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

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Preheating can be done with the serving

tray either barely touching the upper platen

of the press or with a “daylight” gap of up to

0.125". No pressure should be applied in

either case. Usually, the matrix manufactur-

er will specify the length of the preheat

cycle. The duration of the preheat should

not vary by more than 15 seconds to account

for normal changes. During the preheat

cycle the molding press must maintain a

temperature between 3,000° F and 3,100° F.

Closing Rate. At the end of the preheat peri-

od, the thermosetting phenolic resin of the

matrix will soften to a working viscosity and

allow entry of the master into the board with

minimum pressure. The closing rate of the

press is critical and should be slow, about

0.10" every five seconds. Pressure can then

be applied, maintaining a steady closing rate

until the bearers are tight. It is important to

time the close rate accurately and it should

generally take about 30 seconds to com-

pletely close onto tight bearers. Closing too

fast will cause a “splashing” or “ridging” of

the viscous phenolic resin. Closing too slow-

ly can result in precuring of the phenolic

resin, causing a high matrix floor and poor

shoulder formation. Ram movement may be

indicated by a commercially available depth

gauge that amplifies the slow vertical closing

movement of the upper and lower platens.

The fibrous material of the matrix is

hygroscopic (attracting moisture from the

atmosphere) and may require a "breathe"

cycle to eliminate potential problems, such

as blistering. The breathing procedure

involves applying a small amount of pres-

sure and then quickly opening and closing

the press to allow the steam and volatiles to

escape. Using a matrix conditioning unit will

dehumidify the board and help eliminate this

type of problem.

Pressure and Curing Requirements. The

amount of pressure required for any particu-

lar mold will vary, depending on the nature

and total print area of the copy matter in the

master. If the master is plain type form, no

more than 300 lbs. per square inch should be

necessary. If, however, a halftone engraving

is used, pressures up to 1,000 lbs. per square

inch may be necessary. The amount of pres-

sure required to mold an engraving varies in

direct proportion to the amount of solid area

to be molded. The pressure required to mold

a particular image can be calculated using

the following formula:

RAM FORCE PRESSURE

(LBS)



(LBS/IN

)

 PRINT AREA

The phenolic resin materials of the matrix

require a curing (vulcanization) time of 8 to

10 minutes at a temperature of 3,000° F to

3,100° F to ensure a complete cure. In some

molding operations, where production

speed is desired, the matrix material can be

partially cured for 5 minutes in the molding

press, separated from the original and oven

cured at 3,000° F to 3,100° F for the remain-

ing five minutes, producing a total cure.

Cooling. When the molding cycle is over and

before removing the mold from the press,

the position on the serving tray should be

noted to ensure repeatable accuracy during

the plate-molding procedure. The cured

mold is removed from the original and

allowed to cool to room temperature. Once

cooled, the mold should be checked for

accuracy by measuring the matrix floor

thickness (Figure

). To determine the

thickness of the molded floor, a micrometer

or depth gauge with a needle point or tip

should be used to allow precise measure-

ment in the finer areas compressed into the

matrix. If there are inaccuracies in the floor

readings, these should be noted on the back

of the matrix and makeready (doctoring) of

the mold may be required.

Position Molding. Molding presses vary

slightly and the molding surfaces may not be

perfectly parallel. It is therefore desirable to

mold the rubber plates in the same position

on the serving tray in which the matrix was

PLATES 15

16 FLEXOGRAPHY: PRINCIPLES & PRACTICES

originally made. To achieve this, the exact

location of the matrix on the serving tray

should be noted by marking the front por-

tion of the mold with a wax pencil. The

matrix should be placed in the same press

and in roughly the same position every time

a plate is made.

Matrix Mold Makeready. Inaccuracies in a

matrix mold can be corrected (doctored)

without remaking the mold. Makeready is

accomplished by a combination of building

up thin areas of the mold with thin paper or

foil and by sanding down the thick areas of

the mold with a fine-grain sandpaper.

The sections to be corrected can be identi-

fied from the back of the matrix by noticing

the color difference between the high and

low areas. The ultimate goal is to produce an

accurate printing plate; therefore, plates

made from doctored molds should be

checked carefully.

