Study on pre-compensation processing technology of flexographic prepress (Part one)

[Abstract] Today's flexographic printing has developed rapidly and the quality of flexo printing has become increasingly important. This article describes the printing characteristics of flexographic printing and the pre-prepress compensation processing techniques.

Keywords: flexographic printing; pre-compensation technology; prepress technology

Flexographic printing is a type of letterpress printing. Originally referred to as flexographic printing, it uses a rubber plate and is now plated with a photosensitive resin plate. Flexo printing has its own characteristics and advantages. First of all, flexographic printing uses water-based environmental protection inks that are harmless to the human body and are particularly suitable for the packaging and printing of food, medicine and other products. Second, it can be combined with post-processing to form a production line, which greatly improves the production efficiency and reduces the production cost. Once again, once the flexographic process parameters have been determined, the consistency of repeated printing is good, ensuring the stability of the package. In addition, there is a wide range of printing materials, printing pressure is small, the printing plate can be bent, the advantages of high printing speed. therefore. Flexographic printing has achieved great development in the world, especially its unique advantages in the packaging field, making this printing method accepted by more and more people in the packaging and printing industry.

1 Printing characteristics of flexographic printing

Relative to offset printing and gravure printing, the printing characteristics of flexographic printing are as follows:
1) The printing plate is flexible. A piece of flexographic printing plate is made. When it is mounted on a cylindrical roller, the plate will have a bending deformation along the circumferential surface of the roller. This deformation will involve graphics on the surface of the printing plate (see Figure 1). Makes the printed text and document size deviation.

2) The flexographic printing pressure is small, so the change of pressure has a great influence on the consistency of printing quality, and the control of printing pressure becomes very important.

3) Flexographic printing uses anilox rollers to transfer ink, and the number of lines of the anilox roller must match that of the printing plate, otherwise it will cause the dots in the highlight area to be imprinted or moire.

4) The flexographic dot gains are non-linear, and dot gains are more severe. When the outlets are extremely small, it is easy to block the network or increase to 15% or even larger outlets. The flexographic plate has a low hardness, and the graphic and text are easily deformed after being squeezed. The general increase range is 16% to 30%, and the midtone adjustment can reach 50%.

5) Flexo Printing Compared to offset printing or gravure printing, the flexographic density curve is unstable, and the effect of changes in plate thickness on printing quality is large, making the difficulty of flexographic color overlay printing more difficult.

6) Most of the flexo printing equipment adopts the unit type, and the unit generally has 6-8 or more, so it is necessary to reset the color separation parameters and consider the multi-color separation in order to make up for the insufficiency of the performance of printing the Internet. The flexo-printed EO, ~U units are more, and can also be combined with post-processing to form a linkage production line, which requires higher precision for overprinting, so it is necessary to design a reasonable post process.

It is due to the above reasons that if flexographic prepress processing is carried out according to the offset or gravure printing method, many flexographic prints appear dull and monotonous, and gradients in the middle and bright tuning parts are prone to barring, netting, and insufficient transitions. It is smooth and soft, and there are missing or hard edges in the highlights. Therefore, print products with low quality requirements may not be considered, but for print products with higher quality requirements, measures must be taken to compensate for such changes in size and dot gain in order to ensure a rich hierarchy of print products.

2 prepress pre-compensation processing method

Plate bending deformation and dot gain are two main aspects that cause the maximum deviation in flexographic printing. The most obvious feature of the flexographic plate is its elasticity, so that the tensile deformation produced in the circumferential direction of the cylinder after the flexographic plate is mounted on the cylinder cannot be avoided. Therefore, in order to obtain a normal print, it is necessary to compensate for the distortion of the printed image. In actual production, the flexure deformation compensation is generally performed by the following method:

1) In the design of the manuscript, the corresponding value is subtracted from the reversal dimension of the manuscript based on the measured plate elongation.
2) Using computer-based plate printing, you only need to give a one-way zoom instruction before designing ink separation.
3) Using the drum type printing method The plate cylinder has the same size as the plate cylinder.
4) Take a deformed mirror when shooting a printing film to shorten the redirection dimension.
5) Before the film is out, the dot gain is compensated by adjusting the color separation characteristic curve of the flexographic plate.


(to be continued)