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.
The low viscosity, large amount of ink transferred to the PS plate and blanket, and finally reach the paper, will eventually cause the dot area to expand. Therefore, when offset paper is used as proofing paper, its 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. The formula for calculating the K value is: K = density in the field - (75 density) When the field density is used for proofing with 157g coated paper, the best data for the relative contrast K between the yellow, magenta, cyan and black plates is 0.4-0.6. . Keeping the K value as high as possible is beneficial to the dark tone level that embodies the picture.
3. Hue and purity of the proofing ink The hue of the proofing ink is 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. There will be two different situations in some red-based screens, as well as skin color and large red areas: printing with magenta will show bright red; while in the skin tone area, it will not be slightly more or less like 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, the proofing job will always use a fixed set of ink under normal circumstances, and 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. (Finish)

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