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

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PLATES 5

Plate Classes

lexographic printing plates are

divided into two broad classes:

rubber and photopolymer. The

oldest technology is that of hand-

engraved rubber plates, followed

by molded-rubber plates. The

photopolymer plate, in its sheet and liquid

forms, represents a major step forward in the

industry and is the dominant technology in

use today.

HAND-ENGRAVED

RUBBER PLATES

Long before the introduction of molded-

rubber and photopolymer plates, there were

hand-engraved rubber plates. These plates

have limited use today in printing large,

point-of-purchase displays, in applications

requiring very large printing blankets for

solid-color printing, or in the application of

coatings. The material is supplied as a cured-

rubber sheet, either natural or synthetic. It is

usually of a soft durometer and comes in

large rolls roughly 4' wide by 10' long.

Instead of using a film negative for plate-

making, a full-size mechanical tracing is made

for each of the colors to be printed. Each trac-

ing is then “rubbed off” or transferred to the

rubber-engraving material by using a transfer

solution, allowing the pencil tracing lines on

the layout to be tattooed on the surface of the

rubber. If compensation for plate stretch is

required, the rubber material is secured to the

appropriately sized curved cylinder and the

tracing is transferred in the curve.

When the tracing has been transferred, a

skilled engraver cuts the traced image by

hand-ensuring an accurate depth of cut, as

well as the proper shoulders and bevels. The

finished product is a printing plate that is

ready to be mounted and go onto the press.

In some applications, hand-engraved plates

are the quickest, most economical method

of producing flexographic printing plates.

Table 1 summarizes the advantages and dis-

advantages of hand-engraved plates.

Table 1

ADVANTAGES

■ Plates can be used for very large areas of

■ Plates do not require metal or photopoly-

mer engraving

■ Plates are ready to use after being cut

DISADVANTAGES

■ Layout and cutting are hard work

■ Size and intricacy of the characters cut

are limited

■ Plate life is not as long as with molded or

photopolymer plates

■ The engraved image may not have the

same accuracy as that of a molded or

photopolymer plate

HAND-ENGRAVED PLATES

6 FLEXOGRAPHY: PRINCIPLES & PRACTICES

MOLDED-RUBBER PLATES

Molded-rubber printing plates are flexible,

resilient and have the printing image in

relief. They are duplicated from a mold, or

matrix and made from an original or master

pattern plate carrying the image. The master

pattern may be made of metal, either mag-

nesium or copper, and is produced from the

artwork through a photographic and etching

process. Any number of printing plates may

be made from this mold. The printing plate is

pliable because it is made from a flexible

material, either rubber or a combination of

rubber and plastic. These materials have

excellent ink-transfer characteristics, pos-

sessing both an excellent affinity for a wide

variety of inks and the ability to release the

ink onto many different substrates. The

basic production steps for molded-rubber

plates are listed in Table 2.

Most suppliers of liquid photopolymers

provide materials ranging in durometer from

40 to 60 Shore D, designed to manufacture

masters for molding matrix boards, from

which rubber printing plates are made.

These photopolymer masters are manufac-

tured in the same way as direct-printing

plates. The plate masters for compression

molding, as well as deep-relief powder mold-

ing, can be made from these materials. Most

molding techniques dictate that the pho-

topolymer master be sprayed with a release

agent to prevent sticking to the matrix. Most

standard matrix boards can be used for face

molding or powder molding. Molding proce-

dures are comparable to rubber, except for

the need to minimize pressure to prevent

plate distortion.

PHOTOPOLYMER PLATES

The direct photopolymer plate is one of

the major innovations in modern flexo-

graphic printing. It affords the ability to

transfer an image from a photographic nega-

tive directly onto the surface of the printing

plate, thereby giving excellent image fidelity.

Photopolymers are ultraviolet light-sensi-

tive materials and are used to prepare print-

ing plates for flexography, letterpress and

offset, as well as printing resists and proof-

ing films. Flexographic photopolymer print-

ing plates are similar to molded-rubber

plates in that both are flexible, resilient and

have excellent ink transfer. There are many

systems available for producing photopoly-

mer flexo plates.

Raw materials are available as either vis-

cous liquids, ready to be cast to a desired

thickness, or as preformed solid sheets of an

appropriate thickness. Photopolymer mate-

rials, whether liquid or sheet, are converted

The various compo-

nents which make up a

photopolymer plate.

