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

Подождите немного. Документ загружается.

88 FLEXOGRAPHY: PRINCIPLES & PRACTICES

This typical single-color

step scale is used to

measure dot gain and

calculate cutback

curves.

A small section of over-

print patches in a typical

target is used to create

ICC profiles.

times a special color is used on a line station

at a lower screen ruling. In this case, the cut-

back required is less than that used for high-

screen rulings. Details on how to evaluate

the correct cutback curve can be found in

the section on process color.

The second method used to compensate

for press gain is to use color management

techniques, such as the creation and use of

ICC profiles. Rather than using only single-

color step scales, as in the case of cutback

curves, a large number of overprints are

used (Figure

). Different color profiling

software packages use different numbers of

patches, but using over 1,000 patches is com-

mon. The goal is still to modify the dot per-

centages in the output file for the plates, but

this time the modification is generated from

color measurements of all the overprint

patches. Color management techniques can

go one step further than simply matching a

particular press and proof. Because it is

based on spectrophotometric measurement

of color, it is possible to specify a color by

the numbers and match to that. This latter

method, known as device-independent

color, is receiving much attention but is not

yet a mature production method.

back” curve to the file, which is to be output

to film for plate making. This basically

changes the values of the dot percentages so

that the dot percentage on the printed sheet

matches the dot percentage on the proof.

Cutback curves are calculated for each

process color from single-color step scales

(Figure

Cutback curves can also be calculated for

special colors, particularly those used often

and those used in screens opposed to only

line work. It is usually not practical to gen-

erate a cutback curve for each special color,

in which case one of the process-color

curves can be used instead. Also, many

0 3 5 7 10 15 20 25 30 35 40 45 50 60 70 80 90 100

raditionally, film output repre-

sents the actual end product

produced by the prepress com-

pany. While prepress charges for

all items that go into producing

the film, the film is what the cus-

tomer is purchasing. A new type of plotting

device, known as the platesetter, entered the

market a few years ago. This device exposes

a specially treated photopolymer plate,

instead of film, thus eliminating the need for

plate-ready film negatives.

The film output department consists of a

film-plotting device or imagesetter and some

type of processor to develop the films or

plates that come off the imagesetter. Film

plotters are either flatbed or drum models

which vary by size, configuration and type of

film supported (Figure

). A film plotter’s

size is measured by the size of the film that

the film plotter can expose. Plotters range in

film size from 9"x 12", up to 47" x 96". Flatbed

PLATES 89

Film Output/Imagesetting

A typical imagesetter.

Drum plotters are

known for their speed

and are more effective

with larger sheets of

film.

90 FLEXOGRAPHY: PRINCIPLES & PRACTICES

plotters are usually better for registration of

one color to another. Drum plotters are

known for their speed and are more effec-

tive with larger sheets of film.

FILM PROPERTIES

Plate-ready film has several important

properties:

• emulsion;

• orientation;

• film thickness;

• image properties;

• screen ruling and screen angles;

• dot shape;

• stochastic screening; and

• registration and mounting marks.

Emulsion

Film is made of a clear plastic sheet coated

with a light-sensitive silver-halide layer. The

side of the sheet with the silver halide is

called the emulsion side. The other side is

referred to as the base. The emulsion side

can be visually detected on an exposed and

processed sheet of film by its distinctive dull

look when compared to the base’s high-gloss

or shiny appearance. Another method to

identify the emulsion side from the base side

is to scratch an area of exposed film (black

areas as opposed to clear). The emulsion

side will scratch, exposing clear film. It goes

without saying that this destructive test

should be performed on nonimage areas.

Orientation

The film can be exposed as either a posi-

tive or a negative. Positive film has all non-

printing areas in clear or no emulsion, while

negative film is the exact opposite. Non-

print elements are black (the color of

exposed emulsion) with all printing ele-

ments as clear. Film also has an orientation

which is determined by how copy appears in

conjunction with the emulsion of the film.

The film orientation can be either right-read-

ing copy when the emulsion of the film is up

(RREU) or facing the viewer, or right-read-

ing copy when the emulsion of the film is

down (RRED).

Film Thickness

Film thickness is measured by the clear

plastic base of the film in mils or thousandths

of an inch (1 mil is 0.001"). Film comes in 4

and 7 mil thickness. Film that is 4 mil is used

on smaller imagesetters, while 7 mil is the

dominant choice for both large format image-

setters and photopolymer platemakers.

