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68 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Drum as well as flatbed scanners filter the

light through red, green and blue filters and

then use an electronic detector to convert

the light into the separate electronic RGB

channels. Drum scanners use a photomulti-

plier tube (PMT) to convert the light. This

technique allows for capturing a wide range

of color. It also makes the equipment more

expensive when compared to flatbed scan-

ners. Flatbed scanner optics utilize CCDs

(charged coupled devices) to detect the light,

one scan line at a time. CCD technology is

less expensive, but it generally provides a

lower range of reproduction. Recent

advances in CCD technology have greatly

leveled the playing field.

Another difference between the two types

of high-end scanners is the ability to provide

an image compatible with high-resolution

output devices. Resolution outputs of most

high-end devices range from 2,400 to 4,000

dpi (dots per inch) for commercial work.

Both drum and desktop high-end devices

easily meet these requirements. However, for

especially high-resolution output, the drum

scanner far surpasses the desktop models.

Drum scanners can go up to 10,000 dpi, while

desktop models max out at 5,000 dpi.

SCANNING IMAGES

A good scan is as important as a good orig-

inal to successful reproduction of an image.

Digital retouching, either by resampling or

interpolation, or high-quality output can not

make up for an inadequate scan. The quality

of a scan is highly dependent on the number

of pixels per inch (ppi) a scanner can cap-

ture. This is called its resolution.

Before scanning an image, it is important to

know how that bitmap image will be repro-

duced, its printed size and which screening

technology – either stochastic (FM) or con-

ventional (AM) – will be used. The resolution

to use when reproducing images via FM

screening depends on the FM screen used. An

image with a FM dot that is close to the mini-

mum size the printing press can print consis-

tently is considered ideal.

If traditional halftone screening for color

and grayscale bitmap images is used, the res-

olution required is usually dependent upon

the screen ruling and the final printed size.

At actual reproduction size, it is recom-

mended that the resolution be at least 1.5

times the screen ruling. For instance, an

image printing at 120-line screen should

have at least 180 (120 x 1.5) ppi for high qual-

ity reproduction.

During the process of enlarging or reduc-

ing the size of an image, the “effective” reso-

lution is changed. Resolution is changed in

direct proportion to the percentage of

enlargement or reduction. If, for example,

the 180 ppi scan were enlarged to 200%, the

effective resolution is reduced in half to 90

ppi. This scan would now only support qual-

ity reproduction at 60-line screen. This is

why scanning should always be done with

the final printed size in mind. If a scan will be

used for more than one size, or the size is not

known precisely at the time of scan, it is best

to scan at the highest resolution. A scan with

too much resolution can be safely down-

sized, but a scan with too little resolution

can not be upsized (resampled). The missing

data simply can’t be created (interpolated)

to still maintain the quality for printing. With

a resolution of more than twice the line

screen, however, there is no appreciable

improvement in the quality. The following is

the formula to calculate the scanning resolu-

tion required:

Scan Quality Screen

Resolution



Factor



Ruling

 Magnification

Where

Quality Factor = 2.0 is the rule of thumb; 1.5

minimum recommended.

Screen Ruling = Screen ruling which will

be used to print the image,

such as 120 lpi.

Magnification = Magnification of original

image to the printed image.

Example: The image from a 35mm slide

transparency will be printed at 300% enlarge-

ment (magnification of 3) at 120 lpi. Using a

quality factor of 2, the required scanning res-

olution would be 2 x 120 x 3 or 720 ppi.

Note: For a given scan resolution and

quality factor, screen ruling and magnifi-

cation can be traded. That is, a file of a

given size in total number of pixels can be

printed with the same quality level at dif-

ferent combinations of the two.

In the above example, if instead of a mag-

nification of 3, a magnification of 2 is

used, the screen ruling now becomes 360/2

or 180 lpi. That is, the same file could be

printed at the 200% magnification at a

screen ruling of 180 lpi with the same qual-

ity level as before at a 300% magnification

and 120 lpi.

