Crawling

A printing defect characterized by a printed ink film beading up on a substrate which it does

not wet completely. Crawling is also a symptom of poor trapping, the printing of one ink on
top of a previously-printed ink, in which the first ink film repels the second, causing the ink
to form drops. (See also Crystallization.)

Crystallization
A printing defect symptomatic of poor trapping, the ability to print one ink on top of another
previously-printed dry ink, in which the dry ink film—which has been allowed to dry too hard
—repels the wet ink, causing crawling,mottle, or rub-off. Crystallization occurs with inks
that dry by oxidation and when the second ink is printed at a much later time than the first
ink. Crystallization can commonly be avoided by reducing the amount of drier in the ink.
(See Trapping.) As used in printing terminology, crystallization has nothing to do with the
chemical termcrystallization, which describes an entirely different phenomenon.

Cissing

A printing defect of screen printing in which a printed ink film or varnish recedes from
portions of the substratedue to incomplete wetting of the surface.

Trapping

In prepress, the compensation for misregister of successive colors or images. Trapping and

trapping techniques ensure that there are no unsightly gaps or overlaps of successively-
printed colors or images. Trapping in this sense is referred to by many different terms,
ranging from spreads and chokes, fatties and skinnies, lap register, and making grips.

In conventional (i.e., non-digital) lithography, trapping is accomplished using photographic

techniques calledspreads and chokes, also known as fatties and skinnies, respectively. A
spread is a photographic overexposure of an image to make it larger than it is on the
original. Thus, when it is overprinted onto another image, it will "spread" into the other
image by a predetermined amount, eliminating gaps between the two. Spreads are more
commonly used on foreground images or objects; backgrounds are often modified by
chokes, which is a photographic enlargement of the background color or object, which
"chokes" the subsequently overprinted color or image with an overlap of a set amount.
(See Spreads and Chokes.)

In an ideal world, trapping would not be needed. Trapping is only needed because of
misregister during the printing process. The various parts of the process that lead to
misregister almost always involve the fact that paper is too flexible and dimensionally
unstable. Misregister occurs because of paper twisting or bouncing as it moves through the
press, from gripper to gripper, because it can be stretched by the feeding mechanisms, and
it can change size due to its ability to absorb moisture from the environment.
(In flexogaphy, the stretching of rubber plates as they are mounted on the press cylinder is
also cause for misregister. Also in flexographic printing, non-paper substrates, very fast
press speeds, and many differnt spot colors result in the need for special trapping
considerations. Traps for flexography need to be in the range of 0.006:0.01 inch.)
Misregister also results from improper film assembly during prepress, or from a dimensional
instablity on the part of the film used to make negatives. Errors in platemaking also cause
register problems on press.

The amount of trapping required in a particular print job depends not only on the desired
degree of fit between successive color or images, but also on the type of ptess
and substrate used. Sheetfed offset lithographic presses often require 0.003 inch of
trapping, while web offset presses (especially those using highly absorptive newsprint as a
substrate) requiure more. The following chart provides general trap amounts:

[Chart C004]

The above chart illustrates basioc "default" trap amounts by printing process, but the trap
may vary depending on prevailing press and substrate conditions. The amount of trapping
can also be calculated from the line count of the halftone screen being used, and can
essentially equal half the diameter of a halftone dot. A way of determining this is to find out
the screen count of the halftone (for example, 133 lines per inch), divide into 1 inch to
obtain the diameter (in inches) of the halftone dot (or 1 ÷ 133 = 0.0075). Divide the
diameter of the halftone dot by 2 to get the suggested trap amount (or 0.0075 ÷ 2 =
0.004). In contrast, an 85-line halftone screen (commonly used on newsprint) would require
more trapping, or 1 ÷ 85 = 0.012 ÷ 2 = 0.006 inch.

