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:

The Color Atlas - helping designers to specify color

Way back when, because all their "camera-ready" art was done in black and white, designers would specify screen tint color builds by using published reference Color Atlas guides like the generic Kuepper's book which showed examples of hundreds of CMYK screen tint build combinations:Or this poster of screen tint combinations:Because they were based on, loosely defined (at the time) universal standards, the use of a color atlas, in my case, even allowed me to communicate color for presswork in countries, like China, where I did not otherwise understand the language:The prepress shop/printer would use pieces of screened film, according to my specification of X% Cyan, X% Magenta, X%Yellow, etc., stripped into flats to create the image on plate that would result in the requested color on press. Today, that specification happens in an illustration or page layout program – however the principle is the same.
Of course, a generic Color Atlas could not reflect the color performance of an individual printshop. As a result many printers would create, and provide their customers with their own unique CMYK Color Atlas like this one from Agency Press with the same screen tint builds printed on four different types of paper:As long as the creative specified their color builds according to the printer's guide in the Atlas they would have a reasonable expectation as to the color that would happen on the press. Some might argue that this is ancient technology, however, for printers who want to clearly set print buyer expectations - a Color Atlas still represents the reality of actual "ink-on-paper" performance – especially if the printer's presswork is outside of industry norms. For example, Hennegan Press:who wanted to show their color capabilities with 10 micron FM. Or Fort Dearborn in Chicago helping their customers specify 7-color process printing with their HiColour system (also FM screening):or Anderson Litho communicating their ability to mimic metallic paint for car brochures (also FM screening):or Intelligencer Printing demonstrating their metallic print capabilities in this superb brochure (also FM screening):Even with today's sophisticated color management systems, the savvy printer would do well to consider producing their own Color Atlas to smooth the color communications channel with their customers - especially if the print shop is doing work beyond the mundane standards for color printing.

Update December 15, 2009: Heidelberg has just published their own color atlas - click on this POST for details:

Naming Image Files

When I managed a prepress scanning department that handled thousands of images, I used a simple naming convention that anyone (even creatives) can use which practically made the images organize themselves - and avoided incorrectly formatted images from being placed into page layouts or used for the wrong application.

The naming convention followed this coding format:where the first letter of the image mode and the first letter of how the image was purposed are used as the first two letters of the image name. By naming image files this way, whenever a folder containing images is viewed by name, the images will be automatically grouped according to their mode which makes choosing the correct image format very simple.
It also used a descriptive name that was logical enough to be searched on. I used a date code in the name so that as the image went through the editing/revision cycle I would save the updated images with the new date in the name. This meant that I had copies of the image that tracked the revisions done to it with the most recent version of the image being the one with the most recent date in its name. This avoided using the ambiguous term "Final," "Final v2," "Latest Final" etc. in the image name.

Image mode codes:
R = RGB
C = CMYK
G = Greyscale
B = Bilevel/Bitmap
M = Monotone
D = Duotone
T = Tritone
I = Indexed

Destination/purposing codes:
P = Publication/SWOP
S = Sheetfed/GRACoL
N = Newspaper/SNAP
F = Flexo/FIRST
W = Web
D = Display Inkjet
B = Backlit Inkjet

Some examples showing how the codes are used in practice to easily identify/describe the image:

GN_Sydney_Harbor_Australia_201008.tif = Greyscale image purposed for Newspaper reproduction.

RW_Sydney_Harbor_Australia_201008.jpg = RGB image purposed for use on a Web site.

CP_Sydney_Harbor_Australia_201008.tif = CMYK image separated for use in a magazine.

B_Sydney_Harbor_Australia_201008.jpg = Bilevel/Bitmap image.

MF_Sydney_Harbor_Australia_201008.eps = Monotone/single color image prepared for Flexographic printing.

DS_Sydney_Harbor_Australia_201008.eps = Duotone/two color image prepared for Sheetfed printing.

IW_Sydney_Harbor_Australia_201008.gif = Index mode image prepared for posting on a Web site.

In some cases I would add the word "TOSS" to the image name:CP_Sydney_Harbor_Australia_201008_TOSS.jpg = CMYK image prepared for Publication. "TOSS" signified that the image could be trashed/erased if hard drive space was needed.

The Wayback View – Independence Day, July 4th, 1940

While war rages in Europe and Asia, U.S. movie theaters presented this film as part of their Independence Day celebrations.
One year and 5 months later the U.S. will enter WW II – 2 years and 3 months after the actual start of the war.

Please press the play arrow to view the video. Note that it may stop for a moment while the video buffers in the background.

Reducing, or Eliminating, Objectionable Rosettes

When screens of cyan, magenta, and black are overlaid at their respective angles (105º, 75º, 45º) they form a moiré pattern called a "rosette." If the printer is required to use a fairly coarse AM/XM halftone screen (e.g. 85-150 lpi (newspaper & magazine work) ), then, depending on the image color content, the rosette pattern can become visible enough to be objectionable.
One way to reduce the visibility of the rosette structure is to move to a finer AM/XM screen which makes the rosette smaller and hence less visible. However, if that is not possible, then changing the separation method might be a viable option.