Table 2

1. Create a master pattern by exposure

through a photographic negative,

either acid etching a metal engraving,

or processing a hard-durometer

photopolymer plate by water wash

2. Mold a cavity in a phenolic matrix board

from the master pattern plate

3. Produce the rubber plate by molding from

the cavity in the matrix board

BASICS OF MOLDED-RUBBER

PLATEMAKING

Caliper

Total height

of printing

plate

Image Area

The printable

surface

Floor

The non-

printable area

Shoulder

Support for

the printable

area

Relief

Distance

from floor

to top of

image area

Plate Backing

Material on the

back of the

plate to provide

stability

to flexographic printing plates when ex-

posed to ultraviolet light passed through a

photographic negative image of the artwork

to be reproduced. The photopolymer is then

processed to develop the relief image

(Figure

). Table 3 outlines the process.

The negative is the single most important

element in photopolymer plate preparation.

It is a selective, light-blocking stencil that

controls image formation during exposure

of the photopolymer plate. In general, the

guidelines are discussed in the film-negative

section of the Photopolymer Plate chapter

apply to all photopolymers. It is important to

check with your plate-material supplier to

determine the correct negative preparation

for your particular platemaking process.

Liquid and Sheet

Photopolymers Compared

The liquid-photopolymer system uses

detergent and water to process the plate.

The solid-plate process generally uses organ-

ic solvents. Some water-wash sheet pho-

topolymer systems use a mild acid or caustic

solution, depending on material type. Drying

time of a water-wash plate is five to 10 min-

utes because only water needs to be

removed from the plate surface. Sheet-plate

systems using solvent wash require addition-

al drying time to remove solvents that have

been absorbed by the photopolymer materi-

al. In liquid-photopolymer platemaking sys-

tems, the exposure unit includes the setting

of plate thickness.

Customized platemaking techniques can

cast and expose varying plate thicknesses.

Sheet photopolymers are available in a range

of predetermined thicknesses. The overall

platemaking time for liquid photopolymer

plates is generally shorter than that for sheet

plates.

Plates for Process Printing

Much has been said about relative differ-

ences between rubber and photopolymer, in

terms of molecular structure, porosity and

ink-transfer capabilities. Either can achieve

outstanding print results. The inherent

dimensional stability of photopolymer and

direct reproduction capabilities have made

photopolymer more popular for printing

sophisticated designs, halftone screens and

process-color images. Tone-reproduction

curves that accurately compensate for

image gain, can be established for each type

of plate, either through characterizing (fin-

gerprinting) the press or other controlled

methods. Different image-compensation

requirements for rubber- and photopolymer-

plate systems tend to rule out using the same

screened, color-separated negatives for both

processes.

All steps of the “rubber” production process

for making plates (i.e., acid etching, matrix

molding, subsequent rubber-plate molding

operations) contribute to dot-percentage

change and affect printed tonal values.

FILM NEGATIVE REQUIREMENTS

Preparation of the film negatives is one of

PLATES 7

Table 3

1. Back-exposure of base to ultraviolet light

to harden (cure) floor and establish relief

depth

2. Face exposure of surface to ultraviolet

light through the negative to harden (cure)

the relief printing image

3. Wash out in appropriate solvent or water

to remove unexposed polymer and leave

printing image in relief

4. Post-exposure to finally cure floor and

character shoulders

5. Drying of the plate either to remove

absorbed solvent and restore gauge thick-

ness, or remove surface water and render

plate pressready

BASICS OF PHOTOPOLYMER

PLATEMAKING

8 FLEXOGRAPHY: PRINCIPLES & PRACTICES

the most crucial operations in the manufac-

ture of relief-image photo engravings and

photopolymer printing plates. Prior to the

introduction of computer graphics, the fin-

ished black-and-white mechanical artwork

was photographed using an engraving cam-

era to produce the platemaking negatives. In

the engraving camera, special lenses were

used to pick up the finest detail, but any

imperfections in the artwork were also

transferred to the negative. Therefore, in

camera-art systems, the artwork was care-

fully inspected to make sure that the image

elements were clean and had sharp line def-

inition before the art reached the camera.

With the advent of computer graphics,

laser-imaged film and automatic film pro-

cessing equipment, reflection-copy imper-

fections have been eliminated. This does not

imply that laser-imaged platemaking films

are perfect. Poorly maintained imagesetters

can produce minute imperfections in the

platemaking films that may escape the

notice of both the artist and platemaker. This

is especially true of films containing halftone

process screens.

