For mass production with an offset press, sometimes the printed product is difficult to achieve the effect of traditional proofing, and even printed products may be scrapped because they cannot be processed afterwards. There are several reasons.
1. Proofer and printing machine have different structures
The proofing machine uses a round flattening printing method, and the printing machine is a round flattening printing; the water supply and ink supply mechanism on the proofing machine is far less perfect than the printing machine, so in the aspects of water and ink balance, dot reproduction, image level, etc. There will be a certain gap from the printed matter.
2. The printing process of the proofing machine is different from the printing machine
Multi-color prints are printed continuously on the printing machine, and intermittently printed on the proofing machine, proofing one by one, color by color. Continuous printing determines that the printing press is printed in wet press wet mode, and the interval time of the proofer is longer. When the first color is printed, the previous color is already dry, so it is generally carried out by wet press dry mode. These two methods determine that the proofs and prints will have different printing effects. For example, printing gold ink on the ground, printing gold once when proofing may achieve the desired effect, but on the printing machine, arrange two after the field printing unit The gold printing of the three units can not print the proofing effect, and the gold can not be printed until the solid color block is dry. In addition, gold and silver cardboard prints cannot be glazed online, otherwise the front marks will be pulled out by glazing oil.
There is no paper feeding and receiving process on the proofing machine, but on the printing machine. This leads to the fault that the print on the surface of the gold and silver cardboard or the printed product is scratched by the paper conveying mechanism, or the paper is too deformed to be conveyed smoothly. When the paper is collected, if the ink layer is not dry, it will also cause the print to be dirty, or the relative movement between the paper will cause the print to be scratched, and the print on the back of the print will be scratched by the suction wheel. Therefore, the dusting method is often used to prevent sticking when collecting paper. After powder spraying, it may cause the gloss of the printed product to be reduced and the surface is rough; or when the second overprinting is used, the ink color behind is not true; or it may be due to excessive powder spraying, which will give post-processing (such as laminating, Bronzing, etc.) brings hidden dangers. Of course, sometimes the additives in the ink will also affect the printing effect. In addition, some papers are too thin, and after being printed on the printing machine, they will be rolled into a ball and cannot be received.
Proofers usually print at low speeds, while printing machines operate at high speeds. In order to achieve a certain effect, some samples require a very thick ink layer, which cannot be achieved during normal high-speed printing, and an excessively thin ink layer is difficult to control during high-speed printing.
The printing pressure of the printing machine is greater than that of the proofing machine. The same paper can be used normally during proofing, and problems such as peeling of paper may occur on the printing machine.
3. Other reasons
Whether it is proofing on the proofing machine or printing on the printing press, the paper will stretch to a certain extent, but the amount of stretching is different. Generally, the proofing is made on the proofing machine, and the rate of expansion and contraction is small. When printing on the printing machine, the rate of expansion and contraction is large. Therefore, if multi-color overprinting needs to be printed on the machine several times, in the second overprinting, there may be obvious overprinting inaccuracy due to paper deformation, and the paper deformation is irregular.
In multi-color printing, it is often the case that the inks prepared according to the ratio of the proofing ink show a large color difference during printing. This may be because other color inks remain on the ink roller during proofing. Although these residual inks are few, they account for a large proportion of the few inks used for proofing. Therefore, wash the ink roller when proofing. Of course, sometimes the difference in paper color can cause color difference.
The proofing film used for proofing is generally single imposition, while the proofing film used in printing production is generally multi-imposition. When the multi-imposition film is made from the single-composition film, it may be overprinted or the dots may be lost. As a result, the deviation between the printed product and the proof sheet.
When printing on a printing machine, the pH value, additives (such as red and white dry oil, ink adjustment oil, etc.) and the use of the drying device must also be considered.
Therefore, the problems that may occur in printing production should be considered during proofing, and the problems in proofing should also be considered during printing, and appropriate adjustments should be made so that the products printed on the printing machine are close to the samples printed on the proofing machine.
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