This calculation can easily be seen in a pro-

gram such as Adobe

Photoshop. Suppose

an image has a width of 8" and height of 4" at

a resolution of 75 ppi. This means, the file has

a total of 600 by 300 pixels. In the Image Size

menu, if the resolution is changed to 150 ppi

and the resample image box is not checked,

the new width and height will be 4" by 2".

Similarly, changing to 300 ppi decreases the

size to 2" by 1" (Figure

). These examples

demonstrate how the originally available pix-

els have simply been redistributed.

Note: If the resample image box is

checked, the program will interpolate data

to give the same size image at the higher

resolution. Quality will not be maintained

in that case. Taking the original image and

forcing the resolution up by a factor of 4

(from 75 to 300) and then outputting at the

original 8" by 4" size will result in a totally

unacceptable image.

For line art, scanning is not dependent on

the screening method. Instead, line art

should be scanned at the output device reso-

lution, if the output device is less than 1,200

dpi. Scanning at a higher resolution than

1,200 pixels per inch will not yield a better

looking image.

PRODUCING A COLOR

SEPARATION FOR FLEXO

It is important to point out that traditional

methods of producing color separations are

geared toward offset reproduction. The

uniquely different characteristics of flexo-

graphic printing dictate that offset separa-

tions should not be used for flexo printing.

The following describes the differences

between flexo and offset separations.

PREPRESS 69

The same electronic file

of 600 by 300 pixels

results in different size

images at different pix-

els per inch. If the

resample dialog box is

not selected in an

image-editing program,

as pixels per inch goes

up, the image size goes

down, keeping the total

number of pixels avail-

able for output constant.

300 ppi

150 ppi

75 ppi

Electronic File

600 x 300 pixels

70 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Highlight/Shadow Treatments

Highlights and shadows are treated differ-

ently in flexo than in offset. The smallest re-

producible dot on a flexo printing plate is

about a 2% dot. Dots that are 1% do not carry

the same amount of support on the plate,

and in some cases, do not print at all. In

other cases, ink builds up on the dots and is

released onto the substrate in blobs. This is

known as “dirty print.” A scan, then, should

not have anything less than a 2% dot.

A current technique addressing minimum

dot size is frequency modulated (FM) dots in

the highlights. Printed samples have shown

that it is possible to fade to a 0% dot. This

technique not only allows for the reproduc-

tion of cleaner, brighter highlights, but also

results in cleaner or more saturated colors.

Shadows also require a different printing

approach. Flexographic presses generally

record the highest density value at 93% to

98% screens, not on a solid. Solids, especially

when printing in combination with screens,

tend to produce picking. This is when the ink

does not fully adhere to the substrate, leaving

tiny holes. Screen values of 93% to 98% not

only adhere better to the substrate, but also

gain on press to a solid. Due to these factors,

separations for flexo should not be made

where the shadows go to 100%.

Separation Techniques:

GCR/UCR/TAC

GCR (Gray Component Replacement),

UCR (Under Color Removal) and TAC (Total

Area Coverage) are separation techniques

which are used differently in flexo than they

are used in offset.

UCR is the balanced reduction of cyan,

magenta and yellow in shadow areas, with

an increase of the black to maintain the dark

and near neutral shadows. This technique is

not always best suited for flexographic print-

ing. The ideal use of this technique will be

where one can reduce the amount of color in

yellow, magenta and cyan while maintaining

the shape and shadow detail in those three

colors.

TAC is the total of the dot percentages of

the four process colors on the final film in

the darkest shadows. Knowing and compen-

sating for the TAC is important during the

conversion stage. Typical maximum TAC for

flexo runs from 280% to 320%.

GCR is more easily defined by saying that

an unwanted color (cyan in reds or magenta

in greens) can be replaced entirely or par-

tially with black. Under normal conditions in

the flexo process, it is recommended that

GCR be restricted to a single unwanted

color. The use of GCR in flexo separations

allows printers more latitude on press and

prevents printed images from looking gray

and dirty.

