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78 FLEXOGRAPHY: PRINCIPLES & PRACTICES

A side-by-side compari-

son of an acceptable

and unaceptable

vignette. Acceptable

vignettes contain no

banding, while unac-

ceptable vignettes

contain banding.

UPC codes should be

placed in the picket

fence position, in which

the bars run in the

machine direction.

requirements of the printer. It is important

for the job engineer to understand what the

customer expects and translate that expec-

tation into a vignette that is visually appeal-

ing and technically printable. The job engi-

neer must work with the desktop publisher

to determine how the vignette is created.

With this information, the job engineer will,

in most shops with high-end proprietary sta-

tions, recreate the vignette to the customer’s

requirements.

In instances where the vignette will be

stripped on a desktop PC/Mac workstation,

the vignette may still be recreated to pro-

duce the desired effect if the original

vignette is not satisfactory. A vignette is

deemed unsatisfactory if it either produces a

“banding” effect (Figure

) or is specified

below the minimum dot requirements for

flexo printing. Whether vignettes are created

on a desktop or a high-end station, it is a

good practice to output the vignette before

the final film. To output the vignette at the

same time the entire job is output, and to

find the vignette needs adjustments, is a

tremendous waste of resources. Also, when

vignettes will be compensated or cut back

during output, it is advisable to apply that

compensation to the vignette when it is out-

put during the test.

UPC POSITIONING

For optimum reproduction, UPC codes

should run in the direction of the printing

unit. (Figure

). The job engineer should

question a UPC running in the transverse or

ladder direction, in the event that the cus-

tomer overlooked it.

No Banding Banding

Picket Fence

Ladder

esktop departments in a pre-

press environment are com-

monly called the Mac depart-

ment. These Mac departments

grew as an extension of the

Mac departments of the

designers. When designers created art board

mechanicals, there was a need to convert

that mechanical to a format that could be

stripped. That meant making a line shot or

negative of the mechanical’s type elements

and, through the use of goldenrod mask and

manually cutting/stripping in tints, creating

plate-ready negatives. Stripping was done on

a light table and the various flat negatives

and masks were composed together to make

a negative for each printing color.

In the 1980s, designing and stripping pack-

age art shifted from being done by hand to

being done almost exclusively on comput-

ers. Designers began utilizing the Macintosh

computer for designing while the prepress

houses began using various expensive pro-

prietary electronic stripping stations. For

the first time a gap was created between the

designer and the prepress shop. The pre-

press shop had no way of taking the design-

er’s file from the disk and getting that digital

information into its system. The solution

was that prepress companies went out and

purchased the same type of computers used

by the designers, then used the customer’s

disk to create a file that could be recognized.

As the computing power available for

desktop publishing systems increases, the

job assembly and output functions, formerly

done by the proprietary systems, can now

also be performed with Macs or PCs, albeit

with less efficiency.

It is important to note, that while the

Macintosh computer still is the dominant

operating system for graphics reproduction,

IBMs and IBM-compatibles (collectively

called PCs) also have the ability to do the

same job that the Mac can. Software pro-

grams that were once only available on the

Mac, are now available for the PC. In fact, the

cross-platform capabilities on today’s com-

puters have resulted in the desktop depart-

ments consisting of both PCs and Macs.

“READING” FILES

To make an electronic stripping workflow

possible, a common digital format was

required. PostScript became that standard,

universal computer language all computer

makers adopted to allow for the exchange of

electronic documents between varied com-

puters. PostScript files support composite

and separated workflows for vector and

bitmap images, but require that all fonts be

embedded. The prepress shop can easily

convert the electronic file created in any

software program such as QuarkXpress or

Adobe Illustrator to a PostScript file and

“read” that PostScript file on the stripping

station.

The latest development, spurred by the

growth of the Internet, is the Portable

Document Format (PDF), developed by

Adobe Systems Incorporated. This format is

designed as a solution to easily exchange

electronic documents between Mac and

other platforms. This is possible because

PDFs are independent of the original appli-

cation software, hardware, and operating

system used to create those documents.

