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

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Plate Construction

In most common applications, photopoly-

mer material is supported by a transparent

and dimensionally stable polyester backing

sheet. The polyester sheet is generally be-

tween 0.004" to 0.010" thick. In some special

applications, the polyester sheet may be

removable. Some manufacturers of sheet

photopolymer offer a metal backing for use

on magnetic or mechanical lock-up printing

cylinders. These metal-backing materials are

not transparent to ultraviolet light and are

supplied with a preset relief depth. These

materials have a limited shelf life and should

be used soon after receipt.

Special Plate Construction

Both liquid and sheet photopolymer man-

ufacturers offer materials with special con-

structions. The following paragraphs de-

scribe several examples of special-construc-

tion plates.

Image Contrast. In this construction, the top,

or imaging layer, is of a different color than

the base photopolymer. Consequently, once

the plate is processed, the image surface is

of a different and contrasting color from the

base of the plate. This aids in aligning the

plates when mounting on the plate cylinder.

Dual Durometer (Capped Plate). In dual

durometer plates, a thin (0.004" to 0.010"),

harder image surface lies atop a lower-

durometer polymer base. The image surface

may also produce a steeper character shoul-

der. Figure

shows a photomicrograph of

a capped and uncapped plate.

Strippable. This construction allows removal

of the polyester base following plate produc-

tion. These plates are generally used when

the plate is laminated to sheet steel for

mounting on magnetic cylinders and in non-

critical register applications.

Compressible Construction. Compressible

plate construction is available in both sheet

(as supplied) and liquid (as produced) pho-

topolymer systems. These plates have a com-

pressible foam layer within the plate – be-

tween the photopolymer image layer and the

dimensionally stable polyester backing – that

control image gain with printing impression.

Aqueous Processing. Liquid photopolymer

systems utilize water and detergent solu-

tions for plate processing. Most sheet pho-

topolymers are processed in an organic sol-

vent that require special processing equip-

ment with aqueous chemistry.

Jumbo Plate Sizes. The most common

platemaking equipment size, liquid or sheet,

is 30" x 40". Recent demands on manufactur-

ers have extended these capabilities to sheet

sizes of 52" x 110" and even larger for special

applications. Special processing equipment

and handling techniques are required for

plate production of these extraordinary sizes.

Demand for these sizes is driven by corrugat-

ed linerboard, large point-of-purchase dis-

plays, and stepping of multiple repeating

images on plates for general flexo converting.

PHOTOPOLYMER PLATES:

AN OVERVIEW

The following is a brief overview of gener-

ally accepted benefits of using photopoly-

mer plates.

General Factors:

• Better Print Quality. Produces sharp line

PLATES 25

A capped and uncapped

plate. In durometer

plates, a thin (0.004" to

0.010"), harder image

surface lies atop a

lower-durometer poly-

mer base. The image

surface may also pro-

duce a steeper character

shoulder during plate

development.

Capped

Uncapped

26 FLEXOGRAPHY: PRINCIPLES & PRACTICES

images, excellent ink transfer and pre-

dictable halftone results.

• Large Plates. Sizes up to 52" x 110" are pos-

sible.

• Plate Mounting. Efficient process to mount

plates, especially when pin or microdot

registration systems are employed.

• Allows Step and Repeat. Plates can carry

multiple images.

• Filing Space. Film requires less space than

metal and rubber molds. Negatives will

not deteriorate like metal or rubber molds

and can be easily duplicated.

Accuracy Factors:

• Predictable Plate Gain. Plate gain can accu-

rately be determined and compensated for

in art.

• Better Registration. Because of stable

backing, a 90% coverage plate will now

• Accurate Prepress Proofing. Off-press proofs

are accurate reproductions from the film

used for plates. Photopolymer prepress

proofs provide accurate rendition of speci-

fied job.

Time-saving Factors:

• Time Trimmer. A 6-color, 4-up job can be

mounted with pin registered photopoly-

mer plates in about 30 minutes as opposed

to 8 hours or more for rubber.