Shrinkage. Progressive mold shrinkage was

a major problem at one time, but is no longer

a concern. Modern materials and techniques

assure that the molds experience almost no

progressive shrinkage when used again and

again in the vulcanizing process. Although

low-shrink matrix materials have excellent

dimensional stability in both directions, it is

still recommended that the matrix material

be cut in the same direction for each color of

a multiple color job containing fine screens

or exceptionally tight registration to elimi-

nate any dimensional differences.

Molding a Matrix

The following is a general summary of

steps and procedures in molding a matrix:

Temperature. Molding press temperature

should be between 300° F and 310°F (60 lbs.

steam pressure at sea level if press is steam-

heated).

Preheating. Master type form or metal pho-

toengraving should be preheated in the

molding press for roughly five to seven min-

utes to allow for the expansion of the metal.

This helps prevent the pattern from “locking-

up” in the mold as expansion takes place.

Preconditioning. Matrix material in a heated

oven will prevent the hygroscopic matrix

board from taking on moisture and reduce

the need to “bump” the mold to release

gasses. Preconditioning will also soften the

phenolic resin in preparation for molding.

Preparation. Matrix material to be vulcan-

ized is cut approximately l" to 2" larger than

the master on each side. The border should

be fairly uniform to restrict the flow of rub-

ber evenly on all four sides during the sub-

sequent plate-molding process. Spraying the

metal original or the uncured matrix board

with a commercial release agent before

molding the matrix, is a common practice.

Thin-plate/Low-relief. The matrix material is

placed coated-side-up on the serving tray

with the metal engraving face down on the

board (Figure

). Bearer bars of the cor-

rect thickness are placed either side of the

assembly.

Thick-plate/Deep-relief. The type form is

placed face-up on a carrier. The relief cavity

of the master is filled with phenolic powder,

lightly tamped and carefully leveled. The

carrier and master is positioned on the serv-

ing tray and the matrix board placed coating-

side down over the powder filled master

(Figure

). Bearer bars of the correct

After the molding cycle,

the mold is allowed to

cool. Cure before being

checked for accuracy.

Using a micrometer

or depth gauge, the

matrix floor thickness

is measured.

Overall

Matrix

Board

Thickness

Depth

of Impression

Floor of

Matrix

thickness are placed on either side of the

assembly.

Protective Cover. Whether a photoengraving

or type form original is used, the entire

assembly is covered with a protective cover

panel or sheet. The sheet can either be

Holland cloth (starched linen), release

paper, a sheet of graphite-coated steel or an

epoxy-fiberglass laminate.

Preheating. The serving tray is positioned

between the platens and the molding press

closed to a gap of about 9". The entire assem-

bly is preheated according to the thickness

of the matrix material. (The supplier will

generally furnish the recommended preheat

time for a given board.)

Closing. After the initial preheat interval, the

molding press is slowly closed, until the

bearers are tight. This is best determined by

tapping the bearers as pressure is applied

until they are immovable. The amount of

pressure varies with the image area in the

original, but will rarely exceed 1,000 lbs. per

square inch.

Curing.The entire assembly is cured from

eight to ten minutes at 3,000° F to 3,100° F.

Marking. Before removing the cured mold

from the press, identify its position on the

serving tray.

Cooling. Allow the mold to cool to room tem-

perature before inspecting it carefully for

defects and measuring the floor height. It is

recommended that the floor measurement

appear on the back of the mold for future

reference.

Brushing. Brush the inside of the mold with a

soft-bristled brush to remove any foreign

particles while polishing the surface at the

same time. The mold is now ready for rubber

plate molding.

Appendix A covers some common matrix-

molding problems and offers suggested

remedies.

MOLDING THE PRINTING PLATE

Before molding the rubber plate, it is nec-

essary to know the final thickness of the

printing plate in order for the molding press

to be properly set up. The final plate thick-

ness is determined by the control bearers

used to set the molding platens parallel.

There are also many other considerations

in rubber molding, including molding-press

pressure requirements, correct load of rub-

ber, flow characteristics of the rubber, repe-

tition in loading, proper use of release

agents, the preheat cycle, the curing cycle

and the closing speed of the press.