PLATES 79

Desktop/Preflight

80 FLEXOGRAPHY: PRINCIPLES & PRACTICES

PDFs have found a niche in desktop pub-

lishing, with its positive ability to preserve

the original graphic appearance. This file for-

mat embeds all fonts, as well as information

about whether the PDF is trapped or not it

also has the ability to represent bleed and

trim, lossless compression and can insert

ICC profiles about the intended printing con-

dition. Using special software, such as

Adobe Distiller‚ PostScript level 2 and 3 files

can be converted to PDF files. A new format,

PDF/X – the X stands for eXchange – is a

proposed American National Standard

Institute (ANSI) standard being developed

by the Committee for Graphic Arts

Technologies Standards (CGATS), and most

likely to become an International Standards

Organization (ISO) standard. It is a variant of

the PDF, intended for prepress production

and high-end printing, and can handle com-

posite files containing both vector and raster

objects. Two PDF/X specifications are being

developed. PDF/X1allows files to be output

directly; and PDF/X2, which allows modifi-

cation required by the file, such as OPI

image replacement prior to output.

PREFLIGHT RESPONSIBILITIES

Today’s desktop department has two pri-

mary responsibilities: creating files that can

be recognized by the stripping station and

preflighting of those incoming files. The

ever-increasing power of today’s desktop

computers has caused, in some cases, the

desktop department to be responsible for

the film assembly (stripping) of the package.

Preflight is the process of reviewing all

materials for adherence to known specifica-

tions. In the flexo print process, those speci-

fications are entitled FIRST (Flexographic

Image Reproduction Specifications and

Tolerances). The desktop publisher is

responsible for making sure that the ele-

ments of the electronic file comply with

either FIRST specs or the printer’s custom

specifications. Checking the files in the

desktop department, also allows the pre-

press company to notify the customer of

required changes before costly film output

and proofs are made. The following ele-

ments should be reviewed during the pre-

flight process:

• software versions;

• low resolution placed images (FPOs);

• live images;

• imported EPS files;

• fonts;

• line weights;

• font sizes;

• tints and screen builds;

• vignettes and gradations; and

• layers.

The elements listed above and described

below also appear in Appendices A and B in

a checklist format that can be used as a

guide for preflighters.

Software Versions

It is important for the prepress house to

have the correct software and version to

view and output the file. New versions and

updates of software are released, and the

prepress house may not have upgraded to

the latest version. Software versions should

be verified early in the process, allowing suf-

ficient time to either have the customer

resend the job, saved in a compatible ver-

sion, or for the prepress house to purchase

and install the new version without delaying

the project. Even in an environment where

design and prepress, or prepress and print-

ing, are done under one roof, control of soft-

ware versions is important.

Low-resolution Placed Images

Low-resolution images placed in the lay-

out as a place holder for high-resolution

images are for position only, called FPO’s.

The purpose is to make the layout easier to

work with since FPO files are much smaller

and therefore easier to handle. All FPOs

should be sent to the prepress provider. It is

not uncommon for a designer to forget to

copy FPOs to the transfer disk going to the

prepress house. Without these items, the job

assembler will be unable to accurately dupli-

cate the size and placement required for any

high-resolution images to be used for film

output.

OPI and DCS are methods of working with

low-resolution placed images.

An OPI (Open Prepress Interface) work-

flow (Figure

), the design utilizes low-res-

olution placeholder images. The high-resolu-

tion image is stored on a file server and the

FPO is automatically replaced with the high-

resolution image when the file is output to

film. Typically, the prepress provider scans

the artwork, keeps the high-resolution image

on file, and gives the designer a low-resolu-

tion image to use in the design.

DCS, desktop color separation, files are

five-part EPS files. They utilize a low-resolu-

tion display image for placement and high-

resolution separation files for cyan, magen-

ta, yellow and black.

