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76 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

can then be positioned to print acceptably

with minimal makeready by grouping the

two thickest plates on the right (gear) end of

the plate cylinder, the two thinnest ones on

the opposite end and the remaining two in

the center position.

When proofing, the impression is set to

print the two thickest plates with a near-skip

impression. Using the parallel eccentric, the

left (operator’s) end is then moved in until

the two thinnest plates print. With a little

trial and error, acceptable results may be

achieved with no additional makeready.

Note: After mounting, the parallel eccen-

tric must be reset to the original position.

MOUNTING THE FIRST

SET OF PLATES

The first set of plates should be mounted

over the butt seam of the stickyback to pre-

vent the horizontal ends of the seams from

lifting during the run. Rotate the plate cylin-

der until the butt seam is positioned near the

top. Remove a 1"-wide strip of protective

liner from the stickyback in the area of the

horizontal center of the first plate. The cen-

ter lines should be extended on photopoly-

mer plates (before register marks and center

lines are cut off the plates) by aligning a

straightedge with the horizontal and vertical

marks and drawing an accurate extension of

these across the plate in the nonimage area.

Position the plate on the plate cylinder,

making sure that the horizontal and vertical

scribe lines in the nonimage area of the plate

correspond with the lines drawn on the

proofing paper. This is done by looking

through the viewer on the optical-type

mounting and proofing unit. With the

mechanical type unit, use the mounted

straightedge and markings.

When the two scribe lines are in position,

tap the plate gently onto the stickyback to set

the position of the plate while avoiding

excess pressure. Check the alignment of the

plate to ensure that it is in perfect position.

When positional accuracy is acceptable, the

remainder of the stickyback protective liner

may be removed on the portion nearest to

the operator, allowing the plate to fall gently

down. The plate should be smoothed out as

it rests lightly on the stickyback. Take care

not to trap air bubbles between the plate and

stickyback. If air bubbles are seen or felt by

hand, lift the plate free from the stickyback,

wipe out the air bubbles, and smooth down

the plate. Then reapply the plate to the

stickyback. The same procedure is repeated

to lay the opposite side of the plate nearest

the impression cylinder. Before proceeding,

the plate should be checked to make sure it

is in perfect alignment with the layout guide-

lines. Repeat this operation with each of the

other plates for this color.

Note: Mounting plates from the center

first, rather than the top edge, reduces by

half any alignment error that may occur.

The larger plates with a great deal of print-

ing area, specifically solids, should be mount-

ed first because they are generally less prone

to distend or distort as they are laid. This first

plate mounted becomes the key plate provid-

ing the location for all the others. For film

printing, the key plates are usually the white

plates; for paper printers, they are usually the

black. When very tight register is required,

particular care must be taken when position-

ing the key plate. If there is more than one

color with large coverage, it becomes a mat-

ter of selecting the more important plate to

become the key plate.

When mounting thin-molded rubber

plates, no plate with a solid print area should

be “stretched” in order to fit an image.

Stretching of the rubber will create thin or

low printing areas in the plate. Conversely,

with rubber plates that have a scattered or

sporadic image area, it is acceptable to

stretch corresponding colors to fit an image

in the solid plate because the stretching

takes place in the nonprinting areas.

There may be times when it is necessary to

cut plates apart in order to achieve accurate

copy is broken up into small sections, as

with minor colors.

MOUNTING FOR

CORRUGATED POSTPRINT

For flexographic corrugated postprint

applications, it is common practice for sev-

eral small pieces of printing plate (slugs) to

be positioned on a single 0.030"-thick carrier

sheet, which is then attached to the print

cylinder of the press. With changing technol-

ogy, some operators mounting for corrugat-

ed postprint use stickyback to hold plates to

the carrier or backing sheet. Many, however,

still use a brush-on adhesive. High quality

printing requires careful mounting to mini-

mize plate thickness variation on a given

printing cylinder, whatever the cause. If the

film thickness of a brush-on adhesive is not

controlled, it will affect the overall plate

height on press.

Reduce the adhesive to the recommended

viscosity, so that brush marks will flow out,

producing a smooth, even coat. Apply thin,

smooth coats to both the backing sheet and

the plate. Allow both surfaces to thoroughly

dry, in order to achieve the maximum bond-

ing strength, before mounting the plate.

Remove all previously applied adhesive

from the cylinder, backing sheet or used

plates, before re-mounting.

