top-and-bottom printing units

were developed to adapt the printing units to

whatever converting operation already exist-

ed. Top and bottom printing are each applica-

ble in different circumstances. For example:

• On a folder-gluer with an “up-folding”

principle, the printing needs to be on the

bottom of the sheet.

• On a so-called “down-folder”, the print-

ing is performed on the top of the sheet.

• On a rotary die cutter with the rotary cut-

ting die on the top shaft, printing needs

to be on the bottom because the creasing

lines to form a box or container must be

on the inside of the box.

• Conversely, if the cutting die is on the

bottom shaft, the printing must be on

top. The same principles apply for plat-

en-type die cutters.

The quality of flexo printing on corrugated

sheets, whether performed from the top or

from the bottom, is not truly an issue. Rather,

the key question is: Which approach is better

aligned with the overall scope of the total

corrugated manufacturing and converting

process? One way to address this question is

to begin with the corrugator. Consider the

following:

• The corrugator, without exception, deliv-

ers the combined sheet with the print

liner on the bottom.

• The majority of corrugators have auto-

matic down-stackers or up-stackers that

deliver piles of combined sheets with the

print liner on the bottom.

• The majority of pre-loaders deliver the

sheets into the feeders of converting and

printing machines from the top of piles in

batch form or in a shingle stream. Others

deliver sheets from the bottom in a shin-

gle stream, without turning them over.

• Most rotary or platen die cutters cut from

the top and therefore need the printing

on the bottom.

• Stitching machines and tape applicators

insert staples or apply box-joining tape

from the top.

In all of the above cases, printing must take

place on the bottom of the sheet. While all

the above scenarios can be overcome, it

would certainly be at the cost of additional

equipment, such as pile turning devices.

On a top printer, the printing is done above

the board line (where board dust and debris

cannot fall onto the printing plates) and away

from floor-level dirt (Figure

) One major

advantage of top printing is that the printed

image can readily be seen without removing

a sheet from the in-line process.

With a cam roller feeder,

a set of cam-shaped

feed wheels contacts the

bottom blank in the pile

and pulls it into the nip

of the feed rolls.

To Printing

Section

Feed Rolls

Cam Roller

Feed Wheels

Vacuum

Chamber

Plate Mounting

Generally speaking, photopolymer plates

have made tremendous progress in corru-

gated printing and are used almost exclu-

sively today, especially in the value-added

graphic arts field.

On sheet-fed corrugated flexo presses,

plates are very seldom glued directly onto

the print cylinder. However, this practice

does still exist for mounting so-called “slugs,”

i.e., when only a box certificate or a bar code

is needed. For such applications magnetic

cylinders are generally preferred, where the

printing plates are pre-mounted on 0.010"

sheet steel.

Plate mounting methods in corrugated

vary. However, the most widely used method

uses polyester, vinyl or other plastic materi-

al, 0.030" thick, onto which the plates are

glued. This plastic mounting or carrier sheet

is provided with a lead-edge hook, which

locates into a slot on the press cylinder

(Figure

). Slots are provided in the tail

end of the mounting carrier sheet, where

elastic straps are used to tighten it around

the printing cylinder.

Several other mounting methods exist,

such as full wrap-around systems with a

take-up device, as well as steel-backed plates

for mounting onto magnetic cylinders.

In a typical high-production box plant, plate

slugs are mounted on print carrier sheets. The

slugs are designed according to customer

specifications and ordered from an outside

supplier. The designs are often simply com-

pany logos, but can be made for almost any

graphic design, including halftone screens.

Text can also be engraved on the slugs.

The slugs are made of photopolymer or

rubber compounds that easily conform to the

round shape of the print cylinder. When the

slugs have been manufactured and delivered,

they are mounted onto a carrier sheet. A

number of different slugs can be attached to

a single carrier sheet. All slugs required for

printing a single color are mounted on one

carrier sheet and arranged to register with the

panels of the final printed boxes (Figure

A centerline mark is placed on the print car-

rier sheets and on the print cylinder.

Centerline marks are used as a tool for

mounting the print carrier sheets in the cor-

rect location on the print cylinder. When the

machine is running, the centerline of the

sheet should be aligned with the centerline of

the print cylinder. Pin registering systems

may also be installed on the press and the car-

rier sheet to facilitate correct plate-mounting

and registration on a press.