Once the rubber plate has been cured and

cooled to room temperature, the plate must

be inspected, trimmed down and checked for

PLATES 17

In molding a matrix for

thin-plate/low-relief,

the matrix material is

placed coated-side up

on the serving tray, with

the metal engraving

face down on the board.

In matrix molding for

deep/relief plates, the

type form is placed

face-up on a carrier.

The relief cavity of the

master is filled with

phenolic powder, lightly

tamped and carefully

leveled. The carrier and

master is positioned on

the serving tray and the

matrix board placed

coating-side down over

the powder filled master.

Protective

Cover

Relief filled with

Phenolic Powder

Machine Frame

Upper Platen

Lower Platen

Matrix Board

Molding

Ram

Thickness

Control

Bearer

Metal

Carrier

Master

Pattern

Protective

Cover

Master

Pattern

Machine Frame

Upper Platen

Lower Platen

Matrix Board

Molding

Ram

Thickness

Control

Bearer

18 FLEXOGRAPHY: PRINCIPLES & PRACTICES

plate thickness and defects, such as voids or

bubbles.

If it is determined that there is too much

variation in the plate thickness, either the

back of the rubber plate may be ground in an

attempt to bring the plate into acceptable tol-

erance, or the mold may be doctored and the

plate remade. Plate grinding calls for caution.

Too much abrasion of the back of the plate

can cause distortion of fine type or cupping

(dishing) of solids and, in some cases, the

plate can be totally destroyed.

Determining Molded

Plate Thickness

The final rubber printing plate will be

mounted on a bare cylinder that will be dri-

ven by a gear attached at the end of its shaft.

The combination of the bare cylinder diame-

ter plus the thickness of both stickyback and

printing plate, must build up to the pitch

diameter of the gear driving the cylinder. The

pitch diameter of a gear can be determined

from any standard gear publication. The

bare-cylinder diameter can be determined

by measuring its surface. The difference

between the pitch diameter of the gear and

the bare cylinder diameter must be offset by

the combined thickness of the printing

plates and mounting material. The combined

thickness of rubber plate and mounting

material will be half the difference between

the pitch diameter of the gear and the bare

cylinder diameter.

In wide- and narrow-web applications, the

usual practice is to add 0.002" to the plate

thickness to allow for impression squeeze

and to prevent the cylinder drive gears from

“bottoming” during the pressrun. The corru-

gated industry usually adds 0.005" to the

plate thickness.

Determining Bearers. It is necessary to calcu-

late the thickness of the control bearers for

the finished rubber plate to be at its proper

thickness. This calculation is made by

adding the floor thickness of the mold to the

desired plate thickness, including that of the

cover sheet (i.e., Holland cloth, release

paper). As an example, consider the follow-

ing hypothetical situation:

Floor of Mold 0.080"

Cover Sheet* 0.005"

Shrinkage & Deflection 0.005"

Desired Plate Thickness 0.107"

Impression Squeeze 0.002"

Overall Height of Bearers: 0.199"

* There is no need to add the thickness of the

cover sheet if it extends over the bearers on both

sides. If it does, overall bearer height would instead

be 0.194".

A shrinkage allowance, plus press deflec-

tion caused by molding pressures, must be

taken into account when computing the

thickness of bearers required to produce the

plate. These factors are constant for each

press and generally do not change unless the

press is re-shimmed or there is a change in

the rubber compound or desired plate thick-

ness. Typically, an allowance of 0.005" for

shrinkage and deflection is added.

The bearers are placed on both sides of

the serving tray to limit the ram movement.

The bearers must be free of dust and foreign

matter and be well maintained to consistent-

ly produce accurate plates. To avoid sub-

stantial press temperature loss, the serving

tray should be kept in the press whenever it

is not in use for any prolonged period.

Pressure Requirements, Pressure require-

ments for molding rubber printing plates

vary according to the compound thickness

and the plate construction. Pressures neces-

sary to mold rubber plates can run as high as

600 to 1,000 lbs. per square inch. In some

instances, thinner and shrink-controlled

plates need even higher molding pressures.

For the highest degree of plate accuracy it is

important to mold all plates with just enough

pressure to produce tight bearers.