When using stickyback to hold plates to

the backing sheet, take care to ensure that

no pieces of paper, dust or trash are trapped

between either the cylinder and carrier

sheet, carrier sheet and stickyback or sticky-

back and plate.

PROOFING THE

FIRST SET OF PLATES

When proofing the first set of plates, cover

the comprehensive proofing paper, showing

the layout, with clear 0.004" to 0.005" acetate

and tape it in position, making sure there are

no wrinkles or movements. This will enable

a proof to be taken over the laid out com-

prehensive proofing paper, to check the

position and printability of the plates, with-

out destroying the original layout.

Note: When mounting in register and

proofing, the impression cylinder should be

rotated in the same direction for all colors.

If the first plates down are not positioned

correctly, they can be moved and proofed

again. The first proof can be washed off the

acetate with a lint-free cloth and water or

solvent. If all plates print up fully with

acceptable impressions, then the acetate

sheet can be removed and the proof taken

directly onto the comprehensive proofing

paper carrying the job layout. Move the

impression and plate cylinders closer togeth-

er a distance equal to the acetate preproof

thickness, to ensure the same impression on

the comprehensive proof layout paper as

was obtained on the preproof acetate.

A thin, uniform film of ink should be

applied to the plates for each impression

made. This is achieved by first rolling out the

ink using a brayer on a sheet of Plexiglas to

a minimum film of ink – just enough to fill

the grain of the paper. The ink is then

applied with the brayer to the surface of the

plate to be proofed.

Once the first color plates are mounted

and proofed and after the ink has dried, tape

a sheet of acetate over the comprehensive

proof. This will protect the proof during

mounting of the second color plates and also

provides a surface for a trial proof of the sec-

ond color over the first. The first plate cylin-

der may now be removed.

When using an optical machine, the best

into each other. The scribe lines can also be

used, but only as a check for the second

color. With a mechanical-type machine, the

MOUNTING AND PROOFING 77

78 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

same marks and divisions used on the first

color should be followed. The plates should

be inked and the second color proofed on

the transparent sheet overlaying the com-

prehensive proof of the first color.

Next, check the register of the two colors.

(Corrections may be readily made by lifting

and repositioning the plates and pulling

another trial proof if necessary). Re-ink the

plates, remove the transparent sheet, move

the impression equal to the thickness of the

acetate preproof, and pull the proof of the

second color directly on the proof of the first

color. Repeat this procedure for the other

colors.

The recommended proofing ink is a water-

based paste ink that does not readily dry and

harden on the brayer, but dries quickly on

the paper and can easily be washed from the

plates and acetate. The ink rollout slabs may

be washed with water. If necessary, alcohol

may be used to remove dried ink. Cleaner

proofs are obtained when ink slabs and bray-

ers are kept clean.

For a detailed procedure on a specific

make and model proofing machine, consult

manufacturer.

PROOFING FOR PRINTABILITY

Described here is a procedure that can

best be called “proofing for printability” or

“press-related proofing” (Table 16). Skills

developed in the proper use of this tech-

nique make it possible to detect and correct

potential printing problems off press,

achieving a number of benefits:

• Press downtime will be reduced, since

the press operator will not have to do

plate makeready on the press to elimi-

nate high and low spots.

• Waste will be reduced. Less print stock

will be required for setup and less wast-

ed during running because of unsatis-

factory printing.

• Quality will improve. Where multiple

plates (per color) around and across the

cylinder are being printed, conditions in

which some plates are producing satis-

factory printing, while others on the

same cylinder are printing with an

insufficient or an excessive amount of

squeeze or ink, will be eliminated.

• Production will be better. With fewer

press stops, efficiency will increase,

resulting in more production with less

effort.

Steps to Proofing for Profitability

Proofing Paper. Choose a proofing paper with

the smoothest surface and least thickness

variation. The thickness should be sufficent

to increase impression-cylinder circumfer-

ence to match the gear pitch circle. Although

0.003" undercut is common for many

mounter-proofer manufacturers, this dimen-

sion should be checked. Using a 50# super-

calendered paper is suggested.

Securing the Paper. Make sure the proofing

paper is secured to the impression cylinder

very tightly and snugly.

Proofing Ink. Use a water-based, glycol or oil-

based ink, compatible with the plate material.

The ink should be high in color strength, of

thin paste consistency, and moderately fast

drying (it should be fast enough to dry on the

proofing paper within half to three-quarters

of an hour after the impression is made), but

slow enough not to dry on the plates until the

proof can be pulled.

Palette or Roll-out Surface. Spread the ink

onto a sheet of 0.25" acrylic or glass as thin-

Table 16

■ Press downtime will be reduced

■ Waste will be reduced

■ Quality will improve

■ Production will be better

BENEFITS OF

PRESS-RELATED PROOFING

ly and as evenly possible, using the brayer

roll. A sheet of white paper should be placed

under the transparent roll-out surface to

show the ink film applied. Keep the surface

and entire area clean, allowing only a mini-

mum of ink on the surface.

Note: Use a brayer or roller that is con-

centric, with no damage or dirt on the sur-

face. The rubber covering should be about

50 Shore “A” durometer.

Inking the Plates. Practice is required to

achieve a very thin, strong, even coat of ink

on the roll-out glass, brayer roll and the

mounted printing plate. The proper amount

of ink applied to the plate should be suffi-

cient to just fill the grain of the proofing

paper. Too little will show the white of the

paper within an area of supposed ink cover-

age, whether a large solid or a small halftone

dot. Too much will show excess pile-up,

either on the face of solids and dots, or out-

lining the solids and dots. Extreme excessive

inking will cause bridging from dot to dot

and small-type characters will fill in and not

be crisp or sharp. Examine the ink coverage

under a 20x magnifier to see that it is right.

After Inking. After the plates are inked, re-

examine the surface for evidence of brayer

marks, either of stops and starts or of brayer

edges – small ridges of ink. To remove any

brayer marks without adding more ink, roll a

nearly dry brayer over the plates repeatedly,

thereby smoothing out the ink.

Paralelling the Plate Cylinder. To ensure accu-

rate proofing, the plate cylinder must be par-

allel to the impression cylinder. Using a feel-

er gauge equal in thickness to the total of the

ideal plate and stickyback thickness, place

the gauge between the bare plate cylinder

and against the proofing paper that is taped

to the impression cylinder. Bring the plate

cylinder and impression cylinder together

until the gauge can be removed with mini-

mum drag. Gauge the opposite end and the

middle to make sure that drag resistance is

identical at all locations. If it is not, then the

machine must be reset before mounting.

Establishing Zero Contact. Even the most per-

fect plate, stickyback and plate-cylinder

combination will have some minor varia-

tions in concentricity and printing height.

The goal is to have all parts of all plates on a

given cylinder print up with a thin, strong,

continuous film of ink, with no portions

skipping or partially transferring ink, and no

portions so high as to over-impress and dis-

tort the image. When the plates are properly

inked, bring them into gradual contact with

the paper by rotating the print cylinder with

an oscillating movement. When the first visi-

ble ink is transferred (Figure

), roll a com-

plete proof. “Zero contact” is that distance

between the plate cylinder and the impres-

sion cylinder that permits some small

amount of the total plate area to print up on

the proofing paper.

The mounting operator needs both judg-

ment and skill to achieve viable zero contact.

When proofing a cylinder mounted with mul-

tiple plates, a single spot of printing on the

proofing paper must be considered a high

spot and not a zero contact. On the other

hand, four or five large areas beginning to

print up on the proofing paper, may be con-

sidered excessive for zero contact, unless

they are exactly the same area on multiple

plates.

MOUNTING AND PROOFING 79

Establishing zero

contact requires that all

parts of all plates on a

given cylinder print up

with a thin, strong,

continuous film of ink.

No portions can exhibit

signs of skipping or

partial ink transfer, and

no portions can meet so

high as to over-impress

and distort the image.

80 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

Setting the Gauges. Having established zero

contact, set the impression gauges on the

mounting and proofing machines to zero.

Obtaining a Complete Print. When proofing a

set of mounted plates, the goal is to get all

the print areas, on all the plates mounted on

the cylinder, to print completely with the

least possible additional impression of the

plates to the impression cylinder. For

process work, this should be minimal, gen-

erally within 0.001" additional impression.

Impression Tolerances. It is difficult to gener-

alize what constitutes good and bad impres-

sion tolerance because of the diversity of

plate thickness, mounting foam thickness

and other materials used in the vast field of

flexographic printing. Corrugated postprint

presses may use printing plates with 0.280"

of total mounting thickness, while small

label presses may only have 0.067" total

undercut. As a rough guide, a compression

of the plate material and mounting foam

equal to 2% of their total thickness should be

sufficient to proof print the images on the

plate.

On a typical press undercut for 0.125" addi-

tional impression from zero of 0.002" may be

the difference between good, sharp printing

and a completely unsatisfactory job. This

0.002" additional impression may produce

halos and fill-in while the job is being run.

The maximum allowable impression may be

different from one type of plate to another or

from one kind of print copy to another. All

large solids and large type will permit more

impression before distorting than will small

type, mechanical screens or halftone

process work.

Inking should always be consistent in film

thickness. However, if the plates on the

same cylinder contain both larger solids and

type and small type or dot work, then the

maximum tolerable impression is that

amount just before any of the copy becomes

distorted or enlarged with over-impression.

For process work, color keys made from

the finished negatives can serve as a goal for

ideal small-typeface appearances and ideal

halftone dot sizes in highlight, midtone and

shadow reverses.

Areas of poor or no ink transfer, known as

skip-outs, are a possible indication of low

areas that need to be built-up with make-

ready to print up without additional impres-

sion squeeze. False skip-outs may be caused

by incomplete plate ink-up, ink dried on the

plate before transfer, or low areas in the

proofing paper itself (usually not over

0.0005"). These last three possibilities should

be checked before using makeready by re-

inking the “skipping” area, rotating the

impression cylinder to a clean area on the

same paper and pulling a partial proof at the

same impression reading.

PREPRESS MAKEREADY

If several portions on all plates do not print

before the highest plate areas distort, two

options are available: raising low areas, or

lowering high areas.

Note: The following procedure is not rec-

ommended for process work. Plate cylinders,

bearings, stickyback and plate-thickness

variables for process work need to be hand

picked, measured and controlled so that the

whole plate area will print without visible

image growth or distortion.

Lowering High Areas

When using nonfoam or noncushion

stickyback, and with line work of various

configurations, the impression cylinder may

be utilized as an accurate smoothing and lev-

eling device.

Remove all ink from the plates and with

about 0.010" to 0.012" additional impression

from zero, rotate the plate cylinder against

the impression cylinder. This may “set” the

plates at the high points into the stickyback

and level them out.

Clean and re-ink the plates, find the new

zero contact (as it will likely be farther in

than the first contact because higher areas

have been lowered) and reproof the plates.

If the condition has improved but some

areas still do not print, the procedure may be

repeated with more impression, perhaps

0.00l5" without damage to the plates.

If the setting of the plates into the sticky-

back fails to produce acceptable results, the

plates must be removed from the cylinder

and another makeready technique applied.

If the irregularity was not found in pre-

mounting quality control of plates and cylin-

ders, the variation could be due to sticky-

back or plate-cylinder surface variations,

which are not discovered until initial proof-

ing on the mounter. The following procedure

may produce acceptable results.

1. Clearly outline the area needing correc-

tion. Lift the plate from the stickyback

by the end closest to the high plate area

until the backing substrate behind the

area is exposed. Take care not to distort

or crinkle the backing or move the plate

from its registered position. Isolated

high spots in photopolymer plates can

be successfully lowered by as much as

0.002" by manually sanding the backing

substrate.

2. Best results are obtained by removing

the plate from the cylinder and laying it

face or image area down on a smooth,

flat surface. The high spot should be

clearly identified and outlined and,

using a small piece of #400-grit wet or

dry finishing paper, gently sand the

plate backing substrate within the out-

lined area.

3. Finish by feathering the outer perimeter

of the sanded area. Be sure not to

remove too much material from the

backing sheet as this will cause a low

spot. Check your progress periodically

with the plate micrometer.

4. To avoid removing excessive film, peri-

odically lay the plate back down and

reproof it. At best, this is trial and error,

but with practice and observation, judg-

ment and skill will improve. As a result,

otherwise questionable plate/cylinder

relationships may be greatly improved.

This procedure has also proven helpful in

softening dots in vignetted areas of process

work. Although slightly more difficult to

sand, similar corrective action can be

accomplished with rubber and other syn-

thetic plate materials.

Building Up Low Areas

There are several ways of building up non-

printing areas or low spots in the plates.

Large low areas, or areas that do not print

up satisfactorily, after lowering the high

areas as discussed above, can be raised by

partially lifting the plate from the sticky-

back, applying a thin coat of liquid adhesive

to the back of the plate in the low areas and

resetting the plate on the stickyback.

Use a liquid adhesive from the plate manu-

facturer or plate-making material supplier, or

common rubber cement which is satisfactory

for this purpose. Dilute the cement with the

recommended solvent, then apply with a

clean brush in several thin layers, allowing

each layer to dry in turn. There is also the pos-

sible danger of swelling the plates. If the

cement is applied too heavily, remove the

excess adhesive. Use a flat, wide brush on

larger areas and a round brush for smaller

areas. Common shellac can also be used for

the same purpose; the buildup of each coat of

adhesive will vary with viscosity.

Another alternative to raise low areas in

the plate is double-sided adhesive tape,

which is available in various thicknesses,

ranging from 0.0009" to 0.005". Experience

with these techniques will make it possible

to determine very quickly, when looking at a

proof, just how much cement, shellac or

tape is required to build up a low spot.

MOUNTING AND PROOFING 81

82 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

Composite Proof

After each cylinder has been proofed and

made ready as outlined above, a composite

proof of the entire job is made. As a guide to

the press operator, the composite proof

should be marked with the total amount of

impression required for each cylinder to

obtain a complete print up of all plates.

Any failure to get the same results on

press probably means the printing equip-

ment or the mounting and proofing machine

needs maintenance. Proper interpretation of

the handmade proof requires the under-

standing that plate flaws or printing irregu-

larities unacceptable on the press will not be

nearly as obvious. Close inspection is

required. One of the most important func-

tions of the mounting and proofing operator

is to read the final proof for correctness of

copy and freedom from errors. The final

proof should be folded into the finished

product to verify that all copy is in the right

place on all panels before the job is passed

to the pressroom for printing.

Edge Sealing

If the job is mounted for a long run or for

repeated runs, tape down or cement all plate

edges. This will prevent inks or plate-clean-

ing solutions from working under the plate

edges and dissolving or otherwise destroy-

ing the adhesive or cushion. There are spe-

cial edge-sealing cements available for this

purpose. Any good plastic adhesive tape will

also suffice. All edges and ends must be

rubbed down thoroughly.

Cleaning

After completing the mounting and proof-

ing, carefully wash and dry the printing

plates. Wash molded plates thoroughly with

alcohol and photopolymer plates with a mild

solution of soap and water. Grease spots, fin-

ger marks and other foreign matter on a

plate can repel ink from the print surface

and make it appear that the plate is not print-

ing properly. Proofing ink left on the plate

can cause fine type and dot work to build up

ink on press and cause dirty print, especially

if the proofing ink is oil-based and the press

ink is water-based.

Wrapping Mounted Cylinders

One way to assure a firm, complete bond

between the plate, stickyback and cylinder,

and to eliminate captive air pockets, is to

wrap a low density polyethylene film (about

2.5" wide) around the cylinder, completely

covering all plates mounted on it. Wrap the

tape around the cylinder in a spiral manner

from one end to the other, with each winding

overlapping the other, and then reversed to

crisscross back over the first layer of wind-

ings (Figure

). Keep the tape tension tight,

but not too tight to compress the plates, pos-

sibly creating highs and lows. Secure the

tape end with a pressure-sensitive tape or by

tucking it under one of the last turns. The

wrap should remain in position at least sev-

eral hours, overnight if possible, but not

more than 24 hours. Mark the over-wrap

with the color, press number, station or deck

number, plus any other helpful information

to identify the job.

Low density poly-

ethylene film is wrapped

in a spiral manner

around the cylinder from

one end to the other.

Each winding overlaps

the other, and then is

reversed to crisscross

back over the first layer

of winding. Wrapping

the mounted cylinder

this way eliminates

captive air pockets and

assures a firm complete

bond between plate,

stickyback and cylinder.

MOUNTING AND PROOFING 83

ADDITIONAL OFF-LINE

TIME SAVERS

To reduce downtime and waste during a

job set-up on press, there are several addi-

tional steps that can be taken while the job is

in the mounting and proofing machine.

Web-edge Guide Marks

To identify the precise edges of the press

web, mount two small plates in the trim

area. The plates have a printable line about

the thickness of a pencil line (1 pt.) and

about 0.125" to 0.25" long. From these print-

ed lines, the press operator can quickly see

where to establish lateral web position in

relation to the copy and be assured the edge

guide is holding it in place during the run.

Web-trim Mark

When a web-stock slightly wider than that

required for the job is printed, and a selvage

edge must be trimmed, a small plate is mount-

ed (as above) to identify the correct location

for the trim knife. This permits the press oper-

ator to set the knife quickly and accurately,

and to check its position on each roll change,

web break or stock-width variation.

Slitter-knife Marks

The same principle can be applied in locat-

ing multiple-up slitting-knife locations. It

assures quick knife positioning at setup and

is a constant reminder of any knife slippage

or web deviation.

Bag-folds, Former-guide Marks

After multiwall bags are printed, the web

is folded into a tube in which formers are set

to create side folds and gussets. The web-

marking principle can be applied to locating

the formers. The small marks are located at

bag cutoff and will not be visible in the fin-

ished bag. The final proof, containing all of

the above information, is used to check the

job thoroughly for just about every possible

requirement, except the actual color.

84 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

ounting systems designed

and developed for the

flexographic printing in-

dustry, have enhanced

the accuracy of prepress

plate positioning in regis-

tration. Press time has been diminished as a

result. This section discusses the compo-

nents of the different mounting systems on

the market today, as well as the advantages

of the process and the preparation practices

necessary to the success of these latest

technololgies.

COMPUTERIZED MOUNTING

AND PROOFING SYSTEM

Computerized mounting systems were

introduced to the flexo industry for both

wide-web substrates and corrugated pre-

print liner in recent years. These mounting

systems are available in a range of sizes for

plate cylinders, from 60" to 120" with asso-

ciated cylinder-repeat sizes (Figure

The systems do not use conventional pins or

punching of plates and negatives. They use

conventional stickyback for holding plates

to sleeves or integral printing cylinders.

Accurate registration is aided by micro-

video cameras and monitors that magnify

al alignment, making it possible to position

plates across and around a given cylinder to

within a tolerance of 0.002". All preliminary

functions are programmed into the machine,

allowing the operator to complete the plate-

mounting process rapidly.

The following is an abbreviated series of

procedural steps for operating the system:

1. Select the proofing paper or other

proofing material and place it on the

impression cylinder. This may be done

manually or automatically.

2. Bring the printing cylinder into position

and lock in place. Apply stickyback to

the plate cylinder, either automatically

or manually.

3. Lower the plate hold-down table ito a

predetermined position relative to the

plate cylinder.

4. Place the plate on the hold-down table,

allowing a portion of the plate to hang

over the front edge of the table. Turn

the vacuum on to hold the plate in posi-

tion to the table.

5. Move the microvideo camera over a

defined register mark on the plate and

enter the position into the computer.

Locate a second register point on the

plate and enter this into the computer.

6. The computer determines the position

of the plate on the hold-down table and

moves the table to bring the plate square

with the cylinder face. Position the plate

relative to the centerline or any desig-

nated point on the plate cylinder.

7. Two pressure rollers contact the over-

hanging portion of the printing plate,

attaching it to the stickyback and holding

it in place while the vacuum is shut off.

8. As the computer rotates the plate cylin-

Recent Introductions in

Mounting Equipment Systems

der, the pressure rollers gradually apply

the plate to the stickyback and the plate

is mounted.

9. The plate is inked and proofed. The

proofing cylinder moves down and

away, allowing the plate cylinder to be

removed and the next cylinder brought

into position.

These steps are repeated with each plate

to be mounted, substantially cutting mount-

ing time, especially in three- or four-color

process work.

PIN-REGISTER

MOUNTING SYSTEM I

Pin-registration systems have been used

for many years in letterpress and offset print-

ing. Adaptation of these methods to flexogra-

phy has not been easy in the past because of

dimensionally unstable rubber plates and the

large cylinder inventories used for variable

repeat lengths in packaging markets.

Availability of dimensionally stable photo-

polymer plates allowed pin registration to

become a reality in flexographic printing.

Operating Principles

The pin-registration system, used in other

printing technologies, consists of producing

accurately sized and positioned holes for reg-

istration pins. In practice, this usually

involves punching holes in the films, plates

and press cylinders, and positioning these

materials on register pins during all steps of

MOUNTING AND PROOFING 85

Computerized mounting

and proofing systems

were introduced to

provide greater

accuracy and efficiency.

They use conventional

stickyback for holding

plates to sleeves or

integral printing

cylinders, instead of

pins. Punching of plates

and negatives is not

necessary.