After photography or imagesetting, the film

is developed to conform to density specifica-

tions and should be inspected for defects.

Inspection is carried out on a light table. The

light beneath the negative makes any tiny

transparent spots (“pinholes”) or other

imperfections easily detectable. These faults

in the negatives may be corrected by painting

over them with a commercial opaquing solu-

tion. Opaquing should be applied carefully

and with the platemaking process in mind as

each image transfer system may have differ-

ent requirements.

In single-color jobs, only one negative is

produced. For multicolor jobs, a negative for

each color must be made. If preseparated art

is used, each overlay is imaged and a nega-

tive made. With “composite” or “key-line”

artwork, it is necessary to make a composite

negative for each color to be printed. The

color-separation artist then paints out all the

colors but one on each negative, leaving all

details of that one color on its own film. For

example: On a two-color job, say red and

blue, two negatives are made. All the blue

copy is opaqued out of the negative, produc-

ing the red-plate film. On the second nega-

tive, all the red copy is removed, producing

the blue-plate film.

Great care is required to ensure that all

copy is clean and sharp. In addition, imper-

fections such as pinholes, broken letters or

other flaws should be carefully retouched.

Center lines and registration marks should

appear on the negatives and be reproduced

on the printing plates. Small designs (under

24 square inches) can be photographed on

0.004" film, while larger designs should use

0.007" film.

A good film negative is critical to all the

plate-imaging processes with the exception of

laser-engraving and other direct computer-to-

plate processes. Table 4 briefly lists the con-

siderations when producing film for flexo

platemaking

Note: If the film is imaged with poor or

veiled dots, higher-than-normal exposure

times will be needed. Over-exposure causes

reverses to fill in and results in tone com-

pression on the finished plate.

DIRECT-IMAGED PLATES

Direct-imaged plates refer to plates made

directly from digital data output from a com-

puter and usually, but not always, involves a

laser to write the image to be printed.

Laser-engraved Plates

Laser-engraved rubber plates are produced

by engraving rubber with a laser unit similar

to that used when producing ceramic anilox

rolls. The high-energy laser vaporizes

(ablates) the unwanted rubber in the relief

area of the plate, leaving the raised image.

Laser-engraved rubber plates combine the

excellent printing characteristics of rubber

and direct imaging from the computer-gener-

ated artwork, thereby eliminating the need

for negative films. The engraving process is,

however, time consuming, especially in the

deep-relief printing plates used for direct-

corrugated postprint applications.

PLATES 9

Table 4

■ High-contrast film free of dirt, kinks,

nicks and pinholes

■ For process, halftone or screened plates,

the film must be imaged with hard, fringe-

free, round dots

■ The density of the film:

– in the nonimage area (black area)

should be 4.0 or greater

– in the image area (clear area)

should be 0.05 or less

■ Nonmatte film should be used for liquid

photopolymer and metal master patterns

■ Matte film is mandatory for sheet photo-

polymer

■ Image orientation must provide for emulsion

to plate contact

■ The negative must be:

– emulsion side, right reading for

surface printing

– emulsion side, wrong reading for

reverse printing

■ Image correction for distortion of the plate

material being used

■ Film thickness is either 0.004" or 0 .007",

however, 0.007" film is desirable, as it is

easier to handle and store without kinking

■ Opaquing:

– liquid photopolymer systems:

on the emulsion side only

– sheet photopolymer and metal masters:

on the base of the film

KEYS TO A GOOD FILM NEGATIVE

FOR FLEXO PLATEMAKING

10 FLEXOGRAPHY: PRINCIPLES & PRACTICES

olded-rubber plates are

flexible, resilient and have

excellent ink-transfer char-

acteristics. They are manu-

factured by duplicating an

image from a mold, or

matrix, that was generated from an original

pattern plate. The mold can be used repeat-

edly to make duplicate plates carrying the

same image. The plates are made from a flex-

ible material, either natural rubber or a com-

bination of natural and synthetic rubber com-

pounds, giving the plate its flexibility. The

basic production steps in making molded-

rubber plates can be seen in Table 5.

THE MASTER PATTERN

The first step in the molded-rubber plate-

production cycle is making a master pattern.

These could be either metal masters or pho-

topolymer masters.