The majority of RGB to CMYK image conversions use "GCR" as the method (it is the default separation technique in Adobe PhotoShop). This ensures that wherever C, M, and Y inks are used black will be introduced. The result is a very visible rosette structure as seen in the left image below.GCR----------Click image to enlarge----------UCR
To reduce, and even eliminate most rosettes, a better strategy is to use the UCR separation method on problematic images. UCR separations (see image on right above) unlike GCR separations, primarily introduce black only in neutral and near neutral color areas. Since very little, if any, black is introduced in C and M screen tint areas – no rosettes are actually formed in those areas and hence no rosettes are visible. The result is smoother, less grainy appearing color.
While the UCR separation technique can reduce or even eliminate rosettes, there is a downside in that there will be a slight increase in ink usage as well as a slight reduction in color stability through the pressrun. That is why it should be used only for images with problematic colors - primarily dark blues and purples as well as dark skin colors/areas.

Confirming the Applied Calibration Plate Curve - without measuring the plate

Most printers with CtP output use tone calibration curves applied to their plates in order to achieve the print characteristic they've targeted for their shops. Many shops will have multiple curve sets to accommodate different paper characteristics, screening, or solid ink density targets. As a result, making sure that the correct curve has been applied is an important part of the plate imaging quality control process.
This technique creates an inspection target that will be imaged in the gripper area of the plate. It will allow prepress to quickly confirm visually that the correct curve has been applied without having to measure the plate.

1- In PhotoShop create a new grayscale image 3,300 x 600 pixels at 300 dpi. Instead of 600 pixels, you can use a height one half the height available in the gripper area.Then create a 100% black to 0% white gradient ("dither" should be deselected):
2- Apply the "Equalize" adjustment to the gradient: "Image"-->"Adjustments"-->"Equalize" This linearizes the gradient.
3- Posterize the gradient "Image"-->"Adjustments"-->"Posterize" Enter "21" for the levels value. This divides the gradient into 5% tone increments.
4- Make a copy of this gradient and send it through your workflow so that it is halftone screened with the calibration curve you wish to use:This screened bitmap with curve applied needs to be captured from your workflow as an image. If you cannot capture a bitmap from your workflow you may need to call on your workflow vendor's demo facility or customer support to do this for you.

5- Impose the screened gradient with the original posterized linear gradient.
6- When this graphic is imaged to plate the screened bitmap is untouched by the workflow since it is a bilevel bitmap containing no grey levels. However, the greyscale continuous tone part of the graphic will be affected by the calibration.
If the correct curve has been applied you'll see no difference in the top and bottom part of the graphic:If no curve, or the wrong curve, is applied you'll easily see the difference in the tones in the graphic:
This target assumes that the same curve is being applied to all plates in a CMYK job. If individualized curves are applied to separate CMYK channels then copy the posterized linear gradient into each channel of a blank CMYK image that is the same size as the gradient. Then send it through the workflow applying the plate curves, capture the screened bitmaps, recombine them into a single CMYK image file and finally combine it with the original CMYK graphic as was done with the greyscale graphic.

It can help if the screened bitmap image included the name of the applied calibration curve as a further way to ensure that the correct curve(s) were being applied.

If there is no room in the gripper to stack the gradients, you can integrate them by combining the screened bitmap into the greyscale gradient like this:The result is the same. The correctly applied curve would make the graphic appear like this:While the wrong calibration curve would make the graphic appear like this:

R.O.I.

Lack of productivity and high manufacturing costs are often attributed to a failure to install new technologies and the latest types of automation. Indeed, achieving a lean, efficient, and effective print-manufacturing process requires improvement of the process in every feasible way. However, technology, by itself, is rarely able to deliver a return on investment unless the printer also makes an equal investment in innovation. New tools used with old skills, old knowledge, and old attitudes will not extract value from new technologies.

When you think about "Return On Investment" you need to also think about "Return On Innovation."

Hi-Fi color - 8 strategies to implementation

There are basically 8 established ways to print contone images with added vibrancy – i.e. Hi-Fi color. Most require a great deal of testing and experimentation. Many will be problematic from a proofing point of view, however, the testing process can often provide samples that can be used give buyers a good enough idea as to what their specific finished product would look like. Note that the extra vibrancy achieved on press with these processes is dependent on the gamut of the images selected for this process. Images that are already well housed within the standard cmyk color gamut will likely not benefit from the Hi-Fi gamut and therefore show no visible difference compared to a standard four color process image.

