Backtrap stains in the printing of the hottest coa

Back trap stain in coated paper printing

for multicolor offset printing of coated paper, printing stain (mottle, which is usually said to be "printing blooming", "printing speckle", "false ink", etc.) is one of the most common problems, and it is also a major research topic in the international paper industry and printing industry. In this paper, the causes of color spots will be preliminarily discussed, but the solutions still need to be further studied

I. type of printing spots

printing spots usually refer to spots or streaks with abnormal color and uneven depth on a print. It is usually arranged irregularly, and it is not easy to get rid of the "R" words ǔ？ Summary? Class:

1. Back trap mottle (BTM stain)

it refers to the stain produced by the stripping of the blanket (i.e. back trap) of the ink of the previous color of the print in the subsequent color groups in multicolor printing. This stain will be the focus of this article. Since there is no appropriate Chinese translation in China, in order to avoid confusion, this stripping phenomenon - back trap will not be translated in this article

2. Water interaction mottle (WIM stain)

it contains two kinds of color spots. One is "wet exclusion" color spots, which appear in the multicolor printing of coated paper and align with the center. The reason is that the coating on the surface of coated paper is too closed to fully respond to the fountain solution, so that the ink of the next color cannot be separated and transferred to the blank area of coloring to form color spots; Another kind of stain is caused by the problem of fountain solution (such as too much fountain solution, incorrect fountain solution allocation, etc.). The occurrence of this kind of WIM is not limited by the paper grade, and can appear on monochrome

3. Wet ink trap mottle (WTM stain)

it is generally referred to as "reverse overprint" or "color mixing" in domestic data. In multi-color printing, because the viscosity of the latter color ink is greater than that of the previous color ink, the ink of the former color is stuck off by the one color ink of unreasonable customers. The appearance of WTM is not limited by paper grade

in addition, if the paper surface is too rough, resulting in uneven ink transfer during printing, color spots will also be formed. This kind of stain is called "contact mottling". With the progress of papermaking technology, this phenomenon has rarely occurred

at present, the domestic printing industry generally pays attention to reverse overprint and ink balance, while the papermaking industry always pays more attention to wet rejection, and has made a certain depth of research. The national standard of coated paper has also made relevant provisions on the problem of wet rejection. However, there is no report of in-depth research on back trap stain in China

II. Causes of back trap color spots

back trap is a unique phenomenon in the printing process of multi-color offset printing machines (especially models with more than 4 colors). For multi-color offset press, as long as it is wet press printing and there is blanket, the back trap phenomenon is inevitable

in the process of modern multicolor printing, with the increase of printing speed, the longest time for paper to pass between two adjacent color groups is only 1-2 seconds, and the total time for some to pass through several color groups is only 2-3 seconds. In such a short time, ink is impossible to dry show. During printing, the blanket has a certain pressure on the paper. Therefore, during the printing process, when the paper enters the back color group from the previous color group, in addition to the phenomenon that the ink and fountain solution transfer to the paper surface between the blanket and the paper in the back color group, there is also the possibility that the blanket in the back color group will stick off the ink printed on the front color group, which leads to the back trap phenomenon. For the blanket of the back color group, any blank area on the blanket may peel off the wet ink on the paper. This phenomenon will not occur if rubber blanket is not used in dry printing or printing process

during the printing process of the 4-color machine or the 6-color machine, carefully observe the blank area of the blanket of the following color groups, and you will find that there are more or less residual patterns on the front color groups. This is because the blanket of the subsequent color deck has a back trap effect on the ink transferred from the previous color deck to the paper

back trap stain is the product of back trap occurring unevenly on the paper to a certain extent. If the back trap occurs evenly, it will not have a negative impact on the printing. The appearance of back trap color spots is that the printing image is different in depth. It is usually easy to occur in a single color, such as a large area of gray, blue, red or color superimposed areas on a print. Figure 3 is an enlarged view of the back trap stain in the 70% magenta area

back trap stains usually occur only on coated paper, but are not easy to occur on double offset paper. This is because the paper structure of the double offset paper is relatively rough and loose, the absorption of ink is good, and its printing pressure is usually large. Therefore, when entering the lower group, a considerable part of the ink printed by the front color (including ink particles and binder) has penetrated into the paper. Therefore, even if uneven back trap occurs, it will not have a great impact on the color depth of each point on the paper. In addition, double offset printing itself is relatively rough and cannot be used for fine printing, so the impact of back trap on double offset paper can be ignored