Metal Masters

Metal masters are produced from the orig-

inal art using a photographic process. The

image on the negative is first transferred

photographically to a photosensitive coating

on the face of a sheet of metal. The plate is

then etched in an acid bath, leaving the relief

image. The etched metal becomes the mas-

ter pattern from which a matrix mold is

made. For standard-web flexo applications,

the metal sheet is usually 0.064" overall in

thickness, with an etched-relief depth

between 0.030" to 0 .035". Deep-relief plates

used in the corrugated industry are made

using 0.250" or 0.187" metal originals with an

etched relief between 0.140" to 0.150".

Types of Metal Originals. Photo engravings

can be made from magnesium or copper.

Magnesium is an excellent engraving metal

for producing high quality line originals.

Magnesium is sometimes used for coarse-to-

medium (up to 100-line screen) process color

work but magnesium undergoes unpre-

dictable lateral copy loss in the etching

process, making it unsuitable for very fine

work. Copper is used mainly for fine detail

and halftone screen reproduction where fine

tone or process color jobs are involved.

Preparation of Metal and Image Exposure, The

metal for the photo-etching process is pre-

coated with a photosensitive material, ready

for transfer of an image from the negative

film. The photo-resistant coating has a poly-

ethylene protective sheet that adheres light-

ly to the surface. After removal of this pro-

tective sheet, the emulsion side of the image-

carrying negative is placed in direct contact

with the photo-sensitive coated surface. The

two are locked tightly together in a vacuum

frame and exposed to a light source.

When transferring the image to the photo-

Molded-Rubber Plates

Table 5

1. Make the master pattern by exposure

through a photographic negative and

either acid-etching a metal engraving

or processing a hard-durometer photo-

polymer pattern material

2. Make a phenolic matrix mold of the mas-

ter pattern plate

3. Mold the rubber plate from the matrix

BASIC PRODUCTION STEPS FOR

MOLDED-RUBBER PLATES

resist, it is recommended that a step expo-

sure (Stouffer) gauge be used to ensure suf-

ficient light exposure. For surface printing,

the negative should be wrong reading, with

the emulsion down to make plates; for

reverse-printing applications, the negative

should be right-reading emulsion down.

The procedure is similar to printing pho-

tographs, except that the platemaker is

working with metal, instead of photographic

print paper. The vacuum locked assembly is

exposed to an intense light source, rendering

the exposed areas of the photo-resist insolu-

ble when contacted by acid. Developing the

image removes the still-soluble coating from

the unexposed, nonprinting areas, leaving

the acid-resistant coating on the image areas

of the metal.

The Etching Process. The metal, with the

exposed and developed photo-resist, is placed

in a stainless-steel etching machine where it is

splashed with a mixture of nitric acid, oil,

etching additives and water until the proper

etching depth is reached. The hardened coat-

ing, which carries the image of the design to

be printed, resists the action of the acid, while

the unprotected areas are dissolved by the

acid. In this way, the acid bath creates a relief

image on the surface of the metal against a

background that has been etched away.

A concern in the etching operation is to

control the acid action to prevent undercut-

ting the metal from beneath the image. This

would destroy the usefulness of the engrav-

ing as a pattern for molded-plate production.

In the mid-1950s, a chemical process called

“powderless etching” was developed. The

process involved a special filming agent

mixed in the nitric acid bath that acted to

protect the side-wall or shoulder formation

of the image, preventing undercutting during

the etching cycle. The entire etching proce-

dure requires very precise control of the

chemical solution, machine speed, bath tem-

perature and timing. The powderless etching

method is universally used and produces

high quality flexo engravings.

The ideal profile of the supporting sides or

shoulder formation of the engraving should

be almost vertical, with a uniform, smooth

taper (Figure

). Excessive shoulders and

broad, stepped shoulders (Figure

), will

tend to cause ink buildup on the finished

plate, resulting in a smeared and dirty print.

At the other extreme, undercut conditions

(Figure

) would tend to lock the engrav-

ing into the mold and make separation of the

two impossible without irreversible damage

to the mold and possibly the master.

Finishing. After etching, the finished engrav-

ing is cut from the large metal flat and care-

fully finished to remove any imperfections.

PLATES 11

Ideally, the proper

etching should be

almost vertical, with a

uniform, smooth taper.

An improper etching

with excessive

shoulders will tend

to cause ink build up

on the finished plate

resulting in a smeared

and dirty print.

