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

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96 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

mandrel. Once the correct thickness of nick-

el has been achieved, the sleeve formed is

removed from the mandrel and trimmed to

the correct length. By using the electrolytic

method, a completely seamless sleeve with

extremely uniform thickness is achieved.

Composite sleeves are also available. The

definition of composite in the plastics indus-

try (where these materials were developed) is

a polymer (plastic) which is reinforced with a

fiber such as fiberglass or carbon fiber. Many

combinations are possible due to the avail-

ability of different materials and methods for

putting them together. The fiber can be con-

tinuous and woven to give greater strength

and stiffness in both bilateral directions.

Another method for producing sleeves is

referred to as filament winding. This method

lays continuous fibers in specific directions to

give very exact design properties.

Composites can be tuned to achieve many

different properties. For example, print-

enhancing or “cushioned” sleeves are com-

prised of a urethane covering on top of a

base composite sleeve. Sleeves can be made

in different thicknesses to build up repeats.

No matter which materials and manufactur-

ing methods are used, personal preference

often plays a part in which system is adopt-

ed by a printer. Cost is also a factor. One area

of concern to a flexo printer in the design

and use of composites is the weakness in the

Z direction, which can result in the polymer

delaminating from the fiber layer. Fiber

direction obviously affects this property as

does the choice of fiber.

The surface properties of composites are

quite different from metallics. The surface is

dependent on the polymer used and is not as

resistant to knife cuts and gouges as metal.

Another limitation is that most polymers

cannot be used in the high temperature envi-

ronments used to vulcanize elastomers.

Because of this, vulcanizing on composites

can be tricky and must be done within the

temperature limitations of the material being

used. In order to take full advantage of these

material strengths, as well as controlling the

costs, many printers are adding the compos-

ite sleeves as a third part of their system.

They continue with their nickel plate carrier

sleeve and the base cylinder. To this, they

add the composite sleeve, when needed, to

print a repeat for which they don’t have a

cylinder. The same result can be achieved by

vulcanizing a thickness of rubber onto the

nickel sleeve. The built-up sleeve can be

used either with a printing plate or as a

direct printing plate to produce continuous

solids. It can also be used to apply tints or

varnishes.

Another application is to laser engrave the

rubber-covered sleeve making it into a con-

tinuous printing design roll. Previously, laser

engraving was carried out on rubber vulcan-

ized directly onto the plate cylinder. This

made transportation of the heavy printing

roll difficult and expensive. Now, a laser-

engraved rubber-covered sleeve can be

shipped with far greater ease.

Sleeves can be made which are covered

with unexposed photopolymer. The thick-

ness of the sleeve can be varied to achieve

different repeat lengths and the final sleeve

is a seamless sleeve with unexposed pho-

topolymer. The floor of the plate can be

established prior to mounting on the sleeve

or while mounted. Once the photopolymer is

affixed to the sleeve, it can be exposed in

two ways. One is with the use of a negative

contact film with conventional light expo-

sure. Once exposed, the photopolymer is

processed on the sleeve. A second method is

to use direct imaging of the photopolymer

on the sleeve. This is called computer-to-

sleeve or CTS. In this process, the unex-

posed photopolymer has a mask on the top

surface which is ablated by a laser imaging

system. The system and process is the same

as direct-to-plate (DTP), also called comput-

er-to-plate (CTP), except that the photopoly-

mer is a permanent part of the sleeve.

Computer-to-sleeve systems combine the

advantages of sleeve mounting with those of

digitally imaged photopolymer plates, name-

ly lower dot gain and extended highlight

range. In both methods of exposure, con-

ventional or computer-to-plate, no distortion

correction is needed since the plate is

imaged in the round.

Mounting Procedures

The most popular method of ensuring that

the sleeve maintains an aggressive, non-slip

contact with the plate cylinder is to make

the inside diameter of the sleeve slightly

smaller than the outside diameter of the

plate cylinder. Then, by means of com-

pressed air, the sleeve is expanded suffi-

ciently to slide onto the plate cylinder. Once

it is in position, the air is disconnected and

the sleeve clamps tightly onto the cylinder

surface. This is achieved by a simple conver-

sion of existing plate cylinders. Air is forced

into the plate cylinder at one end and exits

through a series of holes that are drilled

close to the opposite end of the plate cylin-

der. The air flow expands the sleeve, which

can now slide over the roll with ease. Care

has to be exercised not to go past the air

holes, because if this happens the means to

expand the sleeve has been lost, as it can no

longer be inflated by normal methods. This

problem can be overcome by butting anoth-

er sleeve of the same diameter against it,

covering the air holes, and wrapping tape

tightly around the joint of the two sleeves, so

that the air flows underneath the sleeve once

again, facilitating its removal.

In the case of solid-plate cylinders or very

large cylinders, such as those used in

preprinted linerboard applications an exter-

nal manifold is used to supply the required

air. Air enters a manifold and exits through

the surface of the cylinder at the same end.

The manifold is connected to the end of the

plate cylinder each time a sleeve is mounted

or demounted.

Sleeves are being used successfully on

cylinders with diameters ranging from about

2" to 22" and in lengths up to 110". Special

handling equipment may be required for

large sleeves.

Sleeve Storage

For sleeve applications to offer the maxi-

mum cost and efficiency, an appropriate

sleeve storage system needs to be used – one

that offers easy access and identification of

the stored sleeves. Some manufacturers sup-

ply the sleeve in a combined shipping and

storage container that converts into a ware-

housing system. Some printers prefer to cre-

ate their own sleeve storage using racks,

rods, or other in-house adaptations. It is

important to give sleeve storage the priority

it deserves to benefit fully from all the

advantages a sleeve system has to offer.

MOUNTING AND PROOFING 97

98 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

composite proof of process

work can be valuable before

going into the production run.

It helps evaluate flexo plates,

inks and substrates, and can

be produced on a proofing

press with a power-driven impression cylin-

der that is suitable for all flexo web-printing

capabilities. The proofing press uses the

basic flexo doctor-blade configuration, and

the impression cylinder can handle any sub-

strate. Plates made for the production run

are mounted on an all-purpose-sized plate

cylinder and are inked by a doctor-blade-

wiped anilox roll. Each color is proofed

individually and additional color plates are

registered to the first-down plate by the use

of two mounted microscopes.

Flexo ink formulas, colors, lacquers and

viscosities designated for the production

run can be used to produce the composite

proof. The result is a near replication of

what can be expected from the press. The

system produces several exact duplicate

composite proofs for as many colors as

needed and is eminently suited to process

color reproduction.

The proofing system will not produce a

proof identical to that achievable on produc-

tion equipment because two production

presses will seldom produce identical

results. Where hand-brayered ink of approxi-

mate colors without over-lacquer, or a single

proof will not satisfy the demand, the result

from this machine will produce multiple pro-

duction-like proofs without consuming time

and materials on production equipment.

MOUNTING THE PROOF

The proofing press system works as fol-

lows:

1. Mount the plates to be proofed. Cut the

substrate to the circumference of the

impression cylinder. Insert both ends of

the material into the gripper-tension

bars and draw the substrate tightly

around the cylinder.

2. Place the gear-driven plate cylinder,

having a grid of plate-positioning sur-

face scores, in the press on its bearing

supports.

3. Apply stickyback to the plate cylinder

in the conventional way, ensuring the

horizontal center of the stickyback is

over one of the grid lines.

4. Remove the 0.75"-wide horizontal strip

of backing across the stickyback

(Figure

5. Lightly draw a horizontal line on the

exposed stickyback, using a straight-

edge aligned with a cylinder score.

(Figure

6. To mount the first plate, align the hori-

zontal plate-register marks with the

drawn line.

7. Press the plate to the exposed sticky-

back.

8. Peel off the bottom half of the sticky-

back backing, ease the plate down and

smooth into position. The top half of the

plate is done similarily (Figure

An Off-line, Nonproduction

Flexo Proofing Press

9. Lock the plate-cylinder gear in position,

eliminating any rotational backlash.

10. Position both microscopes so their

crosshairs are precisely aligned with

the register marks on the mounted

plate. Do not move the scopes for the

remainder of the job, as the register

marks on the plates for subsequent col-

ors are aligned to the crosshairs in the

two scopes.

11. Set the impression of the plate cylinder

to the impression cylinder in the usual

manner, using the impression carriage-

dial indicators or engineering “slip

gauges” (Figure

12. Set the impression of the anilox roll to

the plate cylinder in a similar manner.

The best impression of plate cylinder-to-

impression cylinder and anilox roll-to-plate

cylinder can be refined by trial proofing.

INKING THE PRINTING PLATE

Use the following procedures:

1. The gear-driven anilox has an attached

doctor blade that serves as the ink foun-

tain. Hold the blade against the anilox

roll with enough pressure to contain the

ink.

2. The ends of the blade-to-anilox roll nip

are dammed with small wads of cotton

to form a containment trough wide

enough to cover the plate image

3. Introduce a quantity of ink at the preset

viscosity into the trough (Figure

)

and start the machine cycle.

4. The proofing press makes one full revo-

lution of the impression cylinder carry-

ing the substrate.

5. The first color is printed. Mount addi-

tional precut substrates on the impres-

sion cylinder for any duplicate proofs.

6. Prepare to print subsequent colors.

Back away the anilox roll by releasing

the pressure holding the doctor blade

against the anilox, allowing the unused

ink to fall into a catch tray beneath the

MOUNTING AND PROOFING 99

The first step to mounting

the proof si to remove the

backing from the sticky-

back.

A horizontal line aligned

with a cylinder score is

lightly drawn on the

exposed stickyback.

TThe bottom half of the

stickyback is removed and

the plate is eased and

smoothed into position.

Grid Line

Exposed

Strip

Remove Strip

of Backing

Remove bottom

half of stickyback

and smooth down

bottom half of plate

Grid Line

Exposed

Strip

Remove Strip

of Backing

100 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

Engineering slip gauges,

or impression carriage-

dial indicators, are used to

set the impressionm of the

palte cylinder to the

impression cylinder.

A quanity of ink of

determined pH is

introduced into the trough

before the machines cycle.

anilox, backing out the plate cylinder

and cleaning the entire station.

7. Remove the first plate from the sticky-

back, taking care not to damage it, or

disturb the microscopes.

8. Mount the next and subsequent plates

with the plate-register marks aligned to

the cross-hairs viewed through the two

microscopes.

If the subsequent plates are mounted

parallel and squarely, but the micro-

scopes show them to be a fraction off-

ways and advance or retard to obtain

perfect register without lifting the

plates. Repeat the procedure for each

subsequent color.

If rubber plates without a dimension-

ally-stable backing sheet are being

used, it may be necessary to place a

sheet of acetate over the first-down

color proof in order to make a trial

proof and confirm register, before

proofing directly on the actual sub-

strate. If photopolymer plates are used,

the backing sheet often provides more

dimensional stability, but an acetate

preproof to confirm register is still

advised.

Check register of plate

Lock gear

in position

Doctor

Blade

Cotton

Wads

Ink

Trough

MOUNTING AND PROOFING 101

Plate Mounting

Without a Mounting and

Proofing Machine

For shops that do not

have a plate-mounting

device, the surface of

the print cylinder should

be engraved with a grid

of longitudinal and

circumferential lines to

aid in the premounting

of the cushioned sticky-

back directly to the plate

cylinder.

lthough not recommended, in

many shops it may be neces-

sary to mount plates without a

mounting device. This can be

done on a rack beside the

press or directly in the press,

because of the nature of printing equipment

and other circumstances (pin register tech-

niques excluded). As an aid to accurate

mounting, it is recommended that the sur-

face of the print cylinders be engraved with

a grid of longitudinal and circumferential

lines. Premounting cushioned stickyback

directly to the plate and then mounting the

stickyback and plate to the print cylinder, as

a unit, is not recommended, except for

plates which are less than one quarter of the

cylinder circumference. For larger plates,

there is a possibility of the cushioned sticky-

back bunching, causing high spots under the

plate.

Stickyback is applied to the print cylinder

in the conventional manner. The steps to

mount the plates are as follows:

1. Project scribe lines from the cylinder

onto the stickyback (Figure

). This

may be done with a straight edge and a

sharp pencil or other instrument that

will make a clean fine line on the sur-

face of the adhesive.

2. Bevel the edges of the plate. This can be

done by hand with either scissors or a

knife by cutting on an angle, with a

paper cutter, a foot-activated table

shear, or a commercial plate trimmer

designed for this purpose.

3. Clean the back of the plate thoroughly:

rubber plates, clean with alcohol; pho-

topolymer plates with clean water. Dry

plates thoroughly.

4. Holding the plate with both hands, align

the horizontal scribe line on the plate

with the projected lateral scribe line

made on the stickyback (Figure

The vertical scribe lines on the plate

must also line up with those projected

from the circumference of the cylinder

onto the stickyback.

5. Carefully position the center of the

plate on the stickyback, without

stretching, and gradually smooth the

102 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

plate, pressing down evenly from center

to head and from center to foot.

6. Examine the mounted plate for evi-

dence of trapped air bubbles between

adhesive and plate. Pierce air bubbles

in nonprinting areas or by pressing

down firmly against the adhesive.

Note: When mounting a large plate, place

two pieces of backing on the stickyback, one

above the horizontal scribe line and one

below it, in such a way as to leave about a

1" strip of stickyback exposed across the

area where it is to receive the plate. This

permits positioning the center portion of

the plate more easily and checking its ver-

tical alignment before pressing the plate

down completely. If the stickyback has been

wrapped around the entire circumference

of the cylinder with a butt seam where the

To apply the plate

onto the stickyback,

the plate is aligned to

the horizontal and

vertical lines on the

cylinder and the project-

ed lateral scribe lines on

the stickyback.

two ends join, it is advisable to mount the

first plate over this seam. This serves to

lock the ends of the stickyback and prevent

them from pulling away from the cylinder.

MOUNTING AND PROOFING 103

Miscellaneous Procedures

amage control is a primary con-

cern of every flexographic

printer. It’s always in the press

operator’s best interest to avoid

any type of delay, particularly

one that might be caused by

mishandling of the printing plates. This sec-

tion describes some difficulties that could be

encountered and the steps that should be

taken to minimize their impact. Use of

release agents, hold-down bands and

bounce-avoidance procedures are covered.

REMOVING PLATES

FROM THE CYLINDER

If the plate molded rubber plate is so firm-

ly bonded that peeling it off will damage the

plate or the stickyback, the following is sug-

gested. Release one corner of the plate from

the stickyback, then with a small artist’s

brush, apply naphtha or toluene to the

pealed area in small dabs between the plate

and the adhesive. The plate may then be

gradually pulled away as the solvent softens

and releases the adhesive. Allow the solvent

to evaporate completely before remounting

or storing the plate.

Polyester-backed photopolymer plates

should release from the stickyback with

minimal effort. Remove the photopolymer

plate by loosening the leading edge all along

the full width and pull it “squarely” from the

cylinder. Pulling from a corner will most cer-

tainly buckle the polyester backing and pos-

sibly ruin the plate. Do not use solvents of

any kind to help release the plate. The sol-

vent may get between the photopolymer and

its rigid polyester backing and cause the

plate to delaminate. Some studies have sug-

gested that, for some operations, saving a

plate for reuse is not financially worthwhile.

Using Release Agents

Adhesive systems used in the manufactur-

ing of flexo cushion tapes have to meet strin-

gent and conflicting requirements: They can-

not allow the plate to lift or shift during pro-

duction, but they must release from the plate

and cylinder at the end of the run. Various

solutions and sprays are applied to plate

cylinders and the back of photopolymer

plates to increase or reduce the level of

adhesion. Exercise caution in their use.

Using a release agent, such as shellac, on

the back of rubber plates to facilitate their

removal from the stickyback after the run

requires great care and should be attempted

only by very experienced personnel. To print

properly, a rubber plate must be in absolute-

ly tight contact with the cylinder. Improper

application of release agents can impair this

bond. If a release agent, or cushion adhesive,

allows a plate to be removed too easily, it is

a certain sign that the plate was not held

tightly enough for proper printing. Problems

that often result are misregistration or plate

lift on press.

MOUNTING

METAL-BACKED PLATES

With plain, metal-backed plates, align the

notches along the sides of the metal with the

scribe lines on the cylinder. In the case of

pin-registered metal backs (plates having

holes to locate them in register on the cylin-

104 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

PLATE STAGGERING

Plates mounted in straight banks across

the cylinder are very likely to produce irreg-

ularities in the final printing because of

cylinder and plate bounce. This linear

method of mounting does not provide con-

tinuous nor uniform impression squeeze

during the full rotation of the cylinder.

When the open “valleys” between banks of

plates come around to a position facing the

inking rollers (Figure

), the bank on the

opposite side, which should be printing, is

not receiving the support necessary to sus-

tain the proper impression squeeze. As a

result, the cylinder deflects away from the

web, and the print is irregular. As the open

“valley” between the plates rotates into

position against the impression cylinder, a

“bounce” effect occurs as the leading edge

of the printing surface comes into contact

with the impression cylinder. This, too, can

cause irregularities in the printing.

To prevent plate and cylinder bounce,

stagger plates around the cylinder to pro-

vide a continuous bearing surface through-

out the full rotation (Figure

In some converting situations (for exam-

ple, when printing is followed by a sheeting

operation), it is impossible to stagger the

plates. This is also true when a job calls for

only one plate. To reduce the problem of

deflection in these instances it is advisable

to use cylinders of as large a diameter as

possible and mount two identical plates

around the larger cylinder, giving a double

repeat. The addition of wide bearer-bars to

print on the edges of the stock may also

reduce plate and cylinder bounce, but it also

requires additional stock width.

Plate bounce is more

likely to occur in the

linear plate-mounting

method, as this does

not provide continuous

or uniform impression

squeeze during the full

rotation of the cylinder.

To prevent plate and

cylinder bounce,

stagger plates to

achieve a continuous

bearing surface

throughout full rotation.

ders), position the two holes in the backing

on the two corresponding pins in the cylin-

der.

Either type of metal-backed plate may be

secured to the cylinder by tightly clamping

the hold-down bands furnished for this pur-

pose. Tighten each band by pulling up one or

two notches at a time, alternating from the

band on one side of the plate to the other.

Note: Ensure the clamp is positioned over

the seam of plates to pull the ends toward

each other, otherwise the plate may buckle.

Impression

Cylinder

Jarring

contact

causes

bounce

Unsupported

plate backs

away

Inking Roller

Impression Cylinder

Continuous

bearer surface

equalizes

impression

squeeze

Inking Roller

MOUNTING AND PROOFING 105

Appendix

A: TOOLS FOR MOUNTING AND PROOFING

Adhesive Cement [7]: Various types of adhesives

are available depending on the application –

bonding plates to stickyback, bonding stickyback

to the plate cylinder, sealing the edges of the

plates to the stickyback or building up low areas

in makeready.

Allen Wrenches [14]: A full set is required to adjust

any of the Allen screws on the cylinder gears or

any working parts of the mounting and proofing

machine.

Ball-point Pen [4]: For highlighting scribe lines in

the nonimage area of the plates and for the mark-

ing of areas to be trimmed. A fine point will give

greater accuracy. Darker ink is more easily seen

on all but black plates, where white or yellow ink

is better.

Bulb Syringe [9]:This is a common rubber ear

syringe (or plastic squeeze-bottle) with a very

small opening. Filled with solvent, it is a handy

tool to apply small amounts of solvent between

plate and stickyback to make plate separation

easier.

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