Dot control in the most popular offset printing

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Point control in offset printing

point is the pixel of the printed image and the smallest unit of adsorbed ink in offset printing. It is responsible for transferring the tone and tone of the original. According to the theory of chromatics, color has three attributes: hue, brightness and saturation. The yellow, magenta and cyan primary color dots are superimposed together, and thousands of different colors can be formed by changing the size or coverage of the dots. Therefore, no matter how rich the level of an original is and how the color changes, as long as the appropriate collocation of the three primary color points can simulate the color and tone of the original

when evaluating the quality of color printing, the reproduction quality of dots is the most important index. Among them, many standards related to the reproduction quality of printed matter, such as uniform ink color, no deformation of dots, no ghosting on printed matter, ink bars, etc., are almost related to the control of dots

The change of

points can cause the change of clarity and subtle levels, the change of color, as well as the problems of contrast, ink tone, ink density and ink overprint. Although electronic methods have been used to predict and prevent some point changes, the change of control points is still necessary to ensure the printing quality

the size of dots and the position between them determine the color tone of printing. For example, large dots printed closely will form dark areas, and small dots with large spacing will form highlight areas. Therefore, point increase can greatly affect the reproduction of eye and color photos in these areas, and has a great impact on the middle tone

for the reasons mentioned above, we put forward the concept of point control, that is, to control the quality of points in all links of the whole workflow of printing and reproduction. Due to the close connection between various processes, we should also pay attention to the quality control of mutual coordination

although the term "digital printing" has been put forward for many years, in the traditional printing methods that are relatively widely used at present, analog information transmission is still more used for the conversion of point information between devices, which is precisely because this transmission is realized by using analog information, It is inevitable that there will be unwanted changes in the information of the point itself between devices (such as point increase, point decrease, point deformation, point loss or merging)

now our common point printing (the printing methods mentioned in this article are offset printing) the work flow of reproduction is as follows:

original → color separation (now mainly using scanning or electronic color separation) → image processing (including adjustment and compression of tone, mixing of graphics and texts, color separation, addition, etc., which are mostly completed by computer) → large version making → film production (exposure, development, fixing) → plate printing (exposure, development, and baking plates are also used for large print volume) → machine printing → printing is over

the above is a common workflow. The whole transmission process from the formation of points in the image processing stage to the completion of printing is completed through light, film, plate, printing machine (mainly printing plate and blanket), ink and substrate. However, due to many inherent reasons such as equipment and technology, the nature of transmission itself determines that point changes are inevitable, In addition, these materials and equipment have many uncertainties, which makes it impossible for the point to be accurately copied and transmitted at every step, that is, no matter what we do, it is impossible to completely avoid the changes of the point in these transmission processes (the above-mentioned point deformation, point increase, point decrease, etc.). In actual production, what we need to do is how to control the change of points within a reasonable and predictable range, standardize the production process and reduce the fluctuation of changing parameters in the production process. The changes we can encounter in the process of point replication in production mainly include the following:

1 The point is reduced, that is, the area of the copied point is reduced relative to the upper layer (the previous process) carrier of the point

2. Point increase: relative to the upper carrier of the point, the area of the point obtained after replication increases, and the density of the replica becomes larger

point increase can be divided into mechanical point increase and optical point increase

"mechanical point increase" (also known as physical point increase) is the actual physical increase of point size. The most common form of mechanical point increase is non directional, and excessive increase may cause paste. This increase is caused by the thickness of the ink film, the type of substrate, the number of high lines, or the ink transfer under pressure between the blanket and the printing plate, between the blanket and the substrate

"optical point increase" is a visual phenomenon formed by the light absorption characteristics of ink and the light dispersion characteristics of substrate. When the light shines on the non image area, that is, the "blank area", the light diffuses, and part of the light near the point is suppressed. This light is considered "absorbed" because it cannot be reflected back to the observer's eyes. The point appears to be larger than the actual density and size, and it seems that the point has increased

3. Point deformation: the shape of the copied points has changed, such as the original circular points may become elliptical or points without fixed shape after copying

the three problems mentioned above have a great impact on the whole reproduction stage of printed matter. However, when we can more accurately predict the changes caused by the transmission of points between devices, we can more effectively compensate for the changes that will occur before the point changes. For example, if the following changes occur in the point of a print during the copying process: the positive point of the copy is reduced by 2%; The drying point is reduced by 4%; Increase the printing point by 15%; The point optics is increased to 12%, so after simple mathematical addition and subtraction, we will find that as long as the point is pre compensated by 21% before the point starts to transfer, we will just get the effect we want after the transfer. Although the actual point compensation setting is not as simple as the above, the relationship between this compensation and point change is not just a linear relationship, and the change trend of different percentages of points in the replication process is also different, we only need to understand the above compensation relationship, and tell the software about the change of points (typically the change rate of points at 50%), The rest can be left to the computer