GCR should not be used when the printer is

forced to print line black on the same station

as the process black. It is better to have a

short (skeleton) black for the separation, so

there is more latitude in setting the impres-

sion. The use of GCR also allows items of sig-

nificant color variations to be printed side by

side (Figure

). For example, printers tradi-

tionally stay away from printing an item like

carrots next to a bowl of peas. The results are

usually poor because in an effort to get more

red into the carrots, the increased magenta

makes the peas dirty. The use of GCR

removes the magenta from the peas (and

cyan from the carrots). This allows the print-

er to increase the magenta as needed without

the peas being affected. In conjunction, the

cyan in the peas can be manipulated without

affecting the color of the carrots. Figure

shows a separation with and without GCR.

Cutback Curves/ICC Profiles

Cutback curves and ICC profiles are two

methods of compensating for the particular

print characteristics, mostly the dot gain, on

a flexo press. The methods will be discussed

elsewhere in detail, but depending on the

particular workflow, some, or all of these

measures, can be built in right at the scan-

ning stage. When working with ICC profiles,

for example, the profile of the scanning

device can be generated and used with the

scan. Ultimately, using ICC profiles, each

input and output device is characterized and

the desired color is specified in device inde-

pendent CIELab color space. With current

practice and technology, this workflow has

not been implemented to any large extent.

DIGITAL PHOTOGRAPHY

Digital photography is still in its infancy

when it comes to the flexographic print

process. It is important to recognize the cur-

rent uses and workflows in which digital

photography is utilized and then compare

them to how things should work in today’s

flexo prepress environment.

Digital photography has been an enor-

mous benefit to the offset-print market. This

process captures and saves the image as dig-

ital data during the actual photography

stage. Where traditionally an image is pho-

tographed, a color negative developed and

then a color transparency or print is gener-

ated that can then be scanned; a digital

photo bypasses almost all of those steps.

Once the image is photographed it is trans-

ferred to computers for immediate editing

and output. Generally, the images do not

require separation from an RGB color space

to the CMYK printing color space. The cost

of separating the image is eliminated as is

the time to do so.

The RGB digital capture is easily converted

PREPRESS 71

Colors respond

differently to the GCR

process. When yellow

is swapped out for black

the resulting color

changes are most

noticeable. Replacing

black with cyan or

magenta exerts a

significant, but less

obvious, impact on

the color palette.

The apple image is

compared with and

without GCR. When

GCR is used, there is

an increase in the black

separation.

With

GCR

MAGENTA

0% GCR 50% GCR 100% GCR

MAGENTA and YELLOW

CYAN

YELLOW

0% GCR 50% GCR 100% GCR

0% GCR 50% GCR 100% GCR 0% GCR 50% GCR

100% GCR

72 FLEXOGRAPHY: PRINCIPLES & PRACTICES

by the photograhper to CMYK through color

conversion tables. Digital proofing devices,

available to the photographer, allow the

image to be proofed and submitted to the cus-

tomer for review. If any color changes are

needed, the photography studio can easily

execute the changes and resubmit the image.

This process works well for offset printing

because the conversion tables and proofing

systems have been optimized for that

process. It does not, however, meet the needs

associated with flexographic reproduction

for the same reasons that a scan specifically

created for offset will not print well in flexo.

The following are some of the reasons.

Minimum/Maximum

Dot Requirements

As mentioned previously in the scanning

section, flexo requires a minimum of a 2%

dot and a maximum of 95% to 98% dot. The

RGB-to-CMYK color conversion tables avail-

able to the photographer do not traditionally

allow for these settings. However, new soft-

ware and more sophisticated color conver-

sion programs are quickly closing the gap.

Use of 100% GCR

Today’s flexo separators are using a full

GCR (gray component reduction) approach

more than ever before. This means that sepa-

rations are done predominately with full

range (0% to 100%) in yellow, magenta and

black and a short range (60% or greater) for

cyan. This “short” cyan is used when a green

color is reproduced and to add weight to very

dark shadows. Color conversion tables that

go from RGB to CMYK have been set up to

produce an opposite separation. Those sepa-

rations are done with a long yellow, magenta

and cyan, and a short black. This requires

extensive retouching to make the adjustment

from long cyan to short cyan. New software

entering the market will address this issue

and offer acceptable alternatives.

CMYK vs. RGB Proofing

One of the biggest reasons why digital pho-

tography has not benefited flexo the way it

has the offset market is because of the digital

proofing dilemma. The proofing devices use

an RGB-to-CMYK color conversion table that

is completely different than the one used to

create the color separation for printing. This

is an important fact to consider. The digital

file output by the separator is completely dif-

ferent from the proof supplied by the cus-

tomer as a color target. The separator, then,

has to manipulate the file to match the cus-

tomer’s or the photographer’s proof.

These issues can effectively eliminate the

cost and time savings associated with digital

photography. In short, the file received by

the flexo separator can not be used as is. It

must still require minimum and maximum

dot percentages and GCR applied, and must

be color corrected to match the customer-

supplied proof. Digital photography is a

valid means of capturing an image, but the

customer has to realize that, because of the

unique properties of the flexo print process,

the digital file must be treated as if it were an

original transparency or reflective art.

SCANNING DEPARTMENT SETUP

The quality and variety of equipment

found in a scanning department in a pre-

press house varies from supplier to supplier

(Figure

). Generally, components include:

• Scanners – drum, flatbed, transparency, for

translating hard-copy originals into elec-

tronic files that can be manipulated by

electronic prepress systems. Software

bundled with some high-end scanners

allow sophisticated image manipulation,

or produce separated files in PostScript,

or proprietary formats, which can be out-

put on an imagesetter. As high-resolution

images tend to be large and difficult to

work with on a desktop computer (see

Table 9 for file sizes of CMYK scans),

• Monitors. High-resolution models are capa-

ble of 24-bit color display. Larger screens

usually require a video card to accelerate

the display.

• Software. Programs include those to oper-

ate the scanner, color management soft-

ware, and image processing/color correc-

tion/retouching applications.

• Short-term Storage Devices. Transportable or

removeable media include Zip, Jaz or opti-

cal disks and CD-ROM.

• Long-term Storage Devices. Hard disks, or an

array of hard disks, CD-ROMs and/or mag-

netic tape are needed to handle and archive

the many gigabytes images require.

• Computers. Workstations with a fast CPU

and sufficient RAM are required to run the

software and handle the large files.

• Proofing Devices. Contract-quality and digi-

tal proofing systems are essential to proof

the image prior to the output of film.

These proofing devices, when set up to

conform to actual press characteristics,

are extremely useful tools to the prepress

company as well as the end-user.

PREPRESS 73

many prepress services provide a low-res-

olution of the image to the customer for

use during layout and design, storing the

high-resolution version until the pages are

output. (See the section on low-resolution

placed images for more detail)

PREPRESS 73

A typical scanning

department includes a

file server, scanner,

retouching color work-

station, imagesetter and

proofing device. These

pieces of electronic

equipment control the

flow of data.