Compound Loading Procedures. It is impor-

tant to ensure that the mold is always cold

before charging it with raw plate gum to

avoid partial curing of the raw compound.

Operator experience plays an important part

when laying down plate gum in the correct

proportion to produce the desired plate

thickness. For example, where solid areas

occupy a large portion of the mold, more

plate compound is needed to fill out these

areas compared to areas containing fine type

or small-print areas.

Compound Thickness. As a rule, type areas

should be covered with a thickness of rub-

ber 10% to 20% less than the desired plate

thickness, whereas solid or tint areas require

at least 20% more than the final plate thick-

ness. For example, for a finished 0.110" plate

thickness, type areas must be loaded with

approximately 0.090" thickness material and

solid areas would need 0.130" material.

There must be enough plate gum to properly

fill out the entire plate area with sufficient

rubber density, leaving no porosity or voids.

Compound Flow. As the rubber begins to flow

around the sides of the mold, it hinders the

natural flow of the material in the center. For

best thickness accuracy, the “flow” of the

rubber must be assisted in the center portion

of the plate. If this is not done, the solids and

tints will be thicker in the center compared

with the edges, and will generally require

plate-grinding and/or makeready in order to

print well. The problem can be solved by

carefully loading and doctoring the mold or

by using a fast-flowing rubber.

Repetition in Loading. When molding multi-

ple plates of the same or similar design, the

operating steps must be duplicated in order

to reproduce identical-thickness plates with

the same accuracy. Loading the mold must

be done carefully, using pieces of plate gum

cut exactly the same size and positioned in

the same way for each repeat plate. This will

eliminate any major pressure adjustments

on the molding press for each new plate.

Release Agents. Rubber compounds may

stick to nonprinting areas, especially on the

borders of the mold. The problem can be

minimized by ensuring the smoothness of the

matrix coating and using release solutions

applied to the surface of the matrix prior to

loading. Plate compounds may be dusted on

one side with special talcum powder to im-

prove rubber flow and molding fill-in.

The face of the rubber is checked for

cleanliness and placed dusted-side down on

the mold. The talc on the surface of the rub-

ber acts as a lubricant and assists the escape

of air as the plate gum flows into the

crevices of the mold. The talc will also

absorb any moisture present.

Accurate Plate Molding

General guidelines for accurate plate

molding follow.

Release Sheet. The back of the plate can be

prevented from sticking to the upper platen

of the press by the addition of a cover sheet,

such as Holland cloth, or any number of

appropriate silicone-treated kraft papers. If

Holland cloth is used, it should be free of

pinched folds that may tend to form when

rubber flows during the molding cycle. Any

fabric pleats present will be duplicated on

the back of the plate.

Position on the Serving Tray. To minimize

plate-thickness variations, the assembly

(charged mold) should be placed on the

serving tray in the same position as when the

mold was made, as indicated by the “front

mark” put on the mold .

Preheat. Preheating softens the compound

to enable proper flow to be maintained as

molding pressure is applied. The preheat

cycle is accomplished by contacting the

molding assembly with the upper platen (at

307° F) for a prescribed length of time.

Preheat time can vary, depending on the rub-

ber compound.

Closing. Closing speed can vary according to

the rubber compound being used. Closing

pressure should be applied at the same rate

until the bearers become tight. The depth

PLATES 19

20 FLEXOGRAPHY: PRINCIPLES & PRACTICES

gauge should be used as a reference to deter-

mine speed of closing and degree of tight-

ness. It may be necessary to “bump” the plate

as pressure is being applied in order to fill an

intricate design, or compensate for batch dif-

ferences among rubber compounds.

Bumping is accomplished by first applying

a small amount of pressure and then quickly

opening and closing the press (similar to

matrix breathing). This bumping should be

repeated several times. The final pressure is

then applied until the bearers are tight.

Vulcanizing. Vulcanization of the rubber com-

pound takes about 10 minutes at 3,070° F,

depending on the total thickness of the fin-

ished plate. Once the rubber plate has been

completely cured, the assembly is removed

from the press. The plate is stripped from the

mold while still hot, taking care not to tear

the still delicate plate. The recommended

procedure is to gently remove the plate from

the sides and carefully work toward the cen-

ter. This will help prevent possible stretching

and plate distortion. The plate can then be

brought down to room temperature or

cooled in a chiller.