Live Images

Any “live” images placed should be clearly

indicated. They should also be checked for

proper resolution, color space – RGB or

CMYK – and size before starting the assem-

bly of the job. It is also important to check

that the image is flexo-ready. Often, supplied

images are prepared for offset printing and if

they are not converted for the flexographic

PREPRESS 81

Systems for automatic

image replacement,

such as OPI or DCS,

are sset up so desingers

cn work with smaller

low-resolution versions

of the images, while the

high-resolution images

are stored remotely.

During output, the

low-resolution files are

automatically replaced

withthe high resolution

versions.

Imagesetter

Final Film

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

17 49 85 96 100

IG-28

Original Art

Low-resolution

file for placement

Operations possible:

• Scale

• Rotate

• Crop

• Skew

Operations NOT Possible:

• Mask

• Color Correct

• Edit

High-resolution

CMYK file for imaging

Operations possible:

• Scale

• Rotate

• Crop

• Skew

• Mask

• Color Correct

• Edit

Linked to

high resolution file

Scanner

82 FLEXOGRAPHY: PRINCIPLES & PRACTICES

process, they will not produce a quality

result. The preflight person should check for

minimum and maximum dot values, as well

as the use of GCR. If it is found that the “live”

image does not meet flexography specs, the

image must be sent to the color department

to be adjusted. It is also best to notify and

alert the customer of any additional time and

cost that will be incurred.

Imported EPS Files

Imported EPS files should be checked for

missing fonts as well as flexo readiness. Most

software programs only alert the operator to

missing fonts in an imported or placed EPS

file when it attempts to print it. The desktop

publisher should open up each placed image

in its native program and check that the

appropriate fonts are available, that the reso-

lution is sufficient for the line screen at

which it must be output and that the file is

prepared for flexographic reproduction.

Fonts

The file must be checked to make sure all

fonts are available. Fonts come in two dif-

ferent types: TrueType and PostScript Type 1

or 3. TrueType fonts utilize an outline font

file for both screen viewing and printing.

Type 1 and Type 3 are PostScript fonts creat-

ed by foundaries such as Adobe or

Bitstream. They consist of a separate screen

and printer font. A screen font is needed to

correctly view the font on a monitor, while

the printer font is required to be resident on

the computer to print the document proper-

ly.

TrueType fonts used in the electronic doc-

ument should be replaced with the appropri-

ate Type 1 or 3 font. TrueType fonts have

been found to be unstable and problematic

when used in a PostScript environment.

Line Weights/Font Sizes

The nature of the flexographic printing

plate prevents very thin type or rules from

being reproduced during the platemaking

process. In addition, the relatively large

traps required in flexo printing dictate that

rules need to be of a certain weight to allow

them to be trapped. FIRST and/or the print-

er specifies minimum type sizes and rule

weights that the desktop person should ver-

ify on the incoming electronic file. Any type

or rules falling below the specification

should be brought to the attention of the

customer with the recommendation that

they be increased.

Tints and Screen Builds

The electronic file should be checked to

make sure that any screen tints adhere to the

minimum or maximum dot values required

by either FIRST or the printer. Any screen

builds assigned by the design firm should

also be checked for use of GCR. If a color is

created with a “contaminating color” it

should be changed. The customer usually is

not notified or required to authorize this

change because the resulting color is virtual-

ly identical to what the designer originally

specified. It is also recommended that there

be no color created with more than three

process colors, and two-color tints are high-

ly recommended when possible.

Vignettes and Gradations

The desktop production artist should indi-

cate how vignettes are created in the elec-

tronic file and check that they are done prop-

erly. Vignettes have to adhere to the same

minimum and maximum dot requirements

specified by the printer. Since it is common

to specify vignettes as going from 0% to 100%,

it is not uncommon that there will have to be

adjustments made to the electronic file. The

desktop production artist will usually have to

work with the design firm and the printer to

make the proper adjustments.

In most prepress shops, it is actually cus-

tomary to replace the vignettes at the strip-

ping stations. This is done because the strip-

PREPRESS 83

A typical desktop

department is

comprised of basic

equipment: a file server,

workstation with moni-

tor, and printer.

ping workstations have more control over

the vignette and more often than not are able

to eliminate any banding that may exist in the

electronic file. This is why it is important that

the desktop production artist is able to deter-

mine exactly how the vignette was prepared

so that he/she can accurately communicate

those instructions to a stripper.