Etching

Depth

0.035"

Excessive Shoulder Etch

12 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Ideally, there should be little tooling, routing

or other handiwork necessary with a good

quality etching. Pinholes in the photograph-

ic negative, impurities in the metal or a lack

of proper control during the etching bath

operation often cause pimples or tick marks

(Figure

). If the condition is not too

severe, the marks can be removed by tooling

or routing the finished engraving.

Extra centerlines are sometimes provided

for convenience in locating slug sections

such as price changes, nutritional clauses,

weight or other changes to any part of the

plate. Some printers and converters use

scribe lines to indicate folds, panels, repeat

marks or other data, especially on carton

work. Identification for the job should be

carefully stamped into the background area

of the engraving. Other useful information,

such as curve direction, print position, loca-

tion on bag, color and any identifying data

the platemaker or customer requires would

also be added to the background area.

The photo-resist is removed and cleaned

from the face of the engraving for subse-

quent production of the mold. If all of the

photo-resist is not removed, it can cause a

blistered, uneven print surface and flaws in

the finished plate. After the finished engrav-

ing is proofed and checked for quality, size

and accuracy of color separation, it is ready

to serve as a master pattern for molding the

matrix.

Photopolymer Masters

In the production of metal masters, the

metal-etching acid is dangerous and difficult

to dispose. Therefore, for both environmen-

tal and health reasons, masters made from

very hard-durometer photopolymer material

have become the standard in molded-rubber

plate production.

There are many types of photopolymer

masters for shallow-relief printing including,

photosensitive nylon and metal-backed thin

photopolymer. These masters come in a vari-

ety of thicknesses and with different back-

ings, usually either stainless steel or alu-

minum. Deep relief photopolymer masters

for corrugated plates are usually made using

the liquid platemaking technique and a spe-

cial high durometer master pattern pho-

topolymer.

The masters are produced in the same way

as regular photopolymer flexo plates. Once

the photopolymer master has been made, it

is handled in the same molding procedure as

a metal master.

THE MOLDING PRESS

The molding press, or vulcanizer as shown

in Figure

, is used to make both the

Undercut conditions in

the plate tend to lock

the engraving into

the mold and make

separating the two

impossible without

damaging the mold

and/or master.

Dirty etching can be

caused by pinholes,

impurities and/or lack of

proper control of the

ethcing bath operation.

Dirt Pimples

Undercut Etch

matrix and, in turn, the printing plate. The

matrix molding process uses a heat-set

material and the plate-molding process vul-

canizes the rubber-plate material; both re-

quire high temperature and pressure.

The matrix and printing plates are pro-

duced between two, accurately ground, par-

allel surfaces. A press with accurately

ground platens and precise temperature con-

trol is necessary to produce properly cured

flat molds and rubber plates. Normal press

tolerances are ±0.0015", while critical accu-

racies of ±0.0005" are needed for process

printing plates.

The molding press may be heated by either

steam, electricity or hot oil. The ideal temper-

ature for matrix and rubber molding is 308° F

to 310° F. It is important that platen tempera-

tures are maintained above 200°F. Constantly

cooling and reheating the press can eventual-

ly cause the shims (accurately ground thick-

ness-control bearers that give the press its

accuracy) and platens to become uneven and

put the press out of tolerance.

Molding pressure is generated by hydraulic

pressure applied to the bottom table or plat-

en (sometimes referred to as the ram), which

travels up or down as required. The top plat-

en is stationary in the press. Molding presses

are equipped with a serving tray to allow the

work to be brought into and out of the open

press. An accurate timer is provided to time

preheat and molding cycles, together with

accurate temperature and pressure controls.

The thickness of the matrix and rubber

plates is determined in the molding press

through use of steel shims (accurately

ground steel strips), sometimes referred to

as molding bearer bars.

Auxiliary Equipment

A matrix preheat oven can be used,

instead of preheating the matrix in the mold-

ing press, before applying pressure during

the molding cycle. A matrix preconditioning

cabinet can also be used to keep moisture

out of the uncured matrix sheet before mold-

ing. This will help achieve a full cure of the

matrix sheet and maximize stability, elimi-

nating excessive shrinkage and blistering.

THE MATRIX MOLD

Three components make up modern

matrix materials: phenolic resin (Bakelite),

cellulose fibers and mineral fillers. A pheno-

lic, thermosetting resin first melts, then

cures when exposed to heat and pressure.

