2 Croda test ink
The use of ink coloration tests to study printing printability was not the beginning. The performance of various test inks has been described in detail in related literature [3], including commonly used K&N inks and Croda inks.
Croda ink consists of an insoluble pigment and a solvent (carrier), similar to an offset ink. The difference is that its pigment is colorless, while the solvent is red, and the paper absorbs this red solvent and displays color. This ink is significantly lower in viscosity than K&N inks. Croda ink produces a stain with a Q value [3] that increases abruptly with increasing contact time. The ink is a non-drying ink and the resulting stain is opaque and does not diffuse.
The rubbing method is used during the test. The thickness of the ink layer and the ambient temperature during the test may affect the results. Therefore, this test must be performed under the same conditions. Judging from the color of the paper after rubbing, the color of the paper after coating with Croda is marked red, and the unevenness of the paper absorption can be fully exhibited.
3 test methods
Reference paper ink coloring test method step [3]. Croda ink rubbing method is described as follows:
1 Cut the paper into squares and use scotch tape to stick both sides to the smooth glass and use adhesive tape to reattach the top edge. The tape is preferably about 1 cm wide and has a sufficient bonding area. When the two sides of the tape are pasted, neither the paper nor the tape can be wrinkled, so as not to cause the ink thickness to be inconsistent when the ink is scraped.
2 Add the appropriate amount of Croda ink to the upper tape.
3 Then quickly apply the ink to the paper from top to bottom with a bar (smooth glass rod or metal rod).
4 Using a stopwatch, wait for a certain time interval, use a cotton cloth or toilet paper to clean the ink on the paper. When erasing the ink, pay attention to evenness. Clean the ink as quickly as possible, and wipe the cotton cloth or toilet paper. Rub again to prevent the ink left on the cotton cloth or toilet paper from leaving unremovable marks on the paper (see Figure 3).
Figure 3 Croda coloring test
4 experimental process
A set of experiments was conducted using the coated paper on one side of the market and the coated paper on the other side without printing stains. The Croda test and commercial printing at different time intervals were performed at the same time (the test sample was omitted).
In the Croda ink test experiment, the time intervals of 15s, 45s, 1min, 30s, and 2min were used. The reason is that although the printing press is fast and the ink reaction interval is short, the Croda ink is non-dry and requires a long drying time. If you use a shorter time, it is harder to see the obvious difference. The purpose of this experiment is to test how short the optimal time is, the effect is obvious, and it is similar to the printing result.
Commercial printing adopts Roland 4-color offset printing machine, the printing speed is 10000 sheets/h, and the printing color sequence is black, magenta, cyan and yellow. There is a monochrome dot area for testing the Backtrap on the printed version. Since black is the first color, if a black spot appears on the screen area, the paper will inevitably have a Backtrap stain problem.
Test results are shown in Table 1
Table 1
Table 1 uses 0, 1, 2, 3, 4, and 5 to indicate the degree of stain, with 0 being very uniform, 5 being very non-uniform, and color spots being the most severe. The method of the stain evaluation was visually observed and judged in this experiment. At present, there is a set of software [5] in foreign countries. Combining with a computer with a scanner can digitize the degree of stain, and this assessment is more objective.
Finally, in order to find out the same points of the commercial printing and Croda ink test stains, the Croda test sample (1min30s) and the commercial printed stain area (red 70% screen area) were continuously observed by microscopy and photographed.
(Note: Since the photos and proofs are all in color, this article is printed in monochrome, and the colors of the photos and proofs are not well represented. Please ignore them here. Please understand.)
5 Discussion and analysis of experimental results
5.1 Commercial printing and Croda testing both showed excellent spots on the A side of the paper, and the B side appeared uniform. This shows that the Croda ink stain is the same as the commercial printing spot.
5.2 In the Croda ink test, no noticeable color spots were observed during the short time interval. As the interval time became longer, the color spots became more and more obvious, and the shape and size of the stain did not change much with time. This verifies that the test results of the Croda ink are non-diffusing and that the 1.5-min interval is appropriate for routine inspection of coated paper.
5.3 The shape of the Croda test patch is similar to the shape of a commercial print test specimen.
5.4 Photographs of commercial printing proofs were taken under the same conditions, but the dark areas were dark in color, and the light areas in the shallow areas were light in color, indicating that the occurrence of Backtrap stains was not due to the incompleteness of the halftones, but due to the color of the halftone dots. Depth (the thickness of the ink layer) caused by.
5.5 The micrographs of the dark and light spots on the Croda test proofs and commercial printing show the same pattern. The darker areas of the ink have less coverage than the shallow areas and more fibers can be seen. This conclusion is related to the previous The thickness of the coating affects the depth of the ink [2]. Careful analysis revealed that where the offset color is dark (thickness of the ink layer), the solvent must absorb more, and finally the fixed thickness of the colored pigment on the coating is thick. Here, the sealability and coverage of the coating are not good. To more fibers. Although Croda ink is solvent colored, it is also a coating sealer, where the coverage is poor and where there is a lot of red solvent, so the color is deep and more fibers can be seen.
5.6 In commercial printing, stains are also present in the two-colored areas of the A surface. This stain is also a Backtrap stain. Because on this side, a single color area is blooming, it is not an overprint stain. In multi-color printing areas, Backtrap stains may occur as long as the color is printed (rubber cloth) later.
6 Conclusion
After many commercial printing and contrast tests with Croda ink, the same rules were found, indicating that the Croda ink test can reflect the paper Backtrap stain. The more flower-shaped Croda test results are, the more severe the print spots are. Making full use of Croda inks allows coating manufacturers to quickly determine the problem so that improvements can be made as soon as possible.
Thanks to Mr. Du Haitao for his valuable suggestions on the revision of this article.
Source: Suzhou Zixing Paper Co., Ltd.