the above reason seems to be very simple, but when it comes to practical operation, we will find that it is not easy to truly achieve the standardization of management and operation due to the various types of work involved in point transmission. However, this is an indispensable and important aspect for point reproduction on printed matter. Only by strictly implementing the standardization of management and the standardization of operation, it is possible to achieve a targeted goal in prepress image processing and point compensation, which is precisely what many printing enterprises cannot do. The actual situation is often that plate making and printing go their own way, which is very detrimental to the correct reproduction of points

below, we will discuss some problems that need attention and factors that are easy to cause abnormal deformation in actual production

selection of printing materials: This includes paper, ink, blanket, printing plate, wetting liquid, etc. The choice of these raw materials will often directly affect the reproduction quality of the printing point. A successful printing is not only to get a better quality of our printed matter, but also to achieve the minimum cost within the allowable quality range

At the same time, the choice of plate making process: This is a big and wide problem, but it involves the replication of points, and the most important is the shape of points and the number of lines. The correct choice of point shape can have a great impact on the surface effect of printed matter. More importantly, we should pay attention to the different properties of each point shape. For example, the square dot has a large gradient jump at 50%, and the total dot increase rate of the square dot is also larger than that of other dots. Therefore, square dots cannot be used in the color separation and addition process of high-end prints with rich middle tones, and diamond or oval dots are the primary consideration. The higher the number of lines in the dot, the richer the level that the dot print can show, and the more delicate the picture will appear. But at the same time, because the side length and sum of dots on the whole page will be larger, according to the theoretical condition of dot increase, we know that the change trend of dots is also larger. Many times, blindly increasing the number of dots and lines will lead to a decline in the replication quality of dots instead of improving. Therefore, in actual production, we should weigh the number of dots and lines according to the comprehensive conditions of many factors such as printing paper, ink, machine and so on. The point increase law with the same point percentage and different number of points and lines: when the point percentage is the same and the point increase value is the same, the higher the number of points and lines, the greater the point increase rate; When the percentage of points is the same, and the value of point increase is the same, the higher the number of lines, the greater the area percentage of point increase

print: 1 Exposure. In the process of PS plate exposure, due to light penetration and other reasons, the points on the printing plate are actually smaller than those on the film. Generally, this reduction should be controlled within 3% - 5%. However, this number is not unchanged. Because there are many different situations in the vacuum degree of the printing equipment, the transparency of the printing glass, the control of the printing time, and the quality of the plate itself, it is necessary to often use the printing control strip to test the point changes in the printing experiment process and the measurement, display, analysis, control, etc. are completed by the microcomputer, and then feed back the data to the prepress image processing unit; 2. Development: regularly test the concentration of developer, and determine the development time through the test results. If the same developing time is still used after the developer is tired, the edge of the point will not be clear, causing the point to increase. In the process of printing, it is necessary to regularly detect the working condition of the printing machine and the development conditions. In this process, it is OK to use the special test strip for printing, and timely feed back the results to the scroll spring fatigue tester. Type 1: prepress, so as to respond in advance

influence of fountain solution: a the pH value of fountain solution: the pH value of fountain solution in the printing process should not be too low, and it is appropriate to be about 5 ~ 6, because the low pH value of fountain solution has a great impact on the transfer of offset ink, and will cause plate corrosion, reduce the printing resistance of printing plate, cause the loss of high setting points, or other printing problems; B water volume on the printing plate: the water film thickness of the blank part on the printing plate is generally 0.5 ~ 1 μ m, and the ink film thickness is 2 ~ 3 μ M. If the water content on the printing plate is too large, it will erode the ink film of the graphic part, resulting in a little emptiness; If the thickness of the ink film is very large, the ink will spread to the blank part under the action of pressure, resulting in an increase in dots

influence of ink: 1 Moderate liquidity. The ink with too little fluidity has too strong cohesion, which is difficult to be ground evenly, thus reducing the transferability of the ink; If the fluidity is too large, the adhesion of the ink will be poor, and the printed points will be flat, dim, or even spread around, affecting the quality of the product. In addition, it should be noted that when printing dot products, the ink fluidity should be smaller than the text, so that the printing dot is clear and full. 2. Choose the appropriate ink. The ink is required to be fine, smooth, good flow performance, appropriate viscosity, without the shortcomings of sluggish version, bright and durable color, and the ink color is preferably translucent

reasonable emulsification of ink:

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