Flexo printing is a direct letterpress rotary printing method that uses a flexographic plate to pass through the web, but it is different from letterpress printing.

Flexo printing has some characteristics: the material is a flexible material (a rubber plate was used before the 1960s and a photosensitive resin is used instead of an eraser after 60s). The printing pressure can be very light, reducing the loss of the plate and the machine; Liquid faster than ink; plate, pad version easier than the lead plate; can be adapted to print on some printing materials that can not be printed with gravure or gravure printing, such as rough surface, absorbent material, but also in some Non-absorbent materials such as wax paper, cellophane, aluminum foil, plastic film and its products, glass and its products, paperboard and its products, etc., printing speed faster than letterpress rotary press, commonly used web printing, printing machine's cost is low, Less waste, so the economic benefits are good. According to foreign forecasts, flexographic printing will compete with offset and gravure.

First, flexible plate making method

The flexographic printing plate originally had a carved rubber plate and a rubber shaped plate, and their process flow was as follows.

The engraved rubber plate making process flow is: originals → transfer images to the rubber roller → engraving → correction → printing.

The rubber forming method plate making process is: original preparation → making a template → pressing a plate.

The original can be a movable type, photo relief, etc., special original thermosetting phenolic resin as a template material, after hot pressing into a template, and then to natural rubber or synthetic rubber, vulcanized, die-casting with a compression molding machine and Cured to form a rubber block.

There are various types of flexographic plate products on the market today. The photosensitive rubber layers have different compositions, but all consist of the following components.

The highly elastic polymer body is a polymer elastomer resin. Commonly used are synthetic rubber, styrene-butadiene rubber, nitrile rubber, sodium butyl rubber, cyclized rubber, etc.; polyurethane elastomers such as TDI elastomers, hexamethylene diisophthalic acid ester elastomers; polyester elastomers such as Oxalic acid polyols are unsaturated polyesters; styrene copolymers such as propene copolymers, acrylonitrile copolymer elastomers, and the like. Due to the difference in elastic resins, the elasticity, wear resistance, solvent resistance, and developer of various printing plates are not the same.

Photoinitiators Photoinitiators determine the sensitivity and depth of the plate. Commonly used are benzoin ethers, aromatic ketones, anthraquinones, diazo compounds, and halides.

Crosslinking agents mainly use acrylic monoesters or polyesters. Also useful are polyamine active compounds such as double amino compounds or azide compounds.

Thermal Inhibitors Hydroquinone, gallic acid, and vitamin C are commonly used to prevent thermal polymerization in the system and improve the shelf life of plates.

Sensitizers A variety of dyes, such as sub-blue and blush, are commonly used to increase sensitivity and expand the photometric spectrum.

The Cyrel plate produced by DuPont, USA, consists of a highly elastic polymer of polystyrene-polyisoprene or polybutadiene and trimethylol triacrylate. Develop with a mixture of 1.1.1-chloroethane.

The plate process of the flexographic plate is similar to that of the photosensitive resin plate. After exposing the negative film with the negative image, the relief plate with ammunition is developed. The process flow of the plate making process is as follows: manuscript → close-contacting with an image negative film → developing → drying → post-exposure → correcting → printing.

Exposure first full exposure of the back of the plate, the exposure time should not be too long, and then the front exposure, the length of the exposure time, affect the graphics part of the resistance and the blank part of the sunken depth, the general part of the sunken depth, trademarks, etc. It is 0.4-0.5 mm and the depth of the printed paper bag is 1.0-2.0 mm.

Development In a predetermined developer, the uncured compound remaining on the plate surface is removed to make the surface of the image bright and smooth, and the developed surface embossed slope is 65 to 70°.

Drying can be hot air drying, the temperature is controlled below 60 °, to prevent the image from being deformed due to excessive temperature. It can also be left at room temperature for 24 hours to allow it to dry on its own.

Post-exposure After the plates are fully dried, post-exposure is performed to fully cross-link the polymers to improve the printing plate's resistance to printing.

The printed plate can be printed by using double-sided adhesive tape on the platen cylinder of the rotary machine.

Second, the flexographic printing machine

Flexo printing presses use web transfer machines, which consist of a conveying section, a printing section, a drying section, and a rewinding section.

1. Volume section

The paper roll in the roll-feed section must enter the printing section in a straight line, and it is required that the web tension be sufficient to eliminate wrinkles on the paper and prevent the paper from sagging when the printing press rotates or stops. It is equipped with an air expansion or paper jamming mechanism in the paper loading shaft, and the overprinting of the paper is controlled by the phototube detection head, so that the overprinting of the print is correct.

2. Printing section

Each monochrome unit of the printing section consists of a plate cylinder, an impression cylinder, an ink fountain, an ink fountain roller, and an ink roller.

Since flexo printing uses fast-drying liquid inks, the ink supply mode is different from letterpress printing. The ink fountain roller is a metal or rubber-coated roller that is etched into mesh holes. The ink fountain roller is rubberized or metal rollers. .

In the use of thicker inks for printing or glazing, coating, generally in the ink roller also need to add a flexible squeegee, in order to overcome the ink due to the gap between the two rollers caused by the current limiting effect, The use of ink can spread the ink roller more evenly and well. The use of a squeegee is especially important for chromatic printing because it allows the ink flow to be thinner and more accurate, thereby improving the quality of the print.

The printing part of a flexographic printing press is generally composed of two color units to eight color units. According to its arrangement, there are stacking, satellite, and horizontal linkage.

Cascade is in the host two will be stacked together for printing a single unit, as shown in Figure 6-1, each unit is driven by the host gear chain, the machine can print on both sides, and can be completed before the reverse printing ink drying; Between monochromatic units, there is room for inspection and repair, which makes the replacement and washing of parts more convenient; the accuracy of the toilet sets is somewhat poor, and it is not suitable for printing high-stretch or extremely thin printing materials.

The horizontal linkage type is that the monochromatic units are separated independently, and their positions are kept on the same level and driven by a common axis. As shown in Figure 6-3, this machine is similar to a laminated flexographic printing machine and can perform double-sided printing. Overprinting accuracy is poor, but it is used as a narrow web press. Since the distance between printing units is not too large, better overlay accuracy can still be guaranteed.

In the printing process, the ink of the previous color is dried before the next color printing, otherwise it will be mixed with the previous color, and after the printing is completed, when the rewinding is performed, the ink must also be dried, otherwise the ink will be dirty. To this end, the printer is equipped with a dryer. Desiccator inter-color dryer (not available on all flexo printing presses) and the rear total dryer. After the dryer dries the prints, the web is rolled onto the take-up roll.

4. Rewind part

The rewinding part is the paper receiving part of the printing machine. A shaft is mounted on an ordinary bearing, and the reeling roller is fixed by the iron core chuck to take up the printed matter. Some printing presses also have the function of cutting and slitting to facilitate future cutting.

Flexible printing includes trademarks, labels, paper bags, plastic bags, and aluminum film composites. It can print both text and dot images. In recent years, the use of flexographic printing has continued to expand. The United States has used newspaper printing and printed books and magazines on printing presses.