Finish

Film comes in either gloss, smooth, or

matte finish. Different platemaking process-

es call for different film finishes. For sheet

photopolymer platemaking, matte finish is

usually required. For liquid photopolymer

plates, clear is recommended. The particular

finish required should be determined by con-

sultation with the plate supplier.

IMAGE PROPERTIES

Aside from the film itself, there are prop-

erties of the image on the film: the screen

ruling and screen angles, dot shape, image

distortion, registration and mounting marks.

Screen Ruling and Screen Angles

Films that contain halftones are composed

of dots of varying sizes, based on a particular

screen ruling. The screen is determined by

the number of dots or lines per (linear) inch.

Coarse screen rulings measure below 100 lpi,

while fine screen rulings are 150-lpi and

above. Line screens from 100 to 150 are the

most common screen rulings used in flexo

printing. These same dots are also laid out in

varying degrees or angles, which allow for

multiple colors. When printed on top of each

other the screens should create a rosette pat-

tern, not a moiré pattern. Moiré patterns look

like crosshatches, or in some cases, rings or

swirls when screens print on top of each

other. Conventional color angles are 45°, 75°,

105° and 90°. That is, the four process colors

are printed with the dots running at these

angles. To minimize moiré, it is common

practice to separate the four process colors

by 30°. Since only 90° are available, this is not

possible and only three colors can be sepa-

rated by 30°, with one separated by 15°. In

flexo, there is an additional consideration –

the angle of the anilox roll. The screen angle

and engraving angle of the anilox roll can

interact and cause moiré patterns. In order to

minimize this problem for all anilox engrav-

ing angles (30°, 45° and 60°), screen angles

offset by 7.5° are used. Table 12 shows the

conventional angles and the angles offset by

±7.5°. It also shows a common assignment of

the process colors to specific angles.

Dot Shape

The dots in the film also come in varying

shapes – either square, round, circular, ellip-

tical or star. Each dot shape has its own

characteristics regarding dot gain and the

ability to be reproduced on a printing plate.

A round dot has been found to give the best

reproduction in flexographic printing.

Combination Screening

This method of half-tone screening com-

bines conventional and stochastic screening

and is used specifically in flexography to

address the highlight break problem.

Conventional screening varies the size of the

dots to increase or decrease the amount of

color in an area. That is, the dot density and

hence the dot percentage is determined by

the size of the dot. Conventional screening is

also called AM for amplitude modulation.

In stochastic screening, the size of the dots

remains constant and the density, or dot per-

centage, is determined by the spacing of the

dots (Figure

). Low density has widely

spaced dots, while high density has more

closely spaced dots. Stochastic screening is

also called FM for frequency modulation.

The entire image can be printed using sto-

chastic screening. However, it has been

found that stochastic screening only benefits

the highlight end of the tone scale and con-

ventional screening does a better job at the

midtone and shadow end of the tone scale.

This has led to the combination of conven-

tional and stochastic screening. Stochastic

screening is used in the highlights and then

gradually changes over to conventional

screening for the rest of the tonal range

(Figure

Registration and Mounting Marks

Platemaking films should contain registra-

tion marks. Generally a cross-hair, they

allow the job assembler to place multiple

pieces of film on top of each other for exact

PREPRESS 91

Conventional (AM)

screening varies the dot

size but keeps the dot

spacing (dots per inch)

the same. On the other

hand, stochastic (FM)

screening keeps the dot

size constant (small) but

varies the dot spacing.

Table 12

MAGENTA BLACK CYAN YELLOW

Conventional 45 75 105 90

Conventional

minus 7.5° 37.5 67.5 97.5 82.5

Conventional

plus 7.5° 52.5 82.5 112.5 97.5

SCREEN ANGLES

92 FLEXOGRAPHY: PRINCIPLES & PRACTICES

alignment. Proper placement of register

marks is in the center of the overall dimen-

sion of the film. Figure

shows the proper

positioning for a one-up and step-and-repeat

application. Many printers utilize video

mounting and registration systems. These

systems require small microdots (0.010" in

diameter) on the films in each of the printing

colors (Figure

). The dots are imaged with

a video camera and serve as positive align-

ment locations when mounting the plates.

Combination screening

uses FM screening in

the highlight area and

then transitions to AM

screening for the bal-

ance of the tonal range.

Proper placement of

registration marks in a

one-up and step-and-

repeat application are in

the center of the overall

dimension of the film.