Inspection and Finishing

As the molded plate cools, it contracts or

shrinks in all directions, including the cross

section or thickness. The plate must there-

fore be gauged with a micrometer after it

reaches room temperature. The printing

plate is inspected for complete fill-out, skips,

blisters or foreign matter and absolute fideli-

ty with the mold. The plate is then gauged for

accuracy using a plate micrometer. A stan-

dard hand-held micrometer should never be

used to gauge the flexible printing plates,

because there is no control over the amount

of pressure applied.

When gauging a rubber plate, it is essential

that the foot of the indicator rest squarely on

the plate surface. Gauging for accuracy and

consistent plate thickness in the larger-type

and solid-printing areas usually is satisfacto-

ry. Readings of plate gauge should be taken

every 2" or so throughout a plate to deter-

mine uniformity. Even with a special gauge,

it is very difficult to get an accurate reading

on small isolated areas or small type within

a line of copy, as any pressure applied will

cause deflection of the print surface. Special

attention should be given to the corners and

the center of the plate; if there were varia-

tions in the molding process, this is where

they will show.

Plate Gauge. A thickness range of ±0.001" is

generally accepted for line work and solids.

Plates with halftone process screens should

have a thickness range of less than 0.0005".

Fine type or delicate copy matter, positioned

alongside heavier type or solid areas, should

be lower than the heavy areas by 0.001" to

0.002". This means that the total variation

from the heaviest point of a solid area to the

lightest point of a fine-type section could be

as much as 0.004".

Out-of-Gauge Plates. If it is determined that

there is too much variation in the plate thick-

ness, two options exist: Doctor the mold to

eliminate the variation or ground the back of

the rubber plate in an attempt to bring the

plate into acceptable tolerance.

Remolding with Makeready. If the plate is not

of uniform thickness, it may be remolded

using makeready on the mold. Makeready is

the method of fine tuning the matrix board

before molding the rubber plates. If the plate

is too thin in one area, the back of the matrix

board may be sanded in the offending area to

raise the plate height. Conversely, if the plate

is too thick, makeready tape, foil or tissue

paper may be used under the matrix board in

the offending area to lower a section of the

finished plate.

Plate Grinding. If the plate is only slightly over

caliper it may be recovered by grinding the

back of the plate. Care must be taken when

plate grinding, as too much abrasion of the

back of the plate can cause distortion of fine

type and cupping (dishing) of solids and, in

some cases, can totally destroy the plate.

Plate Cupping. Plate cupping is the result of

taking the rubber plate and arcing it in the

opposite direction to normal, then removing

rubber by grinding. On a drum-type grinder,

this is necessary because the backside of the

plate is being finished, and the printing face

is against the drum of the plate finisher.

When the rubber is removed, the backside

has a greater circumference than the print

side. When the plate is arced back for

mounting on the print cylinder, the print side

is stretched slightly to conform to a greater

circumference than the backside. On large

solids, the edges will tend to print harder

than the center.

TROUBLESHOOTING RUBBER-

PLATE MOLDING PROBLEMS

Rubber-plate molding requires attention to

the process and the materials being used,

since they are perishable and can change

with age. The troubleshooting guide in

Appendix B lists some common plate-mold-

ing problems and remedies.

RUBBER PLATE COMPOUNDS

AND PROPERTIES

There are many different rubber com-

pounds used in the molding of flexographic

plates. Among these are: natural rubber,

Buna N (nitrile), butyl, styrene, ethylene,

propylene, neoprene and a combination of

Buna N/vinyl elastomers.

Properties such as modulus, ozone resis-

tance, durometer, abrasion resistance, stor-

age stability, cure rate, molding shrinkage,

resilience and solvent resistance are all

important factors in plate gums formulated

for molded-rubber flexo printing plates. All

these properties have a direct bearing on the

molding and printing characteristics of the

rubber.

Rubber printing-plate compounds are mar-

keted in many colors. The amount of color-

ing matter is sufficient, seldom, if ever, to

affect the service characteristics of a com-

pound and usually is just a matter of choice.

