Wilco de Groot
During the printing process, a thin ink film is applied on the paper. Under normal circumstances the printer wants as thin as possible film of ink, not only for reasons of cost but also for technical reasons. The thicker the total ink film, which can be several µm if four-color trap is used, the less stable the process and the longer the drying time of the sheet or the higher the drying energy.
The ink penetrates normally slowly but steady into the paper. Before the ink is fully penetrated, the second color is printed on top and splitting of the second ink takes place on top of the wet ink. In this case a good trapping can be reached if the inks are printed in the right tack sequence and there is little chance of picking.
If however the ink has penetrated already completely and the paper surface has been weakened by the liquids in the ink, the tack of the second ink may cause damage to the paper coating such as pick. This can be tested with the multi-pass pick test which is described in a previous article.
Because there are different stages in the printing process with a shorter or longer time between the print and the next process step, there are different methods to test. Also is there a difference between the methods used by the paper industry and the ink industry.
The time differences between process steps in printing can vary from 20 ms up to 2.5 s, depending on the process (from newsprint to business forms). Or from seconds to hours (or even days) for the post press handling.
The process simulation on a printability tester can be done to conform with the different applications: very short time, 15 ms up to 30 s, intermediate times from 3 s to 1000 s and long times up to 6 hours.
The way it is tested is always the set-off method. In this method, a print is made on the paper to be tested with a well known ink film. The ink is allowed to penetrate the selected time and a counter print is made. On the counter print, the ink which is printed but is not dry is set-off, and an ink density can be seen and measured depending on the time and the setting behavior of the ink.
Except for the time variable, for different applications there are different combinations of settings and materials possible or required. The ink industry uses normally a reference (optical) density for the ink to be tested; this density is the maximum or normal density which is used in the printing process for this particular ink. Because the paper type is not known, it is tested on a reference paper type, e.g. APCO.
The paper industry on the contrary does not know which ink is used in practice and uses a reference ink. Also, the counter paper (set-off paper) varies in accordance with the application. For ink testing it is most times APCO again, for paper testing is the back of the same paper type as the paper to be tested. For comparison of different materials, it is always a reference paper like APCO.
Test Description and Advantages of Modern Methods
As can be seen from all the options and variable above, most users of these tests will have to do the tests a number of times to fulfil the needs of all applications of their materials. They have to know the short time behavior as well as the long time behavior, compared to the same paper and to reference materials.
Until recently, the tests would have to be done a great number of times to get a reasonable picture of the setting process from 1 up to 300 s. In particular, in research and trouble-shooting, this was a very time-consuming job. With the new developments on the market, the user now has the chance to perform the test with four or 10 steps of increasing time intervals, resulting in a single measurement from which a full setting curve can be derived.
If the test, as recommended, is repeated three times, there is a time gain of several hours compared to the previous methods, where only a limited number of time intervals were tested to reduce time. An example: a common test runs up to 300 s, the shortest interval is 10s. On the old-fashioned way this takes three times four tests, 10, 30, 60, 300 s, each test takes two minutes plus the time of the interval, so a minimum time of at least 45 minutes with 12 times cleaning the inking system. In the new way, this takes a maximum of 25 minutes with only three cleaning sequences. To determine the full 10 points curve on the older method would take at least 2.5 hours and 30 cleaning sequences.
Once the paper or ink characteristic setting curve is determined, another time reduction can be realized by checking only the four critical times of the curve.
Ing. Wilco de Groot, MBA, is managing director of IGT Testing Systems. After working some years in other industries, he started in 1985 at IGT, where he worked in R&D until becoming managing director in 2002. He is member of a number of organizations, convener in ISO and Tappi working groups and expert in a number of other WGs. Mr. de Groot studied computer science, and finished his MBA in 1995.