The detail shows a

slight misregistration of

the cyan printer.

In a video mounting

system, microdots are

used. This illustration

details the slight mis-

alignment of the four

process colors.

Correct

Incorrect

he proofing department in a pre-

press company is often the most

overlooked. It is assumed that

all of the “real work” is in strip-

ping and film output. Proofing

should be viewed as one of the

most important departments because the

proofs have two very important functions:

• to represent the printed product as

closely as possible and

• to be the last opportunity for the cus-

tomer to make any corrections before

final films, plates and printed samples

are made.

Different types of proofs are made on

many different proofing systems. Even with

the use of good process control and optimal

systems, an exact match to the press will

likely not be achieved.

The degree of color difference can be

quantified but for the last analysis, it is visu-

al judgement that is commercially accept-

able. With conventional techniques, such as

dot-gain control, achieved through cutback

curves, several issues prevent an “exact”

color match. First, the pigments and col-

orants used in proofing systems are different

from the actual printing inks. Second, over-

printing of multiple inks or colorant layers,

creates a different reaction on the proof than

the printed package. Finally, the substrate of

the printed piece can be of a significantly dif-

ferent material and color from the proof.

The use of CIELab color management tech-

niques can overcome these issues. It will

enable better matches to be made because

the colors are matched by measuring them

in the same manner as the human eye per-

ceives them. While CIELab methods can be

used with any proofing system, they are par-

ticularly suited for use with digital systems.

TYPES OF PROOFS

There are different types of proofs to satis-

fy different needs. The types fall into three

categories: concept proof, color target proof

and contract proof. These types of proofs

have been formally defined in the second edi-

tion of FIRST and are summarized below and

in Table 13.

Concept Proof

This proof is used to show the graphic lay-

out of the product, including the type and

sell copy. Images and bar codes can be rep-

resented by FPOs.

Concept proofs are used to communicate

design concepts and layout to others. Color

may be used, but it is not necessary to show

colors that will accurately match to the final

printed job. Low-cost color copies, laser

proofs, ink jets and small dye sublimation

proofs are common examples.

Color Target Proof

This proof has not necessarily been

matched to the particular printing process,

nor the particular press. The color target

proof does, however, represent the cus-

tomer’s desire or expectation for color. The

“proof” may be a previous job printed on an

unspecified press and even a different print-

ing method. It may be the final version of a

concept proof, output on a low-end proofing

device. It may be a proof from a high-end

proofing device, optimized for another print-

PLATES 93

Proofing

94 FLEXOGRAPHY: PRINCIPLES & PRACTICES

ing process, like offset. In these cases, the

color in the proof may, or may not, be achiev-

able on press.

Contract Proof

The most critical proof is called a contract

proof. This proof is output in accordance to

FIRST specifications using a press profile. It

does not have to be a dot-for-dot reproduc-

tion, but it must be an overall visual simula-

tion of the expected print results. A contract

proof is produced at the end of the prepress

process and is what the customer signs off

on. It has all high-resolution images in place

and should accurately predict what the final

printed piece will look like. Remember, some

spot colors, varnishes and metallic inks can

not be represented by color proofs. Within

the FIRST specification, further technical dis-

tinctions are made between different types of

contract proofs. These distinctions address

how the contract proof is made, but do not

change the basic definition of what a contract

proof is. The three types of contract proofs

File preparation correct

• color breaks correct

• copy correct

• fonts correct

• all elements present

• trim and registration marks

present

Made directly from an electronic file, proofs

generated from desktop digital printers usually

use ink-jet or thermal-wax technology to give

all the information available in black-and-white

PostScript output, plus an approximation of

the specified colors. When used with color

management systems, they may provide a fair-

ly close match to press color, but differences

in the dyes and pigments and in the the

PostScript interpreters, can cause differences

between the proof and the film output.

WHAT TO CHECK FOR:

DESKTOP DIGITAL PROOFS

17 49 85 96 100

IG-28

Color images correct

• image colors correct

• copy correct

• fonts correct

• all elements present

• trim and registration marks

present

Made directly from an electronic file, these

composite CMYK color proofs meet industry

color standards for prepress proofing systems,

but cannot proof actual film.

WHAT TO CHECK FOR:

HIGH-END DIGITAL PROOFS

17 49 85 96 100

IG-28

All elements as expected from

imagesetter output:

• copy correct

• fonts correct

• all elements present

• trim and registration marks

present

PostScript laser printouts should provide the

same results as an imagesetter output, but at a

lower resolution. Printing colors as grays or

printing separations can show color breaks for

color jobs.