In order from simplest to most complex:

1) Increase solid ink density.
Solid graphic is 175 lpi gamut at standard SIDs. Translucent graphic is 175 lpi gamut at higher SIDs.
Solid ink densities can often be increased by about 20-30 points from industry standards on presses which have aqueous coaters. Curves are applied to plate to normalize dot gain (restore tone reproduction). Extra saturation affects all color areas on the page. Testing determines the max density that can be achieved before presswork color becomes unstable, or ink slinging or tailing occurs. Uses existing CMYK images. This is the simplest approach to add punch since the only thing in production that needs to happen, once testing is complete, is to have a curve applied to the plates and new SID targets communicated to the pressroom for jobs targeted for the extra vibrancy. A popular strategy because it does not require anything to change other than a curve applied to the plates. It will increase overall color saturation but may not increase the gamut in areas where CMYK is weak (oranges and purples). A more complete explanation of the process begins HERE. A variation on this method would be to run CMYK at normal SIDs and then do a second hit of CMY also at normal SIDs.

2) Use FM screening.
Solid graphic is 175 lpi gamut using standard inks. Translucent graphic is 20 micron FM screening gamut.
Going to a finer screen, either 20 micron FM or greater than 385 lpi AM/XM screening will provide about 10-15% greater gamut volume compared with 175 lpi AM/XM screening. The extra gamut will be available in one and two color screen tint builds only.


3) Big Gamut CMYK
Solid graphic is 175 lpi gamut using standard inks. Translucent graphic is 175 lpi gamut using wider gamut CMYK inks - Toyo Kaleido inks in this example.
This method uses higher pigment load inks, or spectrally purer colorants (and therefore more expensive inks). Examples are Toyo Kaleido inks and (BASF) Flint Novaspace f 2010 inks. Extra saturation affects all color areas on the page. This method uses existing CMYK images, however existing separations may produce unexpected results. From a production point of view, washups and ink change overs will happen when switching from regular CMYK to Big Gamut CMYK jobs.

4) CMYK plus "bump" (touch plate) color.
Solid graphic is 175 lpi gamut at standard SIDs. Translucent graphic is extra gamut resulting from the addition of a special Orange and special Blue.
Adds gamut only where needed (e.g. oranges, blues, etc.). Manual process in Photoshop to create 5th plate. Requires testing and experimenting to establish workflow. Uses existing CMYK images with added spot color channel to add extra vibrancy within specific images only. Note that the extra ink(s) will need to be formulated to wet trap, be screened, and have a dot gain similar to its closest process color. E.g. Red ink would mimic Magenta in lithographic performance. This method is usually used in fine art reproduction, catalogue, and automotive work to bring specific colors into gamut.

5) Swing process colors.
Solid graphic is 175 lpi gamut at standard SIDs. Translucent graphic is extra gamut in the blue range resulting from the substitution of a violet for the standard process Cyan.
This uses a standard CMYK ink set where one of the process colors, usually magenta but sometimes the cyan, is swapped out for an alternate. For example, the standard process magenta might be swapped out for a PMS Red 032, Warm Red, Rhodamine Red, or even PMS 2395. This distorts the entire gamut but can be very effective depending on image content. For example, a photo of an orange against green leaves would really pop if a warm red is used instead of a conventional magenta. This method is best used where there are no skin tones present since skin tones would look quite odd. Note that all image content is affected, including text. Requires a lot of experimentation and documented samples.

6) Big "H" Pantone Hexachrome.
Solid graphic is 175 lpi gamut at standard SIDs. Translucent graphic is extra gamut resulting from the use of proprietary Pantone Cyan, Magenta, Yellow, Orange and Green.
Uses a proprietary 6 color inkset with fluorescing agents in their pigments. Extra vibrancy affects all color areas on the page (images and text). Inks tend to have poor printability. Expensive. Manual process to do separations in Photoshop. It often delivers images that have an "artificial" look. Colors can appear garish rather than natural. Requires testing and experimenting to establish workflow. Complex separated workflow (DCS 2 files).

7) Small "H" Hexachrome.
Solid graphic is 175 lpi gamut at standard SIDs. Translucent graphic is extra gamut resulting from the use of Orange and Green.
Uses standard CMYK inks plus Orange and Green inks to expand Gamut. Uses Pantone Heximage software from Pantone to do manual separations to 6 color process in Photoshop. Extra vibrancy applies to images only. This method is popular in the label and packaging markets. It can be a good compromise compared to process 5. Complex separated workflow (DCS 2 files).

8) CMYK plus "extended" process colors.
Solid graphic is 175 lpi gamut at standard SIDs. Translucent graphic is extra gamut resulting from the use of Red, Violet, and Green "extended" process colors.
ICC profile based workflow. RGB in and separated to CMYKRG or CMYKRV, or CMYKRGB out. Fully automated process. Requires profiling the press using the appropriate inkset. Creating an RGB to CMYKXX separation profile. The profile is used by the workflow to separate the images as part of the process plan, as delivered to the workflow as a preseparated file. Extra vibrancy applies to images only.