Choosing a CtP - some considerations

Focussing laser energy
The laser system that exposes the plate in a CtP system has an impact on the consistency of the halftone dots that make up the image on the plate and therefore the consistency and integrity of the final presswork. The more well focussed, and hence sharper, the beam of exposing energy the more consistent the resulting halftones will be.
The four main ways that laser energy is focussed on a CtP device in order from basic to most sophisticated:

1) Depth of field. This is the method most often seen used in internal drum CtP machines and film imagesetters. It is similar in concept to how depth of field works in a camera. Although a lens can precisely focus at only one distance, the decrease in sharpness is gradual on each side of the focused distance, so that within the depth of field, the unsharpness is imperceptible under normal conditions. Typically requires constant calibration to maintain imaging consistency.

2) Hard focus. The laser is focussed for the particular plate when the CtP system is initially set up. This method is not able to cope with issues of variations in plate media thickness or as plate surface to exposure focus point changes. Hot spots and/or banding may appear.

3) Auto focus. The laser does an auto-focus for the particular plate each time just before actual imaging begins. This method is not able to cope with issues of variations in plate media thickness or as the plate surface to exposure focus point changes. Hot spots may appear.

4) Dynamic auto focus. The laser continuously adjusts focus for the particular plate during actual imaging. This method is able to cope with issues of variations in plate media thickness or as the plate surface to exposure focus point changes. Hot spots are unlikely.

The "tape test"

To get an idea as to how well the focussing system is on a particular CtP device, simply make a large "X" using Scotch tape/Sellotape on the back of the plate. Image a 50% halftone screen across the entire plate. Remove the tape and process the plate.If, after processing the plate, the tape is visible in the 50% flat tone patch then you know that any variations in plate thickness, or plate surface to exposure focus point, that is equal or greater than the thickness of the Scotch tape/Sellotape will result in variation in halftone screen tones and therefore presswork. It's unlikely that in day to day production that you'd stick tape to the back of a plate before exposing it, however it is quite possible for a small piece of debris to land on the CtP drum or back of the plate during pre-exposure handling.

Ideally, the imaging integrity of the best system would not be compromised by the tape on the back of the plate and all you will see is a large flat 50% tone - no visible "X" pattern.

During the CtP manufacturing process the geometric accuracy of the CtP drum on which the plates will eventually be mounted and imaged is measured.
Geometric compenstion
Imagine a grid wrapped around the CtP's imaging drum.

In a perfect world the grid would have perfect integrity.

In reality - the drums are never perfect cylinders.

For those CtP devices that have this capability, geometric correction distorts the bitmap that will be imaged on the plate in order to compensate for differences in the geometry of the imaging drum on the CtP device. Geometric correction also provides CtP device to CtP device integrity so that the plates from different machines will be as identical as possible.

Thermal compensation
Aluminum plates expand and contract with temperature change as much as film does - about 0.5mm across a 1m plate (typical 8-page size) for every 5°C (9°F) temperature change.
That 5°C change in temperature will result in a half row of dots @ 175 lpi misregistration plate to plate which can result in the need for the shop to reimage all four plates in a process job even if only one is actually needed.
Plate imaged twice at different temperatures. Without temperature compensation, the change in plate size due to the ambient temperature change results in a misregistration of the image.
Thermal compensation, for those devices that have this capability, corrects for ambient temperature variations by scaling the bitmap in such a way that it is as if the plates were always imaged at a single temperature.

The Color Bar

Color bars (a.k.a. color control bars, color control strips, or proofing bars) are essentially test targets that are used to measure print and/or proof attributes. Normally, but not always, it is printed in the trim area of the press sheet.Typical placement of a color bar on an offset press sheet - at the trailing edge (back end of the sheet).
However, it can take many different forms - sometimes hard to recognize - but always serving the same purpose.

Sometimes the "color bar" is incorporated within the graphic design of the publication. In this case the color makeup of the title (Cyan) and section headers (Blue, in this example, - Magenta overprinting Cyan).
Sometimes it is hidden in the spine (in this case the grey line running from top to bottom on the front edge of the photo).
While it is certainly possible to measure the color of the actual live image area, the technology is expensive and, as result, few printers are fortunate enough to have it at their disposal. Also, measuring the live image area doesn't provide as much useful information as a color bar can. Color bars therefore act as proxies, or substitutes, for the live image area as well as provide additional data.

The logic behind color bars

1) Unlike the live image area of the press sheet, color bars are consistent job to job. Therefore they are more efficient at providing a benchmark and can be used to track trends in variation over time.

2) Color bars can be tailored to meet the needs and measurement capabilities of individual print shops.

3) Color bars may be used to measure all aspects of the "print characteristic" - solid ink density, overprinting (ink trapping), dot gain, grey balance, as well as issues such as slur and dot doubling.

4) Color bars can reveal issues with ink hue, blanket condition, impression cylinder pressure, etc.

5) They can be used forensically to help understand why a specific job did not meet expectations.