Also of concern in trapping is the question of which color should trap and which should be
trapped. In general, lighter colors should be spread into darker colors, but it is not very
often that the dark/light dichotomy is obvious. Often, a color wheel is utilized. A color wheel
is essentially a circular graph or plot of all the reproducible colors. Around the circumference
of the wheel are all the "pure" hues—red, magenta, blue, cyan, green, and yellow, in order
proceeding clockwise. Each hue gets progressively grayer (or increases in value) towards
the center of the wheel, where black is plotted. Any to-be-reproduced color can be plotted
on this wheel by first determining its primary hue and then moving towards the center until
the precise shade is found. This can then be compared to a second color, and the relative
brightnesses of them can be compared by means of lighter/darker arrows indicated on the
wheel. Although this is a rather simplistic means of evaluating color differences for purposes
of trapping, it is useful in a variety of situations.

One means of trapping (rarely used in conventional film-based prepress) involves the
creation of a screen tint of just the trap line, which avoids the unsightly creation of third
colors when two other colors overlap. This is very difficult and t8ime-consuming to
accomplish photograpohically (essentially, a negative needs to be made of just the trap line,
and a screen tint applied to the negative. Since the trap line is only several thousandtyhs of
an inch thick, this can be hard to manage. However, in digital prepress systems and
programs, it is very simple to accomplish, and the high-quality results include a less
obtrusive trap line. Other solutions inlcude the elinmination of trapping altogether; when
colors overprint, they create a third, secondary color that is the mixture of the two
primaries being combined. Thus, rather than printing a background color with a knockout
and printing the foreground color in the box (which creates the perfect breeding ground for
misregister), the foreground object is overprinted directly on the background. The inks are
chosen such that when the foreground object overprints, its color will mix with the
background color to produce the desired color. Another solution is to bound each separate
color with thin black lines, thus letting black do the trapping. This works only so long as
each separate color can be outlined with black.

Although at one time trapping was effected by the printer during prepress, the advent of
digital prepress systems has resulted in less of an opportunity for printers to be able to
create the trap. Printers once were responsible for shooting negatives, generating color
separations, etc., and thus could trap to their heart's content. In contrast, more often than
not negatives and color separations are generated via imagesetter output from digital
systems, either from a publisher or from a prepress houser or service bureau, with the
printer involved long after the time for trapping has passed. Thus, it is up to the designer to
handle any trapping (or properly communicate to a service bureau the trapping
requirements. However, page make-up and image processing programs make trapping
reasonably simple to effect, and each program has its own means of performing accurate
trapping. There are also specialized programs just for trapping that can be purchased and
utilized.

The term trapping, in printing, has a different meaning from the above definition, and refers
to the action of printing an ink film on top of another ink film, as in process color printing.
Proper trapping results in well-printed materials, while poor trapping results in successive
inks that do not adhere properly and bead or rub off readily.Wet trapping refers to trapping
performed in wet multi-color printing, where one ink is laid down on top of a previously
printed, still-wet ink. If the second ink has greater tack than the first ink, poor trapping will
occur. Dry trapping is a multi-color printing process in which one ink is laid down on top of a
dry ink. (See alsoCrystallization, Crawling, Cissing, and Ink: Printing Problems and Defects.)

Dry trapping is a multi-color printing process in which one ink is laid down on top of a dry ink.
(See also Crystallization, Crawling, Cissing, and Ink: Printing Problems and Defects.)

Wet Trapping

In wet process color printing, the ability to successfully lay down a wet ink film on top of a
previously printed, still-wet ink film. See Trapping (second definition).
MONDAY, JULY 13, 2009

Ink Sequence - 4/C process & beyond

In 4/C process (CMYK) or any multi-

color printing for that matter, different inks are laid down in sequence in order to build the final
image. The sequence that the inks are laid down can significantly alter the final printed result. Ink
sequence can also impact whether the job runs successfully or fails on press.

