When inks are used on a two-colour offset printing press the second ink film is printed over the first ink film while it is still completely wet. For multi colour presses the same can be said for the other colours. In order to do so the inks have to be trapped on tack and/or viscosity. The systems for measuring tack and/or viscosity measure these ones as it is in the tin, without the influence of absorption by the paper.
On the printing press, as soon as the ink is printed on the paper the low viscosity components of the ink will penetrate into the paper and the viscosity and tack of the ink will increase rapidly. The longer the ink is on the paper, the bigger this increase of viscosity and tack will be. The time interval between individual prints differs considerably in various types of presses. In practice these interval times are between about 0.03 and 3 s.
For proper adjustment of ink for wet-on-wet printing a printability tester is essential, operating from very short time intervals upwards. Although in practice water is used and an emulsion is formed, this method gives very good results for testing wet-on-wet printing. The method can be used to test inks and paper as well. There are 2 methods:
W46 for a printing form with coated rubber of 85 Shore A for smooth papers.
W69 for a printing form with coated rubber of 65 Shore A for smooth and rather rough papers
Principle
A set of two colours of offset inks is printed on each other on paper in both the colour sequences and with different interval times. The results are observed visually or with the help of a densitometer. The best print quality and the best ink transfer show the right colour sequence. Also for a four colour series of offset inks the inks always must be tested in sets of two colours.
With the AIC2-5T2000 the tests are carried out with 2 interval times on one strip. With the Global Standard Tester it is possible to make the trials with 2, 4 or 10 different interval times on one test strip. With the settings for 2 interval times it is possible to work with the very short interval times. In most of the cases the short interval times of 0.1, 1 and/or 3 s will be used. This can be realized in the mode of 2 fields at 1 m/s. If the test is used to test inks, it is advised to use the art paper, IGT code Ka APCO II/II.
Method of operation
• It is recommended to execute the test in the standard atmosphere; to most standards it is 23.0 ± 1.0 °C (73.4 ± 1.8 °F) and 50 ± 2% rh.
• For the operation of the AIC2-5T2000, Global Standard Tester, High Speed Inking Unit 4 and ink pipette follow the instructions of the manuals, IGT information leaflet W100 and the displays accurately.
• Handle the samples carefully.
Assessment
VISUALLY
1. Judge the differences between the prints for different interval times of both colour sequences.
1.1 The colour sequence with the smallest difference between the prints of different interval times is the best colour sequence.
DENSITROMETRICALLY
2.1. After > 4 hours after making the print measure the contrast density of all parts (both single colours and both colours) of the strips with the densitometer set for the 2nd colour. NOTE: Calculate the average of the measured densities per part.
2.2. Calculate the % of ink transfer with the formula:
D(1+2) – D1 F = ------------------ x 100 % D2
In which: D(1+2) = density of both colours together D1 = density of 1st colour D2 = density of 2nd colour
2.3. The highest % of ink transfer is the best colour sequence.
(Above) wet-in-wet prints with 2, 4 and 10 fields