• More Efficient. Eliminates the mounter-

proofer operation.

• Economical. Downtime is minimized to

reregister plates.

• Fewer Steps. Process work can be done in

half the time (or less) than it takes to

make copper engravings, molds and rub-

ber-plates, and can be stepped and repeat-

ed in multiple images, so there are fewer

plates to register.

• Easier. When rubber plates are mounted

on a mounter-proofer, they may not regis-

ter on press.

• Faster. Plate production is faster than for

metal, molds and rubber.

Cost-saving factors:

• Longer Plate Life. Lasts about twice that of

rubber plates.

• Eliminates Need for Engraving or Mold.

• Allows Reuse. Plates are more reusable

due to less distortion.

• Better Production Capability. More photo-

polymer can be made per man-hour using

relatively unskilled labor. Process work

will cost about half as much as molded

rubber for initial printing plates to press.

HOUSEKEEPING

Maintaining a clean, dust-free environ-

ment in the platemaking area cannot be

overstated. Photopolymer materials in the

uncured state are susceptible to contamina-

tion and damage by dirt and other foreign

particles.

Materials and solvents used in photopoly-

mer platemaking should be handled carefully.

Physical Hazard of UV Radiation

The photopolymer platemaking sequence

uses several sources of high-energy ultravio-

let (UV) radiation that may present a hazard

to the platemaker. The exposure equipment

supplied provides sufficient protection to the

operator during normal platemaking. Safety

interlocks should be maintained at all times.

Ultraviolet Light. The high-energy lamps used

in the exposure, post-exposure and light fin-

ishing units of the platemaking system emit

ultraviolet energy, as well as visible-light

energy. The proportion of ultraviolet light

energy is far higher than the visible light,

therefore, the human eye is a bad judge of

how bright the light is. Special UV-blocking

glasses are needed to provide adequate eye

protection and the protective quality of

these lenses need to match the wavelength

of the UV light source. Platemakers taking

certain prescription medications should

avoid UV light as those medications can

amplify skin photosensitivity.

Short-wavelength Ultraviolet (UV-C). UV-C

has a wavelength bandwidth of 180 to 280

nanometers. This light is used in the light-fin-

ishing process of photopolymer platemak-

ing. UV-C lamps, generally referred to as ger-

micidal lamps, emit very little visible light,

but can cause severe burns to both skin and

eyes from very short exposures.

Medium-wavelength Ultraviolet (UV-B). UV-B

has a wavelength bandwidth of 280 to 320

nanometers and is not normally found in

platemaking equipment.

Long-wavelength Ultraviolet (UV-A). UV-A has

a wavelength bandwidth of 320 to 400

nanometers – visible light starts at about 380

to 400 nanometers. This light is used in the

back-, main- and post-exposure processes of

photopolymer platemaking. UV-A lamps,

typically found in commercial sun beds, emit

a fairly large amount of visible light.

Prolonged exposure can cause severe sun-

burn and eye damage.

FILM NEGATIVE

PREPARATION AND HANDLING

The film negative is the single most impor-

tant element in photopolymer plate prepara-

tion. Necessity for the care in preparation,

handling and storage of these critical imaging

tools cannot be overstated. Negatives used in

photopolymer platemaking should meet the

following requirements as summarized in

Table 6.

PRINCIPLES OF PHOTOPOLYMER

PLATE EXPOSURES

The basic process of making a printing

plate is similar for liquid and sheet photo-

polymer. In both cases, the back is exposed to

UV light to establish the floor. The face is then

exposed through the film negative, which sets

the printing surface. As a final step in the

platemaking process, the plate is “light fin-

ished” to cure all remaining photopolymer.

PLATES 27

Table 6

■ Negatives must be high contrast film,

free of dirt, kinks, nicks and pin holes

■ Matte-surface films are mandatory for

sheet photopolymers to ensure good

contact between negatives and plate

during exposure.

