Quality evaluation of printed matter

Foreword: Nowadays, flexographic printing has received unprecedented attention in our country. In recent years, many flexographic printing equipments have been introduced. However, on the other hand, these equipments have not achieved the expected results (including product quality, production efficiency and economic benefits). There is a big gap compared with the situation of introducing offset printing and gravure printing equipment. In order to make the healthy and rapid development of flexo printing in our country, we must solve some problems in the development of flexographic and gravure printing from the understanding and practice.

The printing quality mainly includes the following indicators: the number of screens, the amount of ink applied to the ink, the accuracy of overprinting, and control factors for printing quality.

1. Screen number: The gravure printing can reach 175 lines/inch, while the flexographic printing can generally reach 120 lines/inch, and the high-quality level flexographic printing can reach 133~150 lines/inch.

2. Ink amount: In normal printing, the amount of ink on concave, soft and offset printing is gravure 4~8g/m2; flexo 3~5g/m2, 0.8~2g/m2(UV); offset 1~2g /m2. The proportions of gravure, flexo, and offset printing on average are: 400%, 267%, and 100%. From this it can be seen that the gravure print has the greatest brightness.

3. Accuracy of overprinting: As with the gravure printing machine of the unit type, the satellite type flexographic printing press can obtain similar overprint accuracy (color-color), but the unit type flexographic printing press is generally difficult to obtain the above accuracy.

4. Difficulty in quality control: Gravure printing is a relatively simple process with few variables, so quality control is simple and reproducible. Flexographic printing has more influencing factors and is more likely to cause problems. The quality of flexographic printing depends on elements such as color separation, negative film, plate exposure, print mounting (mounting the plate on a roller), tape thickness and quality. All these elements have been continuously improved in recent years, and the gap between the quality of flexo and gravure has been continuously reduced. The use of laser imaging technology for flexographic plates has eliminated the above negative film production process. When using laser imaging platemaking, a clearer image can be obtained: 1% dot and 150 lines/inch can be achieved. Laser engraving can use a rotary platemaking apparatus to make a sleeve-type flexographic plate, which also facilitates quality control.

Since the flexographic printing plate is made of a soft rubbery polymer material, the graphic line points on the printing plate are “embossed” on the surface of the plate base, and the central impression cylinder is a rigid metal surface. The pressure between them will cause deformation of the printing plate's graphic line point. This aspect determines that the pressure must be very small, and the pressure variation range is only about 2kg/cm2. The change of pattern leads to the ever-changing dot-levels, which brings the problem of selection of the anilox roller: The roller of the cable is slightly thick, which causes the small dot of the high-light spot to sink into the ink hole and cannot be printed. The “dumb-bell shape” stencils; too thin At the middle tone point, there was a lack of ink on the screen and the film was thin and easy to color. The tone could not be opened (the density line was unstable, and the non-linear change in the dot was a major obstacle to the quality of flexo printing).

The ink used for flexo printing is water-based ink and has less environmental pollution. However, the printing method used by the flexo printer determines that its ink loading is lower than that of gravure printing and silk screen printing. Although the printing quality of the large field pattern printing blocks is obvious, the quality is obvious. Better than offset printing, but when printing gold and silver ink, due to lack of ink, the printed product is dull and dull and cannot be compared with gravure.

Looking at the gravure again, the printing plate is directly inked or transferred directly to the printing plate by the ink roller. Since the gravure plate is a rigid chrome metal roller, the graphic line point is composed of a number of pits that are in the shape of "holes." After inking, all the ink on the surface of the plate roller except for the pit is scraped clean by the scraper, and only the ink in the pit is transferred to the surface of the substrate for printing under the pressure of the embossing roller (special soft rubber surface). Printing process. The thickness and density of the ink layer depend on the volume and transfer rate of the aperture ink in the printing roller itself. The pressure between the embossing roller and the printing plate is much higher than the flexographic printing (about 40kg/cm2), the adjustment range is large, the control is relatively simple, and in the high-speed printing, the teletext point has almost no enlargement. The disadvantage is that the scraper is continuously scraped on the surface of the printing plate for a long time. Although the plate surface is plated with hard chrome, it will inevitably be scratched, or the printing wire or line will be generated due to impurities and coarse hard particles in the ink, resulting in printing times. Products, affect the quality of printing.

In general, gravure printing is based on the thickness of ink and the density of dots and dots to reflect the level. In all evaluations of the printing process, the gravure level of the gravure is pulled open the most, and the thickness of the ink layer is increased by approximately 3 times than that of the flexo. The public ink is the most vivid and crisp and the most distinct.