EQUIPMENT AND SOFTWARE

In addition to the equipment found in the

scanning department – scanners, long-term

and short-term storage devices and work

stations – the desktop department in today’s

prepress facility also consists of telecommu-

nication devices, server(s) and some type of

printer for proofing purposes (Figure

)

Prepress providers that handle a large vol-

ume of electronic files often have an elec-

tronic bulletin board or mailboxes where

customers can dial up to post their files by

modem. Such services enable clients to

deliver files in a relatively quick amount of

time and at a minimal cost.

Individual workstations, either Macintosh

or PC systems, consisting of a hard drive,

keyboard, mouse and monitor are net-

worked to a server and printer. Software

programs for package design, page layout,

drawing, image manipulation and word pro-

cessing are common additions, while more

specialized products, specifically created for

the flexo packaging market, may reside on

the workstations as well.

The workstations are normally connected

to a host of removable drives. The media for

these drives – Zip, Jaz, CD, DAT, floppy –

allows files to be copied to them from the

hard drive, and then removed for transport

from one workstation to another. Each type

of removable drive has its own unique

advantages in speed, durability or the num-

ber of megabytes it can hold. Those used by

the prepress company is usually dictated by

the drive used by its customers. For this rea-

son, most prepress companies need to have

several different removable drives available.

Printers used in the desktop department

are usually color. They do not have to be

accurate for color, but the most popular

devices can provide an excellent representa-

tion of color and be calibrated to reflect dif-

ferent print processes and substrates. Large

format ink jet, color laser, and thermal wax

transfer are among the different types of

printers, all varying in size, color accuracy,

cost and method of reproduction as well as

resolutions. The right printer should fit with

the type of work to be reproduced.

File Server

Workstation

Removable Drives

Zip, Syquest,

Optical, Tape

Printers

Dye Sublimation,

Thermal, Laser, Ink Jet

84 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Job Assembly/Layout

Typical equipment

found in a job-assembly

work area.

ome prepress shops utilize the

desktop department as the film-

assembly department. Due to

the nature of the work pro-

duced and the sophistication of

today’s desktop systems, it is

possible to produce jobs that were once pos-

sible only on expensive “high-end” systems.

Job (or film) assembly or layout (also

known as stripping) is the process of assem-

bling various elements into a file that can be

used to generate plate-ready films or pho-

topolymer plates. The “electronic stripper”

or job assembly person generally requires

the most technical set of skills in all depart-

ments within a prepress environment.

The primary responsibility of the film

assembler is to combine all elements in such

a way that it is consistent with the cus-

tomer’s expectations. He or she must do this

within the capabilities of the print segment

in which the job will be printed.

Familiarity with the flexographic process

allows the stripper to take advantage of

flexo’s unique strengths while minimizing its

limitations. One method to ensure that the

job is optimized for flexo is throgh the appli-

cation of FIRST. By following these specifi-

cations, and making a commitment to quali-

ty, the job assembler can produce a consis-

tent product. This commitment to consistent

print quality applies to the way graphic ele-

ments are trapped, separations are handled,

logos appear; in short, every element on the

package. If the graphics on the outside of the

package always looks the same, the con-

sumer can feel comfortable that the product

inside the package will always be the same.

HARDWARE AND SOFTWARE

Just a few years ago, stripping was exclu-

sively completed on a light table with

rubylith and goldenrod and composed on a

vacuum frame to produce plate-ready nega-

tives. Today most, if not all, stripping for

flexo packages is done on a computer work-

station Figure

. These workstations can

File Server

Workstation

Monitor

Removable Drives

Zip, Syquest,

Optical, Tape

Printers

Dye Sublimation,

Thermal, Laser, Ink Jet

be either open architecture or proprietary

systems.