Scanner

File Server

Retouching/Color

Workstation

Imagesetter

Proofing Device

Tape Drives/

Optical Drives

Table9

2700 Digital file size image scanned at 266 ppi/133 lpi

3430

Digital file size image scanned at 300 ppi/150 lpi

12345678

FILE SIZES OF SCANNED IMAGES

277 553 830 1080 1350 1620 1890 2160

352 704 1030 1370 1720 2060 2400 2750

553 1080 1620 2160 2700 3240 3780 4320

704 1370 2060 2750 3430 4120 4810 5490

830 1620 2430 3420 4050 4860 5670 6480

1030 2060 3090 4120 5150 6180 7210 8240

1080 2160 3240 4320 5400 6840 7560 8640

1370 2750 4120 5490 6870 8240 9610 11000

1350 2700 4050 5400 6750 8100 9450 10800

1720 3430 5150 6870 8580 10300 12000 13700

1620 3240 4860 6480 8100 9720 11300 13000

2060 4120 6180 8240 10300 12400 14400 16500

1890 3780 5670 7560 9450 11300 13200 15100

2400 4810 7210 9610 12000 14400 16800 19200

2160 4320 6480 8640 10800 1300 15100 17300

2750 5490 8240 11000 13700 16500 19200 22000

2430 4860 7290 9720 12200 14600 17000 19400

3090 6180 8270 12400 15500 18500 21600 24700

2700 5400 8100 10800 13500 16200 18900 21600

3430 6870 10300 13700 17200 20600 24000 27500

74 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Preflight Quality Control

uality Control (QC) reviews

are conducted prior to manu-

facturing and the release of

materials to a converter, print-

er or customer. In the prepress

environment, the job engineer

is responsible for reviewing each project for

manufacturing issues prior to actual execu-

tion. This is done soon after the arrival of the

desk-top mechanical or laser proof.

The job engineer looks for issues that could

cause printing problems if not handled prop-

erly, and plans each job in order to maintain

consistency between operators. All of this is

done with the customer-supplied laser proof

as a reference point. This is what separates

the function of the job engineer from the pre-

flighter. Where preflight reviews the actual

electronic file, the job engineer only reviews

the laser proof supplied by the customer.

In actuality, the preflighter and the job

engineer work very closely together. The job

engineer identifies potential issues based on

the laser proof and the preflighter confirms

how the electronic file is set up. The follow-

ing section describe what the job engineer

checks for on the incoming laser proof to

confirm that the information for the job is

accurate. Table 10 summarizes that process.

SIZE/DIMENSIONS

One of the initial checkpoints is the actual

size of the job. A low-resolution or laser

proof supplied to the prepress provider

should either be at full (100%) size or, if at a

reduced or enlarged size, it should be clear-

ly indicated. The dimensions can be checked

with a ruler to confirm their accuracy.

SCANNING TECHNIQUES

Both the job engineer and scanner opera-

tor should review the actual scanning tech-

niques required for an image. Sometimes it is

possible to eliminate one of the process col-

ors through the use of GCR. This informa-

tion, if realized up front, can help in deciding

how many colors the job actually needs. For

instance, when separating a field of peas,

magenta may be eliminated altogether, since

it is a contaminating color. If there is no

other magenta required on the package, the

customer and printer have freed up an addi-

tional deck, which they can decide to use for

another color.

INKS REQUESTED VS.

INKS REQUIRED

The inks requested by the customer could

be different than the inks actually required

for optimum flexographic reproduction. The

job engineer has to take into consideration

many factors when trying to decide what

Table 10

■ Size and dimension

■ Scan techniques required

■ Inks requested vs. inks required

■ Spot colors or process match

■ Ink rotation and trapping

■ Tint builds

■ Screening requirements

■ Vignettes, gradations and blends

■ UPC positioning

A JOB ENGINEER’S CHECKLIST

colors will produce the best looking pack-

age. More often than not, this discussion is

done with full cooperation of the printer.

Some of the issues to consider are:

• The existence of corporate or logo colors.

Colors signifying a brand name or corpo-

rate entity are almost always specified as

line color to ensure print consistency

from press run to press run.

• Repeating colors in a product line. When deal-

ing with multiple items in a product line, it

is important to consider colors that repeat

on each of the different packages. When

the products share a common printing

color, the usual approach is to print that

color as a line color. This is to ensure con-

sistency between all the packages.

• Utilization of a “code color”. When a cus-

tomer has products in a line that are very

similar, a “code color” may be used to dif-

ferentiate between items. For instance, a

line of three packages could have identical

separations and layouts, but the customer

chooses to print the flavor description

copy in a PMS 287 blue on package “A”, a

PMS 327 green on package “B” and a PMS

872 gold on Package “C”. In order to save

films and plates, the job engineer would

not want to print those flavor colors in

process matches. Instead of having to

make four process-color films for each

package, a common set of process films

would be used and a new line color made

for the flavor description.

• The color’s ability to be reproduced in screens

versus a line color.

An example would be

when a customer has seven of eight decks

chosen and has to decide between a logo

color of PMS 327 green or some other “sell

copy” that prints in PMS 287 blue. While,

the initial reaction would be to put the cor-

porate PMS 327 green on the line deck, this

might not produce the optimum results. In

this scenario, the job engineer might opt to

print the PMS 287 blue as the line color.

The reason: a PMS 287 is a much more dif-

ficult color to match in process than a PMS

327. Some customers have gone as far as

assigning a delta (∆) E

to help make the

decision. Whichever process-match-to-

line-color value has the lowest ∆E-value is

put in process.

• Text size This issue is closely related to the

previous one. If, in the previous example,

the “sell copy” to be printed in the PMS

287 blue is small text, this would by itself

dictate the use of a line color. With larger

type, it might be feasible to use a color

matched with process.

• The amount of ink coverage. Colors that have

heavy ink coverage are better served by

being printed on a line deck. Also lighter

process match colors, like yellows, light

oranges, pinks, pale blues, pale greens and

light grays, are better put on line decks

because they tend to be a little more diffi-

cult to control on press. This is especially

true for near-neutrals, where a small shift

in one of the constituent colors makes a

large visible color difference.

SPECIAL COLORS:

SPOT OR PROCESS MATCH

The job engineer should confirm how all

colors are to be reproduced, especially

whether they are spot or process match. A

simpler case is when the designer specifies

all the colors used as PMS colors and indi-

cates if they print as a “line” color or a

process match. In this case, the task is to

assign the proper tint values for those colors

that will be matched with process.

A more difficult case and potential prob-

lem comes about when the file has a color

assigned as a tint-build only. This may seem

to be clear, but the problem lies in knowing

the real intent of the designer. Most likely,

the tint values were copied from the process

PREPRESS 75

1 ∆E is a numerical measure of color difference in CIELab color space. Refer to

the chapter on process color for more information.

76 FLEXOGRAPHY: PRINCIPLES & PRACTICES

swatch book and the real desire is to match

that particular color in the swatch book. The

problem is that the tint values given in the

swatch book for that particular swatch are

not guaranteed to produce the given color

when printed flexographically. As a matter

of fact, it is almost certain that the printed

color will be a poor match to the swatch.

Swatch books that show process-match

builds are printed using offset specifica-

tions. For example, the specifications for

PMS 485 red is 100% yellow and 100% magen-

ta. For offset, this means that a process

match of PMS 485 red is printed with a solid

ink density of approximately 1.40 for magen-

ta and 1.00 for yellow. When the same 100%

magenta and 100% yellow is printed in flexo,

the solid ink density for yellow is typically

also 1.00, but the magenta is less – around

1.20. Because of this, the resulting color is

significantly more orange; the magenta con-

tent has been reduced when compared to

original yellow content. The knowledgeable

prepress facility will reduce the percent of

yellow to regain the balance between the

yellow and the magenta found in the original

offset PMS swatch. It is worth pointing out

that typical flexo solid-ink densities will usu-

ally result in a color that is a little “weak”

when compared to an offset swatch of the

same process build. Of course, only the den-

sity has been considered thus far. Besides

the density, there is the issue of the hue of

the inks used.

In general, the hues of flexo inks are not

identical to offset inks, leading to yet anoth-

er cause of color difference.

Note: It might be pointed out that even in

offset printing, the process-match builds

specified in the swatch books often produce

unacceptable results and the builds need to

be modified. One solution to the problem,

which can be applied to flexo as well, is to

use a spectrophotometer and special soft-

ware to calculate the required process-tint

values. The spectrophotometer measures

the desired color. Then the software, using

data stored for the particular printing

process, calculates the closest match possi-

ble using process tints. Using this tech-

nique, any color can be specified with

process colors. The software program gives

the degree of match possible in terms of the

above mentioned ∆E value.

INK ROTATION AND TRAPPING

Ink rotation can determine how a job is

eventually constructed or trapped. For

instance, when a customer uses a very

opaque ink, such as a PMS 872 metallic gold,

the job engineer must know what the exact

rotation will be. The ink rotation will be

determined by the printer, taking into

account the particular press and complexity

of the job. In the case of the metallic gold, if

there is solid-black type printing over the

gold, the black can be set to overprint, if it

prints after the gold. If the black prints

before the gold, then a knockout must be

applied to the gold to allow the black type to

show through (Figure

). This is because

the opacity of the metallic gold is such that it

will hide any color that it prints over. In all

cases, and with all colors, the relative opaci-

ty is one of the determining factors when

deciding how a job is to be trapped.

The other key factor is the actual colors

involved. With transparent inks, no matter

what the rotation, proper trapping must be

applied or unwanted results can occur. In

general, dark colors can be successfully

overprinted onto light colors, but the deci-

sion of whether to overprint or knockout

needs to be made by considering the partic-

ular colors involved. Figure

shows an

example where the green type in the yellow

circle can overprint the yellow. However, in

the red square, the green type must be

knocked out. Any potential issues that may

arise when two colors require that they be

trapped to each other should be reviewed

and decided on before film assembly or

stripping takes place. Objectionable traps

can be discussed with the designer or cus-

tomer up front and suggestions can be made

to alter the design if necessary.

TINT BUILDS – THREE-COLOR

TYPE OR TINTS

Any tint builds in a package should have

no more than three colors. Print reproduc-

tion is better controlled using two colors;

however, this is not always practical. The job

engineer has to work with the desktop per-

son to determine if any colors that need four-

color tints exist or if a three-color tint can be

reduced to two colors. For example, some-

times a three-color tint calls for a very small

dot percentage for one of the colors. In this

case, the customer might approve the slight-

ly cleaner color that results when that small

component is removed.

SCREENING REQUIREMENTS

It is common in flexo to print process work

(screens) separate from solid line copy. This

is due to the cell counts of anilox rolls being

used on press. Process printing, which is

often at 100- to 133-line screen, requires

anilox rolls with a higher cell count. Typically

these rolls have cell counts of 600–800. Line

decks usually carry solid-line copy – done

with rolls that have a cell count of 400–550.

The job engineer must be aware of the par-

ticular screening requirements when a cus-

tomer requests that a screen be printed on a

line deck. When this request is made, the job

engineer must inform the customer, that to

get an optimum reproduction, it is best to

print that screen in a coarse line screen. The

line screen that is generally used to print

screens on a line anilox roll is 65 to 85. The

customer must be aware of this, because

depending on the screen used, there may be

a dot pattern that the customer will find

objectionable. If the customer is notified up

front, it is possible to come up with an alter-

native before the expense of films and proofs

are made. The customer will usually opt to

convert the screened color of the line deck to

process printing, or use the coarse line

screen on the line deck.

VIGNETTES/GRADATION/BLENDS

The execution and handling of vignettes

(also called gradations or blends) warrant

detailed discussions during the job engineer-

ing stage. The way the vignette is created in

the electronic file is not necessarily how the

customer expects it to print. Engineering of

vignettes requires that the values of the

vignette meet the minimum/maximum dot

PREPRESS 77

An example of ink rota-

tion where the black

prints before the opaque

gold. With a knockout,

the black type is visible.

However, if the gold

overprints, the black

type will not be visible

through the opaque

gold.

Darker green type is

set to overprint in the

yellow circle, but is

knocked out in the

darker color of the

square.