Offset proofing and printing
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Mass production with offset printing machines, sometimes printing products are difficult to achieve the effect of traditional proofing proofs, and even printed products can be scrapped due to the inability to carry out subsequent processing. There are several reasons for this.
1. The proofing machine is different from the printing machine structure
The proofing machine adopts the circular flat printing method, and the printing machine is round and round printing; the water supply and ink supply mechanism on the proofing machine is far less perfect than the printing machine, so in the ink balance, dot reproduction, image level, etc., proofing proofs There will be a certain gap between the print and the print.
2. The proofing machine is different from the printing process of the printing machine
Multi-color prints are continuously printed on the press, while on the proofer are intermittent prints, proofing one by one and one color. Continuous printing determines that the printing machine is printed in wet-wet mode, and the proofing machine has a long interval. When the color is printed, the previous color is dry, so it is generally carried out in a wet-drying manner. These two methods determine that the proofs and prints will have different printing effects. For example, printing gold ink on the ground may result in the desired effect when printing the proof. However, on the printing machine, two units are arranged after the field printing unit. The three units of the printing gold can not print the effect of proofing, must wait until the solid color block is dry before printing gold. In addition, gold and silver cardboard prints cannot be glazed online, otherwise the front print will be removed by glazing.
There is no paper feeding or receiving process on the proofing machine, but there is on the printing machine. This leads to the failure of the gold-silver cardboard surface or the imprint on the printed product to be scratched by the paper feeding mechanism, or the paper is too deformed to smoothly feed the paper. When the paper is not received, if the ink layer is not dry, the printed product will be dirty, or the relative movement between the papers will cause the printed matter to be scratched, and the back print of the printed product will be scratched by the suction wheel. Therefore, the dusting method is often used to prevent sticking when the paper is delivered. After the powder can be sprayed, it may cause the gloss of the printed product to decrease and the surface to be rough; or when the second time the machine is overprinted, the back ink color is not printed; or because of excessive dusting, the post-process (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 of the paper is too thin and will be rolled up after being printed on the printing press, and the paper cannot be taken.
The proofer usually prints at low speeds while the press is running at high speeds. Some samples require a very thick ink layer to achieve a certain effect, which is not possible at normal high-speed printing, and an excessively thin ink layer is difficult to control in high-speed printing.
The printing pressure of the printing press is larger than that of the proofing machine. The same paper can be used normally during proofing, and paper peeling and the like may occur on the printing machine.
3. Other reasons
Whether it is proofing on a proofing machine or printing on a printing press, the paper will have a certain amount of expansion and contraction, but the amount of expansion and contraction is different. Generally, the proofing is performed on the proofing machine, the expansion ratio is small, and the printing is performed on the printing machine, and the expansion ratio is large. Therefore, if the multi-color overprinting needs to be printed on the machine several times, in the second overprinting, there may be obvious overprinting inaccuracies due to paper deformation, and the deformation of the paper is irregular.
In multi-color printing, it is often the case that inks formulated according to the ratio of the proofing inks exhibit a large chromatic aberration during printing. This may be because other inks remain on the ink roller during proofing. Although there are few residual inks, they account for a large proportion of the ink used for proofing. Therefore, the ink roller should be washed when proofing. Of course, sometimes the difference in paper color can also cause chromatic aberration.
The printing plate used for proofing is generally a single imposition, and the printing film used in printing production is generally multi-imposition. When multi-ply film is copied from a single piece of film, there may be inaccurate overprinting, and even the dot is lost. This causes deviations between the printed product and the proof.
When printing on a printing press, consideration should also be given to pH, additives (such as red and white dry oil, varnish, etc.) and the use of drying equipment.
Therefore, the proof of printing production should be considered in the proofing, the printing should also take into account the problems of proofing, and make appropriate adjustments, so that the products printed on the printing machine and the proofs on the proofing machine are close.