Fourth, the determination of BACK TRAP stains At present, for the coating of paper BACK TRAP stains commonly used laboratory test IGT test method, VANDERCOOK printing test method, CRODA ink test method.
The IGT test method uses an IGT printability tester to transfer a layer of ink to a paper sample. After a specific time, the paper is rolled on the paper with a special pressure roller 4 times. Then the ink on the paper surface is observed. degree. If the paper absorbs ink unevenly, the amount of ink sticking off by the rubber roller will also be uneven, and a noticeable color spot will appear on the paper surface. This measurement method uses a rubber roller to simulate the blanket during the printing process.
The instrument used in the VANDERCOOK print test method is the VANDERCOOK printability tester. The method is to first print a low-viscosity ink on the paper sample. After a certain period of time, the ink is printed on the high-viscosity ink again to observe the printing result. The color ink has a high viscosity, so that part of the first color ink is peeled off by the second color ink. If the paper absorbs the ink of the first color unevenly, the amount of peeled off is also uneven, and a stain appears on the surface of the paper. This method of measurement uses high-viscosity inks to simulate blankets during printing.
The CRODA test method is relatively simple. A red ink called “CRODA” is applied to a paper pattern. After a certain period of time, the ink on the paper surface is wiped off with a cloth. If the paper absorbs the ink unevenly, then the paper The color of the face is different. CRODA ink is a kind of non-drying solvent ink. Its color is produced by red solvent, so it will appear red as long as the solvent penetrates. The place where the ink penetrates is deep in color; the place where the ink penetrates is light in color. This does not directly reflect the situation of the BACK TRAP stain, but only the degree of uniformity of the absorption of the coated paper. Whether or not the coated paper is absorbed uniformly is a determinant factor for the generation of BACK TRAP pigmentation. Therefore, by observing the depth distribution of the color after the CRODA test, the extent of BACK TRAP pigmentation can be judged interveningly.
The above-mentioned laboratory measurement of BACK TRAP reflects only the possibility of appearance of BACK TRAP stains, and is a test performed under a sniffing ideal condition. In fact, whether a BACK TRAP stain on a defective product is serious is affected by factors such as the uniformity of paper absorption, but also by factors such as printing conditions and screen composition. For example, if there is a similar degree of BACK TRAP stain. In the next, prints with a single screen color are more prone to problems than colorful prints on the screen. The cyan and magenta two colors are more prone to problems than yellow due to the strong visual effects. (This is not only applicable to the BACK TRAP color. Spots also apply to other stains). In addition, the ever-changing printing conditions also affect the severity of the BACK TRAP stain. Therefore, even if the laboratory test results are not satisfactory, it does not mean that the effect of BACK TRAP stains cannot be reduced. Below we discuss how to reduce the effect of BACK TRAP pigmentation by changing the printing conditions.
5. How to reduce the effect of BACK TRAP stain If you find that BACK TRAP stains appear on the coated paper during the printing process or the possibility of this problem is found through experiments, how can you improve by changing the printing conditions? In general, the following methods can be used:
1. Strictly control the amount of water and ink In the premise of ensuring that the ink printing density meets the requirements and the color reproduction is good, the least amount of water and ink is used. When the amount of ink is reduced, the time needed to dry the ink is shortened accordingly, and the amount of ink sticked by the next color blanket is accordingly reduced, and the severity of the BACK TRAP stain is reduced. At the same time, reducing the amount of water and ink also helps improve overall print quality.
2. Reduce printing pressure Under the premise of ensuring ink printing density and color reducibility, try to use less printing pressure to reduce the amount of BACK TRAP stain. Since the surface of the coated paper is mostly smooth, the appropriate reduction of the printing pressure does not have too much effect on the reduction of dot and color. The smoother the paper surface, the greater the pressure can be reduced.
3, improve the printing speed In the study, we found that, while maintaining the same other conditions, with the increase in printing speed, BACK TRAP stain severity is gradually reduced.
Based on this phenomenon, we speculated that the difference in ink absorbency caused by the uneven structure of the paper coating after the ink was transferred from the blanket to the paper surface was gradually enlarged over time. In other words, after the ink is transferred to the paper surface, in a very short period of time, the difference in the absorption of the ink on each point of the paper surface is small, and the viscosity of the ink on each point on the paper surface is approximately equal. At this time, the BACK of the blanket is affected. With TRAP, the amount of stripping is also essentially the same everywhere. Only with the increase of time, the differences in the absorption of paper points gradually increased. At this time, after the BACK TRAP of the blanket, the peeling amount will show a relatively large difference at each point on the paper. Therefore, the severity of the appearance of the BACK TRAP color patches will gradually decrease with the printing speed. At the same time, to improve the printing speed, the evenness of the ink roller is also improved, which also helps to improve the printing quality.
In the CRODA test of the paper pattern shown in FIG. 5, the results obtained by changing the dwell time of the ink on the paper cloth exhibited similar trends as those of the printing experiment described above. If this inference is correct, then the satellite-type, semi-satellite-type offset press will greatly reduce the generation of BACK TRAP color patches.
Since the increase in printing speed is limited by the conditions of the equipment and the conditions of the paper samples, the effect of this method in actual operations will vary depending on the paper and equipment and should be carefully selected.
Since the last color will not be affected by the BACK TRAP, if a BACK TRAP color spot is found to be particularly severe during printing, and other methods are ineffective to eliminate it, you may consider putting the color to the last printing. Usually this method can effectively eliminate the color BACK TRAP stain.
For example, when a print is printed in the KMCY color sequence, a severe BACK TRAP stain appears in the C color. At this point, the C color can be placed on the final print, ie printed in KMYC color sequence.
When printing with this method, care should be taken to determine whether or not the color sequence will affect the normal overprint of each color ink. If there are adverse effects, continuous multi-color printing can be divided into two. Although this is a little troublesome, it can completely solve the problem of BACK TRAP.
The problem described in the above example can also be used to print KMY first, then print it again after the ink is dry.
For coated paper with BACK TRAP pigmentation problem, the above measures can generally solve the problem.
VI. Concluding remarks In the multicolor offset printing process of coated paper, BACK TRAP is inevitable as long as it is wet-press wet printing and the blanket is used. However, BACK TRAP does not necessarily produce BACK TRAP stains. The BACK TRAP stain appears only when the coated paper shows a large inhomogeneity in the ink communication. Through certain experimental methods, we can predict the severity of BACK TRAP stains. In printing, we can achieve the goal of reducing the effect of BACK TRAP pigmentation by changing the printing conditions.

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