6) They are efficient since, unlike the live image area, they are a constant made up of well defined elements that continue from proof to press sheet.

Solid ink density
A printing press is essentially a complex machine for laying down a specific film thickness of a specific color of ink onto a substrate. The ink is metered out in zones across the width of the press sheet according to how much ink coverage is required for each color in each zone.Therefore, for most press operators, the minimum requirement for a color bar is that it contains solid patches of the inks that will be printing since solid ink density is the only thing on press that an operator can adjust while the press is running.Those solid patches are then repeated over the width of the press sheet so that each ink zone is represented by at least one complete set of patches - containing one patch for each color being printed.
Information provided by only using solid ink density targets in the color bar
In this example, cyan is misregistered while the black printer is over emulsified (fountain solution breaking down the ink).
1) Provides a solid ink density value, measured using a densitometer, to determine if the press sheet is conforming to published industry, or shop specific targets.

2) Is an indirect, but practical, method of determining optimum ink film thickness and hence the balance of maximum color gamut without introducing image degrading inking issues such as slinging/misting.

3) The balance of the primary solid densities determines the hue of the overprints - i.e. the SID of magenta and SID of yellow determine the hue of the resulting red.

4) Indicate misregistration which can then be examined in the live image area.

5) Reveal defects such as slinging/misting/tailing, over emulsification, slur, doubling.

6) If records are kept, the hue of the ink currently on press compared with the hue of ink used in the past to determine if there is any contamination, change in color due to ink batch differences, etc.

Forensic targets on color bars are image elements that are typically not measured by the press operator unless there is a problem in aligning presswork to the proof. If that happens then these targets may provide useful information as to the cause of the problem.

Two-color overprint ink trapping targets
Ink "trap" is an objective indication of the ability, or inability, of a printed ink to accept the next ink printed compared with how well the substrate accepted that ink. Poor ink trapping results in presswork color shifts in reds (magenta plus yellow), greens (cyan plus yellow), and blues (cyan plus magenta) as well as a loss in total color gamut.
The two-color overprint solids allows for the objective measurement and evaluation of ink trap efficiency as well as the overprint hue error and greyness.
Typical trap values for three print conditions running a CMY ink sequence with Black first or last down:
Offset sheetfed: R=70, G=80, B=75
Heatset web offset (publications): R=70, G=87, B=72
Coldset web offset (newspaper): R=50, G=89, B=50

Slur and doubling targets
Slurring and doubling are print defects that occur when halftone dots and type blur as a result of a slight second contact or movement between press cylinders or the paper and blanket. (More about slur HERE and doubling HERE)
There are many different styles of slur and doubling detection targets. Here are two of the most popular:Of course, every halftone dot or letter character on the printed sheet will reveal slur and doubling, however the targets in the color bar signal the defect easier and quicker.
Grey balance targets
Grey balance targets are made up of a patch of three screened process colors that are balanced so as to appear as neutral grey under standard printing conditions. They are typically printed adjacent to a black screen tint of a similar value to allow for a quick visual, or measured, evaluation of how grey balance has shifted.Grey balance targets can be useful since variation in any of the three process colors because of dot gain, slur, doubling, density, trapping, and registration will be reflected by a shift in hue away from neutrality. The 3/C patch will take on a bluish, reddish, or greenish color cast.The idea behind this target is that any grey balance color shift away from neutrality suggests a possible color shift in the live image area. However, in production printing the grey balance target may not be a reliable indicator of presswork issues.

Other targets
Other targets that may be included in the color bar are:

Dot gainThese targets are intended to capture dot gain variation information. The dot gain targets may consist of just two patches for each process color to measure the dot gain a one location on the tone scale, or, with the addition of more patches, to measure the dot gain at the quarter, mid, and three-quarter tone values. Dot gain can be useful because issues like slur, doubling, or incorrect solid ink density, will be reflected by a variation in the measured dot gain.

Brown balance targets
Brown balance patches are similar to grey balance patches in function except that they are made up equal percentages of cyan, magenta, and yellow. Unlike grey balance patches which allow the press operator to make a subjective visual assessment of hue shift, brown balance patches can only be evaluated objectively with instruments.

ProprietaryProprietary targets such as that used by System Brunner are typically used to drive on-press closed loop color control systems.

Spot colorIf a spot or brand color is being used then it will warrant at least a solid patch in the color bar so that its solid ink density can be measured. Space permitting, the solid patch will be adjacent to a screened back patch so that dot gain information can be measured.

For process control, color bars should be included on every proof and press form of every job. If that is not possible because there is no room on the sheet (as often happens in newspaper work) then there are several options;

1) Run color bars on occasion by including it in the live image area.With the publisher's permission if required.

2) With the print buyer's permission, incorporate color bars test elements into the graphic/editorial design of the printed piece (see the USA Today example in Part 1).

Color bars are not a requirement for quality printing, however, they are key to making proofing and printing more efficient and effective while reducing overall production costs.

Presswork should be run "to the numbers" i.e. the solid ink density aim points, at which time the presswork should align to the signed-off proof. At that point the press operator should be free to make any needed ink key adjustments to refine the match. The color bar then becomes a record of initial match and needed adjustments. That information can be used in statistical process control to spot any trends, or issues, revealed by the kind of ink key moves that are made over time.

Color bars can be placed anywhere that they fit on the press form, including the lead and trailing edge as well as across the center of the form. In fact, placing it in the center of the form parallel to the inking rollers is ideal, since there is less likelihood of seeing the variation that occurs at the lead and trailing edges. Color bars can even be placed in the gutter inline with the direction of the sheet through the press, although doing so is not optimal since it provides information from only one ink key zone.

Ideally the color bar should use the same halftone screening as the live image area and have had the same press curve applied.

You can take it with you! - (How to be at the printshop - when you're not.)

Sometimes you just can't be at the printshop. Perhaps you're on holidays, relaxing at home in the evening, or maybe just goofing off on a "customer visit." Well now there's a way to not be away when you're away: just carry the sounds of the pressroom with you.

You can play them when you're pining for the shop when traveling to remind you of how good your job is:

The chugga chugga of a Heidelberg press

Or simply play them in the background when you get that unexpected phone call from the boss:

The satisfying rhythm of a productive press

You can acquire the appropriate press sounds that meet your shop's equipment from the online sound effects library HERE to use and enjoy as needed.

Implementing FM Screening

The foundations of successful FM screening implementation include the following steps:

1) Make sure that everyone in the shop that is involved with print sales and production understands why the company is changing the print process. All should want the change to succeed and should see a payback specific to their area. Sales will have a new story to tell about the company's ability to meet buyer expectations. Prepress and pressroom will be able to stabilize and optimize their areas. Management should see a clear differentiator, reduced manufacturing costs and more consistent product.

2) Make sure that the plates have the resolution to support FM at the chosen micron size (typically 20-25 micron for general commercial, 25-35 micron for publications and newsprint). Contact your local service or sales representative for the current list of qualified media.

3) Make sure that your CtP has the capability to do FM at the chosen micron size. Again, contact your local service or sales representative.

4) Communicate your intent to print FM with your ink vendor. Ask them about the ink series you are using and whether it is appropriate for FM. They may suggest a different series based on their experience. If they don't have experience with FM, then involve them in your initial start up and learning. If they are not interested then start looking for a different ink vendor.

5) Your shop should have an existing reliable and consistent print manufacturing process. I.e. the press is not used as a color correction device. One simple test is to have the press operator do their make ready without seeing a proof. In a well run shop, the press can be brought up to color and when then compared to the proof there will be very little difference. If the press operator is lost without the proof then that might indicate that the process is not stable and in control.

6) Build dot gain curves to be applied to plate imaging to bring tone reproduction in line with your current AM/XM printing.

7) Optionally, fine tune your proofing so that it reflects the extra gamut you'll see with FM.

8) Make FM your standard screening - not an exception for special projects. The goal should be that AM/XM screening only be used for special applications like reprints. That way the press operator gain experience and understanding of how FM reacts on press compared to what they are currently familiar with.

9) If you have more than one press/press crew - choose one to be your pioneers and champions of FM printing. They can then teach the other crews the keys to their success.

Generally, most of the problems encountered with implementing FM screening result from print shop culture - not the technology itself.

Fountain Solutions
Due to the over all small dot sizes, ink and water balance is more critical with FM screening (and very high lpi AM/XM screening) than with conventional AM/XM screens at industry standard lpis. So, before switching to FM screening, you should resolve any ink and water issues you have with your current screens.

There are no set rules about fountain solutions and FM, except to use a good product that is recommended by your ink and plate manufacturer. In general, less is best. Where possible, run the minimum dosage in your fountain solutions that the manufacturer recommends.

While FM microdots are less sensitive to physical dot gain from over-inking, they are more sensitive than the larger AM/XM dots to surrounding water levels and fountain conditions. Therefore, it is important to control pH, temperature, conductivity, and contamination in the water pick-up tray and on the chrome or ceramic pick-up rollers. The temperature of the fountain can be controlled in the re-circulation system (keep in mind that temperature in the tray may be 5 to 10 degrees warmer). Fountain temperature should be controlled within 0-5 degrees of manufacturer’s specification. Temperature can be further controlled in the press, using chilled rollers. Buffered fountain solutions help to stablize pH levels; however, pH should still be measured periodically to ensure that it stays within the manufacturer’s specifications. Processed water is recommended to further stabilize conductivity and pH. Check with your supplier to find a water treatment system suited to your printing needs.

A certain level of ink contamination in the fountain is unavoidable; however, excessive contamination is an indicator of other issues. In fact, it may first be noticed in your presswork as all-color "rain," or water streaking. Look for excess ink coming back through the roller train. This is sometimes referred to as chrome roller feedback, pick-up, ink belled back or reverse emulsification. Numerous factors affect this condition, including temperature of ink and fountains, emulsification, press speed, and the condition of the water pick-up roller. Simple remedies include better maintenance of chrome roller surface (no micro-cracks), temperature control, and desensitizing the chrome roller to improve the water pickup and keep ink away.

Back trap piling in the non-image area of the plate or blanket is generally caused by excessive ink in the fountain, low conductivity, and non-image area that is not desensitized – attracting ink instead of repelling it with water. Remember that fountain solution is a mild solvent. As such, if it is over aggressive it may overpower the ink film on the microdots and cause a loss of ink density.

Alcohol and alcohol substitutes help lower surface tension and ink viscosity, which promotes better release of FM's microdots. The amount of alcohol and alcohol substitutes used will depend on environmental restrictions in each locale.
When implementing FM screening it sometimes helps to view the screening in the context of AM/XM halftoning. There are two basic ways to look at how they compare.

The first is related to their resolution, or detail rendering equivalency. That is usually determined by running a line diagonally through the FM screen and then measuring the number of dots per inch along that line. The more dots per inch that the line intersects the higher the effective lpi.

The second is their lithographic equivalency. Basically how the FM screen reacts on press to ink, water, etc. That is usually determined by counting the number of transitions (dot edges) in a given screen tone area and comparing that to the number of transitions in the same tone area in an AM screen.

Here are the numbers for several popular FM dot sizes - micron size compared to lpi:

Detail rendering equivalency:
FM 20 = 500 lpi
FM 25 = 325 lpi
FM 35 = 325 lpi
FM 36 = 275 lpi

Lithographic equivalency:
FM 20 = 385 lpi
FM 25 = 240 lpi
FM 35 = 240 lpi
FM 36 = 205 lpi

The above equivalencies are for a typical second order FM screen - one that, close up, looks like this:The vast majority of FM screens in use today are second order using similar dot patterns so the equivalencies should be close enough for practical application. These equivalents are not absolute values since they will differ somewhat according to the design of the specific screen and how the engineer calculates equivalency.


Curve Management
The dot gain characteristics of FM require the application of dot gain compensation curves to plates. For predictable tonal response on press, keep the mechanical and chemical conditions on your press well maintained, and only build tonal compensation curves when the press is in a stable condition for your current AM/XM printing.

The basics of building tonal compensation curves to align your AM and FM presswork

1. This will take two press runs - one to get data for your current screening, and the second to get the response of the FM screening. If you have confidence in your current AM/XM presswork you may choose to only do the FM screening pressrun and compare the values it delivers to the target tones of your AM/XM presswork.
2. Your press should be in a stable condition that accurately reflects your pressroom environment, including documented standard ink density (SID) for your shop.
3. Your existing AM/XM presswork should be the tonal target for your FM screened presswork.
4. Image plates for your AM screening using whatever plate curves you currently apply.
5. Image plates for your FM screening without applying any plate curves (uncalibrated plate).
6. Bring your presswork to to standard solid ink densities.
7. Pull several good sheets and measure the CMYK tone scale response of the AM/XM and FM screens. The more sheets you
measure, the more representative the average will be of your printing conditions.
8. Use the measured data to build tonal compensation curves for your FM presswork.
9. Set up your RIP and workflow to manage the application of the FM curves.
10. Create and impose a test target for you FM presswork to confirm that the workflow has image the plate with the correct dot gain compensation curve.
11. Go back on press for a verification run to confirm that the curves for the FM screening are correct. Make adjustments as required.

Color Management
You may optionally decide to profile your FM presswork in order to reflect the added FM gamut so that your proofs are a closer match to your presswork.
1. To color manage a proofing device, build profiles by characterizing your FM presswork proofs with ICC targets or equivalent.
2. Gather the characterization data from your FM verification press run and use the resulting measurements to build an ICC profile to be used in a color-managed workflow to drive your proofer.
3. This is rarely done, however, if desired, you may choose to build a separation profile to convert RGB images to CMYK in order to take advantage of FM screening's larger gamut.

Image Management
1. When printing with FM 20 or FM 10 on coated paper on sheetfed and web presses, scan at resolutions above 300 dpi to take advantage of FM's ability to render and capture very fine details and texture. There is no need to scan higher than 600 dpi.
2. When printing newsprint with FM 36 or FM 25, you do not need to scan images higher than 300 dpi. Typically, images may be scanned at resolutions as low as 150 to 200 dpi. To determine the lowest resolution at which you could scan images and print successfully, do a press test. When the resolution is too low, you may see stair casing around the edges of image objects.
3. Make sure that images are not down sampled when going from native file applications to PDF.4. Make sure that there are no settings in the RIP that will cause images to be resampled at a lower resolution.

The Pressroom

Presses
Older coldset webs may have problems with FM screens and AM/XM screens finer than 110 lpi. Dampening systems running bareback with a durometer reading harder than 34 may have difficulty running FM.

Older presses with conventional dampening systems that use covers, sleeves, socks and/or wraps anywhere in the system may find that the increased detail resolution of FM results in the fabric weave/grain of the dampener material being resolved on the press sheet.

Older webs that can run 133 lpi screens can run 35 micron FM and coarser, while older webs than can run 150 lpi screens can run 25 micron FM and coarser. Older webs without an in-feed can also run 35 micron FM. In-feeds are important for web control and can reduce plate wear, web movement, and linting.

On newspaper webs, printing on paper with a recycled content of 75% or greater may create problems because the paper carries a high amount of silica, which causes piling and plate blinding.

Ink
For all press types, before using FM screens, speak with your ink supplier as they may have identified an ink series that works best with this type of screening. Remember that it is not so much whether the screening is AM/XM or FM that's important, it's the size of the dots through the tone scale that helps determine the formulation and performance of the ink.

When using 25 micron FM, you should be printing with inks that flow well with 133 line AM screens, and, for 20 micron FM you should be printing with inks that flow well with 175 line AM screens.

Ideally, FM inks should have low viscosity and high flow. Low viscosity helps ink shear and transfer to the sheet. Printers can drop viscosity by changing the pigment vehicle (oil), by increasing the water pickup, or by increasing ink temperature. High-pigment, low-gain inks are problematic with FM screening because they have a propensity to pile and print inconsistently.

Remember that each press is different. Settings and inks that work well on one press may not work well on another press.
1. Use progressive-tack as opposed to common-tack inks for FM. Generally, you do not need to change ink tack values or sequence from those used with AM/XM settings.
2. Metallic and fluorescent inks have poor transfer, even with conventional 175 lpi screens. The pigments in these inks are coarse, and little can be done to improve the vehicle. Coarser screens such as 25 or 35 micron FM are better suited to these inks. Check with your ink manufacturer to see what other options are available.
3. Some PMS colors (spot or special color) lead to inconsistent coverage on sheetfed presses. For example, solid tints can appear mottled or look like the texture of an orange peel, and this can impact FM screen tint builds. The pigment in each PMS color can vary considerably therefore you should work your ink manufacturer to optimize performance for the dot size you are printing with. If mottle occurs, it may be necessary to mill/refine/grind the ink a second time to reduce the size of the pigment particles.
4. FM performs best on press when using lower levels of water and just enough ink to achieve desired densities. Use water levels rather than ink density to control FM microdots. Adjusting density to control midtones should be the exception and not common practice with FM.
5. Generally speaking, 175 lpi sheetfed printers are typically well suited to run 20 micron FM with the inks they use for their AM/XM printing; however, these printers may still need to adjust ink flow and viscosity to optimize the ink for FM’s microdots.
6. Use low-viscosity, high-flow inks for heat-set webs. Chilled or water-cooled oscillators keep roller temperatures constant and maintain ink viscosity and performance at ideal levels. If your press doesn’t have this capability, ask your ink manufacturer to help change viscosity by altering water pickup or oil.
7. On web presses, use process black ink instead of book black inks where possible. Process black ink
offers the best results with FM as it behaves like cyan, magenta, and yellow inks.
8. On heat- and cold-set webs, book black and recycled black inks have poor transfer rates, and they behave more like fluorescent and metallic ink. Book black ink is typically used for high-density requirements while recycled, black ink is used in high-volume applications such as publications, newspaper, flexography, and packaging to meet cost and environmental requirements.
9. Heat-set and cold-set web inks are generally suitable for 25 and 36 micron FM and typically do not require changes to formulation or vehicle.
10. For 20 micron FM on web presses, work with your ink manufacturer to optimize performance.

Undesirable Patterns in Presswork
Proper press maintenance should be the policy no matter what halftone screening is being used. However, the finer the screen the more critical proper maintenance becomes since the microdots used in high lpi AM/XM and FM screens can reveal flaws in the presswork that were previously hidden by large AM dots.

Thin Lines in the presswork

Flaws and grind lines on press rollers can cause visible lines to appear in press work when printing with fine AM/XM and FM screens. They are characteristically oriented in the direction of paper travel through the press.

Grind lines are imperfections in the rubber rollers caused by grinding in manufacturing and wear on press. They show up as dark lines in the direction of paper travel with a frequency of one to four millimeters. They can be seen by using a loupe and examinging the solids. They are sometimes visible to the eye between the 50% and 75% FM tints. When water film on press is too thin to fill the grind and wear lines on rubber rollers, the resulting water film is uneven and transferred to the plate. Under 200 lpi, the uneven water film has little effect. However, finer screens can render inconsistencies in the water film, and these flaws show up as thin, dark lines in the presswork.

Roller grind lines are typically found on the water, form, or metering rollers. You can visually monitor the quality of water film by examining your chrome rollers and by examining your presswork. When there are visible fine lines and patterns on the chrome rollers, you have inconsistent water transfer, and the source can be either roller grind lines or overly sensitized rollers. White and milky water film on chrome rollers can identify overly sensitized rollers and can indicate that the rollers need desensitizing to help water move smoothly through the press. Ask your press and/or ink vendor for a suitable desensitizing agent.

Short thin "rain—lines" that vary over the sheet and from sheet to sheet may be caused by water lines. They can be found in solids and screen tints and look somewhat like a rainstorm. When water lines are dark, look for ink/water emulsification problems. When water lines are light (looks like lots of short scratches), water and ink levels are generally too low.

Micro patterning and micro detail

FM screening can resolve both desirable detail in images as well as undesirable detail created at various stages in the reproduction process. In particular, fine artifacts such as photographic grain, paper texture, and image manipulation artifacts may not show up with 150 lpi screens but may be resolved with AM/XM and FM microdots. Careful analysis by measuring the pattern, frequency, and angle of micro structures can help identify whether the source is screening, imaging, processing, or presswork. Rotating the plates and images can also help to narrow down the sources of unwanted patterns.

Mottle

Mottle results in botchy, uneven, flat tints with an orange peel texture and is most evident when you look at solids under a loupe. Mottle may be caused by ink breaking down making solids appear as if the were printed with flexography. It is most commonly seen in Pantone Matching System colors with coarsely ground pigments. It’s also common with high pigment loads when emulsification leads to uneven ink films being transferred to the sheet. Low water levels can cause PMS colors to bleed in FM tints, leading to excessive gain and further mottle. Flat-tint mottles are especially bad in blues, greens, and alkaline browns.

Mottle can also be caused by the substrate if it does not have a consistent ink accepting surface. Sometimes the paper's potential for mottle can be seen by holding the sheet up to a light and seeing if there is a blotchy quality of shiny and matte areas.

Run length

A few printers may experience shorter run lengths with FM screening on heat-set web presses (but not with positive plates or sheetfed presswork). This may be the result plate wear or blinding. On negative plates, calcium carbonate crystals from the paper (two to three microns in size) may accumulate on small dots and cause blinding - the plate image is visible on plate but does not carry ink. The smaller dots in both AM/XM and FM printing are also susceptible to paper piling, which can exacerbate plate wear or cause blinding on poorly maintained web presses.

Conducting The Press Check

Printing is a completely different imaging process than proofing. As a result, it is not always possible for the presswork to perfectly "match" the proof. Unfortunately, there is no objective, practical, way of defining what is an acceptable variation from the proof that still constitutes an acceptable pressrun. The role of the press check is to enable the customer, or their representative, to directly communicate their presswork concerns and acceptance, with the press operator, so that a successful pressrun can be achieved.

The customer should be able to go to the press and speak to the press operator directly. Working through an intermediary like the pressroom manager only slows down the process and causes communication errors especially when if they are managing multiple press checks at the same time.

Press check basics for the printer

• The customer should not be press side during initial make ready. Ideally you would have a comfortable holding area/lounge where customers can wait until the press operator has a sheet ready for inspection.

• An initial make ready sheet marked clearly "For Content Only" could be given to the customer so that they can check the sheet for content issues like substituted fonts, low resolution images, missing graphics, etc. This reduces time wasted at the press when doing the color approval. Also, if content errors are discovered then the job can be halted soon enough to avoid excessive make ready wastage.

• The press operator should be told ahead of time, by the CSR or sales representative about issues, concerns, and critical success goals the print buyer has for the job. For example, if a particular blue in a product image is important to the customer - the press operator should know that fact. Having this knowledge shows the customer that the shop is personally involved in the success of both the press run and the customer. It also helps establish better communications, if any color adjustments need to be made.

• When the press operator is satisfied with the color then a press sheet should be pulled and filed as "first press operator OK." Press operator OK'd color sheets can be used later by the printshop to identify and evaluate print manufacturing issues from prepress through the pressroom.

• When the customer arrives at the press, the press operator should introduce themselves.

• When the customer is at the press the press operator should stand aside and allow the customer "breathing room" to examine the press sheet against the proof.

• Tools such as Pantone swatchbooks, ink draw downs, loupes, note paper and pens for customer use should be readily available.

• The press operator should try and use correct terms and use them consistently. For example, do not use the term "blue" if you mean "cyan."

• Encourage the customer to explain what concerns they have rather than tell you how to fix them. Their role is to identify the problem - your role is to know whether, and how the problem can be fixed.

• If you make a press adjustment to fix an issue, tell the customer what you will be doing and how you feel it will fix the problem. This helps confirm to the customer that you correctly understood their issue and it also helps educate them which, in turn, will make future press checks go quicker.

• If there are any issues that you have with the press to proof match - let the customer know that right away rather than have them discover it themselves. Doing so tells the customer that you are not trying to hide anything from them. It also helps establish a mutually respectful environment.

• Be aware of time - every minute the press is idle the company is losing money. Remember, you can get press sheet sign-off with notes to cover some issues (e.g. "OK for color - but must remove all circled spots and hickies). Don't rush the customer, but don't let them dally at the press either.

• Some shops will have a "light room" where customers can take the sheet to view it under "standard" office lighting. This helps mitigate metamerism and substrate fluorescence issues.

• Make sure that the OK press sheet is signed and dated by the customer, the sales representative/CSR, and the press operator. Any continuing issues to be dealt with later should be noted on the sheet.

• Have a cardboard tube ready so that OK'd press sheets can be taken away by the customer.

• Thank the customer for attending the press check.

Press check basics for the print buyer
The role of the print buyer at the press check is to directly communicate their presswork concerns and acceptance with the press operator in the presence of the sales person to help ensure that any color issues are dealt with according to the customer's requirements.

• Once you get the call from the printer, gather up any material related to the job, samples, proofs, spot color draw downs, paper samples, mock-ups, folding dummies, etc. Also, make sure to bring loupes, color swatchbooks, scissors/x-acto knife, pens and notebooks.

• It's a good idea to get an imposition proof from the printer so that you can check three things; are there any potential inline color issues, are pages on the press form imposed the same way as they were on the imposition proof, and finally when the sheet is backed up on press do the pages back up correctly.

• Arrive at the printshop on time. Identify yourself at reception and explain why you are there. Then wait for the sales rep or CSR to be escorted to the waiting lounge or press floor.

• If you are part of a group attending the press check, identify which single individual will be the lead. That is the one person who consolidates the opinions of the folks attending the press approval and therefore the one who speaks for the group to the press operator.

• While waiting to go out to the press floor, ask for an initial make ready sheet that can be checked for content issues like substituted fonts, low resolution images, missing graphics, etc. This will reduce time wasted at the press when doing the color approval.

• At the press, introduce yourself and your team to the press operator. Wait to be invited anywhere near the press console.

• Do not touch or use any equipment at the press unless you specifically ask permission first.

• When you are offered the sheet for examination, ask the press operator if they are happy with the sheet and if they have any concerns/issues with it.

• Engage your sales representative for input and guidance with any thoughts/concerns you have. Tap into their experience.

• Recognize that time equals cost so be focused on the task at hand. If you are working with a team, assign checking roles to each. For example, someone checking registration, another checking for hickeys/specs, low resolution photos, swapped or dropped fonts, etc.

• If you are alone, have a written, organized, step by step yes/no, pass/fail procedure to checking the press sheet . Typically the process goes like this:
1) Is it printed on the correct paper? If a specific paper grain direction was required ensure that it is running in the correct direction.
2) Is it in register?
3) Over all, does the press work color align with the proof? Are there any obvious color issues?

4) When critical color alignment is required, cut the press sheet through the important color and overlay that section of the press sheet on the proof. Colors that may appear correct when compared side by side may appear different when directly overlaid.If you cannot clearly see where the press sheet ends and the proof begins you know you have a critical match.

5) Use your reference material to confirm the correctness of special/spot/brand colors.
• If you have any color concerns/issues, try to describe them clearly and unambiguously. Then describe just as clearly and unambiguously what you want to see. Do not tell them how to fix the issues. Your role is to identify any problems - it is the press operator's role is to know whether, and how any problems can be fixed. You can ask whether a solution you thought of might solve the problem. For example, you could say: "I think this area is too red. Would reducing the Magenta a touch fix it?" Phrasing a suggestion as a question can also help your press operator better understand your meaning according to how you describe the problem.

• Try and use correct and unambiguous terms and use them consistently. For example, do not use the term "blue" if you mean "cyan." Try to avoid terms like: "This area is too hot" or "Can you punch it up a notch?"

• If a press adjustment is made to fix an issue, ask the press operator what they are doing and how they feel it will fix the problem. This helps you to better understand the print production process, and its limitations, better.

• Most press operators will try very hard to achieve what you’re looking for, however, once they've made their press moves and you are still not satisfied it will be up to the sales rep to authorize trying anything else. If it's a really serious issue, the sales rep may stop the press and pull the job.

• Keep in mind that the start/stop/start/stop press cycling during a press check means that the press is not yet running in a stable fashion. Once the press OK is complete and the press is running at optimal speed, some small color issues will clear up by themselves.

• Be aware of time. Respect the printshop's need to maintain their production schedules. Remember, you don’t need to remain until the sheet is absolutely perfect. Just mark it as “OK with changes as noted.” (e.g. "OK for color - but must remove all circled spots and hickies).

• Some shops will have a "light room" where you can take the sheet to view it under "standard" office lighting. This helps mitigate metamerism and substrate fluorescence issues.

• Make sure that the OK press sheet is signed and dated by the person in your team who has authority to take responsibility. Any continuing issues that are to be dealt with later should be so noted on the sheet.

• Ask for a few copies of the OK'd sheet to take away with you for your records.

• Thank the press operator and crew for their performance during the press check. They really appreciate it and will remember you in a positive light during your next press check.

How CtP plate readers read plates

Dedicated plate readers take a different approach to measuring dot area than densitometers. What they do it use an internal CCD camera to take a continuous tone photograph of the plate area at high resolution and magnification:

Depending on the instrument's software and display technology, this original image may or may not be shown to the user.

In order to calculate the dot area, virtually all plate readers use a thresholding algorithm to determine what is the non-printing plate and what is the printing dot.* Put another way, the software decides that a pixel of X tone level and lighter in the captured image is the plate while levels darker in the captured image than X tone level are identified as ink carrying dots.

However, because the halftone dots in the photo will have a slight softness to them, the result is a slight ambiguity as to where the transition from non-image plate to ink carrying dot occurs. Change the threshold and you change the size of the dot the instrument "sees" and hence the tone the instrument reports.

For example at a threshold value of 139 (from a range of 0-255), the software sees this as the printing dots:
and reports a dot area value of 50%

If the threshold is a tone value of 175 the software sees this as the printing dots:
and now reports a dot area value of 55%

So which is best, or more accurate, for measuring halftone dots on plate? Well, in a GATF study of products for measuring the dot area on CTP printing plates (RTR 27: Plate Reading Technologies and Their Performance on CTP Plates) a spectrodensitometer gave very similar overall results compared to the best CCD-based solution for plates.

In their defense, dedicated plate readers can provide more information, such as halftone frequency and screen angle, than a densitometer when measuring plates.

* The extremely sophisticated and expensive Jandel planimeter requires the user to trace where they believe the dot edges are in order to differentiate between plate and dot areas.

Using a densitometer to measure plates

Despite what you may have heard from vendors - yes, you can use a densitometer to read offset plates. This can be especially helpful for the smaller printshops who may not have the resources to purchase dedicated plate readers.

Densitometer basics

Color reflection densitometers are designed to accurately measure Black, Cyan, Magenta, and Yellow colorants on substrates like paper. However lithographic plates usually don't have C, M, Y, K images on a white substrate, instead the plate material is usually brushed aluminum with a grey color. The color of the image itself will vary according to the plate type and vendor.

You need to be aware of these reflectance and color properties since they effect how you use your densitometer when evaluating your plates.

Contrast

Because of plate color properties, the contrast between image and and non-image areas of the plate is typically less than half that of ink on paper, as a result it may not be possible to calculate dot area for some densitometer/plate combinations.

Aperture size

The densitometer aperture (measuring window) should be the largest possible for your brand of instrument - as long as it's not larger than the image area you are measuring. This helps average out the reading and minimizes the effect of random plate patterns.

Densitometer Status

The instrument should be set to "Status T" unpolarized in part because this setting has a wider response to the various plate coating colors.

Consistency is more important than absolute accuracy

Conventional and digitally imaged plates typically use the same aluminum base material. However the different types of mechanical or chemical graining that prepares them for lithography results in a different surface texture which in turn scatters light differently. In order to achieve consistent dot values. To help achieve consistent dot value measurements, you should try to maintain a consistent alignment of the densitometer to the plate. Perhaps using the base of the densitometer aligned to the edge of the plate. Also, zero the densitometer and read the 100% area on the same spot each time and take readings from the center of each target patch.

If you know that a patch on the plate represents a 50% tone (i.e. it is a checkerboard) but your densitometer reports the patch being, for example, a 54% tone while the plate prints correctly on press then 54% becomes the target for that patch on the plate. The idea is that you are controlling your plate imaging process by monitoring and minimizing variation. Your priority is to maintain consistency in the measured dot areas.

Alternatively you could change the dot gain calculation "N" factor. Densitometers use one of two formulas for measuring dot area; the Murray-Davies or the Yule-Nielson equation. They are the same equation however the Yule-Nielson equation utilizes an "n"-factor to "factor" out the optical dot gain (the Murray-Davies equation is equivalent to the Yule-Nielson with n set to 1.00). The n-factor is experimentally determined by adjusting it until the densitometer reads the ‘desired value’ at a known dot percentage. Typically a 50% tone is used because it is easy to spot. You'd print a number of tone values close to 50% (e.g.45% and 55%) then use a loupe to find the tone patch that ‘looks like' a 50% tone - a checkerboard. Next adjust the n-factor on the densitometer until that patch reads 50%. The n-factor is typically applied when a densitometer is used to measure printing plates since printing plates are assumed not to have any optical dot gain.

Printing prepress - literally

A fascinating insight into how production printing was done before mechanized printing presses that is still being done today.

Founded in 1729, The Derge Parkhang is the last remaining Tibetan printing temple of its kind. The Parkhang stores more than 300,000 woodblocks that are used to publish sutras, commentaries and histories of Tibetan Traditional Buddhism.


Please press the play arrow to view the video. Note that it may stop for a moment while the video buffers in the background.
More information can be found on the Derge Parkhang YouTube channel by clicking HERE.

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: