Flexo printing ink transfer (1)

The flexographic printing plate is a general name for a letterpress made of an elastic solid such as a rubber relief plate or a photosensitive resin plate, so the flexographic printing is a relief printing. However, from the aspects of the hardness of the plate, the ink transfer system of the printing machine, and the printability requirements, the flexographic printing and the ordinary letterpress printing are quite different. This chapter mainly describes the use of high-resolution, high elasticity (like rubber elastic), flexographic plate thickness of 1.7 ~ 7mm, using anilox roller to transfer low-viscosity solvent volatile ink, overprinting accuracy of high flexographic printing ink transfer problem .

The ink transfer mechanism of flexographic printing is very simple. The ink with low viscosity and high fluidity is filled into the fine ink holes of the anilox roller. The excess ink is scraped off by the doctor blade, leaving the ink in the ink hole of the anilox roller. Transferred to the graphic relief of the flexographic version, when the ink area on the printing plate touches the substrate, a “kiss” is formed, and the transfer of the ink is completed by light pressure kiss printing.

Although the mechanism of the flexographic printing ink transfer is simple, the flexographic plate has a small elastic modulus, is easily deformed, and has a high dot gain value. The process conditions are particularly important when printing fine images. In addition, the material of the anilox roller, the shape of the ink hole, the viscosity and dryness of the ink directly affect the transfer and transfer of the ink. Therefore, to obtain a high-quality flexographic print, there are still many technical problems to be solved.

First, the reproduction of the price of flexographic printing

A highly flexible, high-resolution flexographic plate consisting of a protective film, a photosensitive resin layer, a polyester film base, and the like. Due to the large elasticity of the printing plate, the printing effect, especially the reproducibility of the tone, is different from the gravure printing using the solvent-volatile ink, and is also different from the general letterpress printing using the higher-viscosity ink. Comparing the left and right diagrams in Figure 6-11, it can be seen that the flexographic printing has a larger dot enlargement from the highlight to the midtone, the printing contrast is small, and the image looks beautiful enough; the edges of the dot are easy to be tampered with ink and appear “hollow”. Dots cause the sharpness of the image to drop. In order to prevent this kind of defect from happening, try to adopt the low printing pressure while printing, choose the appropriate anilox roller, avoid to supply the ink quantity to the printing plate too much.

Flexographic printing is the same as other printing methods. Under the same original dot coverage, different dot lines and dot gains are different. In general, the higher the dot line number, the larger the dot gain value. From Figures 6-12, 6-13, 6-14, and 6-15, it can be seen that with a flexographic printing of 150 lines/inch, the original 30% dots have a dot gain of about 35% on the printed matter; 133 lines/inch and 120 lines/inch flexo printing, original 50% dot, print dot

The increase is approximately 20%; with 100 lines/inch flexo printing, the dot gain is approximately 18%. From the point of view of the expansion of the four types of outlets, the use of flexographic printing with a low number of outlets can result in sharp and clear printing quality, thereby obtaining prints with a sense of contrast, but when the number of outlets is too low, prints The subtle level and clarity are poor. Therefore, when making a flexographic printing negative, select a line number of 100 to 133 lines/inch, adjust the level of the original so that it is compatible with the range of the flexographic reproduction, and use a lower printing pressure. Get the basic conditions for high quality prints.

Second, the effect of anilox roller on ink transfer

The vast majority of flexographic printing presses are rotary presses for printing web printing materials, with few single sheets. The flexographic printing press is mainly composed of four parts: unwinding feeding parts, printing units, heating and drying systems, and rewinding and receiving parts. According to the arrangement of the printing units, there are three basic models: stacked, satellite, and unit. The new type of flexographic printing press is compact in structure and has various on-line post-press processing functions such as feeding from the reel to printing, coating, laminating, rolling gears, slitting, cross-cutting, and single-sheet finished or box-making. A one-stop production line for packaging and printing processing.

The printing unit is the core of the flexographic printing press. It consists of an ink supply system, a plate cylinder and an impression cylinder. The ink supply system is also the core of the printing unit. Currently, flexographic printers use ink fountain roller-anilox roller systems or anilox roller-squeegee systems to supply ink to printing plates.

Ink fountain roller - Anilox transfer roller ink system as shown in Figure 6-16 (a), ink roller from the ink fountain to the anilox roller. The amount of ink supplied by the ink supply system is affected by the pressure between the ink fountain roller and the anilox roller, the position of the ink deposition area between the ink fountain roller and the anilox roller, the ratio of the speed of the ink fountain roller to the anilox roller, and the hardness of the ink fountain roller rubber. And ink viscosity and other factors, especially in high-speed printing conditions, often appear too much ink transfer failure. The ink fountain roller-printer ink transfer ink inking system is difficult to guarantee the uniformity of the small ink transfer, generally cannot meet the high dot line number or the quality requirement of the color image printing, and the use range is limited.

However, since the ink fountain roller transfers ink to the anilox roller, the roller surface generates rolling friction, the abrasion is light, and the service life of the anilox roller is long, so the ink fountain roller and the anilox roller are required to print at a medium speed or in general. Flexo printing presses are still used.

The anilox roller-squeegee ink inking system is shown in Figure 6-16(b) and consists of an anilox roller and a metal squeegee. The blade of the squeegee is directed at an obtuse angle with the anilox roller at the surface linear velocity of the squeegee pressure contact. It is a reverse squeegee (a positive squeegee for a gravure printing machine). The squeegee is only needed. With a very light pressing force, the ink on the surface of the anilox roller can be removed. At present, high-speed and high-quality flexographic printing presses mainly use such an inking system to supply ink to printing plates.

1. Anilox roller - ink transfer performance of the squeegee ink system. The flexo printer uses an anilox roller-squeegee to infuse the ink. The squeegee blade uses a reverse mounting angle. As the printing speed increases, the dynamic pressure on the surface of the anilox roll increases, forcing the squeegee against the surface of the anilox roll. Therefore, the amount of ink supplied to the printing plate by the anilox roller mainly depends on the volume of the ink supply hole of the anilox roller and the condition of ink separation between the anilox roller and the plate surface. However, the relationship with the operating speed of the inking system is very small, that is, the amount of ink can be kept constant at various printing speeds. The above characteristics can well meet the needs of high-speed flexographic printing presses, and at low speeds, according to the print quality requirements, the amount of ink to be printed during normal printing can be precisely set, which is conducive to the stability of printing quality, and also to the stability of printing quality. The operator's work brings great convenience.

The ink volume of the anilox roller-squeegee ink inking system is independent of the dynamic pressure of the ink, so it is not affected by the viscosity of the ink. Figure 6-17 is based on experimental results. The thickness of the ink layer in the figure is the measured value of the ink layer on the print. The viscosity of the ink is measured with a No. 2 Jenfer cup and is the viscosity that is recycled into the inking system. It can be seen from the figure that the ink volume of the ink fountain roller-anilink roller inking system is very sensitive to changes in the ink viscosity, while the anilox roller-squeegee ink infusion system does not affect the ink transfer due to changes in the ink viscosity. Amount, can adapt to a variety of viscosity inks and coatings.

The anilox roller-squeegee ink inking system has a small amount of ink and is not easily changed. The squeegee is directly in contact with the anilox roller. Therefore, the quality of the ink, such as the material of the anilox roller, the ink hole, and the number of lines, is uniform. influential.

A- Ink roller - Anilox roller ink system B - Ink roller - Squeegee ink system