Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1997-01-13
2002-06-18
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S303000, C101S453000
Reexamination Certificate
active
06406833
ABSTRACT:
The present invention relates to printing and has for a more particular object an original use of the frequency-modulated screening in wet or waterless offset, as well as films and plates modified depending on said screening.
It is known that wet and waterless offset printing techniques use supports or plates which comprise surfaces for absorbing the ink, so-called inkphilic surfaces, and surfaces for rejecting said ink, so-called non-inkphilic surfaces. In wet offset, the ink is rejected by a very thin film of water which adheres on the hydrophilic surfaces. In waterless offset, the inkphilic surfaces are based on a coating of silicones (on which the ink does not adhere). Said inkphilic and non-inkphilic surfaces are generated on said plates by techniques of exposure which employ films or directly from software.
On the printing machine, rollers coated with ink deposit said ink only on said inkphilic surfaces of a plate. A blanket, rubber-coated fabric, takes the ink of said ink-philic surfaces of said plate and deposits it on the material to be printed. It may be question of paper, cardboard, metal, etc.
It is therefore of prime importance, in order to obtain a correct result, that the ink catches successively on the ink-philic surfaces of the plate and then on the rubbery surface of the blanket and that it is finally deposited on the material to be printed.
Within the framework of wet offset, the ink should, furthermore, be mixed with a little of the water used for wetting the hydrophilic (inkphilic) surfaces.
The images are produced between two values which are, on the one hand, the “zero” value, i.e. the colour of the material to be printed, blank, and, on the other hand, a “maximum” value, i.e. the solid tint made with the chosen ink (black for black ink). All the intermediate values are obtained by using a screen, which is constituted by thousands of small squares. These may be empty—they represent a white surface, without ink—partially filled, or filled up to solid tint. All the shades are thus reproduced by solid tints of more or less large area. Said solid tints generally consist of dots, equidistant from one another, of an increasing diameter. Amplitude-modulated screens are spoken of. With this conventional screening, the reproduction of a half-tone image passes by the arrangement at equidistance of dots of which the size varies proportionally to the value of the tones of the original.
The screens are qualified by their number of lines to the linear inch.
Depending on the printing process employed and the nature of the material to be printed, more or less fine screens are used, such as screens 65, 80, 100, 120, 150, 175.
Ordinary, cheap papers such as newspaper paper, require a film of fluid, thick ink to cover their rough and absorbent surface. To print such papers, a large screen will have to be used: 65 or 80, for example. Good quality papers, which present a smooth, homogeneous surface, may be printed with a film of ink which is thin and concentrated in pigments. In this context, a very fine screen, 150 for example, may be used.
On carrying out the conventional techniques of which the principal characteristics have been recalled hereinabove, problems of the type set forth hereinbelow are frequently encountered.
The water-ink mixture, in wet offset, is not always made satisfactorily. It may contain too much water or, inversely, not enough water. The quality and productivity of the print are then penalized insofar as said ink may drip, dry poorly, remain on the blanket, etc.
The material to be printed, such as paper, does not always take all the ink deposited on the blanket. Said ink accumulates, “thickens”, on said blanket. It is then necessary to interrupt printing and to wash said blanket.
The surface of the material to be printed, such as paper, leaves on the blanket residues, in the form of dust. The volume of residues is all the greater as the quality of the material to be printed is lower. When said volume attains certain limits, the blanket should also be washed.
The ink deposited on the blanket catches on the material to be printed. If the latter is fragile (case of paper), it will tear from time to time. Pieces of it then remain adhering on said blanket. Said pieces must then be removed and the blanket washed.
On sheet machines, there is also a particular problem insofar as, in the same way, the ink deposited on the blanket “sticks” or, “draws” the material to be printed (the sheet of paper). The latter, held by clips, then slides very slightly. Any slide, even of very small amplitude, brings about a deformation of the print, called double printing.
In wet offset, the reproduction of the screened images systematically undergoes an enlargement, of more or less great amplitude, qualified as “fattening”. Such “fattening” affects the quality of the print a great deal, and it weighs it down, particularly on cheap papers (papers which oblige the printers to work with fluid inks).
In waterless offset, the inks used are more solid. Consequently, the “fattening” is less. On the other hand, said solid inks tend to “draw” the material to be printed, particularly paper, and therefore can to be used only on papers of a certain quality.
Applicant, faced with the problems set forth hereinabove, has recommended lightening said print, particularly in order to reduce the “fattening” and to facilitate print, more precisely to create small, non-inkphilic surfaces in the inkphilic surfaces. This technique of lightening has been described in Patent Application FR-A-2 660 245. It has given good results particularly on matt and/or ordinary papers, but it is delicate to employ on an industrial scale, particularly on coated and/or smooth papers.
In fact, by using a screen 120 and a value of 5% for example for said lightening, there are generated in the inkphilic surfaces non-inkphilic surfaces of 2240 &mgr;m
2
. Such a lightening causes the disappearance, partly and even totally, of too large a number of the dots which constitute the inkphilic surface representing the document to be printed. Print is removed and this may be seen in the details. There is also a loss in density. This has proved unacceptable in too numerous works.
By using, for the same lightening of 5%, a much finer screen, a screen 600 for example, there are generated in the inkphilic surfaces non-inkphilic surfaces which are very small (88 &mgr;m
2
) and very numerous (about 25 times more). The inkphilic surface of the plate is thus riddled with these small surfaces which unfortunately render it fragile. Furthermore, it is very difficult to produce said tiny surfaces on an industrial scale at reasonable prices.
In general, the lightening thus carried out with a conventional screen—so-called amplitude-modulated screen—modifies the print too much, which is regrettable for numerous works. Moreover, precautions should be taken for said implementation in order to avoid the phenomenon of the moiré effect.
The implementation of the lightening, as recommended in Application FR-A-2 660 245 has therefore not, at the present time, given full satisfaction.
An original, particularly high-performance implementation thereof is proposed in accordance with the present invention. Such implementation employs a frequency-modulated screening or stochastic screening.
Such a screening does not use more or less large dot surfaces to reproduce the different values of shades, as is the case with conventional so-called amplitude-modulated screens, but a number of dots, all of the same surface, which varies for example from 1 to 100 to reproduce values of 1 to 100%. According to this screening, said dots of the same surface are distributed at random. In other words, with the frequency-modulated screening, all the dots have the same size, generally very small, but their number per surface zone (their frequency) varies depending on the value of the tones to be produced, and their distribution in space obeys a precise calculation of the assignment addresses (process of randomization). Unfortunately, such a distribution comprises the presence
Hamilton Cynthia
St. Onge Steward Johnston & Reens LLC
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