WHAT TO CHECK FOR:

BLACK-AND-WHITE LASER PROOF

PROOFING OPTIONS

COLOR ACCURACY

PRICE

PAPER TYPE

RESOLUTION

LEGEND

COLOR ACCURACY

Excellent

Good

Fair

COST

Inexpensive

Moderate

Expensive

Table 13. Adapted from Agfa Educational Publications 1999.

defined are: contract analog proof, contract

digital proof and profiled contract proof.

Contract Analog Proof. This proof is made by

using an analog proofing system. Exposing

and processing the proof, as per the manu-

facturer's recommendations for that analog

proofing system, is profiled according to

FIRST specifications. The color match,

whether using dot-gain compensation or ICC

profiles, is to target values for the particular

flexo process, but not to the specific press.

Contract Digital Proof. This proof is made by

using a digital proofing system. Exposing and

processing, as per the manufacturer’s recom-

mendations for that digital proofing system,

is profiled according to FIRST specifications.

The color match, whether using dot-gain

compensation or ICC profiles, is to target val-

ues for the particular flexo process, but not

to the particular press.

Profiled Contract Proof. This proof is profiled

on a specific date, using a specific color man-

agement system, to match a particular press.

Ideally, the press should be running in accor-

PREPRESS 95

All elements correct:

• copy correct

• fonts correct

• all elements present

• trim and registration marks

present

As the name implies, press proofs are run on a

printing press, using the same inks and sub-

strate that will be used in the final print job.

WHAT TO CHECK FOR:

PRESS PROOFS

17 49 85 96 100

IG-28

Film correctly assembled:

• color breaks correct (2- and

3-color)

• all elements present

• imposition correct

Blueline proofs are made by exposing final fim

to a thin-gauge, light-sensitive paper. Bluelines

show only a single-color image, but a second

color can be shown by varying the exposure

time for the second-color film.

WHAT TO CHECK FOR:

BLUELINE PROOFS

Color images correct:

• color match correct

• color balance correct

• registration correct

• no moiré problem

• traps and overprints correct

Also called single-sheet proofs or composite

proofs, these are created by exposing the film

separations for a job in contact with C, M, Y

and K proofing film and laminating the resulting

color sheets onto a single sheet of substrate.

WHAT TO CHECK FOR:

LAMINATE PROOFS

17 49 85 96 100

IG-28

Separations correct:

• color breaks correct

• all elements present

• traps and overprints correct

Proofs are made up of layers of acetate attached

in register to a backing substrate. Each piece of

film carries the image from one piece of separat-

ed film. Distortion caused by loose registration

and by refraction through the proofing film

makes color inaccurate. Can show color breaks.

WHAT TO CHECK FOR:

OVERLAY PROOFS

17 49 85 96 100

IG-28

PROOFING OPTIONS

COLOR ACCURACY

PRICE

PAPER TYPE

RESOLUTION

LEGEND

SUBSTRATE

Custom

Diverse

RESOLUTION

Low

300 dpi to 600 dpi

Medium

600 dpi to 1200 dpi

High

based on halftone

Table 13. Adapted from Agfa Educational Publications 1999.

96 FLEXOGRAPHY: PRINCIPLES & PRACTICES

dance to FIRST specifications. This type of

proof represents a final tweak or correction

to the contract digital proof because it is

press specific. This type of proof could be

done using an analog proofing system but, in

most cases, a digital proofing system is used.

PROOFING SYSTEMS

To produce a proof, whether a concept or

a contract proof, different proofing systems

are available. They fall into three categories:

analog, press and digital. Proofing systems

that make contract proofs must meet two

broad requirements: repeatability and quali-

ty. Repeatability means that the proofing sys-

tem must produce the exact same proof with

each print – proof after proof, day after day,

month after month. This applies particularly

to the color output of the proof. Given con-

sistent color output, there is potential for a

color matching system to match the proofer

characteristics to a printing press. If color

output varies randomly, no color-matching

system can match the proof to the press.

Analog Proofs

Analog proofs, the dominant format, can

be either:

• “overlay” proofs, such as Color Keys or

Cromacheck;

• “laminate” proofs, which include

Cromalin, Matchprint and Fuji Color

Art; or

• “single-color exposure” types, such as

Dylux or Bromides.