Open architecture refers to software solu-

tions that can be purchased and loaded onto

the computer of your choice, with the limita-

tion that the software must be written for the

particular operting system of the computer

(such as Mac or PC). The biggest issue with

open architecture software is choosing the

right software to produce packages for flexo.

This is an area of rapid change, with new pro-

grams being offered and existing programs

being continually upgraded.

Proprietary systems require the purchase

of specific hardware as well as software. For

years, these systems were the only way to

produce quality graphics for print reproduc-

tion. High-end systems dominated the pre-

press markets with their super-fast proces-

sors and enormous hard drives. The emer-

gence of the desktop systems has eroded that

dominance and, in several cases, has caused

a shift in how these systems are marketed.

These systems have shifted to more of an

open architecture format with an unbundled

software component, allowing prepress com-

panies to purchase less expensive hardware.

Whichever system, the workstations gen-

erally utilize some type of hard drive for

temporary storage as well as a removable

media drive for archival and retrieval of

completed packages. The equipment used

and process of archiving and retrieving vary,

but tapes, CDs and optical media are the

most popular formats used and offer excel-

lent stability and relatively long shelf life. In

addition, the job assembly department has

some type of digital proofing device to

check the accuracy of the stripped file

before the output of films.

TECHNICAL RESPONSIBILITIES

Whether working on an open or propri-

etary system, the job assembler must be able

to perform the following functions.

Using Layers

Layers are an important tool to streamline

the production process. The main responsi-

bility of proper layer use rests with the

designer (see Design chapter). The layering

should be reviewed to make sure the proper

layers are turned on and that the design fol-

lows good practice in the use of layers.

Placing High-resolution Images

When digital photos or images are required

on a package, the job assembler must take

care to duplicate the placement of the image

per customer instructions, as shown on the

FPO. Responsibilities include rotating, crop-

ping or scaling – either enlarging or reducing

– the image. The job assembler may also be

required to “warp” or anamorphically scale

an image to fit. For instance, the height can

be enlarged at 120%, while the width is

enlarged at 105%.

Silhouetting of Images

Silhouetting involves the creation of a

mask to eliminate unwanted parts of an

image. Using an image-editing program, it

creates a clipping path. The stripper will, for

instance, mask out the scenery or back-

ground behind a person, so that only the

image of the person is used on the package.

Thin objects are especially difficult to cap-

ture. Instead of appearing as intended, the

thin objects resemble strands of color, and

when they are trapped, they all but disap-

pear. Hair, flora and certain foods not sil-

houetted properly may contain spots of

unwanted background image, or have an

unnatural outline or shape about them.

Assignment of Screen/Tint Values

And Color Information

Print requirements or the number of print

stations available for a project dictate the

assignment of screen/tints values and color

information. The color assignments are

either 100% (solid) of a color, a screen mix or

PREPRESS 85

86 FLEXOGRAPHY: PRINCIPLES & PRACTICES

combination of multiple colors, or a “knock-

out” from actual printing colors. The knock-

out (KO) copy appears punched out of a

color to allow the substrate beneath it to

show through.

Trapping (Spreads and Chokes)

Trapping is accomplished through the use

of chokes and spreads. This technique is

used when two colors are adjacent to each

other and prevents a gap of non-color

between the two colors. The need for trap-

ping arises from the inevitable misregistra-

tion on press. In general, light colors are

spread into dark colors. Because trapping

requires the operator to make colors that are

meant to touch, or actually overlap each

other, an objectionable edge can result. For

instance, a green box trapping to a red one

will result in a thin, dark line equal to the size

of the trap where the two boxes meet. This

is because the yellow and cyan of the green

combine with the magenta of the red to

make a three-color (black) rule. Sometimes

this is unavoidable, depending on the colors

requested by the customer. In some cases,

the printer may be willing to accept less of a

trap in that particular area, or the customer

may allow a rule to be placed around the

boxes to “hide” the trap. Sometimes a

designer will use a rule that will not support

the trap that the printer requires.

For example, a printer requires a 0.004"

trap allowance and a designer has a 0.003"

rule butting to a colored panel. To satisfy the

trap requirement, the assembler needs to

spread the colored panel into the rule by

0.004". Of course, once this is done, the col-

ored panel will actually print inside the rule

(Figure

). The best solution in this case is

to have a rule that measures 0.008" and trap

to the center of the rule. This allows for mis-

registration in both directions.

Bar Code Creation/Placement

The job assembler may also be responsi-

ble for the creation and placement of UPC

bar codes. He/she will need to know the type

of bar code (EAN or UPC-A), the size (100%,

125%, etc), the bar-width adjustment (usual-

ly dictated by the printer) and the actual bar

code digits.

Application of Distortions

Photopolymer plates stretch or distort in

the repeat- or machine-direction (Figure

This occurs when they are mounted on the

plate cylinder. As such, film used for pho-

topolymer platemaking must be scaled in

the repeat direction to compensate for this

stretch of the photopolymer plates. The dis-

tortion is a reduction of the original file size.

If, for example, a photopolymer plate

stretches by 1%, the original file size needs to

The drawing in circle A

demonstrates the case

where the rule is thinner

than the trap allowance.

Consequently, the dark

red shows through on

the inside of the rule.

The image in circle B

shows proper trapping

with the rule wide

enough for the trap

allowance.

Photopolymer plates

stretch in the machine

in the repeat direction,

producing a distorted

image. This distortion

must be compensated

for in prepress.

A: Rule thinner than trap allowance

B: Rule adequate for trap allowance

Normal Image

Distorted Image

be set to 99%, so that it stretches back to the

original 100% size.

The distortion can be computed mathe-

matically from the repeat length and plate

thickness, using the formula:

% reduction  K  100

Where:

K = a constant supplied by the plate mate-

rial manufacturer

R = the printing circumference (repeat

length) of a cylinder (in inches)

Table 11 lists K factors for some common

plate thicknesses. The values are given in

inches and centimeters because the K factor

changes with units of measurement. As an

example:

What is the distortion needed in the film

negatives for a 0.067" plate with 0.004"

backing and a repeat length of 8"?

From Table 11, the K factor for this exam-

ple is 0.396. Putting this value and the repeat

length into the formula gives a percent-

reduction of (0.396  8)  100 or 4.95%. This

means the film used to make the plate must

be output at 95.05% of original size to print at

full size.

This formula is not used on a daily basis

because distortion factors have been deter-

mined for most common repeat, pitch and

plate sizes.

In the case of rubber plates, two distor-

tions are required. Rubber plates shrink in

both directions during their manufacture. In

addition to this shrinkage, there is also the

same wrap distortion as occurs for pho-

topolymer plates when they are mounted on

the plate cylinder. In principle, distortion

factors could be calculated for rubber plates

also. In practice, the distortions are usually

determined empirically.

Dot-gain Compensation

Flexo-printed jobs require that they be

compensated for flexo-specific dot gains on

press. Dot-gain compensation is done in

order to match the press and the contract

proof. There are generally two ways to

accomplish this, depending on the particular

workflow a prepress company is using.

The traditional method is to apply a “cut-

PREPRESS 87

Table 11

INCHES CENTIMETERS

PLATE K FACTOR PLATE K FACTOR

THICKNESS 0.004 BACKING 0.007 BACKING THICKNESS 0.004 BACKING 0.007 BACKING

0.030 0.163 0.145 0.076 0.415 0.367

0.045 0.258 0.239 0.114 0.654 0.606

0.067 0.396 0.377 0.170 1.005 0.958

0.080 0.478 0.459 0.203 1.213 1.165

0.090 0.540 0.522 0.229 1.372 1.325

0.100 0.603 0.584 0.254 1.532 1.484

0.107 0.647 0.628 0.272 1.644 1.596

0.112 0.679 0.660 0.284 1.724 1.676

0.125 0.760 0.741 0.318 1.931 1.883

0.155 0.949 0.930 0.394 2.410 2.362

0.187 1.150 1.131 0.475 2.921 2.873

0.250 1.546 1.527 0.635 3.926 3.878

K FACTORS