■ Nonmatte films should be used for liquid

photopolymer platemaking

■ Optical density of the film should be

– 4.0 or greater in the nonimage

(opaque) areas

– 0.05 or less in the image (clear) areas

■ Image orientation must be correct and

provide for emulsion-to-plate contact

■ Images must be

– right-reading, emulsion side

for face printing

– wrong-reading, emulsion side

for back or reverse printing

■ Images should be clean, fringe-free

and sharply defined with no broken letters

or lines

■ Colloidal black opaques are recommended.

Avoid red opaques which have a tendency to

flake.

■ Opaque on the

– emulsion side of negatives only

for liquid polymer platemaking

– back side of negatives only

for sheet polymer platemaking

■ Opaque only in the nonimage areas no closer

than 0.1" to an image area

■ Prevent kinks from developing in the image

areas. Film thickness of 0.007" is preferred

■ Prevent scratches in image areas. Scribing

or scraping of the emulsion must not rough-

en the surface

MAKING NEGATIVES FOR

PHOTOPOLYMER PLATEMAKING

28 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Back Exposure

This simple step is necessary to fully

attach the floor layer of the plate to the poly-

ester backing and establish the relief depth.

No negative is used during back exposure

and the photopolymer is cured (cross-

linked) by exposure to high intensity ultravi-

olet light through the polyester backing

sheet. During the back exposure, the poly-

mer is cured to form a solid in a progressive

migratory manner. The longer the exposure

or more UV-A energy absorbed, the thicker

the floor becomes. Variables that can effect

proper exposure include differences in UV

sensitivity for the photopolymer, and UV

energy output, especially as the UV lamps

age. Back-exposure tests should be conduct-

ed regularly to establish the rate of cure for

particular combinations of photopolymer

materials and exposure equipment.

Back-exposure Test

This simple test exposes a sample of the

plate material through the polyester back-

ing sheet on the exposure equipment. The

exposure times are stepped according to

the plate material and the equipment manu-

facturer’s recommendations (Figure

The resulting thickness steps produced in

the plate material are recorded and charted

to form a back-exposure guide for the par-

ticular material and machine.

Face or Image Exposure

This exposure transfers the image from

the photographic negative to the printing

face of the photopolymer. This is done by

selectively curing the photopolymer with UV

light through the clear areas of the negative.

As with the back exposure, the curing is pro-

gressive and the rate of cure is dependent on

many factors: clear area in the film, trans-

parency of the film, sensitivity of the pho-

topolymer, image detail and ultraviolet ener-

gy output of the exposure unit. The depth of

photopolymer to be cured during the face

exposure is dependent on the overall plate

thickness and the amount of floor that was

cured during the back exposure (Figure

During back exposure,

polymer is cured to

form a solid. The plate

floor is thickened via

absorption of UV-light

energy. Relief depth is

charted to measure

curing rate. Data

collected in this

process form as a

back-exposure guide

for the material and

machine. Depth of the

photopolymer cured in

the face-exposure test

depends on plate thick-

ness and results of the

back-exposure test.

Capped plates are

relief printing plates

composed of two levels

of photopolymer. The

cap layer may be of

harder durometer and

different photosensitivity

than the base layer.

Advantages include

wider exposure latitude

and less distortion.

684 1012141618202224

Cover

Sheet

Seconds Back Exposure

More exposure = Less relief

Polyester

Backing

Sheet

Relief

Depth

Cured Photopolymer

Polymer removed

during processing

Floor

Thickness

Cover

Sheet

UV Back Exposure

UV Face Exposure

UV Post Exposure

1. Proper Back Exposure

Relief Layer

Backing Layer

Stable

Polyester

Backing

Sheet

Negative

2. Proper Face Exposure

Acceptable Plate

3. Proper Washout

4. Proper Drying

5. Proper Post Exposure & Light Finishing

These complex relationships can only be

resolved by conducting a face exposure test.

Figure

illustrates some of the problems

with incorrect face and back exposures.

Face- or Image-exposure Test

Image-stepped test negatives containing a

variety of copy detail and tonal values are

available from the various plate material

suppliers. Once the desired back-exposure is

established, these images are face-exposed

for various periods to establish the times

necessary for plate production.

