Chemistry: analytical and immunological testing – Condition responsive control
Reexamination Certificate
1998-08-31
2001-02-27
Soderquist, Arlen (Department: 1743)
Chemistry: analytical and immunological testing
Condition responsive control
C073S597000, C422S062000, C436S040000, C436S050000, C436S131000, C436S149000, C436S150000, C436S183000
Reexamination Certificate
active
06194215
ABSTRACT:
The invention relates to a method and a device for continuously measuring and regulating the composition of a dampening medium solution for offset printing, the said solution consisting of water and of at least one organic solvent miscible with water, such as, in particular, short-chain alcohols, glycols or glycol ethers.
In the offset printing method, the printing form consists of a thin metal plate which is covered with a photosensitive layer. This layer may be hardened according to the particular image, so that a flat printing form is obtained. The printing and nonprinting parts lie virtually in one plane and differ only in their wetting capacity. The nonprinting hydrophilic regions can be wetted particularly well with water or a mixture of water and water-soluble organic solvents, the dampening medium solution, and very poorly with oily fluids. Preferably, 2-propanol is used as the main organic component in these dampening medium solutions in concentrations of between 3 and 15 percent by volume.
During the printing operation, the dampening medium solution is transported out of a dampening unit as a thin liquid film onto the printing plate. The nonprinting regions of the printing form take up some of the dampening medium solution there, whereas, on the oleophilic printing regions of the printing form, the dampening medium solution is repelled and therefore remains on the dampening applicator rollers and, in the dampening medium fountain of the printing machine, is mixed again with the dampening medium solution located there.
The dampening medium solution therefore has to be constantly supplemented by an amount corresponding to the quantity taken up by the printing plate. This supplementing of the volume is carried out, by means of a dampening medium treatment system. In this system, the dampening medium solution is cooled, supplemented in its volume and recirculated again to the dampening medium fountain. In addition, this system has the task of supplementing the fraction of 2-propanol in the dampening medium solution, the content of 2-propanol decreasing during the continuously running printing operation on account of its rate of evaporation being markedly higher than that of water. The metering of 2-propanol during the treatment of the dampening medium solution is usually controlled by checking the density of the dampening medium solution and by a metering means connected to the density checking means. A system of this kind is known, for example, from DE-A 41 36 263.
Controlling the composition of the dampening medium solution by means of a density check leads, in practice, to pronounced differences between the measured or regulated value and the actual content in the solution. Deviations of 50% or greater are not exceptional. The reason for this is the slight variation in the absolute density values in the concentration range which is relevant for practical use. Moreover, falsifications of the readings due to dirt on the buoyancy bodies cannot be entirely ruled out. Tests to minimize this problem by enlarging the buoyancy bodies have led to an improvement, but have not been able to compensate the basic weaknesses of the method. Density variations basically remain highly susceptible to the infiltration of salt from the additives and papers used and to temperature fluctuations. These influences are, known and have been accepted in the past. In the case of a 2-propanol content of 12 to 15%, they have constituted a high, but nevertheless acceptable measured value deviation.
Printing studies in recent years have shown, however, that it is possible, without restricting production reliability, to print with a 2-propanol content of 7-8%. Specially designed dampening units in combination with optimized additives have made it possible even to use markedly lower 2-propanol contents. In these concentration ranges, the basically unavoidable measured value deviations in the density check are no longer acceptable.
Alternatively to the density check, a series of other techniques are described, which make it possible to check the 2-propanol content. Such known methods are, for example, the measurements of refractive index, viscosity and conductivity. Moreover, EP-A-0,159,412 describes measuring the surface tension in order to determine the 2-propanol content. Another technique for accurately determining the 2-propanol content is spectral absorption, such as is known, for example, from DE-A-43 24 141. In DE-A-43 40 026, the measurement of the concentration of 2-propanol is carried out in the gas phase.
None of these methods has proved successful in practical uses. In some cases, the corresponding sensor technology was not sufficiently robust in light of the soiling occurring in practice. In other cases, the production of the sensors and of the associated control units incurred costs which were too high.
The object on which the invention is based is, therefore, to specify a method and a device, as a result of which particularly accurate measurement and regulation of the composition of dampening medium solutions for offset printing are ensured.
This object is achieved by means of the features of the various embodiments of the present invention.
According to the invention, the sound propagation velocity in the dampening medium solution is measured and a regulating signal is formed from this, in order to keep the composition of the dampening medium solution constant.
The tests on which the invention is based showed that measuring the sound velocity makes it possible to detect changes in concentration of less than 0.1% with reproducible way. Moreover, the percentage change in the measurement signal when the sound velocity is measured is higher by a multiple of that obtained using density measurement.
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Dietzel Stephan
Rauh Wolfgang
Baldwin Grafotec GmbH
Ostrolenk Faber Gerb & Soffen, LLP
Soderquist Arlen
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