The black compounds are the only plate

gums that are both colored and reinforced

by the fillers. All other colored compounds

are simply reinforced with white fillers and

tinted by the addition of a colorant.

Material properties are obtained by modi-

fying the rubber with various compounding

ingredients, such as carbon black, zinc

oxide, barytes clay, plasticizing oils and oth-

ers. Unmodified compounds generally are

unsuited for printing plate use. Most com-

pounds used for flexo plates require rela-

tively high proportions of reinforcing fillers

to increase hardness and resistance to tear,

abrasion and solvent attack. There are also

fillers that are nonreinforcing agents, added

to act as processing aids, or to obtain specif-

ic physical traits, such as shrinkage control.

Thickness

Compounds used for molded printing

plates must be smooth, of uniform gauge,

properly dusted with talc and free of

entrapped air. The thickness of uncured rub-

ber compounds normally used in the fabri-

cation of printing plates are: 0.040", 0.060",

0.090", 0.110", 0.125" and 0.187".

Storage

All rubber printing-plate compounds are

perishable and should be refrigerated by the

distributor and platemaker to ensure ulti-

mate molding performance. Flow character-

istics of the compound decrease with aging,

making it difficult to mold large plates of uni-

form thickness. As a general rule, fresh rub-

ber compounds will completely cure in 90

days at 700° F. Storage temperatures lower

than room temperature (below 55° F) retard

the action of the curatives in the compounds,

thus making it easier to maintain the stock in

prime molding condition. Shelf life of a plate

PLATES 21

22 FLEXOGRAPHY: PRINCIPLES & PRACTICES

compound is effectively doubled for every

15° F reduction in storage temperature. It is

imperative, however, to bring rubber to room

temperature before molding. The plate mold-

er must exercise sound judgment.

TYPES OF MOLDED PLATES

The choice of compound for the printing

plate depends on the type of ink and solvent

being used. There are various constructions

available: plain-backed plates, shrink-con-

trolled plates, metal-backed plates.

Plain-backed Plate. This is the most widely

used type of flexographic printing plate. It is

supplied without any special inserts or back-

ing and usually is molded between 0.105"

and 0.112" for wide web and 0.067" for nar-

row web. Plate height usually depends on

press specifications and thickness of the

mounting material. The corrugated postprint

industry generally uses 0.25" molded plates

with a fabric insert sandwiched between

two layers of rubber to maintain dimension-

al stability.

Shrink-controlled Plate. Shrink-controlled

plates are typically used in applications

where accuracy of print size and color-to-

color register is critical, or subsequent die-

cutting or other in-line operations require

accurate print register. The shrink-con-

trolled plate is made by sandwiching a piece

of fabric between two layers of rubber dur-

ing the molding procedure. The fabric mini-

mizes shrinkage during molding and gives

the plate its shrink-control characteristics.

Thickness normally ranges between 0.135"

and 0.165" for this type of plate.

Camera-ready art should allow for elonga-

tion around the cylinder for shrink-con-

trolled plates. This type of plate exhibits neg-

ligible shrinkage across the cylinder.

The shrink-controlled plate can be han-

dled and ground in the same way as a plain-

backed plate, as long as it is done before the

plate is precurved. This type of plate is usu-

ally permanently curved to hug the cylinder

by rolling, placing into a cylindrical fiber

tube and post-heating the plate to 250° F for

about one hour.

Metal-backed plates. The metal-backed plate

is molded and permanently vulcanized to a

metal sheet. The metal backing is usually a

thin (0.008" to 0.012") sheet of mild steel or

half-hard brass. Because this plate is metal-

backed, it cannot be ground on a plate

grinder. Therefore, whenever necessary,

molds should be prepared using makeready

techniques. The inherent dimensional stabil-

ity of this type of plate makes it ideal for

close registration requirements of milk car-

tons, paper cups, boxes, egg cartons and tis-

sue paper.

Metal-backed plates may be integrated to

the plate cylinder either by a mechanical

lock-up system, or, when steel-backed, using

magnetic plate cylinders. The plates may be

secured to conventional cylinders with ten-

sion hold-down bands that fit over exposed

metal edges on either side of the plate.