Post-exposure or Light Finishing

During the plate processing, areas of par-

tially cured photopolymer are exposed on

the floor and flanks of the relief image. These

tacky areas are rendered tack-free by further

exposure to high energy ultraviolet light. In

most photopolymers, the presence of oxygen

inhibits the curing action from the standard

exposure lamps. Therefore, special short-

wave UV-C lamps are used to “light finish”

the plate’s surface. The combination of light

finishing, and/or post-exposure, ensures the

entire plate is fully cured and has the opti-

mum physical properties for printing.

Light Intensity

Exposure times effected by light intensity.

Light intensity falls as lamps age and conse-

quently, exposure times must be increased

to achieve the correct amount of energy.

Exposure lamps should be checked at 20-

hour intervals and replaced when the inten-

sity falls below the recommended level, usu-

ally about 75% to 80% of peak emission.

Main- and back-exposure lamps should be

replaced as a set to maintain uniform light

intensity over the exposure area. New lamps

require a burning-in period of 15 to 20 min-

utes to stabilize output before plate expo-

sures are made.

LIQUID PHOTOPOLYMER

PLATEMAKING

The liquid photopolymer platemaking sys-

tem is versatile, reliable and efficient for

plate manufacturing. Liquid photopolymers

are washed out with a water and detergent

solution, which makes them environmentally

safe and user-friendly. Most liquid photpoly-

mers are designed for use with water-based

ink systems, but there are liquid systems that

can be used in solvent-printing applications.

Systems are available to manufacture

plates in sizes from 18" x 26" up to 52" x 110".

They can be used to produce direct-printing

plates for flexographic, as well as letterpress

and molding applications. The platemaking

system uses a viscous liquid photopolymer

resin, which is cast in the imaging unit by the

plate operator and then processed into a fin-

PLATES 29

Exposure problems

caused by incorrect face

and back exposures.

Too Much

Back Exposure

Radiation overcured the

relief area. Washout

produced too little relief.

Inadequate

Back Exposure

Properly cured face is

riding in a soft layer of

uncured material. Plate

will swell during wash-

out and printing.

Inadequate Face

and Back Exposure

Plate has an uncured

midsection, vulnerable

to both washout sol-

vents and ink solvents.

Inadequate

Face Exposure

Fine type distorted and

highlight halftone dots

are highly susceptible

to damage and may

disappear.

Too Much

Face Exposure

Plate will fill in,

especially noticeable in

reverse areas.

30 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ished plate. Under ordinary lighting condi-

tions, liquid photopolymer is stable and safe

to handle at room temperature

The plate-backing material is a manufac-

tured polyester sheet, specially coated on

one side for bonding to the photopolymer.

This provides a dimensionally stable base for

the finished printing plate. No solvents are

used in the process, so plates can be manu-

factured and press-ready in under one hour.

Equipment

The standard system includes four or five

pieces of equipment, described below, that is

used with some types of liquid polymers.

The Exposure Unit. Casts the polymer in a pre-

cise thickness over the protected film nega-

tive and exposes the photopolymer material.

The exposure unit consists of a pair of preci-

sion-ground glasses which are responsible

for the accurate plate tolerance. Exposure

units are available in both state-of-the-art

computer-controlled and manual models.

The Reclaim Unit. An automated device

which removes the unexposed (still liquid)

photopolymer and collects it for reuse dur-

ing another platemaking cycle.

The Washout Unit. Cleans the plate and

removes the residual unexposed polymer

from between the image elements.

The Post Exposure/Dryer Unit. Finishes the

plate with ultraviolet light to cure the floor

of the plate, and the dryer evaporates the

water from the plate.

The Light Finishing Unit. Provides a final cure

to the plate, leaving a tack-free, press-ready

printing surface.

THE LIQUID

PLATEMAKING SEQUENCE

The section on principles of photopolymer

plate exposures covered the basic theory

and steps involved in exposing any pho-

topolymer plate. This section will detail the

steps for the liquid plate.

