Process and device for evaluating the quality of a printing ink

Details Process and device for evaluating the quality of a printing ink Process and device for evaluating the quality of a printing ink

2002-04-11

2003-05-06

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Liquid analysis or analysis of the suspension of solids in a...

Viscosity

C073S061430, C073S061440, C073S862080, C073S073000

Reexamination Certificate

active

06557397

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains to a process and a device for evaluating the quality or the analysis of a printing ink, especially for evaluating the suitability of a printing ink being investigated for printing, especially offset printing.
BACKGROUND OF THE INVENTION
Besides the printing materials, the ink used as well as the behavior of the ink in conjunction with water greatly affect the printing result as well as the stability of the printing process running over a rather long time and under different operating conditions. A printing ink used must be able to take up water to a certain extent especially for offset printing.
The so-called Surland test, according to which 50 g of ink are stirred with 50 g of water, is known for determining the water uptake capacity of an ink. The excess water not taken up by the ink is poured off after 5 minutes. If the ink has taken up 10 to 15 g of water during these 5 minutes, it is considered to be good according to the Surland test, whereas the ink is considered to be unsuitable for offset printing if it has taken up more than 20 g of water.
FIG. 11
shows the moistening agent uptake behavior of different inks A through F used over time, where it can be seen that the inks A through D and F have taken up approximately equal quantities b of moistening agent after exposure to a water bath for the duration a of 5 minutes, so that the corresponding characteristics all pass through the equilibrium point P. The ink E shows a relatively poor water uptake behavior and is therefore unsuitable for offset printing.
Moreover, manual stirring tests are known for determining the saturation limit of the moistening agent uptake capacity of an ink; whether the amount of the stirring effort needed is great or small is assessed in these manual stirring tests by instinctive feeling only. In particular, it is not possible to predict whether a certain printing ink can indeed be used in offset printing or not.
A device for determining the emulsification behavior and the saturation limit of an ink is available commercially from the firm of Novocontrol under the name “Lithotronic Emulsification Tester.” This device has a container for an ink sample, into which extends a stirring device in order to stir an ink filled into it, and the torque occurring during the stirring is measured. According to the test procedure proposed by the manufacturer of the device, the ink sample is first stirred at 1,200 rpm until the torque measured becomes constant. Water is then added continuously at a rate of 1.5 to 3 g per minute. When the torque collapses, the saturation limit of the ink is reached, i.e., water added additionally is no longer taken up by the ink, and it thus permits an easier run of the testing device.
FIG. 12
shows a test result obtained with this device, where two different inks A and B were introduced into the device and stirred continuously for 5 minutes, so that the torque was constant at about 230 mNm and the temperature was constant at about 40° C. The continuous addition of water was started after 5 minutes. As can be seen from
FIG. 12
, the torque to be applied for stirring increases in the case of ink A for about 11 minutes after the beginning of the addition of water and then gradually collapses. The amount of water taken up by the ink sample can be determined from the amount of water added during this time, i.e., the saturation limit of the ink can be determined. The torque to be applied during stirring remains constant in the case of the ink B tested for about 4 minutes after the beginning of the addition of water and then decreases continuously. Thus, a saturation limit can also be determined for ink B, and it can be qualitatively read from the diagram that the water uptake capacity of ink B is lower than that of ink A. As another parameter, it can be read from the diagram in
FIG. 12
that ink A has an increasing wet viscosity, i.e., the viscosity increases during the addition of water, whereas ink B has a decreasing wet viscosity, i.e., it is becoming less viscous during the addition of water. Even though this ink testing process makes possible a certain standardization during the testing of different inks, the parameters determined, namely, the wet viscosity behavior and the saturation limit, are essentially unsuitable for making statements on whether an ink being tested leads to good or poor results in the printing process.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a process and a device with which parameters for evaluating the quality of an ink can be determined. In particular, processes and a device shall be provided according to the present invention which make it possible to make a statement on whether an ink is suitable or unsuitable for a printing process and to determine where problems will occur during the use of that ink.
According to the invention, a process is provided for evaluating the quality of a printing ink. A defined quantity of the printing ink is stirred. A force or energy applied during stirring or a torque applied is measured. A defined quantity of moistening agent is added to the ink at a defined time, preferably all at once.
According to another aspect of the invention, a process is provided for determining the moistening agent content in an ink sample. The saturation limit of a fresh ink is determined. A moistening agent is added to the ink sample during ongoing stirring. The point in time at which the force to be applied for stirring or the torque to be applied drops below a predetermined limit is measured. The moistening agent content is determined from the determined saturation limit of the fresh ink and the time period determined.
According to still a further aspect of the invention, a device for evaluating the quality of a printing ink is provided including a container for the printing ink, a stirring device, with which an ink filled into the container can be stirred, a measuring device for measuring the force to be applied by the stirring device or the torque to be applied and a moistening agent supply. A control actuates the moistening agent supply such that a preset quantity of moistening agent is introduced into the container at a defined point in time.
The present invention is based on the following discovery, which will be described below with reference to FIG.
13
.
FIG. 13
schematically shows the surface of a printing plate, where an ink-attracting, smooth area Z and a water-attractive, rough or porous area Y are shown next to one another. Water is first applied to the printing plate during the printing process prior to the printing operation proper, and this lies on the smooth area Z as a thin film of water and is taken up to a comparatively greater extent by the rough or porous area. An ink is subsequently applied to the printing plate. The ink takes up the thin film of water lying on the ink-attracting areas Z of the printing plate and is thus deposited on these areas. However, the rough area Y has taken up more water than the smooth area Z and acts as a reservoir of water because of its uneven surface finish or porosity. As a consequence, the ink applied to the water-attracting area Y cannot take up the total amount of water being stored, as a consequence of which it cannot be deposited on area Y of the printing ink and cannot be washed off from the printing plate in a subsequent step.
Since only a certain, defined time period is available during the printing operation, during which an ink applied must take up the film of water lying on the smooth layer Z, but the ink must not take up so much water that it will be deposited on the water-attracting, rough area Y, it was recognized by the inventor that to determine the quality of a printing ink, it must be determined, on the one hand, whether a printing ink does take up water too rapidly, because this ink would otherwise possibly also be deposited on the water-attracting area Y, and the initial water uptake also must not last too long, because the printing ink is otherwise unable to take up

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