Metal-backed plates can be provided with

prepunched holes that fit over accurately

positioned pins located in the face of the

plate cylinder. They may also have a bent

lead edge for plate cylinder groove type lock-

up systems. The punched hole or lead-edge

slot systems enable rapid plate changes and

accurate registration of multicolored jobs.

Special Considerations

for Process Plates

Molding procedures used to produce both

line plates or plates containing halftone

screen are basically the same. With plates

containing process screens, special care

must be taken to ensure that the mold

releases fully from the engraving, without

tearing isolated highlight dots or plugging

small reverse dots located in the shadow

areas. The mold should faithfully reproduce

every detail in the engraving.

The most critical element in the production

of a molded-rubber plate with process

screens is the engraving. This has to be

proofed and carefully examined to make sure

it retains all the finest details in a form that is

reproducible during the molding operation.

The molding compound most widely used

for process printing plates is a blend of Buna-

N and polyvinyl. This compound can release

ink from the plate and run cleaner, with less

tendency to fill in. It has good abrasion resis-

tance, which helps prolong plate life and

reduces changes in tonal values due to wear.

The most important consideration for any

molded plate containing halftone process

screens is gauge accuracy. It must be pro-

duced to extremely close thickness toler-

ances by applying local makeready on the

back of the original, or mold, if necessary.

Grinding process plates is not recommend-

ed; however, if plate grinding is unavoidable,

only a very light “polish” or “dressing” is

acceptable. The amount of plate grinding

should not exceed 0.001". Grinding should

never be used to salvage a bad plate.

PLATES 23

24 FLEXOGRAPHY: PRINCIPLES & PRACTICES

he direct photopolymer plate is

one of the major innovations in

modern flexographic printing. It

affords the ability to image the

printing plate directly from a pho-

tographic negative, thereby prov-

ing excellent image fidelity. Photopolymers

are ultraviolet, light-sensitive materials used

to prepare letterpress plates, offset plates,

printing resists, proofing films, pattern mas-

ters for molded rubber flexographic printing

plates and direct flexographic printing plates.

Photopolymer printing plates are similar to

molded-rubber plates in that they are flexible,

resilient and have excellent ink transfer.

There are many systems available for pro-

ducing photopolymer flexo plates. The pho-

topolymer materials used to make the plates

are either viscous liquids ready to be cast to

the required plate thickness, or solid sheets of

appropriate thickness. Photopolymer materi-

als, whether liquid or sheet, are converted to

flexographic printing plates when exposed to

ultraviolet light through a photographic nega-

tive of the artwork to be reproduced.

The film negative is the single most impor-

tant element in photopolymer-plate prepara-

tion. It is a light stencil that controls image for-

mation during exposure of the photopolymer

plate. In general, the guidelines discussed in

the film negative portion of this section apply

to all photopolymers. It is important to check

with the plate supplier to determine the cor-

rect negative preparation for the particular

plate material and printing application.

CHARACTERISTICS

There are a number of manufacturers who

supply various types and constructions of

material used to produce photopolymer

printing plates. Each material is designed to

meet the requirements of a specific flexo

market segment. Two basic categories of

photopolymer printing-plate materials are

liquid and sheet. While finished plates in

both categories are very similar, the plate-

making processes are very different and may

create different physical properties.

When selecting a photopolymer material

for a particular application, it is important to

know the printing system. Not all photopoly-

mers are compatible with all inks. Different

materials from the same supplier may have

different applications and chemical compati-

bility characteristics. Each manufacturer has

specific recommendations with regard to ink

and solvent compatibility. Those recommen-

dations should be followed, assuming that a

photopolymer is compatible with a particular

ink or solvent.

Durometer

Hardness or durometer of the printing

plate has a large effect on the printing char-

acteristics. Durometer is measured by using

a Shore gauge and measurements are report-

ed as either Shore A or D – depending on

hardness.

Photopolymer-plate materials are available

in a range of cured-plate durometer reading

from 25 to 70 Shore A. Most plate materials

for general film and paper converting have a

cured plate durometer hardness of 45 to 60

Shore A. Rough and uneven substrates, such

as corrugated board, require lower-durome-

ter materials of 25 to 40 Shore A.

Photopolymer Plates