All of the above proofs are made from

actual film negatives (or positives) through

some type of exposing, registration and/or

lamination process. Another type of analog

proof is often made when the plates are

mounted on the plate cylinder. These proofs

are discussed in the mounting and proofing

section and are part of the production

process after prepress.

Overlay Proofs. This process involves using

the film and exposing a photosensitive mate-

rial, which will hold the cyan, magenta, yel-

low or black colorant. These colorants are

then processed to remove the noncolored

areas and overlayed on top of each other.

These proofs are accurate for content, trap-

ping and to check the integrity of the film.

They are not accurate for color approval and

they are relatively inexpensive.

Laminate Proofs. This process involves taking

the film and exposing a photosensitive mate-

rial, which creates a carrier or “latent image”

to which a liquid ink or toner powder can

adhere. This system allows toners to be

mixed, thus producing a proof showing spot

colors. The color image created can then be

laminated to some type of substrate. Some

systems allow freedom of choice for sub-

strates, others require that specific ones be

used. Traditionally, these proofs are extreme-

ly accurate for color, trapping and verifying

the integrity of the negatives involved. The

systems are not very expensive, as the hard-

ware is often “given away” in exchange for a

guaranteed purchase of consumables, such

as toners, inks, colorant sheets or substrates.

Single-Color Exposure Proofs. These proofs

are also made by exposing a photosensitive

sheet. However, these proofs can produce

only a single color. Dyluxes are bluish, hence

the term “blueline”, while bromides are

black and white. Exposure of a Dylux pro-

duces an image immediately, while bromides

must be processed to reveal its latent image.

Press Proofs

Press proofs are made on an actual print-

ing press from the final plate-ready films.

These proofs provide almost an exact dupli-

cation of the actual production run. They

are, however, the most expensive proof to

make because they require a great deal of

time and materials, including photopolymer

plates and press time.

Digital Proofs

There are different types of digital proof-

ing devices available to serve various needs.

The key feature of all of them is that the

proof is produced directly from an electron-

ic file, without the use of film. This feature

makes digital proofing devices less expen-

sive to use than their analog counterparts

and is a strong driving force in the adoption

of digital proofing. There are basically five

print-engine technologies used for digital

color output:

• drop-on-demand ink jet;

• electrophotography;

• wax transfer;

• dye sublimation (heat); and

• continuous ink jet.

Print-engine technologies can be catego-

rized according to the colorant (the equiva-

lent of the ink in printing), the signal, and

how the colorant is applied to the substrate.

Drop-on-Demand Ink Jet. Ink jet is a process

where ink is sprayed onto the substrate

(Figure

). The colorant is a water-based

ink and the substrate is theoretically any-

thing which is water receptive. With drop-

on-demand ink jet, the image signal tells the

drop when to spray. To accomplish this, the

signal in some way causes a change in pres-

sure. When the pressure hits a certain point,

a drop flies out of the ink-jet nozzle and onto

the substrate. One way to create the pres-

sure change is for the signal to create a bub-

ble in an ink chamber. Blowing up the bub-

ble forces ink to fly from the nozzle.

The advantage of drop-on-demand ink jet

is its low cost. Relatively inexpensive print

engines can be assembled, making the price

suitable for the desktop office market. The

relatively low-cost color ink-jet printers,

which are so ubiquitous in the office and

home, are of this type, as are the wide-for-

mat poster printers. The disadvantage is

quality and consistency. By nature, this

start/stop approach produces fairly large

drops of ink, limiting effective resolution.

Also, color consistency and repeatability are

not suitable for color proofing – tint values

will be different in areas of large ink cover-

age compared to areas of little ink coverage.

Electrophotography. Electrophotography is

more commonly known as xerography

(Figure

), which in its conventional form,

involves a metal, selenium drum being given

an electric charge. Light reflecting from an

original through a lens, discharges the drum

in non-image areas. The colorant is given the

opposite charge to the drum, and when

applied, sticks to the drum in image areas.

The colorant from the drum is transferred to

the substrate, where it is heated to fuse into

the paper.

PREPRESS 97

In a drop-on-demand

ink jet proofing system,

ink drops are metered

only as needed for the

image area.

Electrophotography,

commonly called

xerography, uses a

laser to write the image

information onto the

drum. The drum picks

up the powdered toner

of the image areas and

deposits it onto the

substrate. The toner is

fused to the substrate

by heat.

Colorant Signal Substrate

Colorant

Laser

Signal Substrate