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

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66 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

he following section will outline

the minimum mechanical re-

quirements needed to achieve

satisfactory results from mount-

ing and proofing equipment.

Machinery used for mounting

and proofing halftone process color work

requires the smallest mechanical tolerances

achievable.

EQUIPMENT CALIBRATION

The following inspection checks and cor-

rective adjustments should be carried out in

the order listed to ensure proper mechanical

function.

Leveling the Machine

Accurate leveling avoids proofing prob-

lems. The machine must be level along the

plate cylinder pedestal-support beam and

transversely across the side frames. An out-

of-level machine will have a twist, causing the

plate cylinder to be out of parallel with the

impression cylinder. As a result, the optical

mirrors will show error from left to right and

the reference straightedge will not be true.

To level the equipment correctly, all for-

eign matter, dust and dirt must be cleaned

from the machine. For side-to-side leveling,

a properly calibrated and adjusted machin-

ist’s level should be placed on the top of the

impression cylinder and square with the

scribe line. For front-to-back leveling, the

level should be placed on each of the end

support frame (in many models, the level is

built into these frames). The adjustable feet

are raised or lowered as necessary. On older

models, it is important to specifically check

the level of the plate cylinder compared to

the impression cylinder.

Impression Cylinder Concentricity

The impression cylinder usually cannot be

adjusted within the end-support frames

which hold it, and becomes the object

against which all other parts must be judged.

The impression cylinder must be as nearly

perfect in concentricity and surface condi-

tion as possible with zero taper.

Once the impression cylinder is absolutely

level, it should be checked using a dial indi-

cator. A dial indicator, capable of showing

deviations of 0.0001", mounted on a magnet-

ic base, should be used to take concentricity

readings in the middle and about 5" from

each end of the cylinder.

Total concentricity run-out should not

exceed 0.0005". If the excess run-out is con-

stant, it may be possible to rotate or change

impression-cylinder bearings to improve the

condition.

Using an outside diameter micrometer, the

cylinder is measured for taper, with readings

taken every few inches along the cylinder

length. Deviations should not exceed

0.0005".

Condition of Plate Cylinders

Plate-cylinder walls must be clean and free

of foreign matter, including ink and grease.

They should also be free of cuts, nicks, dents

Preparing for

Mounting and Proofing

or other surface damage. Plate-cylinder jour-

nals, as well, must be clean and free of sur-

face damage. Check the concentricity of the

plate cylinder using a dial indicator. The

total indicated run-out (TIR) should not

exceed 0.001" for line work and 0.0005" for

process work. Surface nicks and scratches

can be buffed out with fine 400- to 600-grit

emery paper.

Note: Each time a plate cylinder is used

it should be checked for concentricity with

a dial indicator before mounting. Damage

which affects cylinder concentricity could

have occurred since its last use.

Check the plate-cylinder diameter and

taper following the same procedure as

above for checking taper of the impression

cylinder. Deviation should not exceed 0.001"

across the entire face of the cylinder.

Normally, the diameter and taper check

need only be made as new cylinders are

received or as used ones are re-machined or

resized to correct surface damage.

When a sleeve-cylinder system is used, the

concentricity of the sleeve, the shaft and the

complete assembly must be checked very

closely. Each assembly and subsequent

change of assembly can affect final concen-

tricity of the plate-mounting surface.

Plate-Cylinder-to-

Impression-Cylinder Relationship

The mechanical relationship between the

impression cylinder and the plate cylinder

must be such that repeated proofs under the

same settings will yield the same results.

This requires the elimination of lost motion

by adjusting screws, plate cylinder and bear-

ings, as well as any devices that hold the

plate cylinders and the impression cylinder

in position. Older equipment can develop

excess wear and warrants being inspected

for possible refurbishment.

The plate-cylinder journals and bearing

supports, or pedestals, must always be par-

allel to the impression cylinder. Parallelism

is essential to detecting nonuniform areas in

the plates, such as low spots or nonprinting

areas, during proofing.

A suitable method of testing parallelism on

adjustable-plate cylinder models uses a

mechanically correct plate cylinder, set in

the normal proofing position, without a gear

attached. The eccentric journal bearings on

the plate cylinder pedestal are turned to the

neutral position, ensuring the cylinder sup-

port pedestals are firmly locked into posi-

tion and allowing no fore-and-aft play. Three

2"-wide strips of cellophane are inserted

between the printing and impression cylin-

ders, one strip at each end and one at the

center. The impression cylinder is brought

into contact with the printing cylinder. If

plate-cylinder pedestals are motor activated,

the plate and impression cylinders should be

brought into contact cautiously and slowly

to avoid damage.

Under these conditions, each cellophane

strip should pull out with the same amount

of resistance. If not, the equipment must be

corrected mechanically until satisfactory

parallelism is achieved.

The verticality and/or height of the plate-

cylinder support pedestals may be the cause

of unsatisfactory parallelism. In some rare

instances, they may require shimming or re-

machining. If this is the case, contact the

equipment manufacturer for recommenda-

tions on remedial action.

For fixed-plate cylinder support models

with adjustable impression cylinders, the test

can be conducted in a similar same way, as

long as the impression cylinder is in a neu-

trally “square and parallel” position.

Condition of Gears

Make sure the gears are clean and free of

damage or missing teeth. The plate-cylinder

gear must fit the cylinder journal snugly with

no more than 0.002" total tolerance. The gear

must not be misaligned or “cocked” when

the securing set screw is tightened. It is rec-

MOUNTING AND PROOFING 67

68 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

ommended that a gear be purchased for

each cylinder repeat and only used for

mounting. This will help to eliminate anoth-

er variable in the process.

CARE OF EQUIPMENT

The mounting equipment, impression sur-

faces, plate cylinders and gears are precision

tools and must be handled with care and

protected from abuse. It is essential that all

equipment used for mounting process work

is kept as clean as possible in order to main-

tain sharp, accurate proofs.

Daily attention should be paid to the fol-

lowing instructions:

• Keep all surfaces clean.

• Make sure the plate and proofing cylin-

der surfaces are free of any surface

damage, including knife cuts. The dis-

placed metal makes it more difficult to

take sharp, accurate proofs. If an

impression surface is damaged, contact

the equipment manufacturer for recom-

mendation of corrective action.

• Lubricate all metal parts that mate and

move. Apply a light film of oil to all

unpainted portions of the machine to

prevent rust and corrosion.

• Carefully handle and clean mirrors.

Note: The semitransparent mirror

coating can be destroyed by oily fin-

gerprints and by improper cleaning.

Do not use solvents that may dissolve

the coating. Use only a soft, lint-free

cloth lightly dampened with a mild

soap and water solution or any pre-

pared glass cleaner. Caution should be

taken to avoid getting moisture

between the optical bar and mirrors.

Rust may develop and effect mirror

accuracy. If rust is present, it must be

removed.

• Every few months, or more frequently,

depending on the average hours of

machine operation per day, add one or

two charges of standard machine

grease into the machine’s grease fit-

tings. Clean the fittings with a cloth, and

remove all foreign matter before apply-

ing the grease.

• Make it a good practice to periodically

wash the machine and remove all old oil,

grease, ink and other substances. Any

solvent that will cut the oil, grease, ink

and foreign matter may be used, but take

care to keep the solvents from coming

into contact with the mirror viewer on

the optical mounting and proofing unit.

• Check all electrical connections and

wires. The frequency of these checks will

depend on how often the machine is used.

UNDERSTANDING THE

MOUNTING INSTRUCTIONS

A great deal of planning has gone into any

job before it reaches the mounting and

proofing department. It is necessary for the

mounting operator to understand each step,

and make certain everything is ready before

mounting plates (Table 15). The production

order must be studied carefully to ensure

that the following are correct:

• The plate-cylinder repeat size is the

same as the design repeat size or multi-

ples of it.

• The reading direction of the plates (ver-

tical, horizontal, right side up, upside

down, etc.) is specified in relation to the

web at the rewind. The “rewind figure”

shows the possible copy positions on a

given web.

• The gear-pitch diameter is the same as

the plate-cylinder diameter with mount-

ed plates in place.

• The positioning of the plates across and

around the plate cylinder, i.e., the loca-

tion, size and color of the eye spots,

guidelines and register marks are cor-

rect.

• The side of the substrate to be printed,

either surface printing or reverse printing

(i.e., printing on the underside reversed

for proper reading through the film) is

stated.

• The number of colors, color sequence

and the key color to determine which

set of plates all others will register to,

and therefore, the first set of plates to

be mounted is identified. The same

applies to the sets of plates to be mount-

ed second, third, etc.

• A study made of the layout, digital

proof, color sketch, printed sample,

blueprint, die-line layout or finished art-

work to determine the positioning of

plates as to bag, over-wrap or other

package, sealing, trimming of stock or

copy bleed.

• Special bag, carton or other package

construction specifications, such as spe-

cial lip, header, bag, double wall, special

seams, folds, slots and special perfora-

tions and opening instructions must be

noted and adhered to.

TOOLS NEEDED

The mounting and proofing department

should have the following tools on hand to

help in professional mounting. Of course,

some tools are more essential than others,

and these include but are not limited to:

• smooth-surfaced proofing paper, approx-

imately 0.003" thick ;

• four ink braying rollers;

• sheet of Plexiglas for ink rollout;

• set of proofing inks;

• set of Allen wrenches,;

• feeler gauges;

• dial indicator and magnetic base;

• steel rule;

• ball-point pen;

• makeready tape of various thickness;

• cleaning rags;

• magnifying glass; and

• razor blade knives.

A more comprehensive list of tools

appears in Appendix A.

MOUNTING AND PROOFING 69

Table 15

■ Plate-cylinder repeat size

■ Reading direction of the plates in relation to

the web at the rewind.

■ Gear-pitch diameter.

■ Positioning of the plates across and around

the plate cylinder.

■ Side of the substrate to be printed

■ Number of colors, color sequence and the

key color

■ Plate position as to bag over-wrap or other

package, sealing, trimming of stock or copy

bleed

■ Special construction specifications, special

perforations and opening instructions

PRODUCTION ORDER CHECKLIST

PRIOR TO PLATE MOUNTING

70 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

ith clean, level, mechan-

ically true mounting and

proofing equipment that

is in good repair, the

specific mounting in-

structions of the job

understood and the necessary tools on hand,

the operator should be ready to begin plate

mounting.

Mounting and proofing a critical process

job is very similar to a critical line job.

Helpful hints for accomplishing the process

job will be found in appropriate steps

throughout this dissertation on line work.

PLATE-MOUNTING PROCEDURES

1. Assemble all plate cylinders, gears and

bearings necessary for the job and

check each for correct size, mechanical

fit and cleanliness.

2. Position the plate cylinder by measur-

ing the distance between the two plate-

cylinder journal bearings – center to

center. Position the plate-cylinder sup-

port pedestals equidistant from the

plate-cylinder bearings and far enough

to the left to permit the plate-cylinder

gear to align with the gear on the

impression cylinder.

3. Place the plate cylinder on the cylinder

supports. When possible, mount the job

using the same plate-cylinder bearings

to be used in the press. Move the cylin-

der supports inward to allow the small

cylinder-support bearings to touch the

plate-cylinder walls on each end. The

entire assembly must allow the plate-

cylinder gear and impression-cylinder

gears to align. Lock the supports firmly

into position. Avoid too much forward

pressure as this will reduce the advan-

tage of the anti-backlash gears of the

mounting equipment.

4. When the final position of the plate

cylinder is set, lock it in position allow-

ing no free movement – either side-to-

side or up-and-down. The plate-cylinder

gear should now be engaged with the

impression-cylinder gear; check that

the plate-cylinder gear is also locked in

place.

5. Clean the surface of the plate cylinder

again to make sure it is free of oil, ink,

grease and foreign matter.

6. Position the dividing head on the plate-

cylinder journal opposite the gear end

for jobs requiring multiple repeats

around the cylinder

7. Apply proofing paper with the proper

thickness to bring the impression cylin-

der on gear pitch. Most manufacturers

undercut 0.003" for proofing paper.

(Consult the manufacturer for the cor-

rect thickness.) The paper should be

white with a machine glaze (MG) or

machine finish (MF) as minimum

smoothness. The surface of bleached

kraft is generally too rough and irregu-

lar in caliper to disclose plate variation

for high quality plate makeready. An

extremely clean and unmarred surface

Mounting and Proofing

a Complete Line Job

on the impression cylinder is important

to professional proofing.

8. The sheet of proofing paper should be

precut to size. Knife cuts and proofing-

paper trimming must not be done on the

impression cylinder.

9. Apply proofing paper to the impression

cylinder with masking tape, aligning the

edge of the paper with the scribe line on

the impression cylinder. The paper

should be pulled tight and securely

taped to the impression cylinder in sev-

eral places to eliminate the possibility

of movement or buckling.

10. Draw a complete plate layout on an opti-

cal or mechanical mounting machine to

ensure an accurate, press-ready job.

Take time to mark all the information

given on the job order or printing

instructions. This is also another way to

check plate layouts, avoiding possible

mistakes. Figure

and

, are exam-

ples of layouts, typical of a web press,

properly drawn on a mounting and g

machine using the built-in scriber, tape

and dividing head. An example of a lay-

out for a corrugated box is shown in

Figure

The job shown in Figure

, called for four

design repeats, a minimum of four plates –

two across the cylinder and two around the

cylinder – offset or staggered 180°. The

plate-mounting operator begins the layout

by setting the scribing pen to line up with the

center of the plate cylinder. The scribing pen

MOUNTING AND PROOFING 71

An example pf a

properly drawn layout

on the impression

cylinder. This job calls

for four design repeats.

Start of Repeat Left Side

Cut Off First Imprint-Left Side

Cut Off Second Imprint-End of Repeat

Plate 1

Plate Scribe Line

Plate 3

Plate Scribe Line

Start of Repeat Right Side

Cut Off First Imprint-Right Side

Cut Off Second Imprint-End of Repeat

Plate 2

Plate Scribe Line

Plate 4

Plate Scribe Line

Eye Spot

Position

Eye Spot

Position

Eye Spot

Position

Eye Spot

Position

Edge of Web

Center of Plate Cylinder

Cylinder End

Edge of Web

Cylinder End/Gear Side

72 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

A properly drawn

layout on an impression

cylinder.

A properly drawn layout

for a corrugated box.

All sections of the box

are drawn to exact

dimensions and the lead

edge of the box is pos-

tioned at the top.

Lead Edge of Plate Mount

Machine

Center Line

Front Panel

Back Panel

Glue Flap

Bottom

Top

Panel

Center Lines

is jogged up and down to make a broken ver-

tical line that will help to differentiate

between plate centerlines and other lines

drawn to complete the layout.

From the first plate-cylinder centerline

drawn, the operator sets the tape measure

and moves the scriber to the correct mea-

surement to draw the rest of the vertical

lines that form the layout from measure-

ments given on the specification sheet.

These lines include a line to identify the end

of the cylinder, the edge of the web, the plate

scribe lines and the eye-spot position.

Note: To avoid confusion, the only solid

lines are those to which the plates will be

mounted.

The horizontal lines are now drawn to

complete the layout. This is done by using

the dividing head mounted on the journal of

the plate cylinder. No matter what the repeat

of the cylinder, the dividing head will auto-

matically space the horizontal lines on the

layout to preset angular relationships

around the plate cylinder. The first horizon-

tal line represents the start of the printing

repeat (broken line). This is drawn with the

dividing head set at zero.

On this sample job, two plates are spaced

equally around the cylinder and the second

set is staggered. Therefore, the plate-cylin-

der repeat is divided into four intervals, 90°

apart. The plate cylinder is rotated to the

first number four on the dividing head and

the next line is drawn representing plate 1

centerline, left side (solid) and the start of

repeat right side (broken). The cylinder is

rotated to the next number four on the divid-

ing head to draw the last line or cut off the

second imprint end of the repeat. Additional

lines for eye-spot positions are measured

and drawn, and the gear side is marked on

the layout. This completes the layout. The

first plate cylinder is ready for the applica-

tion of stickyback.

Impression Cylinder Layout for

Corrugated Postprint

Drawing the job layout for plate- (die-)

mounting in the corrugated post-print indus-

try involves drawing the layout of the actual

box on paper covering the impression cylin-

der. (Figure

)

It is important to place the lead edge of the

box drawing at the top. All sections of the

box must be drawn with exact dimensions.

On mounting and proofing machines for cor-

rugated post-print, the plates (dies) are

mounted on a plate cylinder that matches the

print repeat of the press, thus eliminating the

stretch compensation factor in the layout.

In some operations, when the box to be

printed will be cut and creased, an acetate

plot of the cutting die, or a full-size digital

color proof of the images, may be simply

aligned and taped to the impression cylinder.

This eliminates the need to draw the layout

by hand, increasing productivity and accura-

cy. It is important to tape the predrawn tem-

plate “squarely” on the cylinder, with the

lead edge of the box at the top and the emul-

sion-side down against the paper to mini-

mize any effect on the cylinder’s diameter.

Cleaning the Plates and Cylinders

The back of photopolymer plates should

be cleaned thoroughly, removing any foreign

matter or particles that, if trapped between

the plate and the mounting tape, would

effect adhesion or caliper. Prepare the plate

cylinders and sleeves in the same fashion.

When cleaning cylinders and plates, it is

important to allow adequate solvent dry-

time before applying the stickyback to the

cylinder or plate. The same holds true for

flexographic adhesives. Isopropyl alcohol

works well on the cylinders or sleeve

because it leaves no residue.

Use of an incompatible cleaning solvent

can lead to plate-lift on press or make plate

removal very difficult. If the composition of

the adhesive is unknown, consult the tape

MOUNTING AND PROOFING 73

74 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

supplier to ensure that the cleaning solvent

is compatible with the cushion adhesive.

Photopolymer plates processed in new

“safe solvents” must be cleaned thoroughly

before being light finished and prior to

mounting on the cushion adhesive. These

wash-out solvents can leave an oily residue

on the back of the photopolymer plate,

which will cause plate-lift on press.

Trimming and Preparing

the Plate Edge

Whether rubber or photopolymer, finished

plates must be trimmed accurately before

mounting. The preferred method is to use a

plate cutter or foot shear. If a knife, however,

is used, the cut should be made from the

polyester backing side of the plate. Edges

must be smooth and free of nicks and burrs.

Beveling all sides of the plate is desirable

because it helps keep the plate from lifting

during the pressrun, in addition to providing

a wider area to apply edge sealant. Beveling

may be done by securing a rigid material on

the bed of the plate cutter or foot shear

(Figure

). Place the finished plate on top

of the rigid material and press firmly to trim.

This flexes the edge of the plate allowing a

neatly beveled cut edge. The position and

height of the rigid material determines the

bevel angle.

Applying the Stickyback

Double-sided adhesive film, or stickyback,

is used to secure the plates in position on the

plate cylinder or carrier in either of two

methods. In the first, if the job calls for only

a few small plates – each no longer than one-

fourth the plate-cylinder circumference – the

stickyback may be applied directly to the

plate and then the backed plate may be

applied to the bare cylinder. If compressible

stickyback is used, it should be permitted to

extend beyond the plate about 1" on all sides.

When multiple plates, spread over much of

the cylinder are being mounted, it is expedi-

ent to cover the entire layout area of the

cylinder with the stickyback and then posi-

tion and apply the plates to it.

Stickyback should be applied to the cylin-

der first when mounting individual plates

longer than one quarter of the cylinder cir-

cumference. This will prevent plate buckling

caused by the stickyback bunching when the

plates experience curvature growth at differ-

ent rates. This is especially important in

process work.

The second method is to apply the sticky-

back around the entire cylinder. If using dou-

ble-backed stickyback (protective sheet on

both faces), approximately 6" of backing

along the leading edge should be removed;

then, holding the entire piece with the side

edge as square with the cylinder as possible,

position the leading edge on the cylinder

about 0.5” above the horizontal lead-edge

scribe line. Gradually remove the remainder

of the backing sheet from below the sticky-

back. While rotating the cylinder, smooth

and press the stickyback firmly into position

all around the cylinder. If using single-

backed stickyback (protective sheet on one

face only), use a clean, undamaged piece of

discarded coversheet to prevent premature

adhesion to the plate cylinder.

Using the grooved scribe line in the plate

cylinder as a knife-blade guide, trim the lead-

ing edge of the stickyback straight. Then

Using a modified sheer,

beveling plate edges

creates a wider

surface area to apply

edge sealant and also

prevents the plate from

lifting during a

pressrun.

Plate-Edge Profile

Plate Edge Profile

Feed Board

Hold

Down

Cutting

Knife

press the overlapping trailing end down

firmly over the leading edge groove and trim

the overlap to make a perfect butt joint. The

dividing head, or the impression cylinder

drag brake, can be used to hold the impres-

sion cylinder in position while trimming the

stickyback. At this point the protective

cover should still be on the top side of the

stickyback.

Stickyback and plate-trimmng devices are

available which attach to the mounter and

can produce accurately angled butt joints.

When a single-roll width of stickyback is

not wide enough to cover the cylinder, a sec-

ond or third width can be butted to the first,

all along its length and completely around

the plate cylinder. Working from one end of

the cylinder, the first width should be laid

with its side parallel to the cylinder end to

avoid an increasing spiral. A circumferential

scribe line, or a measured and drawn line,

may be used as a guide.

The lamination of the stickyback on the

plate cylinder should be thoroughly inspect-

ed to ensure foreign particles or air are not

trapped between the cylinder and adhesive.

Small air pockets may be pierced with a pin

to release air and then smoothed out.

Zoning

When printing repeats with large coverage,

“zoning” the mounting tape prior to remov-

ing the liner facilitates easy tape removal

from the plate cylinder once the print is com-

plete. Simply put, zoning involves using a

precision knife to turn one large section of

mounting tape into several smaller sections

by following a select number of horizontal

and vertical scribe lines in the plate cylinder.

These recessed scribe lines facilitate accu-

rate incisions and prevent the formation of

burrs on the surface of the plate cylinder.

Framing and Priming

Framing and priming the back of the

plates helps in eliminating plate lift. Framing

is accomplished by masking the outer 1" bor-

der of the plate with a removable tape (prior

to applying a release agent to the back of the

plate). Once the release agent has been

applied and sufficient dry time has been

allowed, the framing tape is removed from

the back of the plate to expose the uncoated

section. This uncoated border section of the

plate will have a higher level of adhesion to

the mounting tape.

Priming consists of applying a proprietary

solution to the underside lead and trailing

edges of the plate. This facilitates a very high

level of adhesion along the edges, greatly

reducing the plate’s tendency to lift on press

Matching Plate Thickness

For jobs requiring the mounting of two or

more complete design repeats across or

around a cylinder, it is necessary that all

plates mounted on the same cylinder be pre-

cisely the same thickness. Variations of just

0.00l" to 0.002" can make a visible difference

in the final printed product, especially when

printing critical line work on smooth sub-

strates and certainly in process work. This

tolerance in thickness cannot always be

achieved among plates to be mounted on

one cylinder, especially within economic

constraints. In these cases, makeready tech-

niques need to be used to achieve the

required thickness uniformity. Thinner

plates need to be built up with makeready

tape to match thicker plates.

An alternative makeready technique is

illustrated by the following example:

Assume the job calls for a total of six

plates – three images across and two

around. Each plate has less than 0.001"

thickness variation. The total plate-to-plate

variation is 0.002", more than the normally

acceptable tolerance. The six plates should

be carefully gauged. This means taking read-

ings in many places (for process plates, per-

haps every square inch). The average gauge

is then calculated for each plate. The plates

MOUNTING AND PROOFING 75