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

Подождите немного. Документ загружается.

176 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Doctor Blades

Many flexographic printing presses are

equipped with metal doctor blades. This thin

metal strip is sharp when new, and can

become razor sharp after being used. Extreme

care must be taken when cleaning or installing

new blades in the blade holders. Wearing steel

or fiber mesh gloves will prevent cuts. Plastic

gloves are a poor alternative.

Doctor blades should be properly dis-

posed of after removal. Since they are

extremely sharp, blades should never be put

in any container that a coworker may reach

into unknowingly.

Extreme caution must be used during

press wash-ups. Usually wash-ups are done

with cloth or paper rags that are easily cut by

the doctor blade material.

Use of Rags

Rag use and the storage of soiled rags raise

both personal safety and environmental con-

cerns. From a safety stand-point, rags should

be folded neatly when used for cleaning.

Folding the rags helps prevent loose ends

from being entangled when wiping a rotating

anilox roll.

Soiled rags should only be placed in desig-

nated containers with tight closing lids

(Figure ).

Color-coded rags are sometimes used to

distinguish ink clean-ups from general shop

use. These rags should always be kept sepa-

rate from each other. General-purpose rags

often contain metal filings which may dam-

age rolls or plates if used carelessly.

Die-Cutting Safety

Care must be used when handling and

installing dies onto the press. Wearing gloves

will help prevent cuts to hands or nicks to

the die. Extreme care must also be taken

when the press is running and the dies are

rotating. Personnel must keep hands and

loose clothing clear of the rotating die. Stop

the press completely before clearing any

items that are caught or stuck in the die nip.

etting up a narrow-web press

is different from machine to

machine. This variation is due to

the wide variety of presses used

in the market today. For this rea-

son, this section will focus on

the basics of set up that are common to most

narrow-web printers.

The first step to setting up any job on the

press is to read the job jacket (Figure ).

A job jacket is a folder or envelope that

stores valuable information about the job to

be run. In many cases a prepress proof, art

or a sample from a previous pressrun will be

in the job jacket. The operator must visualize

the final product and how it will be used by

the customer by reviewing the information

on the job jacket and sample. Afterwards,

the operator can develop a set-up plan.

PRESS SETUP

In the next section, we’ll discuss the pro-

cedures for setting up the press:

Select the Print Stations

Generally, art is designed to print the light-

est color first and darkest color last when the

job is to print on the face of the substrate. In

order to hide the areas where two colors

overlap for registration (trap), the darkest

color must print on top of the lightest. The

operator must keep this technique in mind,

and also consider the following items, when

determining which print stations to use:

Existing Colors in the Press. It is important to

think ahead and plan a strategy for all the jobs

being printed, not just the present one. In

some cases, existing colors from a previous

job will meet the requirements for the next

job. If planned from job to job, color strategies

will alleviate unneeded wash-ups and reduce

changeover time.

Anilox Roll Requirements. If the operator can

set up a job to take advantage of existing

anilox rolls, less time will be spent on

changeovers.

Drying Capacity Between Ink Stations. With

large solid ink coverages, it may be neces-

sary to leave an open print station between

colors. This additional web travel length

allows extra drying time for the ink to dry

thoroughly.

Prepare the Dies

The first step of set-up is to locate all tool-

ing required for the job. This process begins

by locating the necessary dies. Care must be

taken to avoid nicking the sharp cutting edge

of the die. To avoid blade damage do not

wear rings when handling dies.

Operators should wipe the shaft and bear-

ers clean and check the blades for built-up

adhesive, ink and dust before the die is put

into the press.

All foreign material must be removed prior

to use. Extreme care must be used when

cleaning the blades on a die. A clean rag and

solvent may be used.

A wooden or plastic paint stick may also

be used to scrape away any debris. A metal

object should never be used to clean a die.

