Control of several major data for analog proofing

In order to ensure the stability and accuracy of proofing quality, it is absolutely necessary to make certain working data for proofing.
The main data indicators that affect the quality of proofing are as follows:
The actual field density of the proofing field reflects the condition of the paper accepting the amount of ink. When the ink is transported from the plate to the blanket and then pressed onto the paper by the blanket, part of the ink is absorbed by the fibers of the paper, and the other part of the ink forms the film surface of the ink layer on the surface of the paper. When an offset paper is used as a printing horse paper, since paper fibers are loose, its ink absorbency is large.
Therefore, the amount of ink required during printing will increase, and the surface of the ink layer formed on the surface of the paper will be thinned. In addition, the surface smoothness of the offset paper is low, so even if it absorbs ink even more, the reflection density of the ink does not appear high. Under normal circumstances, we use the ink reflection density on coated paper as the data control standard for the proof density.
The optical reflection densities of the yellow, magenta, cyan, and black four-color inks are generally in the range of yellow 9-1.1, magenta 1.3-1.4, blue 1.5-1.6, and black 1.8-1.9.
The above control data should ensure that the normal outlet status is valid when proofing. In the case of an unfavorable state of the network, its actual density is meaningless. At the same time, the brand of ink used is different and its reflection density is also different. Under normal circumstances, the hue, magenta and cyan can be set to lower reflection densities when the ink has a high hue.
2. Proofing network expansion values ​​Since analog proofing is a simulation of the printing state of a printing press, it is a round flat printing method after all. Even if the printing pressure is not as large as the printing press when the ink is delivered, it still has a printing pressure, so that there is inevitably a dot expansion. Dot gains will vary depending on the paper. Because the offset printing paper needs a large amount of printing ink, and in order to adapt to the rough and loose surface structure of the offset paper, the ink viscosity must be reduced. Low-viscosity, high-volume inks are transferred to the PS plate and blanket, and eventually reach the paper, which will eventually cause the dot area to expand. Therefore, when offset paper is used as proofing paper, the dot gain value is 20%.
The network will undergo excessive expansion due to abnormal changes in printing pressure, blanket lining, etc. However, it is difficult to observe whether the dot enlargement conforms to the data control standard by using a magnifier. For this reason, it is useful to select the relative contrast value, the K value, as the measurement method for the proof dot gain value. The expanded value can reflect the changing status of outlets in the tightly-expanded area 75. This can reflect the dot-spreading status representatively. The larger the value of K, the better the condition of the dot in the shadow zone. Damage to quality caused by network expansion during the proofing process will be the most serious in the dark tone and midtone areas. K value calculation formula is:
K = field density - (75 density)
When the solid density is used for proofing with 157g bond paper, the best relative contrast K for each of the yellow, magenta, cyan, and black plates is between 0.4 and 0.6. Keeping the K value as high as possible is beneficial to the dark tone level that embodies the picture.
3. The hue of the proofing ink and the hue of the purity ink are directly related to neutral gray balance and color formation. At present, the four-color version of quick-drying inks for lithography in China can be roughly divided into two sets of standards: one for yellow, magenta, sky blue and black; the other for yellow, magenta, sky blue and black.
In these two sets of inks, magenta and magenta are the main differences. In some red-based screens, as well as skin color and large red areas, two different situations will occur: printing with magenta will show a vivid red color, while in the skin tone area, it will not be a little more or less than magenta. One point will be particularly sensitive to reflect; and when using a set of ink magenta, the magenta curve will exist in the neutral balance curve slightly higher than the yellow curve.
The difference between medium yellow and transparent yellow ink is that the hiding power of yellow ink is weak. Therefore, when the use of translucent yellow on the last color overprint, the performance of the color is favorable, but also in line with the four-color machine's printing color sequence.
When using different inks for printing, it will be different when the same dot area forms a color, so the neutral gray balance will also bring difficulties to scan quality and data. For this reason, in the general case, the proofing job always uses a fixed set of ink, and it will never change the brand and model of the proofing ink.
At the same time, the ink in the process of color formation will also affect the normal performance of the print color due to its purity. The main reason that affects the purity of the ink in the proofing process is that during the process of changing the color of the proofer, the ink roller cleaning is not clean, so that the hue of the ink is affected by the residual ink of the previous color. Therefore, it is very necessary to maintain the original hue and purity of the ink during the operation. The use of a reflection densitometer to determine the relevant data for the ink is an effective way to control the hue of the ink. The density of R, G, and B reflection color densitometers is used to determine the density of the other two colors of pure ink at the standard proof density. In fact, the color error of the ink is calculated based on the data obtained from three different color filters of one color ink. , grayscale and color efficiency. However, during the actual operation, in order to facilitate the operation, the hue purity of the ink can be controlled by controlling the minimum density of two of the color filters. In the determination of cyan ink using G, B filter; measurement of yellow ink using R, G filter; magenta ink when used B, R filter.
Simulation proofing is a special technique in the prepress area that simulates the printing process and requires a certain amount of empirical operation. In particular, the proofer is manually added to the amount of ink and the amount of water is adjusted. The adjustment of the printing pressure of the proofer needs to be performed by an experienced operator. At the same time, in the proofing process, it is necessary to use reflection densitometers and high power magnifiers to control the ink density and dot quality to ensure the stability and accuracy of the proofs. The above table shows the disadvantages and reasons that arise during the proofing process.