Pull Bands

In addition to the print carrier sheets, pull

bands are also placed around the print cylin-

PRESSES AND PRESS EQUIPMENT 107

With its print

components positioned

above the board line,

a top printer allows

visual inspection of a

job without having to

remove a sheet from the

line.

A popular method of

platemounting for

corrugated involves use

of a plastic mounting or

carrier sheet with a

lead-edge hook that

locates into a slot on

the press cylinder,

corresponding with the

slots on the tail end of

the mounting sheet.

The elastic straps are

then tightened around

the printing cylinder.

Anilox Roll

Print Cylinder

Printing Plate

Impression

Roll

Wiper Roll

Pull Roll

Collar

Pull Roll

Shaft

Pull Roll

Ink

108 FLEXOGRAPHY: PRINCIPLES & PRACTICES

In this mounting

scheme, plate slugs

have been mounted

about a center register

line, which is used to

aid in locating the

corner sheet on the print

cylinder.

Pull bands, placed

outside the width of

the print blanket, will

grip the outside edges

of the sheets as they

pass through the print

station, holding the

sheets in position

during the printing

process and advancing

them to the next station.

der (Figure

7 ^

). The pull bands are placed

outside the width of the print blanket, where

they will grip the outside edges of the sheets

as they pass through the print station. On

large, full-cover images, the pull bands may

be incorporated with the printing plates on

the same carrier sheet. Using pull bands will:

• eliminate skewing for square boxes;

• eliminate shallow slots for better-

closing boxes; and

• eliminate floating in the print station

for better print register.

Pull bands are used when small sheets

(less than 22") are run through the machine.

The pull bands help to hold the small sheets

in position during the printing process and

advance them to the next station. They are

also used when there is not enough printing

plate in contact with the sheet to hold the

sheet square as it moves into the next nip

point. Pull bands are necessary when only

one end of the sheet is being printed.

Any time that a print cylinder does not

have a print blanket mounted on it, pull

bands that wrap all the way around the cylin-

der should be used. Use of wrap-around

bands eliminates the setup time of trying to

Counter-impression Roll

The counter-impression roll is a smooth

steel cylinder located directly opposite the

print cylinder, forming the impression nip.

The impression roll imparts a light pressure

on the back face of the blank as it passes over

the printing plate. The light pressure exerted

ensures a positive contact of the liner with the

printing plate, necessary for transfer of ink.

The nip with the printing plate also assists in

pulling the blank through the press.

Permanent-mesh Coupling

The position of the impression roll with

respect to the print cylinder is important to

the efficient operation of the machine and

the print quality on the blank. The gap

between the print cylinder and the impres-

sion roll should allow contact between the

blank and the printing plate without crush-

7 %

7 ^

ing the blank. The range of nip adjustment

necessary to accommodate various calipers

of combined board, i.e., 0.040" to 0.300" (1

mm to 7.6 mm), is achieved by the use of a

permanent-mesh drive coupling mounted

between the driven gear and the counter

impression roll drive (Figure

7 &

). This cou-

pling allows the gears in the drive system to

remain in a constant mesh position while

allowing the impression roll to be moved.

These permanent-mesh couplings are also

used on other shafts such as the anilox roll,

pull rolls and slotter shafts, where a large

range of nip adjustment is required.

Inks

In flexographic postprinting, water- r e -

ducible inks are used almost exclusively.

The biggest challenge in corrugated post-

print for ink suppliers is to overcome the

wide range of substrate variables. There-

fore, wide volumetric differences of anilox

rolls are used to try to conform the box or

display to consumer specifications. In corru-

gated postprinting, ink consumption may

vary from run to run: from as little as 2.5

B C M / i n

(billion cubic microns per square

inch) on a high-holdout liner to 10 BCM/in

on a high-recycle content kraft linerboard.

It is still a trend today to supply the corru-

gated industry with fast-drying inks (one of

the attributes of flexo inks) even though dry-

ers are installed on many presses.

Anilox Rolls

There are no limitations as to what type of

anilox roll may be used for corrugated print-

ing. Because of economics, mechanically

PRESSES AND PRESS EQUIPMENT 109

Permanent mesh-drive

coupling between the

driven gear and the

counter impression roll

drive allows the gears in

the drive system to

remain in a constant

mesh position, ensuring

proper contact between

the printing plate and

blank.

7 &

110 FLEXOGRAPHY: PRINCIPLES & PRACTICES

engraved rolls are the most commonly used;

h o w e v e r, use of laser-engraved ceramic rolls

is increasing.

Since process printing on corrugated is

finding more and more applications (even on

old machines), improvements in print quality

are needed. The first improvement necessary

is to obtain better ink metering through the

use of finer (laser-engraved) anilox rolls. In

many cases, retrofits of reverse-angle doctor

blades or even chambered doctor-blade sys-

tems can improve print quality. The long life

span of laser-engraved ceramic-coated rolls

with blade-doctoring systems seems to offset

the additional purchase cost for laser-

engraved versus mechanically engraved rolls.

Because of the very wide spectrum of

graphics printed onto even more variable

substrates, almost any shape of anilox cell,

widely varied volumetric capacities and

probably any available screen-angle (en-

graving angle) can be found on different cor-

rugated flexo presses.

SHEET TRANSPORT SYSTEMS

In order to print correctly, each sheet

transported through a flexo press must trav-

el at the exact same speed as the circumfer-

ential speed of the print cylinder. Since the

inception of flexo printing, sheet con-

veyance has been assured through pull

rollers, also called pull collars, from the

feeder through the printing units and other

converting elements.

Pull-roller

On a flexo folder- g l u e r, two steel pull-roll

shafts are located in each printing station

(Figure

7 *

). The purpose of the pull rolls is

to control the blanks and, on the last print

station, to crush the fluted edges where the

two ends of the box are glued together.

Thus, when the blank is folded into a box,

the overlapping parts will not be thicker

than the walls. This procedure allows the

boxes to stack flatter when folded. If the

fluted edges are not crushed properly, the

finished boxes will bow in the middle and

will not stack evenly.

The upper roll has two laterally adjustable

collars on the shaft, one for each end of the

blank. The pull collars on the printing station

are serrated, whereas the collars on the last

station are designed to crush the flutes on

the edge of the board so that the board edge

will not spring back into shape.

The nip formed by the collar and lower pull

roll is used to grip and pull the blanks

through the printing station. The nip, or gap,

between the pull collar and the bottom pull

roll is adjustable. The gap must be set

according to the caliper of the blanks being

used. If the gap is too wide, the blanks will

not be held securely during the printing

process and registration will not be held. If

the gap is too narrow, the blanks will be

crushed, resulting in reduced box strength.

The distance between pull roller nips dic-

tates what minimum size sheet can be

processed in the throughput direction of the

machine. On a machine with pull roller nips

18" apart, the minimum sheet throughput

dimension needs to be 18.375", so that it

remains in control between nips at all times.

Any time a print cylinder does not have a

print blanket mounted on it, pull bands that

Pull rolls control the

blanks and, on the last

print station, crush the

fluted edges where the

two ends of the box are

glued together.

7 *

wrap all the way around the cylinder should

be used (See Figure

7 ^

). Wrap-around pull

bands eliminate the setup time of trying to

positioning pull bands on the print cylinder,

they should be set on the edges of the sheet to

provide an equal pull across the sheet.

Pull-roller pressure must be adjusted to

hold each sheet firmly in order to avoid slip-

page or skewing. Diameters of pull collars

must be extremely accurate and should be

checked frequently for wear or ink and fiber

buildup. Even with all parameters in good

working shape, pull roller machines gener-

ally yield register tolerances of ±0.0312"

between two printing units.

Since most pull-roller machines are gener-

ally linked from gear to gear, cumulative

units may be severe. However, several thou-

sands of this type of machine are success-

fully used throughout the world to satisfy

printing criteria in the corrugated field.

Vacuum and Belts

Several pull-vacuum belt transport systems

for sheet transport through flexo units (and

other elements) can be found today on cor-

rugated sheet-fed printing presses. One such

system, used on a top printer, has a long, end-

less timed belt with vacuum holes. The belt

reaches from the feeder, starting after the in-

feed pull rolls, through two, or up to four,

printing units (Figure

7 (

). This sheet trans-

port belt actually travels over the counter-

impression roll, holding the sheets while they

are being printed. The belt is of uniform

thickness to assure even ink transfer onto

the board it transports. The belt is kept in

correct timing and at the correct speed by

timing belts vulcanized to its underside.

This system assures improved register

compared to a pull roller machine. How-

e v e r, the belt is prone to becoming dirty and

clogged if ink spills into the inking units. Ink

spillage is one of the disadvantages of top

printing, since the sheets and transport sys-

tem are below the inking units.

Another vacuum-belt transport design

incorporates vacuum belt sections between

printing units, with several belts side by side

reaching from the exit of a printing nip to the

entrance of the following one. In certain

cases, such belts are movable across the

width of a machine, according to sheet size.

The more common design has multiple per-

manently positioned belts spaced across the

machine width, with a vacuum strong

enough to hold any size sheet.

One design incorporates speed adjust-

ments, by servo drives, of any transport belt

section between printing units to correct for

possible register inaccuracies. Vacuum belt

transports are a significant improvement

over pull roller sheet transports.

Rollers and Vacuum

The latest design for sheet conveyance

through a printing press incorporates close-

ly spaced precision rollers (Figure

8 )

) .

Large vacuum fans pull the sheets against

these rollers with a force equal to about 2" of

water column. The transport rollers have a

circumferential speed corresponding to the

print cylinder’s surface speed. Such an

arrangement assures an extremely high-pre-

cision sheet transport without skipping or

PRESSES AND PRESS EQUIPMENT 111

A vacuum-belt

transport system

assures improved

a pull roller machine.

The timed belt reaches

from the feeder through

the printing units.

This sheet transport belt

actually travels over the

counter-impression roll,

holding the sheets while

they are being printed.

7 (

112 FLEXOGRAPHY: PRINCIPLES & PRACTICES

skewing. On some of the latest presses a vac-

uum is even created around the counter

impression cylinder, assuring that the sheets

are pulled perfectly flat. This method

assures so-called “kiss” contact between the

printing plates and the corrugated sheets.

This type of vacuum transfer is generally dri-

ven by a line shaft through as many printing

units as are necessary.

The many advantages of such a sheet trans-

port system are: register accuracy of ±0.008"

through six-color stations can be achieved

reliably; full-coverage printing, side to side

and front to back, without any trim, is easily

achieved; no setup time is needed; and no

contact on or near any printing is ever made,

in contrast to pull-roller machines.

PRINTER-SLOTTER

The printer-slotter was probably the first

machine to be equipped with in-line flexo

units, replacing the letterpress printing units

from which flexo-printing units have evolved.

The printer- s l o t t e r, as a self-contained enti-

t y, still exists today. However, it is mostly

used to produce very large boxes in small

quantities. Most of these machines are fed

manually and have either a kicker bar or

chain feeder. Almost without exception, a

very simplel a y - b o ybelt stacker is used to col-

lect the finished printed-and-slotted blanks.

In a few cases the printer-slotter is equipped

with more automation and may fall into the

category of “jumbo” flexo-folder- g l u e r . Such

machines can handle blank sizes up to 104" x

210" for the production of furniture boxes and

other bulk boxes or containers. In most

cases, one or two flexo units, in their simplest

form, with roll-to-roll metering, are in-line

with such machines. While the printing may

not be very sophisticated, the need for graph-

ics with text or symbols can be achieved as an

in-line operation.

PRINTER DIE CUTTERS

There are two categories of printer die cut-

ters: the printer with rotary die cutting, and

the printer with platen-type die cutting. In

North America, the printer/rotary die cutters

outnumber printer/platen die cutters by a

ratio of about 10 to 1. In Europe, this trend is

exactly the opposite. Rotary die cutters,

known for their simplicity and high speed, are

today available in various degrees of sophisti-

cation with up to six flexo printing units in-

line. Any ink metering system, from roll-to-

roll to combination systems of roll-to-roll plus

chambered or reverse-angle doctor blades,

are offered by a growing number of manufac-

turers. Print register accuracy of ±0.020"

through all colors and process printing up to

85 and 100 dpi are achievable. In its more con-

ventional form, the printer/rotary die cutter

(soft anvil die cutting) is considered a “work-

horse” in brown box plants, where 15% to 18%

of production consists of die-cut blanks.

FLEXO FOLDER-GLUER

An entire book on the flexo folder-gluer has

been written by Joel J. Shulman and was first

published in 1986. It took Shulman six years

to gather technical input on flexo folder- g l u e r

operations alone, without getting into in-

depth printing techniques on corrugated

Closely spaced

precision rollers with

circumferernetial speeds

corresponding to the

print cylinder’s surface

speed assure extremely

high-precision sheet

transport without

skipping or skewing.

8 )