Dot Philosophy Challenging Press

(Author: Mariana Tsekos - Graphic Technologist)

?Published @ www.prepressmag.com - 5th editionPubliced

New modulation philosophy is challenging the print shop as more types of raster are now available; Stochastic, Hybrid, Spiral and Concentric are some of those.

The quest:

Printing presses can be stable, but not consistent; Solid ink densities will naturally vary through the press run, and when they do, dot gain and ink density on the halftone dots varies. Consequently, we face a variation in tone and color reproduction through the press run.

The tone variation happens, because the large midtone dots in an AM/XM screen can accept more ink than the smaller ones in the highlights and deep shadows; though, as solid ink density varies, midtone color goes darker and lighter.

-???AM is based on a geometric and fixed spacing of dots, vary in size depending on the tone color represented, usually 10-200 microns. Color curve helps dots to vary in size.

-???Conditional dot shapes are Circular, Elliptical, Square, Rugby, and Round FOGRA;

see differences between Round FOGRA and Circular Euclidean dot, ESKO’s Double Circular and Square dot.

The proposition:

FM (frequency modulation) screen model have different ink profile; still dots, small and same size throughout the tone range (about 25 microns), but distribution density varies depending on the tone.

The color remains more consistent through the run; acting much like the highlight dots of an AM/XM screen, do not take on as much extra ink as SIDs naturally vary, they remain more stable.

Furthermore, since FM screening utilizes very small dots, very fine details are possible, with significant ink saving.

How?it works?

In FM screening, the?half tone?dots are fixed in size. The dots are arranged randomly according to the tone value; darker tonal values need more dots = higher dot frequency in the area, while lighter tonal need?less?dots = lower dot frequency.

This is a "pseudo-random" distribution using frequency modulation to change the density of dots. So we get a frequency depending structure and not a numerical distribution. The dots vary in distance, simulating the technique of Chalcography (Copper Engraving).

Thus, FM screen creates a greater color gamut than the conventional AM/XM halftone screen frequencies.

The feature is very favorable for printing on rotary machines where the misregistration is very common due to effects such as web growth.

-???Also known as web stretch. It is a physical distortion occurring to paper printed on a web press thus the name web growth. The deformation (stretch) of the paper when printed on the various color units of a typical web press occurs as the paper absorbs ink and water causing the paper to stretch. This stretching and/or shrinking combination becomes accelerated due to the existence of pressure and tension of physically moving paper at a high speed through the various cylinders of a web press.

-???The use of FM screening allowed a UK publisher named Archant, to switch his production to fonts with "tiny holes" permitting a reduction in ink, without turning the fine text to grainy. The result was a significant reduction in production costs.

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Good news for the print shop!

The printer can get LPI Integrity. Rosette can often create moiré, so is very unstable;

The visibility of the rosette can change, then misregistration is possible, creating a numerous issues, i.e. in sensitive skin colors; may not be visible when in register, becomes annoyingly visible with a slight misregistration or misposition. Since FM screens do not produce rosettes, they maintain their LPI image integrity.

Misregistration alternate overprint and ratios of wet / dry trap; while dots overlap (even so when are mispositioned / misregistrated), they normally alternate colors. This can be particularly troublesome in grays, blues and light colors. Again, since FM screens don’t have a rosette, color and tone are better preserved. FM screening can help deliver more presswork color. It succeeds color accuracy and extended color gamut, giving the industry a strong competitor.

Finally, FM offers Faster Ink Drying helping the presswork dry faster; tones are built with many-many small dots. Those microdots carry a thinner ink film and are distributed in a smooth pattern, allowing the ink to pop off its volatiles.

Faster drying benefits:

°??Easier to align presswork with proof, closer to digital proof - they are both dry ones

°??Reduced setoff

°??Better printing on uncoated substrates, newsprint, foils and plastics

°??Less spray powder

°??Jobs go faster to bindery

°??Heatset presses can lower a few degrees their drying ovens - less power consumption, less energy costs

°??Better delivery terms

The new philosophy is challenging the print shop

As far the "Printer’s Devil" exists, even so the "Fake of the Novice" does; it is necessary to understand that if FM screening is only used for "special jobs" then press operators never get used to how it performs on press.

Prepress must cooperate and help the printer to give its best. Many tests, trials, charts, measurements and curve corrections will be required to achieve the best result. Furthermore, prepress need to understand the function of each screening, the reason and the convenient way of processing for each artwork and simulate it to the press.

Extended process control is needed as the dots get smaller. If issues in a 175 LPI print process occur, then high frequency screening will maximize their presence. Those are the most common reasons that cause printers to fail. Thus, all propositions would not concern specific tasks but the whole production.

Keep aware of these:

°??Grainy tint areas; usually doesn't occur with AM/XM screening. The inconsistent plate imaging is a common possible cause, or the plate doesn't have sufficient resolution for the required dot size.

Graininess can also occur when screening back a spot color (spot colors are normally not formulated to be halftone).

°??Poor image; do not resample a high res images. The problem happens when high dpi images are resampled to a lower resolution; when the PDF is created (or resampled or the image goes through the RIP), then, a "softer", a poor image can be created.

Another question is, when the LPI of the screen (AM-XM-FM) are higher, then the detail rendering are higher too. Hence, bad details and flaws of the original image become visible and "better" reproduced.

°??Presswork color and industry standard (SWOP, GRACoL7, etc) alignment;?the larger color gamut is true, appearing more vibrant colors than the same screen tint build in a lower LPI AM/XM screen (it usually effects vectors and normally does not show up in images). So, it is possible to be needed a gamut reduction with color management / new image separating, in order to taint color builds.

-???Spot colors are normally not formulated to be halftone screened. Hence the impact of using a high frequency screen with increased gamut, can lead to an unexpected color appearance.

°??Piling; may occur on heatset web presses and is usually caused by not following protocols for proper blanket wash-up or poor ink rheology. Using inks formulated for large dots of AM/XM screen, rather are formulated for smaller FM dots, too. Thus, an ink that opperates with a 200 LPI AM/XM screen will equally do with a 20-25 micron FM screen (piling is rare on sheetfed presses).

°??Mottle; is usually the result of poor ink transfer, wrong ink composition, fountain solution condition, paper surface and coating. It can also occur on back screening of a spot color. Thus their pigment grind may be too coarse for the dot size being used.

Individual results and benefits

STOCHASTIC

From the Greek "stokhos" which in mathematical terminology refers to a random probability distribution.

Due to the random distribution of microdots on the surface, the composition with more colors is possible, without the risk of moiré and without the phenomenon of overlapping dots and misregistration during printing. Color transition is smooth and offer excellent sharpness in images, especially in very small elements, lines and edges are straight and no grainy. Skin tones and true-to-life color are better reproduced. Color stability is possible. SIDs and dot gain naturally vary during press run. A 10-15% of ink reduction is achieved with an FM screening (depending on tone content).

Summarizing them:

°??Does not create rosette patterns

°??No screen angle moiré, thus no moiré at the artwork

°??4+ color printing is possible

°??Photo-like reproduction - Fine details (halftone dot sizes can be as fine as 10 microns, offering a quality comparable to that of photographic prints)

°??Greater tone (even on misregistration)

°??Color stability (even on misregistration)

°??Halftone dot structure stability on misregistration

°??Larger color gamut

°??Faster drying

°??Reduced ink usage

°??Cost effective

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As per disadvantages we could name:

°???????Special care and cleanliness required in press, because of the small dots used (especially when plates are film depended)

°???????Amplifies dot gain - FM microdots on press resist to added ink densities caused the difficulty of color and tonal adjustments

°???????Printing stability is very important; lot of dot gain occurs

°???????Long run length stability is harder, because of the microdots (new FM screen generations are improved)

°???????Hard transfer from film to plate during the plate making process (before CtP technology)

°???????Harder midtone control and depended of dots cluster; noise or mottle can occur, when dots connect or overlap.

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HYBRID

From the Greek word "hyvridio" which means the creation by 2 different species.

Hybrid is a combination of the FM and AM screening. Thus, different type, larger and smaller dots are used to cover the area, permitting better printing stability, improved press run length, smoother flat tints and higher contone (continuous range of tones from white to black) density. The dots that are used in the majority of the job's tonal values areas are generated using the conventional AM screens, while the highlights (maybe the deep shadow areas also) use FM screening. This allows highlight areas to be printed without too small dots to be held on press.

While in midtones uses AM screening and in highlights / shadows uses FM screening, is possible for the press to get 240 LPI without extra efforts, adding 280 LPI - 340 LPI (on selected systems) and a?complete tone range 1 – 99% on paper.

Thus, one can select the right minimum dot for highlights and shadows and balance selection to get the maximum quality for the expected run length. Hybrid is dedicated per plate type and CTP. These small dots are supported in the highlights by non-printable microdots that result in higher stability during run length. Also, one can grow the dot size and the random placement, achieving a better connection and printing stability on large gaps. Finally, the dot gain is more under control. It is also possible to specify the size in pixels of the FM dots to be used in the highlight and the shadow areas of the job.

The reader will experience the difference, not the printer!

How it works?

AM halftone dot shrinks, from the midtone tints, till smallest reproducible dot is reached. Then highlight / shadow tints are obtained by handling these smallest printable dots in FM; no more random dot placement.

No stochastic but still AM; highlight and shadow details just look stochastic. Rather, they are FM implementations of dots placed at the continuation of traditional angles defined in midtones.

AM to FM transition point is line frequency depended.

The advantages namely are:

°??Better Printing stability

°??Dot gain more under control because dots close better

°??More clustered dots in the highlights to midtones, more robust than standalone small dots

°??Non printable little dots supporting neighbor printable dots

°??Enhanced print quality with a better detail reproduction

°??Improved registration accuracy

°??Excellent color reproduction

°??Run length improved

°??Reduced ink consumption up to 15%

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-???Below, hybrid printing, adding spot color with transparency on CMYK image; with ADOBE all colors under spot are converted to FM while in ESKO, the image keeps AM and the spot is converted to FM, giving a better result

Benefits for FLEXO

Hybrid is very popular with flexo printers as it allows recovering highlight and shadowing tones that would otherwise be lost.

The most popular benefits are:

°??Vibrant colors

°??Better printing stability

°??No breakout of small dots

°??Smoother tints

°??Surprisingly high line ruling - Up to 170 LPI on corrugated - Up to 212 LPI on plastics

°??Anilox with lower cell count [1/6 ratio over ruled: 212 LPI on 900 anilox (> 150) - 175 LPI on 600 anilox (> 100)]

SPIRAL

From the Greek word "spira", the winding shape that combines symmetry with golden ratio.

-???In the prism of symmetry and the golden ratio, spira reflected for the ancients the sense of harmony and beauty.

-???The definition of the spira was made by Archimedes around 220 B.C. where it is specified that the width between two successive windings remains constant. Spira was used as a decorative motif; Illustrations have existed in Greece since prehistoric times (around 1200 B.C).

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Is a new shape of dot, looking like spire. The innovation belongs to AGFA and is included in the AM screening range. It uses degrees and a rosette like the conventional AM raster. Those dots are replaced by the spiral dot which is "custom" in relation to the thickness of the line depending on the print, allowing the reduction of ink uptake.

-???Due to the shape of the dot and because of the white areas inside it, this new technology achieves an average ink reduction of 10%.

The concept and aim are to make prepress and printing operations cleaner, more cost-effective and easier to manage and maintain; Spiral lets you hold a 1-99% dot, vibrant colors, reducing image slur and does not require expensive upgrades for CTP or printing press.

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The benefits of spiral dot shape are:

• Very large reduction of moiré in printing
• Improved color reproduction, especially in blue shades
• Clearer images, especially in dark tones
• Less transparency, allowing the use of thinner paper
• Cost effective

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-???To avoid moiré, try to change the Y ruling in Advanced Screen Setting and lift it by 15%. Keep in mind that you have to use the Clear Center Rosette pattern (an ESKO AE option).

CONCENTRIC

The word comes out of the Greek "kentro" (in latin "centrum") and refers to the point from which all points on the circumference of a circle or the surface of a sphere are equidistant.

ESKO is featuring the Concentric dot, where the solid AM dot is divided into thin concentric rings.

Concentric screening doesn’t offer additional gamut - possibly less - comparing with AM/XM screens at the same lpi, because of the difference in ink coverage area. As an AM screen there is still the chance for screening and moiré, although it seems being less problematic, because of its finer shape.

The thinner ink films and the need for tone reproduction curves for plate imaging leads to ink reduction.

1-2 pixel width Concentric dots could be problematic for imaging systems.

In conclusion, the use of the above mentioned screenings requires a prudent choice and knowledge of strong points and weaknesses of each one.

Technology goes on, giving us great results and many options, but the best choice is up to us to be found. You can be part of the development and help your company to be a leader in the field.

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Source: Agfa – Esko – The Print Guide