Digital Screening Technology in Gravure Printing (3)

Gravure plate making method and requirements for outlets

The conventional method in the gravure platemaking method is a gravure plate making method. The method produces a plate with the same mesh area and expresses the level of printed matter with the depth of the mesh. Its original version has no outlets, and is a continuously adjusted film. The image on the original plate is imaged by UV irradiation on a corrosion-resistant film (film used to corrode the plate cylinder coating) or on carbon paper. The film is then wrapped on a roller, developed in warm water, and chemically passed through ferric chloride. Corrosion, the formation of different shades of mesh. Now we do not use the continuous adjustment of the original carbon paper. We mainly use the method of separating the color film, that is, the method of making the screening color separation film in exactly the same way as the offset printing; the other method is to perform full digital processing on the image without the film. How to make a plate cylinder. In the field of packaging printing and special gravure printing, the filmless production method is rapidly being promoted. There has been real progress in the absence of filming in the field of gravure printing. The modern gravure platemaking process methods mainly include:

(1) Dot gravure. Gravure dot screens are used for photographic screening or electronic screening to produce positive (or negative) images of the screens. They are directly printed on the photopolymer material plate cylinder or the copper roller coated with photoresist. The former only It needs to be washed and developed. The latter is then eroded (or electrolyzed) to make a gravure plate. It is also called direct etching. This plate-making has a simple program, data is stable, easy to operate, made of gravure plates, consistent depth of network sites, can be unified to change its depth (15 ~ 35μm). The dot size varies with the image level, and the ratio of the line wall to the dot can vary from 1:3 to 1:6 (up to a maximum of 13, but not tolerated) with the original coordination contrast requirement. Although it can only express a dot scale of 56% to 73% (up to 79%), due to its thick printing ink layer and high contrast, as well as the expansion of printing tone levels, its tone reproduction is still better than offset printing. . And the picture is flat, uniform and soft, suitable for decoration and printing of building materials and trademarks.

(2) Corrosion gravure. It is one of the main methods of gravure printing. Developed from photogravure, it is directly exposed to the rollers coated with photosensitive emulsions using dot pictures (photographic or electronic color separation), and gravure plates are made by etching. Removed the unstable factors of overprinting carbon paper, simplifying the process. And can take simple offset proofing (or fast proofing) to verify the exact quality of the plate-making picture, to achieve the corresponding gum gravure conversion. The copied color image is not much different from the electronic engraving gravure, and it is conducive to the stable operation of gravure printing.

(3) Electronic engraving gravure. The electronic color separation screen is used to print originals (color separation signals) as originals. According to the density of the originals, an electronic machine (diamond engraving cutter) is driven by the image scanning signal by an engraving machine to directly and directly engrave gravure roll dot printing plates. The dots of the electronic engraving gravure are not overlapped in principle, and only the solid part is liable to produce non-uniformity during printing, which is one of the reasons for gravure to produce water marks. The traditional gravure dots will also produce gradation jumps between the field and midtones. The reproduction of highlights can also be visually perceptible jumps due to different printing conditions. If hierarchical networks and frequency conversion network points are used, sudden mutations can be avoided. The engraved gravure dots are changed at the same time as the size of the dots in the image layer and the depth of the darkness of the net, and there are simultaneous changes in the dot area and the depth of the net points as the tone is deepened. Hierarchical resolution is very fine, image coordination levels and contrast can be reproduced well, and it is more suitable for gravure printing of fine color images. Use should be as good as possible in the use of good clarity, in the electronic color separation should pay attention to their interconnection and influence, the rational use of various functions of the electronic color separation machine to compensate and correct the impact of various factors in the color image reproduction, To ensure good image reproduction quality. The engraved gravure dot enlargement is similar to that of etched gravure. It is approximately linearly enlarged, and the halftone tone is slightly longer than the etched gravure. In the electronic engraving machine is provided with a variety of dot system and multiple sets of engraving level curves (including changes in the gray balance ratio), according to the original selected from the appropriate curve, carved into a fine-grained image suitable for different gravure printing requirements. The color separation screening chart used by the electronic engraving machine can use the offset proofing or digital proofing to check the color separation version to achieve accurate gum gravure conversion.

In addition, there are electron beam spiral scanning screens. It uses the electron beam of the cathode ray tube to perform spiral scanning to obtain dot sites that change by area ratio, and then carries out electron beam blackening to realize image screening. Now, the technical environment of gravure printing, such as prepress digitalization and non-filming, has achieved significant development. The CTP process of cylinder manufacturing has not only been limited to electronic engraving, but more advanced laser methods have also been developed and applied.

Lin Qishui China Printing - 2005 Issue 12

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