for coated paper, because most of the coated paper is coated for many times and subjected to super light capturing treatment, ultrafine calcium carbonate and porcelain clay are widely used as pigments in the coating, and synthetic latex is used as adhesive, so the coating is relatively closed, resulting in the slow penetration of binder in the coating after printing, and a large number of pigment particles and binder are regulated on the surface of the paper. Although the relatively closed coating on the surface of the paper can produce high printing gloss, on the other hand, when the paper moves from the former color group to the latter color group, back trap will inevitably occur in a large area. Moreover, due to the smooth surface of coated paper and the uniform contact between blanket and paper, all inking areas of the previous color in multi-color printing may have a back trap phenomenon in the subsequent color groups

why do some coated papers produce uneven back trap, which leads to back trap stains? This is because the root cause of back trap stain is the uneven absorption of coated paper. If the absorption of ink by the coating is very uniform, the peeling of ink on the paper by the blanket is also very uniform, which will not have any adverse effect on the effect of the print screen, so as to print a perfect print. However, if the coating absorbs the ink unevenly, it will lead to the uneven drying speed of the ink on the paper. When entering the subsequent color decks, the amount of ink stripped by the blanket in the slow drying area is more than that in the fast drying area (see Figure 2). This difference in quantity may be caused by one stripping operation or by the accumulation of multiple stripping operations. When the difference in peeling amount is large enough to be observed by human eyes, there is a situation of different printing colors, that is to say, back trap color spots occur. In the area where the back trap stain occurs, the light color area indicates that the paper absorption performance is poor, and the dark area indicates that the paper absorption performance is good

it should be noted that when using offset quick drying ink, the peeling amount of paper ink from the blanket is relatively small compared with the amount of ink remaining on the paper. These trace inks transferred to the blanket have not been found to have a significant impact on printing. Practice has proved that under normal printing conditions, it is impossible to judge the Backtrap degree from the shape of the point. Even when back trap stains appear, their appearance is also macroscopic. At this time, although the rubber blanket peels off the ink, the amount of ink left on the paper is only a small part. From the point morphology, there is no obvious difference in the depth of different areas, but the depth of some serious back trap stains may be different. The reason why these points can remain intact may be that during the period from the last printing color group to the first printing color group, many contraceptive binders and pigment particles have penetrated into the coating of the paper to varying degrees. Therefore, even under the strong effect of the blanket, point breakage is impossible

the reasons for uneven absorption of coated paper are very complex. It is believed that genomatica company can flexibly use various raw materials in the coating process, which makes the uneven distribution of adhesives and other substances in the coating of coated paper, and finally leads to the uneven absorption of ink by the coating; It is also believed that the coating thickness affects the absorption of ink. At present, the international paper industry is still in-depth research on this topic

III. identification of back trap color spots

in printing, if there is a printing color shift, how to determine its cause? For the several color spots mentioned at the beginning of this article, people are familiar with the color spots caused by too much fountain solution and reverse overprint, and have accumulated rich experience in dealing with such problems. For back trap color spots and wet rejection color spots, because they have many similarities in macro performance, they are not very easy to distinguish. However, there are still some differences in the following and aspects. Since four-color offset printing machines are mostly used in China, we will discuss the k-m-c-y color sequence of four-color machines below

1. Surface form

back trap color spots are macroscopic, usually appear on large-area layouts with relatively single color, but there is no obvious difference from the microscopic point morphology

for wet repellent stains, the reason is that the fountain solution of the previous color hinders the transfer of the next color ink. Therefore, when wet repellent occurs, not only a large area of macro blooming will appear on the print sample, but also the microscopic point is usually incomplete

2. The order of color spots

because the back trap color spots are caused by the ink on the paper being peeled off by the blanket of the subsequent color group, it is possible to occur from the first color to the penultimate color, that is, K, m, C uselessly appear black trap color spots. If color spots are found on the black line during the actual printing process, special attention should be paid to whether it is Backtrap color spots. Due to the final printing of yellow, it is impossible to have back trap stains

for wet rejection, its appearance is... C. Three colors of Y appear, while wet rejection is impossible on the K side of the first color

3. Occurrence area

all the areas on the paper that have been transferred to graphics and text, except the points that have been overprinted with subsequent ink, other points may be affected by back trap. As long as the coating has a large degree of uneven absorption of ink, it is possible to show back trap stains

for wet rejection, it can only occur in the blank of the previous color. For example, magenta stains caused by wet rejection appear in the area with fountain solution on the black version. If you overprint on the field of the previous color, there will be no wet rejection spots$ Page break $

IV. determination of back trap stain

at present, IGT test method, vandercock printing test method, Croda ink test method, etc. are commonly used for the laboratory determination of back trap stain on coated paper

igt test method is to use IGT printability instrument to transfer a layer of ink to the paper sample first, and then roll it on the paper surface with a special rubber roller at a certain pressure for 4 times after a specific time, and then