The Print Guide: June 2010

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.