Inspect the Mounted Plates

Mounted-plate cylinders should be brought

to the press area and readied for placement in

the assigned print stations. Plates should be

checked against the design on the prepress

PRESSROOM PRACTICES 177

Narrow-Web Procedures

178 FLEXOGRAPHY: PRINCIPLES & PRACTICES

proof or the previously printed sample, as

well as to the information on the job jacket.

Plates must also be inspected for damage, ink

buildup left from previous runs, and any other

defects. The plate edges should be checked to

make sure there is no lifting from the sticky-

back. Repairs or remounts of the plate should

be done before placement in the press.

Change Anilox Rolls

The operator may need to change anilox

rolls for the job if the rolls are not present in

the press or in the correct ink station. The

appropriate anilox needed to deliver the

required color density must be determined

by either reading the job jacket or job history

sheet, or viewing the prepress proof or a

A job jacket

is a folder or envelope

that stores valuable

information about the

job to be run.

SHIPPING

OTHER OPERATIONS

PRESSROOM

PLATEROOM

STOCK

JOB JACKET

SAMPLES TO:

PACKING SLIP

SHIP VIA:

BILL/LADING

OLS

REGULAR

PLAIN

OTHER

SPECIAL INSTRUCTIONS:

SHIP TO:

NEXT DAY 2ND DAY

REWINDING

BURSTING

TRIMMING

COLLECT

OTHERSHIP FROM:

PREPAID

OLS

COD

SPECIAL INSTRUCTIONS:

CARRIER

AMOUNT PER ROLL

FINISHED SIZE

CORE SIZE

CYLINDER SIZE

NO. ACROSS

NO. AROUND

DIE SPECS

GRAPHICS SPECS

MARG. LEFT

MARG. RIGHT

HORZ.

VERT.

PERFS:

FANFOLD ATPINFEED:

NO. PER SHEET

REWIND

POSITION

HORZ.

VERT.

SLITS:

DIE NO(S)

PERF/SHEETER

PRINT CYL(S).SIZE

ORDERED

REC'D

SPACING

CYLINDER SIZE

NO. ACROSS

NO. AROUND

SPACING

DATE SENT

CARTON SIZEOTHER

AMOUNT PER PACKAGE

SPECIAL INSTRUCTIONS:

AMT. PER CTN.

INITIALS

JOB NO.

PLAINOLSSHIPPING LABEL

SHRINKWRAPPING

FLEXO

LEFT RIGHT

LETTERPRESS

SAME

MACHINEHAND

BUTT CUTDIE CUT

SHEETEDOTHERCARRIERCARDLABEL

FRONT

ROLLS

CONTINUOUS

SIZE ACROSS

CORNER RADIUS

CUSTOMER NO.

CUSTOMER

DESCRIPTION

REQUESTED SHIP DATE

RUSH NO.

SALES PERSON

PREP’D BY

ACTUAL SHIP DATE % OVERRUN % UNDERRUN

QUOTE NO.

PO NO.

REPEAT NO.

DATE ORDERED

JOB NO.

TOTAL QTY.

PRESS NO.

10)

BACK

AROUND

PRESS DRAW

LINER SIZE

COLORS

NO. OF NEW PLATES

HERE

VARNISH: OVERALL SPOT

SPECIAL INSTRUCTIONS:

YESOVERLAMINATE NO

NO. OF REMARKS MADE BY MOUNTED BY

printed sample if the job has been run before.

When handling an anilox roll, operators

must remember that the engraving on the

surface is very fragile and can be easily dam-

aged. Although a ceramic coated anilox roll

surface is very hard, it is also brittle, and can

be chipped if the roll comes into contact with

anything made of metal. Roll covers protect

the surface from damage during handling.

Die Installation and Setup

The following procedure details a typical

die installation and setup:

1. Apply spacers on the journals of the

gear side of the die. Spacers are usually

nylon, brass or fiber washers.

2. Slip on a bearing block on the gear side

of the die. A bearing block has an inner

diameter that matches the diameter of

the journal (Figure ).

3. Slide the die journal with the bearing

block into the slot on the gear side of the

press. Care must be taken not to let the

die strike the anvil roll. Place the die

gently against the anvil roll.

4. Check to see that the die is pushed firm-

ly against the gear side of the press.

5. Check the alignment of the gears and

bearers on the die with the gears and

bearers on the anvil roll. If the gears and

bearers are not aligned, add or subtract

spacers until alignment is reached.

6. Slide spacers and the bearing block on

the operator’s side die journal. Once

again, add or subtract spacers until the

die slides smoothly into the press with

no side to side play. Adjustable bearing

blocks can be used to eliminate the need

for spacers. The procedure might vary

depending on the press manufacturer

and the type of press used.

7. Install appropriate pressure assemblies,

using one of the downward-pressure

methods detailed in Table 1.

8. Install felt bearer wipers. The wipers

should be positioned to ride on the nip

and should be in contact with the bear-

ers on each roll in the die station. The

wipers keep the bearers clean and free

of dust, which may change die settings

during the press run. Oil is placed on the

wipers to lubricate the bearers of the die

station assembly.

9. Run several feet of web through the die

station. Strip away the waste matrix

from the web and attach it to the waste

rewind or bring it through to the vacuum

system. This method will vary depend-

ing on the initial setup.

10. Examine the liner to see if the die is cut-

ting with the correct pressure. If the

liner does not appear to be damaged, the

die is cutting with the correct pressure.

If the liner is damaged or if the label is

not releasing properly, the die needs to

be adjusted for the proper cut depth.

To assist in stripping the matrix, it is often

necessary to underscore the material to pro-

vide extra support to the matrix ladder.

Underscoring the edges of the stock places a

slight “U” shape on the back side of stock –

making the matrix ladder much stronger

than a flat ladder. Many times an 0.125" to a

0.025" extra substrate must be allowed on the

edge of the web to allow room for under-

scoring. This may seem like wasted material,

PRESSROOM PRACTICES 179

Die installation

begins by applying

a spacer, then fitting

on a bearing block.

180 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Table 1

There are three methods of exerting downward force to hold the die bearers firmly against the anvil roll.

Each method uses a bridge that is held on top of the press frame at each side by pins or bolts. Threaded

rods run through the bridge and apply pressure to the die when screwed down.

Method 1: The screws apply pressure directly to the die bearing blocks. The pressure, in turn, presses

downward on the shafts of the die to hold the die against the anvil roll (Figure ).

Method 2:

An assist roll is added to the assembly. An assist roll has some form of bearing on each end. Some

are removable and some are in a fixed position (Figure ). The assist roll assembly is placed in the same

slots as the die blocks, and its bearers ride on top of the die bearers. The bridge is fixed in position and the pres-

sure screws are screwed down to apply pressure on the assist roll shafts. The pressure exerted tends to be dis-

placed through the bearings on the assist roll. This displacement puts pressure directly on the die bearers.

Method 3: This method utilizes a tractor-style hold down and is similar to the assist roll method. The main

difference, however, is that the pressure is brought directly over the bearer on the die, not through the bear-

ings (Figure ). Hydraulic die pressure gauge assemblies are available to use in place of the pressure

bridge. This system allows operators to observe the force applied to the rotary die, calibrated in pounds of

force. The assembly also enables the operator to make adjustments more consistently, as well as maintain

the minimum die pressure throughout the press run (Figure ).

METHODS OF EXERTING DOWNWARD FORCE

Pressure Screw

Rotary Die

Anvil Roll

Bridge

Bearing

Block

Pressure Screw

Rotary Die

Bearer Wipers

Anvil Roll

Bridge

Bearing

Blocks

Assist Roll

Pressure Screw

Tractor

Assembly

Rotary Die

Bridge

Bearer

Tractor method of applying pressure.

Assist roll used to apply pressure.

Pressure screw applies pressure to the die bearing blocks.

Pressure Screw

Hydraulic

Pressure

Gauge

Rotary Die

Bearer Wipers

Anvil Roll

Bridge

PSI

400

800 1200

1600

2000

Bearing

Blocks

Assist Roll

Hydraulic-pressure gauge and felt wipers.

but the benefits of more effective stripping

will offset the extra stock substantially.

If a problem occurs with predispensing

labels, and more die pressure cannot be used,

strip the matrix directly from the die using no

assist roll. By stripping directly from the die,

the adhesive on the pressure sensitive materi-

al will not flow back to cause this problem.

Use Setup Stock

When setting up the press, substitute a

cheaper, lower-grade stock, called set-up

stock, for the more expensive stock that may

be specified on the job jacket. This substitu-

tion prevents the wasting of valuable material

when setting up the printing stations. The set-

up stock should be wide enough to cover the

print area and be of similar gauge to the actu-

al job. The substrate is one of the more costly

portions of a job, making it important to min-

imize stock waste when possible.

Set Edge Guides

The edge guide keeps one edge of the

web in the same position at all times to pro-

vide side to side register during production.

The sensor that monitors the position of the

web must be positioned at the edge of the

substrate.

Set Auxiliary Stations

At this point in the setup process, the auxil-

iary stations should be set up. Auxiliary station

functions may include slitting and laminating.

Dry Registration

In order to eliminate waste when setting

up a press, it is advisable to dry-register the

print stations. Dry registration of the print

stations simply means the process of setting

up the plate cylinders in the print stations as

close as possible to the required position for

color-to-color registration, and setting the

ink and impression settings before inking

the plates. The following procedure is a gen-

eral dry-register technique used by some

printers. There are many other methods to

accomplish this task.

First, use check gauges to establish paral-

lelism between the anilox to plate cylinder

and plate cylinder to impression cylinder.

This gauging also establishes a starting point

for ink and impression settings. Check

gauges are wheels that temporarily slide onto

the plate-cylinder shaft in place of the plate

cylinder. The outer diameter of the wheel is

the same as the plate-cylinder repeat length.

One check-gauge wheel is placed on each

side of the plate-cylinder shaft. Adjust the

press settings so that the wheels barely touch

the anilox roll and the web on the impression

cylinder. This adjustment is done by using a

thin piece of paper, such as a piece of release

liner, as a feeler gauge. The piece of paper

used as the feeler gauge must be thinner than

the stock being run. Keep adjusting the set-

tings until some pressure on the piece of

paper is felt from both the web and the

anilox roll. This setting ensures that the plate

roll is parallel to the anilox and impression

cylinder, and also establishes a start point for

ink and impression settings. After all adjust-

ments are made, remove the two check

gauges from the shaft and prepare to install

the plate cylinders.

Second, install the plate cylinder for each

color in the correct station in approximately

the same location. If this step is done correct-

ly, the location of the printed images will be

very close to alignment from color to color.

This step minimizes the number of register

adjustments needed during setup, and

reduces the amount of wasted stock. Each

print cylinder or gear should be marked in the

same location. This marked tooth or spot on

the cylinder is used when mounting the plates

to line up the center marks of each plate to

the accompanying cylinder. Therefore, the

images on the plates should be in close regis-

tration from cylinder to cylinder in relation-

ship to that mark. When loading the cylinders

in the press, this mark can be used to install

PRESSROOM PRACTICES 181

182 FLEXOGRAPHY: PRINCIPLES & PRACTICES

A paper fountain pan

liner reduces cleanup

time short runs.

Ink pumps lift the ink

from the buckets to

the pans in a steady

manner, thus ensuring

color consistency.

When using an ink

pump, a stand pipe will

ensure constant ink

levels in the ink pan.

