BACK TRAP stain in coated paper printing

For multi-color offset printing of coated paper, printing stains (MOTTLE, commonly known as "printing," "printing," "ink misrepresentation," etc.) are one of the most common problems that arise today. A major topic for research in the international papermaking and printing industries. This article will initially discuss the causes of the stain, but its solution still needs further study.

First, the type of printing stains

Printing stains usually refer to the appearance of unusual, dark, uneven spots or streaks on the print. It is usually arranged in an irregular manner.煞治韵? categories:

1, BACK TRAP MOTTLE (BTM stains)

It refers to the color spot produced by the peeling of the blanket (ie, BACK TRAP) in the subsequent color group in the multi-color printing. This blemishes will be the focus of this article. Due to the fact that no Chinese translation has been found in China, the divestment phenomenon, BACK TRAP, will not be translated in this article to avoid confusion.

2. WATER INTERFERENCE MOTTLE (WIM stains)

It contains two kinds of stains, one is the "wet repellent" stain, which appears in the multi-color printing of coated paper. The reason for this is that the surface coating of the coated paper is too closed and the dampening fluid cannot fully respond. , resulting in the next color of ink can not be divided into color space to form a stain; the other kind of stain is caused by the problem of fountain solution (such as fountain solution is too much, fountain solution is not properly configured, etc.) The appearance of this type of WIM is not limited by the type of paper and can occur in a single color.

3, WET INK TRAP MOTTLE (WTM stains)

Domestic data is generally referred to as “reverse overprint” or “mixed color”. In multi-color printing, since the viscosity of the latter color ink is greater than the viscosity of the previous color ink, the ink of the previous color is stuck by the ink of the latter color. Spots. The emergence of WTM is not limited by the type of paper.

In addition, if the paper surface is too rough to cause uneven ink transfer during printing, stains may also be formed. This stain is called "ConTACT MOTTLINT". With the advancement of papermaking technology, this phenomenon has rarely happened.

At present, the domestic printing industry generally attaches great importance to anti-overprinting and ink-water balance. The papermaking industry always pays more attention to wet repulsion and has made a certain level of in-depth research. The national standard for coated paper has also made relevant provisions on wet exclusion. For BACK TRAP pigmentation, there have been no reports of in-depth studies in the country.

Second, the formation of BACK TRAP stains

BACK TRAP is a unique phenomenon in the printing process of multi-color offset printing presses (especially models with more than 4 colors). For multi-color offset presses, the BACK TRAP phenomenon is inevitable as long as it is a wet-press print and a blanket exists.

In the modern multi-color printing process, with the increase of printing speed, the time for the paper to pass between adjacent two color groups is as long as 1-2 seconds, and the total time for passing through several color groups is only 2-3 seconds. In such a short period of time, the ink is not dry. While printing, the blanket has a certain pressure applied to the paper. Therefore, during the printing process, when the paper is moved from the previous color group to the subsequent color group, the blanket and the sheet of the subsequent color group are replaced with each other. In addition to the transfer of ink ink and fountain solution to the paper surface, there is also the possibility that the blanket of the subsequent color group sticks off the part of the ink printed on the front color group, thus creating a BACK TRAP phenomenon. For blankets of the subsequent color group, any blank areas on the blanket may peel off the unbleached ink on the paper. This phenomenon does not occur when blankets are not used during dry printing or printing.

In the printing process of the 4-color machine or the 6-color machine, after carefully observing the blank areas of the blankets of the following color groups, it can be seen that more or less of the patterns on the preceding color groups are transferred. This is because the blanket of the subsequent color group causes a BACK TRAP effect on the ink transferred from the front color group to the paper.

The BACK TRAP stain is a product of BACK TRAP that does not occur uniformly on paper. If BACK TRAP occurs evenly, there will be no adverse effect on the printed product. The appearance of BACK TRAP stains is different shades of print. Usually occurs easily in a single color, such as a large area of ​​gray, blue, red or color overlay area on the print. Figure 3 shows an enlargement of the BACK TRAP stain in 70% magenta.

BACK TRAP stains are usually produced only on coated paper, but not on offset paper. This is due to the fact that the paper structure of the offset paper is rough and loose, and its ink absorption is good. In addition, its printing pressure is usually larger. Therefore, when entering the jaw group, the ink (including ink particles and binders) of the front color is printed. ) A considerable portion has already penetrated into the paper. Therefore, even if uneven BACK TRAP occurs, it will not affect the color depth of each point on the paper. In addition, the double offset printing itself is rough and it is impossible to use it for fine prints. Therefore, the impact of BACK TRAP on offset paper is negligible.