The fibers consist of cellulose derived either

from cotton or wood pulp. Fillers typically

are finely ground, high-temperature minerals

that give the matrix resistance to the condi-

tions of plate-molding. Matrix mold is pro-

duced from the metal or photopolymer mas-

ter. The two main types of plates produced

are thin-plate/shallow-relief and thick-plate/

deep-relief, each using a slightly different

matrix-molding technique.

Thin-plate/Shallow-relief Molding. Matrix-

molding materials for producing thin plates

with shallow relief come in fibrous sheets,

coated-one side with the phenolic resin. The

wide- and narrow-web fields traditionally

work with shallow-relief/thin masters and

use this sheet form of matrix. Sheet-matrix

materials come in various thicknesses, sizes,

durometers, coating thicknesses and floor

PLATES 13

The matrix molding

process uses a heat-

set material. The

plate-molding process

vulcanizes the rubber-

plate material. Both

require high tempera-

ture and pressure.

Thickness

Control Bearers

Machine Frame

Upper Platen

Lower Platen

Molding

Ram

14 FLEXOGRAPHY: PRINCIPLES & PRACTICES

specifications, depending on the application

and printing plate requirements. Relief

potential in sheet matrixes ranges from

0.020" to 0.125".

Thick-plate, Deep-relief Molding. When a fin-

ished plate with reliefs over 0.125" is re-

quired, phenolic or Bakelite fill-in powder is

used in conjunction with the sheet matrix to

achieve the extra relief depth. The sheet

matrix is used as a backing sheet, which pro-

vides support and added mechanical

strength. Powdered Bakelite is used mostly

in platemaking for corrugated postprint

because of the greater etching depth

required. The powder is contained in the

mold by providing a frame around the image

in the master. This process is called deep-

relief powder molding or DRPM.

Making the Thermosetting

Mold or Matrix

Matrix Floor. The matrix floor is the point of

measurement from the back of the matrix to

the lowest point of impression. The recom-

mended floor measurement furnished by

suppliers of matrix material is the safe limit

of compressibility for a given original sheet

thickness. As a general rule, the matrix floor

thickness represents 50% to 60% of the origi-

nal thickness, when molded at pressures

ranging from 200 to 1,000 lbs. per square

inch. Over-impressing and reducing the floor

of the matrix may cause cupping in the print

surface, especially when photopolymer mas-

ters are used.

Determining Thickness-control Bearers. Various

thickness bearers, or accurately ground steel

shims, are used to stop the movement of the

molding press ram and control the final thick-

ness of the molded product. To compute the

thickness of the bearer required for molding

the matrix, the total thickness of the master is

added to the desired matrix floor thickness.

The thickness of a cover sheet, starched linen

(Holland cloth), release paper, metal panel or

other sheet is also added if it does not cover

the bearers. If the cover sheet extends over

the bearers, then the thickness of the cover

sheet is not included in the calculation.

For example, to calculate bearer height:

Engraving Thickness 0.064"

Desired Floor Thickness 0.080"

+ Cover Sheet Thickness* 0.005"

= Bearer Thickness 0.149"

*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 be

0.144" instead. In either case, it is important to veri-

fy that both sets of bearers are exactly the same

height on each side of the press.

Note: Unequal bearer height can destroy

originals and damage the molding press.

Making an accurate thickness matrix is the

key to successful plate molding.

Occasionally, the calculation used to

deteremine bearer height, does not give the

exact matrix floor, due to the characteristics

peculiar to the molding press, the materials

used and the nature of the graphics. Some

experimentation may be needed to arrive at

the correct bearer thickness for a particular

press and floor thickness, but once the floor

is established, it rarely changes. Forming the

matrix in the same press, and in roughly the

same position every time, also produces

consistent results.

Preheat Function. The type form or metal

photoengraving must be preheated in the

molding press for roughly five to seven min-

utes to allow for the expansion of the origi-

nal. This helps prevent the pattern from

“locking-up” in the mold as expansion takes

place. Preheating the matrix is perhaps the

single most important step in producing a

good mold. It softens the phenolic resin and

prepares it for molding. Preheating involves

heating the uncured matrix material and pat-

tern plate (original) without applying pres-

sure before molding. An accurate timer, or

clock with a sweep-second hand; should be

used to time the preheat cycle.