Printing always involves a level of compromise and the choice of ink sequence is no exception.
However, while there are few "rules" and surprisingly, virtually no documented information on this
topic, there are several notable factors to consider when determining the appropriate ink sequence
to use for any specific application, namely:
1 - Conformance to an industry standard. For example, ISO 12647-2:2004 for process control in
offset lithography standardizes the chromatic ink sequence to CMY – however, black is acceptable
as either first or last down.
2 - Ink tack - the stickiness of the ink that allows an over-printing ink to stick to an already printed
layer of ink.
3 - Paper absorption - both smoothness and tightness of the surface affects ink tack.
4 - Time - wet ink sticks/traps to dry ink better than wet ink traps to wet ink. E.g. unitized press
(each unit lays down one ink) or common impression cylinder press (one unit lays down multiple
inks) or single color presses where the next layer of ink is applied much later than the previous.
5 - Ink opacity - opaque inks hide underlying inks.
6 - Ink transparency - transparent inks combine with underlying inks.
7 - Ink coverage - the higher the coverage of an ink the less following inks are able to trap efficiently
with it.
8 - The RGB to CMYK separation technique that was used.
9 - The printing method being used - i.e. sheetfed offset, flexography, gravure, etc.

In multi-color presswork the first ink down "traps" the one that follows. Tack - the stickiness of ink -
is a major factor enabling inks to adhere to, or trap, one another.
In the proper sequence, the first ink down must have the highest tack. Subsequent colors have
lower effective tacks, with a descending 2-to 4-point spread between them.

In this first example, a layer of Magenta was printed on a single color press. Then an overlapping
layer of Cyan was printed in a second pass after the Magenta ink had dried (i.e. wet-on-dry or "dry
trapped"). The ink film thickness of both colors was the same.

The result is very good ink film

trapping with a blue where the two colors overlap not having a bias towards Cyan or Magenta.

The second example was produced on a multi-color press. Again Magenta is printed first down onto
dry paper (i.e. wet-on-dry). Then a layer of Cyan was printed onto the still wet Magenta ink (wet-on-

wet or "wet trapped"). While the

Magenta ink film was trapped well by the dry paper, the ink trapping for Cyan was not as good due
to the fact that the Magenta ink was wet, and so the resulting blue where they overlap has a
decided reddish cast.

In the third example, the wet-on-wet printing method was used again, but with the ink order

reversed: The result is that the blue,

where Cyan and Magenta overlap, now has a decided Cyan cast. Note that this particular ink
sequence is the standard for CM inks in CMYK process color printing.

Ramifications
Modern offset presses print all four (or more) inks in rapid succession, 'wet-on-wet'. The first inks
down usually adhere to the paper better than later inks. In some cases the later inks can actually
remove some of the earlier inks, depending on the relative tack of each ink in the sequence. Either
way, the amount of one or more inks remaining on paper is usually less than would be achieved on
a "dry trapping" press, or with a drying system between each unit as is accomplished by some form
of inter-unit drying system, such as IR (Infra-Red) heat for conventional inks, or UV (Ultra-Violet)
light for UV-curable inks.
Wet-trapping can also introduce an unstable performance in darker tones and is often cited as one
of the main problems in matching multiple presses to a standard characterization data set, even
when each press uses the same paper and ink. 

So:

1 - Dry-trap printing processes can achieve a greater color gamut than wet-trap printing.
2 -The sequence of the primary CMY inks helps determine the color integrity of the secondary
colors (RGB)
3 -Changing the sequence of CMY inks can be used to enhance/favor specific secondary colors.
4 - Poor ink trapping in 4/C printing will be revealed in a loss of gamut, color bias, lack of
vibrancy/chroma, and a mottling/splotchy appearance in the secondary colors (RGB)
5 - With graded tack inks, the tack must be adjusted to reflect the ink's new position. E.g. If first
down Cyan tack is 14 followed with an overprint of Magenta with a tack of 11 then, to maintain
good trap, if Magenta becomes first down its tack must be adjusted to 14 and Cyan's tack adjusted
11.
6 - The further apart two ink units are on press, the better their effective trap should be. I.e. In a
KCMY ink sequence, C and Y (forming Green) will trap better than C and M (forming Blue) or M and
Y (forming Red).

