Progressive Dot Sharpening and Image Loss

Progressive Dot Sharpening and Image Loss
Printer Tech Tips
Progressive Dot Sharpening and
Image Loss
Problem
During the course of the press run, the quarter-tone dots progressively and
prematurely sharpen until color visibly diminishes and eventually disappears.
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Description
Alternative terms may include vanishing dot, plate sharpening, or plate blinding.
These are problems that can affect both sheetfed and web, and in extreme cases
can be detected within 5,000 impressions. Usually, the problem is first noticed in
quarter-tone screens, more predominant in stochastic printing, and more
problematic when printing light ink coverage. Once the problem is detected and
temporarily mitigated by plate and/or blanket washing, dot sharpening may reoccur
with increasing frequency. Progressive dot sharpening or dot loss is usually reported
more as a web nuance due to its repeated manifestation on longer press runs but
shouldn’t be confused with image blinding from normal plate wear.
There are both chemical and mechanical causes for progressive dot
sharpening, but the shared characteristic is incomplete ink transfer, which can
usually be traced back to blanket piling, plate piling, or plate contamination.
— I mage Area Blanket Piling is the most common form of dot loss. If a simple
washing of the blankets returns print to normal reproduction then the problem is
image area piling, primarily the accumulation of non-transferring ink sludge and
light paper debris. There are two types of image area blanket piling; image area
piling within the same unit of print and image area piling of first-down colors in
downstream units of print. If the image-area piling is concentrated on the
trailing edge of the image, see Sappi tech tip on Image Area Piling/Tail-edge
Pick.
—N on-image Area Blanket Piling is typically a preponderance of paper-related
debris with lesser amounts of ink sludge across the blanket’s non-image
surface. This type of piling, often referred to as “milking”, can accumulate to
where it eventually creates an image area recess which can diminish ink transfer
through poor plate-to-blanket and blanket-to-paper contact. In extreme cases,
a non-image whitening may be visibly apparent, which is an indication that the
paper coating may be weakly bound or breaking down due to abrasive action or
over-aggressive and/or low pH fountain solution solubility.
—P late Piling or Plate Blinding can be the result of either a build of nontransferring ink components or deposits of hydrophilic substances that reject
ink receptivity and transfer. If the image is intact and quickly returns after a
routine plate wash, the problem is most likely simple ink piling or hydrophilic
interference. If the intact image area demands heavy scrubbing or won’t fully
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Printer Tech Tips
Progressive Dot Sharpening and
Image Loss (continued )
return after washing, then the problem may be related to heavier deposits such
as calcium carbonate, fountain solution gum, or surfactants from an
accumulation of plate cleaners, blanket washes, and/or roller washes. Calcium
deposits are particularly hard to remove and usually deposit around the outside
diameter of the dot, slowly constricting the dot and sharpening the image as the
build increases; a condition that may demand frequent plate remakes. For more
specific information on the effects of calcium carbonate in the printing system,
see Sappi tech tip on Calcium Carbonate.
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Causes
—I nk is over-emulsified.
—L ight ink coverage; slow fresh-ink displacement.
—C umulative contamination of debris or surfactants in the fountain solution and
ink rollers (ie: calcium carbonate, blanket wash, gum, plate cleaners, silicone
additives, algae, mold, etc.).
—H ighly acidic or overly aggressive fountain solution is breaking down ink
resulting in over-emulsification and poor ink transfer.
—H ighly acidic or overly aggressive fountain solution is breaking down the paper
coating resulting in a white, milky blanket glaze.
—I nconsistent or poor water quality.
—U nder-packed plates or blankets impeding ink transfer.
—H igh-force or inconsistent blanket release.
—C oarse or poorly ground ink causing ink pigment separation.
—H igh unit-to-unit ink tack build through the press.
—I nk film on rolls and at transfer points is too thin.
—D ynamic ink temperature too high; problems may first appear on the hotter
gear-side of the press.
—Paper lacks sufficient coat strength.
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Printer Tech Tips
Progressive Dot Sharpening and
Image Loss (continued )
Sappi Printer Technical Service
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Options and Solutions
— Minimize water to the plate wherever possible.
— Check ink/fountain solution compatibility and ink water pick-up characteristics.
— Use ink take-off bars for light ink coverage whenever possible.
— Clean and flush the circulating system; check fountain solution dosage and
replenish. This procedure should be a weekly maintenance consideration.
— Rinse, de-oxidize, and thoroughly final-rinse ink rollers with water-miscible
roller wash. This procedure should be a weekly maintenance consideration.
— Monitor fountain solution for both pH and conductivity. High conductivity
indicates high levels of contamination. The fountain solution mix should be
buffered to a pH of 4.0 or higher.
— Check that water-miscible blanket wash is not contaminating the fountain
solution during the auto-wash cycle. Also avoid using high water-miscible
solvents for metering or slip-roll cleaning.
— Perform a microscopic analysis of plate surface for adhered deposits if the
problem is determined to be plate blinding.
— Hard tap water is inconsistent and contains high levels of calcium which can
adversely react to fountain solution causing calcium soaps. De-ionized water
may be too aggressive and incompatible with fountain solution. Consider
reverse-osmosis ( RO ) water treatment or a treatment system specifically
designed for the lithographic print process.
— Check plate and blanket over-bearer height and adjust packing or blanket
thickness to achieve unit-to-unit consistency.
— Try a rougher-surfaced, quick release blanket to reduce the force of blanket
release, especially when running smooth-surfaced coated papers.
— Check ink grind; coarse-ground inks such as black, reflex blue, or metallics can
be especially problematic when running through subsequent units of print with
no fresh ink overprint.
— Check dynamic ink tack. If the ink is piling in its unit of print, dynamic ink tack
may be too high. If the ink is piling in downstream units, it may be too “fast”
( setting and building tack too fast on the paper ).
— Thin ink films, often the result when printing stochastic, run higher in tack.
Adjust for optimum ink film or consult ink supplier for formulation modification.
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Printer Tech Tips
Progressive Dot Sharpening and
Image Loss (continued )
— Dynamic ink temperature affects ink tack and transfer. Most ink suppliers
recommend dynamic ink temps of 75-83° F. ( 24-28° C.) as measured and
averaged by a hand-held non-contact IR thermometer from various points
across the ink train away from chilled oscillators. Consult with ink supplier and
maintain chill temps accordingly. Fountain solution should also be chilled to
maintain a pan temp of 65-68° F. (18-20° C.).
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— Try a different production run of paper.
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