In the case of coated papers, since most of the coated papers are now coated with multiple coats and subjected to super light treatment, ultra-fine calcium carbonate and china clay are used as pigments in paints, and synthetic latexes are used as adhesives, so coatings are compared. Closed, which leads to slower penetration of the binder in the coating after printing, a large number of pigment particles and binders are regulated on the surface of the paper. A closer coating on the surface of the paper can, of course, produce a higher gloss of printing, but on the other hand, when the paper enters the latter color group from the former color group, BACK TRAP will inevitably occur in a large area. In addition, because the coated paper surface is smooth and the blanket is in uniform contact with the paper surface, the BACK TRAP phenomenon may occur in the subsequent color group in any multi-color printing.

Why do some coated papers produce uneven BACK TRAP, which in turn causes the generation of BACK TRAP stains? This is because the root cause of the BACK TRAP stain is the uneven absorption of the coated paper. If the coating absorbs the ink evenly, then the blanket will peel off the ink on the paper very evenly and will not have any adverse effect on the effect of the print screen, thus printing a perfect print. However, if the coating absorbs ink unevenly, it will result in uneven ink drying speed on the paper surface. When entering the succeeding color group, the ink is peeled off by the blanket in a slow drying area more than in the fast drying area (see Fig. 2). This difference in quantity may be due to a stripping action, or it may be due to multiple stripping effects. When the difference in the amount of peeling was so large that it could be observed by the human eye, there was a case where the printing color was different, that is, the BACK TRAP color was generated. In the area where the BACK TRAP pigmentation occurs, lighter areas indicate poor paper absorption performance and darker areas indicate better paper absorption performance.

It should be noted that when using offset fast-drying ink, the amount of stripping of the ink on the paper surface of the blanket relative to the amount of ink remaining on the surface of the paper is relatively small, and these trace amounts of ink transferred to the blanket have not yet been found to be printed. It has caused obvious effects. Practice has proved that under the normal printing conditions, it is impossible to judge the degree of BACK TRAP from the shape of the dot. Even in the presence of BACK TRAP stains, it is now macroscopic. At this time, even if the blanket has more or less stripping of the ink, the volume of ink remaining on the paper is still only a small part. There are no obvious differences in the shades of the different areas in the shape of the dots, but some of the more serious BACK TRAP spots may have different shades of dots. The reason why these dots can be kept intact is probably due to the fact that during the time that the paper entered the printing group from the last printing group, many contraceptives and pigment particles have penetrated into the coating of the paper. Therefore, even if it is strongly affected by the blanket, it is impossible for the dot to break.

The reason for the uneven absorption of coated paper is very complicated. Some people think that due to various factors in the coating process, adhesives and other substances in the coated paper coating are unevenly distributed, which eventually leads to the absorption of the ink by the coating. The unevenness; there are also views that the coating thickness affects the ink absorption. At present, the international papermaking industry is still deeply studying this topic.

Third, the identification of BACK TRAP stains

In the printing, if there is a printing class, how do you determine the cause of it? For the several stains mentioned at the beginning of this article, the stains caused by excessive fountain solution and reversed overprinting are well-known and have accumulated rich experience in dealing with such problems. For BACK TRAP stains and wet exclusion colors Because of its many similarities in macroscopic performance, speckles are not well distinguished. However, they still have some differences in the following aspects. Due to the fact that many domestic four-color offset presses are used for printing, we will discuss the KMCY color sequence of four-color machines below.

1, surface form

The appearance of BACK TRAP stains is macroscopic and usually appears on a single page with a large area of ​​color, but no obvious difference is observed in the microscopic dot patterns.

For wet-repellent stains, the reason for this is that the fountain solution of the previous color hinders the transfer of the ink of the next color. Therefore, when wet repulsion occurs, not only macroscopic large-area flowers but also microscopic appearances appear on the printed samples. Look at outlets are usually incomplete.

2, the order of appearance of stains

Since the BACK TRAP stain is caused by the fact that the ink on the paper surface is peeled off by the subsequent color group blanket, it is possible that from the first color to the penultimate color, that is, K, M, and C appear to be BLACK TRAP stains unfavorably. If black spots appear on the black screen during the actual printing process, you should pay special attention to whether it is a BACK TRAP spot. However, because of the final printing of yellow, BACK TRAP spots cannot occur.

For wet repellency, its appearance is ...C. Y three colors appear, and the first color K side can not appear wet repulsion.

3. Occurrence area

All areas on the paper that have been transferred to the text, except for the dots that are superimposed on the ink in the sequence, may be affected by BACK TRAP. As long as there is a large degree of non-uniform absorption of the ink by the coating, BACK TRAP spots may appear.

For wet repulsion, it can only occur in the space of the previous color. The magenta stain due to wet repellency is the area on the black plate with fountain solution. If superimposed on the previous solid, no wet repellent stains will appear. $Page break $

Fourth, the determination of BACK TRAP stains

At present, the IGT test method, the VANDERCOOK print test method, the CRODA ink test method, etc. are commonly used for the laboratory determination of coating BACK TRAP stains.

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 non-drying solvent type ink, its color is