Casting the Plate

With the liquid photopolymer system, the

plate operator casts the raw photopolymer

material to form the finished plate thickness.

The equipment supplier sets the exposure

unit during installation and provides the nec-

essary information to manufacture the range

of desired plate thickness. The following are

steps necessary in casting a plate:

• Enter the desired plate thickness into

the system and set the machine to spec-

ification.

• Place the film negative emulsion-side

up on the bottom glass and cover with a

thin protective cover-film (Figure

• Turn the vacuum on to draw the air out

from between the lower glass, negative

film and cover-film.

• Cast the photopolymer over the

protected film negative to the appropri-

ate thickness.

• Laminate the dimensionally stable back-

ing sheet to the upper surface of the liq-

uid polymer. Doctor the cast polymer to

a controlled thickness (Figure

• Lower the upper glass until it makes

contact with the backing sheet and the

thickness gauging system.

• To ensure good tolerance at the thick-

ness required, apply the vacuum to the

upper glass and backing sheet.

Back Exposure

The back, or T1, exposure is responsible

for establishing the relief depth and floor

thickness of the finished plate, increasing

adherence to the polyester backing sheet,

and presensitizing the material for shorter

main-exposure times. A negative is not used

during back exposure. The exact back-expo-

sure time needed to obtain the desired floor

thickness in the plate is determined by using

a back-exposure step-test procedure. The

longer the T1 time, the thicker the floor of

the plate and consequently the shallower the

relief (Figure

Face Exposure

The face, or T2, exposure is responsible

for the imaging characteristics of the plate.

The length of the T2 exposure is determined

by the type of copy on the negative. Fine

lines and screens require more exposure

than do solids and fine reverses.

When the plate material is exposed through

the negative with UV light, the areas corre-

sponding to clear areas on the photographic

negative are hardened. The areas, corre-

sponding to the black areas in the negative,

are not exposed and remain in a liquid state

(Figure

Exposure-control Guides

It is essential to use exposure-control

guides to determine the proper amount of

face exposure, especially when making

process-color plates. These guides, supplied

by the material manufacturer, are small-test

negatives available in different line screens.

They are imaged using a calibrated glass

screening process for accuracy. The guide

incorporates highlights and reverses, as well

as lines and reverse lines. To use the control

guide, the plate is exposed only until the

smallest desired image on the control guide

is held. Exposure times and data from the

test is recorded for future reference. Since

the output of lamps in the exposure unit will

decline with age, exposure control guides

assist in ensuring consistent imaging.

Reclaim

After exposure, the plate is removed from

PLATES 31

Casting the plate.

the film negative is

placed emulsion-side

up on the bottom glass

and covered with a thin

protective cover-film.

The dimensionally

stable backing sheet is

laminated to the upper

surface of the liquid

polymer. The cast poly-

mer is doctored to a

controlled thickness.

The exact back-expo-

sure or T1 time needed

to obtain the desired

floor thickness in the

plate is determined by

using a back-exposure

step-test procedure. The

longer the T1 time, the

thicker the floor of the

plate and consequently

the shallower the relief.

When the plate material

is exposed through the

negative with UV light,

the areas corresponding

to clear areas on the

photographic negative

are hardened. The areas,

corresponding to the

black areas in the nega-

tive, are not exposed

and remain in a liquid

state.

Lower Optical Glass

Plate-making

Film

Cover

Film

Relief Exposure Lamps

Lower Optical Glass

Polyester

Backing

Sheet

Relief Exposure Lamps

Doctor

Blade

Backing

Sheet

Applicator

Polymer

Supply

Contain-

ment

Dams

Lower Optical Glass

Upper Optical Glass

Relief Exposure Lamps

Back Exposure Lamps

Hardened

Photopolymer

Exposed

Hardened

Photopolymer

Unexposed

Liquid

Photopolymer

32 FLEXOGRAPHY: PRINCIPLES & PRACTICES

the exposure unit and placed on the reclaim

unit. The cover-film, which protected the

negative, is removed at this point. The

reclaim unit collects the unexposed liquid

material for reuse, which offers substantial

cost savings, and minimizes the amount of

waste going into the environment.

After reclaim, the plate is ready for further

processing in the washout unit.

Plate Washout

This unit washes the plate with a warm,

mild detergent-and-water solution to remove

any remaining unexposed resin.

Post-exposure/Plate Drying

After washout, a post-exposure step using

UV light hardens the floor of the finished

plate. At this point, the plate is placed in the

dryer to remove rinse water from its surface.

Light Finishing

After the plate is dry, it is moved to the

light-finishing unit, where it is exposed to

shortwave UV (germicidal) light. This step

gives the plate a final, tack-free surface.

SPECIAL LIQUID

PLATE-MAKING TECHNIQUES

Special techniques for liquid photopoly-

mers include makeready, capping and

image-positioned plates.

Prepress Makeready

Prepress makeready is a technique that

allows the platemaker to selectively reduce

the thickness in isolated areas within a sin-

gle plate. Using this technique, press impres-

sion can be optimized when printing fine

type adjacent to large solids by reducing the

plate caliper of the fine images. It can also be

used to compensate for thickness loss dis-

tortion, which occurs when the plate made

in the flat is wrapped around a cylinder.

In the liquid platemaking system, the thick-

ness of the film negative directly affects the

plate thickness. If the film negative thickness

is increased in certain areas by transparent

shimming, the additional thickness will dis-

place liquid resin and cause the finished plate

to be thinner in those areas by a like amount.

Any transparent shim material may be used

as long as the optical density is satisfactory

for UV-light transmission. Shim material

should be placed on the back (nonemulsion)

side of the negative, so that the emulsion

remains as close as possible to the pho-

topolymer (separated only by the protective

cover-film over the negative). Figure

shows an enlarged area of Figure

with the

makeready in place and Figure

shows the

final plate with typical dimensions.

Capping

Capped plates are relief-printing plates

composed of two layers of photopolymer.

The cap layer may be of a harder durometer

and different photosensitivity than the base

layer. As plates are made, two layers of dif-

ferent liquid photopolymers are either man-

ually or automatically cast – one on top of

the other. Advantages of the capped plate

include wider exposure latitude, less distor-

tion on the printing surface, deep reverse-

etch depth and lower press-gain from plate

to printed product.

Image-positioned Plates

Image-positioned plates are large, one-

piece plates with all images in register. The

plates are assembled directly on a 10-mil car-

rier sheet. This eliminates later mounting of

several smaller plate pieces on 30-mil PVC or

similar carrier sheet that is typically used in

corrugated printing.

A full-size, one-piece negative is made for

each color to be printed. Each color is pre-

pared in register to the others, and the nega-

tives are produced with register marks that

are in perfect parallel with the required

plate-trimming line.

A back-mask negative may also be used to

prevent background buildup in nonimage

areas of the plate, thus increasing the

amount of polymer reclaim. Using the pre-

press software, a “white plate” or trap is eas-

ily created around each image area. The

back mask can then be made in any conven-

tional film imagesetter, or cut on a sample

die table from any opaque film or paper. The

back mask is placed between the plate sub-

strate and the background exposure source

during plate exposure, thus preventing poly-

mer cure and background buildup except in

the image area. Plate exposure and process-

ing is the same as for conventional plates.

After platemaking, each one-piece printing

plate is registered and trimmed in position.

Lock-up strips are attached, and the plates

are ready for press. Advantages of image-

positioned plates are excellent registration,

reduced time to press, elimination of plate-

mounting materials, and light, flexible plates

that allow easy handling and storage. While

image-positioned plates can be made with

sheet photopolymer, the ability to reclaim

and reuse uncured polymer makes their

manufacture more conducive to liquid-pho-

topolymer systems.

SHEET PHOTOPOLYMER

PLATEMAKING

The sheet photopolymer system offers high

quality plates for direct flexographic printing

applications. Many sheet photopolymers are

washed out with a solvent system. Water

washout sheet systems are available, offering

environmental and operator benefits.

