Offset Control in Offset Printing (I)

The dot is the pixel of the printed image and is the smallest unit of ink that is absorbed in the offset. It is tasked with passing the tone and tone of the manuscript. According to the color theory, colors have three properties of hue, brightness, and saturation. Yellow, magenta, and blue primary dots are superimposed together. By changing the dot size or coverage, thousands of different colors can be formed. Therefore, no matter how rich a layer of manuscripts are, how the colors change, as long as the appropriate collocations of the three primary dots can simulate the color and tone of the manuscript.

When evaluating the quality of color prints, the copy quality of outlets is the most important indicator. Among them, many standards related to the quality of print reproduction, such as uniform ink, no deformation of dots, no ghosting on prints, ink sticks, etc. are almost all associated with the control of outlets.

Changes in the dot can cause sharpness and subtle changes, color changes and contrast, ink hue, ink density, and ink overprint. Although electronic point methods have been used to predict and prevent some dot changes, the changes in control points are still necessary to ensure print quality.

The size of the dots and their position relative to each other determine the color tone of the print. For example, a densely printed dot will form a dark tone region, and a small dot with a large pitch will form a highlight. Therefore, dot gain can greatly affect the reproduction of these regional meshes and color photographs, and has a great influence on midtones.

It is because of the reasons mentioned above that we have proposed the concept of network control, that is, the quality of the control network at all stages of the entire workflow of printing and copying. Due to the close connection between various processes, we must also pay attention to the coordination of quality control.

Although the term of digital printing has been proposed for many years, in the relatively widely used traditional printing methods, the conversion of network point information between devices still uses analog information transmission, precisely because this transfer is achieved using analog information. , it will inevitably cause the network itself to appear in the equipment between the changes we do not want to see (such as the increase in network points, network outlets, network distortion and loss of network points or a merger).

Now we have more common dot-printing (the printing methods mentioned in this article are all offset printing). The workflow for product replication is this:

Manuscript → Separation (mainly used now is scanning or electronic color separation) → Image processing (including adjustment and compression of gradation, graphic mixing, color separation, screening, etc. This process is mostly done by computer. )→Imprinted version→Out of sheet (exposure, development, fixing)→Stencil printing (exposure, development, and baking for large prints)→Printing on the machine→End of printing.

The above is a relatively common work flow. The dots are formed from the image processing stage through to the printing process, through the light, film, plate, printer (mainly plate and blanket), ink, and substrate. To complete, but due to many inherent reasons such as equipment and technology, the nature of the transmission determines the changes in the network is inevitable, plus these materials and equipment itself have a lot of uncertainty, resulting in the network can not be done at every step Accurate copying and transmission means that no matter how we do it, we can't completely avoid changes in the network during these transmissions (the above-mentioned network distortions, network enlargement, and shrinking). In actual production, what we need to do is to control the changes of outlets within a reasonable and predictable range, and at the same time standardize the production process and reduce the fluctuation of the changing parameters in the production process.

In the process of production, we can encounter the following changes in the process of network copying:

1. The narrowing of network points means that the area of ​​the dots obtained by copying is reduced relative to the upper layer (upper step) carrier of the dot.
2. Dot enlargement: With respect to the upper layer carrier of the dot, the dot area obtained after copying increases, and the resulting replica becomes larger in density.

The dot gain can be further divided into mechanical dot gain and optical dot gain.

"Machine dot gain" (also known as physical dot gain) is the physical increase in the dot size. The most common form of mechanical dot gain is non-directional, and if it is excessively large, it may cause stencils. This increase is caused by the ink film thickness, the type of substrate, the number of screens, or the transfer of ink under pressure between the blanket and the plate, and between the blanket and the substrate.

"Optical dot gain" is a visual phenomenon formed by the light absorption characteristics of the ink and the light scattering properties of the substrate. When light is irradiated on a non-image area, that is, a "blank area", light is diffused, and a part of the light near the halftone dot is suppressed. This light is considered "absorbed" because it cannot be reflected back into the viewer's eyes. The dots appear to be larger than the actual density and size, and it seems as though dot gains have occurred.

3. Dot distortion: The shape of the dot after copying has changed. For example, an original circular dot may become elliptical after copying, or there may be no fixed dots.

The three problems mentioned above have a great influence on the entire copying stage of the printed matter. However, when we can more accurately predict the transmission of the changes made by the outlets between devices, we can more effectively compensate for the changes that will occur before the outlets change. For example, if an outlet of a print product changes during the copying process, the copy of the positive image network is reduced by 2%; the ban network is reduced by 4%; the printing network is increased by 15%; and the optical point of the network is increased by 12%. Then after a simple addition and subtraction of mathematics, we will find that as long as the network points are pre-compensated for 21% of the network points before they are delivered, we will just get the effect we want after delivery. Although the actual dot compensation settings are not as simple as those mentioned above, this compensation and dot change is not just a linear relationship, and different percentages of dot changes in the replication process are not the same, but we only need Knowing the above compensation relationship is sufficient to tell the software what changes will occur (typically, the rate of change of the network at 50%), and the rest can be given to the computer.

The above reasoning seems to be very simple, but on actual operation, we will find that due to the numerous types of work involved in the transmission of outlets, it is not easy to truly standardize management and standardize operations. However, this is an indispensable and important aspect of the copying of prints on the printed matter. Only by strictly implementing the standardization of management and the standardization of operations, is it possible to achieve a definite purpose in pre-press image processing and network point compensation. This is precisely how many Printing companies can't do it. The actual situation is often that the platemaking and printing do each one, which is very unfavorable for the correct reproduction of the outlets.

In the following, we will discuss some issues that need to be addressed in actual production and the factors that are likely to cause abnormal deformation of the dots.