Ink Pan

Stand Pipe

Ink Return Hose

the cylinders very closely (within a gear

tooth) to each other in each station. This

alignment is accomplished by drawing a mark

on the web. The mark on the web is aligned

with the mark on the cylinder or gear in the

first station. The web is then jogged forward

to the next station, where the next cylinder is

aligned to the mark. Even with 360° registra-

tion control, this method can reduce the

amount of wasted substrate and setup time.

Set Ink Distribution Unit

The ink distribution unit must be set up

before the press run can begin. The follow-

ing procedure details a typical ink distribu-

tion setup:

1. Clean all fountain pans and ink wipes.

2. Install wiper assemblies and pans into

the press following the manufacturer’s

specifications.

3. Install paper or plastic fountain pans

or liners if the press can accommodate

their use (Figure ). Using the paper

or plastic liners saves on changeover

time during short run work.

4. Install ink pumps if the job requires a

fairly long press run. Ink pumps

ensure color consistency throughout

the press run (Figure ). Pumps

come in different styles, but all per-

form the same function of getting the

ink from the ink bucket to the pan.

5. Place ink into the fountain. This pro-

cedure varies depending on whether

an ink pump is being used, or if the ink

will be poured directly into the foun-

tain. When an ink pump is being used,

place a stand pipe in the drain hole of

the fountain (Figure ). The stand

pipe keeps the ink at a constant level

in the fountain pan. Then place the ink

into the pumping unit and the supply

hose into the pan. Start the pump to

begin circulating the ink through the

ink distribution unit.

6. Check the ink’s pH and viscosity if using

water-based ink, or viscosity if using

solvent-based ink. This step should be

done to ensure that the ink is at its run-

ning parameters (Figures ).

For short runs when the ink is placed direct-

ly into the fountain, the ink must be mixed in

its container for a time. After mixing, pour a

sample of the ink into a disposable paper cup.

Check pH and viscosity in the cup to ensure it

is within its running parameters. If not, the ink

needs to be adjusted and rechecked to speci-

fications before the pressrun begins.

Set the Fountain Roll

and/or Doctor Blade

Once the ink is in the fountain and is with-

in its recommended running pH and viscos-

PRESSROOM PRACTICES 183

Testing the pH of

a water-based ink

ensures that the ink

is at proper running

condition.

Another method of

ensuring that the

ink is at running

parameters is to test

the viscosity using a

Zahn cup.

184 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ity parameters, the anilox roll may be inked.

When using a two-roll system, the fountain

roll is adjusted to squeeze against the anilox

roll. Once the press is started, the rubber roll

is adjusted to evenly squeeze the ink off the

anilox using the minimum amount of pres-

sure. This pressure setting is checked by back-

ing the fountain roll off until the operator can

see the nip flood with ink. Then the operator

squeezes back in with both sides of the rubber

roll, making sure they are even, until the flood-

ing on the surface goes away. By doing this

several times, the ink wipe is set. On many

narrow-web presses, a doctor blade is used in

conjunction with the rubber fountain roll. In

this case, the doctor blade must be set to

meter the anilox roll (Figure ).

When setting a station with a doctor blade,

the fountain roll pressure should be set a lit-

tle loose. This setting allows for extra ink to

go through to the doctor blade where it will

be metered off. The doctor blade assembly is

set into its holder and then slowly tightened

down until it touches the anilox. As the

blade is tightened to the anilox, the operator

will notice that the ink will meter from the

center of the anilox roll outward to the ends.

When the ink is metered off of the ends of

the anilox roll, the wipe is set. Similar to the

procedure for setting fountain-roll pressure,

the doctor blade should be backed out and

brought back in to the anilox several times

in order to set the wipe with the minimum

amount of pressure. Over tightening the doc-

tor blade can cause the blade to fold under

the pressure, causing the ink to dry due to

the friction and heat buildup behind the

blade. If ink drying occurs, the dried ink pig-

ments can cause damage to the anilox roll.

Once the ink is metered correctly, the ink

fountain covers should be placed on the foun-

tains. The covers reduce the amount of amine

or solvent evaporating out of the ink and help

keep the ink more stable during the pressrun.

If the press is equipped with a chambered

doctor-blade assembly, install the assembly

before the pump is turned on. The ink wipe

should be set in the same manner as a single

blade, and there is no rubber roll to set

(Figure ).

Set Impression, Inking

and Registration

The following procedure details a typical

impression, inking and registration setup:

1. With the press running at a slow speed,

start with the last color to be printed

and achieve the correct impression

and ink settings. First, turn down the

impression slightly so the plate comes

into contact with the web.

2. Turn in the anilox-to-plate adjustment

to begin inking the plate. While looking

at the print on the web, make adjust-

Maintaining proper

blade angle is impor-

tant to effective

metering.

A chambered doctor-

blade eliminates the

need for a fountain roll.

Anilox Roll

Plate

Cylinder

Fountain

Roll

Doctor

Blade

Holder

2°–10°

Plate

Cylinder

Anilox Roll

Reservoir

Ink “Out”

Return

Ink “In”

Supply

Containing

Doctor Blade

Metering

Doctor Blade

ments to the impression and ink set-

tings until there is a clear print with no

images missing.

3. Similar to the setting of the rubber roll

or doctor blade, back in and out on the

settings to achieve both ink and im-

pression settings with the minimum

amount of pressure.

4. Set the remaining ink and impression

settings, working backward to each

print station in order from last to first.

5. After all the settings are complete for

each print station, adjust for color-to-

color registration. Start with the first

station (front of the press) and set reg-

ister for each station in order.

6. Once all the colors for the job are in

to-die registration. Register the die cut-

ter and slitter to the printed image. In

all flexo press settings, it is important

to use the minimum amount of pres-

sure to do the job.

Check Colors to Standard

Once the press is set up and all settings are

complete, the printing must be checked for

accuracy against a color standard approved

by the customer in the design stage of the

job. A color standard may be a previously

printed sample, a Pantone

®10

sample, an ink

drawdown or a spectrophotometer reading.

All visual color-matching should take

place in a viewing booth designed for this

purpose. This type of booth provides a stan-

dardized light source with a neutral gray

background to check color, eliminating the

color variations during the color-matching

process caused by lighting conditions.

A sample of the print should be compared

to the color standard to ensure that the print-

ed job meets the customer requirements. The

operator should check to make sure the color

is printing at the correct density (lightness

and darkness) either visually or with the use

of a densitometer. If the color is not printing

at the correct density, the anilox may be

changed to achieve a density match with the

standard. This change is especially useful

when working with ultraviolet inks. For slight

density changes that are needed when work-

ing with conventional inks, the ink viscosity

may be changed by adding solvent, or the ink

may be extended to reach the color standard.

The ink hue should also be checked visual-

ly in a light booth or with the aid of a spec-

trophotometer. If changes in hue are needed,

the operator should add the correct ink bases

to reach the color standard. All of the infor-

mation from the color-matching process

should be documented so the job can be

reprinted without additional downtime.

Approval Form

Many companies use a checklist approval

form that is filled out after the job has been

set up and approved (Figure ). This form

helps ensure that all the details for printing

the job have been covered and that the job

matches the specifications on the job jacket.

PRESSRUN PROCEDURES

Next, we’ll discuss the areas that must be

monitored during pressrun:

Ink Viscosity and pH

Maintaining viscosity, or pH and viscosity

in the case of water-based inks, is essential

for producing consistent print throughout

the production run. If an ink is color-matched

at a 25-second viscosity, a higher viscosity

will make the color print darker. A higher vis-

cosity occurs because the solvent or amines

evaporate out of the ink during the pressrun,

causing the customer’s product to become

inconsistent as the run progresses. In order

to maintain the same density of print

throughout the press run, viscosity (solvent-

PRESSROOM PRACTICES 185

Pantone, PMS and the Pantone Matching System are trademarks of Pantone,

Inc.