Platemaking systems can be purchased to

manufacture plate sizes up to 52" x 80" of spe-

cific thicknesses for each application. They

can be used to produce direct-printing plates

for both flexography and letterpress.

The plate material (Figure

) consists of

three layers: a polyester backing sheet, a

photopolymer layer, to which the backing

sheet is bonded, and a cover sheet to protect

the printing and image face.

The polyester backing sheet provides a

dimensionally stable base for the finished

printing plate. The photopolymer layer is a

super-viscous liquid which, under normal con-

ditions, is dimensionally stable. Under either

heat or pressure, the polymer may be perma-

nently deformed producing low spots in the

finished plate.

Unexposed plate material should be stored

and handled with care. Boxes of material and

individual plates should be stored absolutely

flat – never on end. Smaller sheets or boxes

should not be stacked on larger sizes. All

platemaking materials must be stored away

from sources of heat

The cover sheet provides protection to the

PLATES 33

Lower Optical Glass

Plate-

making

Film

Large

Solids

Area

Fine Type

Area

Large

Solid

Area

Fine

Type

Area

Premake-

ready

Polyester

Backing Sheet

0.067"

0.035"

0.004"

34 FLEXOGRAPHY: PRINCIPLES & PRACTICES

image surface of the plate material. When

the cover sheet is removed prior to placing

the negative in position, a thin “slip film”

remains to ensure that the negative does not

bond to the polymer during exposure. Large

sheets and narrow strips of plate material

should be handled with care to prevent pre-

mature delamination of the cover sheet.

The sheet photopolymer is stable and safe

to handle at room temperature in a safe-light

(UV-screened) environment. The precast

sheet is exposed and developed by the

platemaker into a finished plate. Because

solvents are used in the development of

these plates, they may take a few hours to

manufacture, due to the long drying times.

Equipment

The standard system includes four pieces

of equipment: an exposure unit, a processing

or washout unit, a dryer unit and a light-fin-

ishing unit. The exposure unit exposes the

photopolymer sheet and transfers the image

from the negative or exposure mask.

Exposure units are available in both manual

and state-of-the-art, computer-controlled

models. The processing or washout unit

cleans the plate and removes the residual

unexposed polymer between image ele-

ments. The dryer unit removes the solvent,

which has absorbed into the surface of the

plate during washout, and dries the plate.

The light-finishing unit eliminates the sur-

face tack, and then post-exposes the fin-

ished plate with UV light to cure the floor of

the plate. On newer machinery these two

steps can be performed simultaneously.

SHEET PLATEMAKING SEQUENCE

Steps for making sheet photopolymer

plates follows.

Material Preparation

Unexposed plate material should be cut

carefully to minimize waste. Typically, on a

sheet of raw photopolymer, there is a small

border of cured material around the edges of

the sheet.

The film negative size is transferred to the

plate material, which is then placed face-up

on the sheet-cutter board. Smooth, clean

cuts should be made either with a sharp

knife or a “hot knife”, allowing a 1" border

around the copy to provide a clamping edge.

It is more practical if several negatives can

be grouped together to form a single sheet

exposure, thus eliminating the necessity to

cut individual sheets of raw material. When

grouping negatives together, it is recom-

mended that the negatives do not overlap.

UV-opaque adhesive tape should be used to

eliminate gaps and to ensure that the nega-

tives are kept flat.

Back Exposure

The back exposure is completed first. The

sheet material is placed base-side up on the

exposure unit and exposed to UV light.

Some automated systems are equipped with

dual light sources. In that case, the sheet is

placed base-side-down over the bottom set

of lamps. The back exposure is responsible

for the relief depth and floor thickness of the

finished plate, increasing adherence to the

polyester backing sheet, and presensitizing

the material for shorter main exposure

Plate material consists

of three layers: a poly-

ester backing sheet, a

photopolymer layer to

which the backing sheet

is bonded, and a cover

sheet to protect the

printing and image face.

Polyester

Backing Sheet

Polyester

Cover

Sheet

Photopolymer

Layer