Addendum
Below are the typical trap values (Status T, Preucil formula) for different types of presswork as well
as the CIEL*a*b* values according to ISO 12647-2:

As noted, the ISO standard specifies that the chromatic inks are laid down in CMY sequence with K
being either the first or last ink down. Traditionally, for most offset applications, the ink sequence
has been KCMY. The preference for
this ink sequence is likely the legacy of the image separation methods used in the past. Prior to
today's desktop image editing applications, the conversion of RGB scans into CMYK images, was
performed by software in the scanner itself. The conversion method utilized UCR (Under Color
Removal) techniques to optimize the image for the press as in this example:

Note that in a UCR separation,

there is very little Black ink coverage compared to the C, M or Y plates. Note also that there is a
large amount of Y coverage in the image, in part, because the Y component in the separation is
being used instead of Black ink to grey, or darken, the image. This means that laying Black ink down
first provides a greater area of dry, non-inked, paper for the Cyan ink to trap to. In a KCMY ink
sequence, running the transparent, high area coverage, Yellow ink last down also has the benefit of
acting somewhat like a gloss varnish to add depth to the reproduction.

In contrast, for newspaper production, Yellow is usually the first ink down with Black ink the last

down: The reason that this ink

sequence is preferred over a KCMY sequence is both related to the use of UCR separation
techniques as well as the inks being used. For newspaper work:

laying Yellow down first helps to

seal the paper thus providing a better surface for the Cyan ink to trap to. Black ink last down
benefits from the three previous inks sealing the paper which helps the Black ink deliver maximum
blackness and contrast. Maximizing the coverage of the chromatic C, M, and Y inks and minimizing
the use of Black ink in images also helped images to maintain as much of their vibrancy as possible
given the poor quality of paper being used. Finally, the Black ink used for newspaper work tends to
be of very poor quality compared to the C, M, and Y inks. If it was first down, it could travel down to
the next printing units and contaminate them.

"A wrench in the monkey works" - GCR separations

In today's image processing workflow, the default separation method uses GCR (Grey Component
Replacement) rather than UCR techniques. In addition, many newspapers and publication printers
are reseparating incoming image files in order to apply GCR techniques in order to reduce ink usage
and increase color stability on press (more information is available HERE). A GCR separation, like

this example: maximizes the use of

Black ink in order to reduce the amount of the more expensive chromatic C, M, and Y inks while
delivering virtually the same final color appearance in print. For newspaper work, the use of GCR
separated images in a YCMK ink sequence may lower the effectiveness of the Yellow ink to seal the
paper and hence reduce the potential color gamut.

Ramification

When evaluating the optimal ink sequence for a specific application, particularly the position of the
Black printer, it is important to consider the type of separation techniques that were used to
prepare images for press and how those separation methods impact both ink trapping and
printability.

Addendum - Ink Sequence for a two-color press

The recommended ink lay down sequence for a two-color press is: first pass CM, second pass: KY:

This sequence makes CM inks wet

trap which helps align the color result with that of a four-color press. Also, because the primary
chromatic colors are laid down together, it facilitates color assessment. Black, being achromatic
does not affect color. Yellow, being the greying component of C + Y does not effect color as much as
C and M.

Adding a 5th, 6th, or more inks to the sequence also adds a degree of complication.
Note that most presses with more than four colors are run with the first units empty and available,
with the KCMY inks in the last four units. However there is no standard practice, so which units are
available will vary from shop to shop.

The general guidelines are:

1 - If possible, keep the process colors together in their standard order (KCMY). This is to avoid color
shifts that might occur if their inter-press unit distance changes. This also helps avoid wash-ups.
2 - If possible do not move the position of the KCMY inks. This is to avoid having to do a complete
wash-up of press units.
3 - If the extra color(s) will not be over-printed by process colors it would preferably be last down.
4 - If the extra color(s) are opaque and will be over-printed by process colors it would preferably be
first down.
5 - If the extra color(s) are transparent and will over-print, or be over-printed, by process colors, its
position should be furthest away from the color it will trap with. This is to help make it a dry-trap
situation increasing print-ability and reducing mottle.
6 - Metallic inks, whenever possible, should be last down unless they are specifically formulated to
be first down in order to be over-printed (e.g. MetalFX printing). Note that metallic inks are usually
varnish coated and that many printers keep the last press unit available for the varnish. This may
then require that the metallic ink be first down even though it won't be over-printed.

Some examples:

Spot color will be over-printed by process colors:

Opaque spot color will over-print process colors:

Metallic silver:
Hi-Fi color printing where transparent extra process color inks will be trapped with standard process
inks: