The status quo and new technology of flexographic printing

Flexographic printing is a direct rotary printing method using a flexible raised image plate, the plate is fixed on a variable-length plate cylinder, and ink is applied by a metal ink roller engraved with ink holes; The metal ink roller (anil roller) is controlled by another ink roller or squeegee to control the amount of ink delivered; liquid and grease inks can be transferred to various substrates. In this process, the non-printing area of ​​the printing plate is not in contact with the substrate, and the ink supply to the printing plate is mainly performed by a doctor blade and a ceramic anilox roller. Flexo inks are very similar to gravure inks and have the same viscosity range. For some substrates, the two inks are even identical.

First, the advantages and technical analysis of flexographic printing

Computer-to-plate (CTP)

In the field of flexo printing, the engraving rubber plate process was used very early, which enabled direct transmission of image data to printing plates. For many years, rubber has been engraved with a thermal radiation carbon dioxide laser in various continuous printing and seamless printing processes. Modern heat-radiation YAG lasers have shorter wavelengths (only 1 μm compared to the traditional 10.4 μm), enabling more precise screen lines to be printed on the plate, improving the plate quality of the photosensitive resin plate. The shorter the wavelength of the laser, it means that the focus is clearer, which is good for carving finer patterns on rubber or synthetic rubber materials.

At the same time, the photosensitive resin plate does not require film as an intermediate information carrier. Its plate material does not differ much from the plate material used in traditional, film-based printing processes. However, unlike the traditional plate, it has a top layer structure and its optical performance is equivalent to fully exposed and processed film (optical density D=3-4). Now using a thermal radiation YAG laser, this layer is removed. Therefore, ultraviolet rays need to be exposed. Subsequent plate making processes (such as rinsing, drying, post-processing, and re-exposure) are no different from traditional processes.

Recently, the quality of flexo printing has been greatly improved. Eliminating an intermediate link, the manpower, material consumption and error rate are greatly reduced, and it also has other advantages of the CTP process.

From our printer's point of view, the details of the image are individual, tiny, closed elements, such as small dots with a diameter of 30μm, or a thin line with a width of 0.1mm, or font size 6P, font Time New Roman is the English alphabet. The details in Chinese characters are much larger than the last example above.

Comparing the digital platemaking production mode with the traditional film-based production mode (we also call it the analog production mode), we can see first that in digital platemaking, the number of lines has a much smaller effect on the tone value. Even if the selected number of lines is high, it can be printed accurately. Second, the analog platemaking process has certain limitations in low-level adjustments (ie, cooking areas for halftone images). 1%-2% of outlets will become 10-15% after printing. In the traditional platemaking process, a tone correction method is needed to overcome this disadvantage. In the digital platemaking process, even if the tone value is less than 5%, there is no problem and it is not related to the number of lines. This is the most important advantage of flexo printing with digital platemaking and other processes such as gravure or offset printing.

When copying fine image elements, the digital printing plate performs much better than the analog printing plate. The factors that affect the quality of the simulated plate are present during the entire plate making process, especially during exposure of the printing plate. These factors are:

â–  The traditional printing plate has a so-called release layer, which is in contact with the film. During exposure, the film may adhere to the photosensitive resin plate and be damaged during separation. This should be avoided as much as possible.

â–  Flexographic film is matte on the contact surface, so that the air between the film and the plate escapes due to the formation of vacuum before the plate is exposed.

â–  To produce a vacuum, the film and plate are covered with a diffuse plastic film.

Second, sleeve technology

The sleeve is actually a tube, it is set on the roller, with the air film as a lubricant between the roller. The drum is usually hollow and has small round holes that allow air to run from the inside of the drum to the drum surface. Sleeves are commonly used as pads for printing plates and also as carriers for engraved ceramic layers on anilox rollers.

There are many advantages to using a sleeve, including:

â–  By using different thicknesses of sleeves, it is possible to reduce the number of different types of roller stocks. With a single roller, it is possible to print a variety of web products.

â–  If the plate can be made on the sleeve, the operation of installing the plate is eliminated, which is possible in the direct engraving plate and CTP process.

â–  If the plate can be made on the sleeve, both the print length and the print registration are in compliance with the requirements for multicolor jobs.

â–  If the bearing and the side panel can be removed from the machine and a roller with a conical bore is installed on the other side of the machine, the replacement of the job can be done quickly by one person in a few minutes.

â–  If the sleeve has a flexible top layer, the plate can be installed with simple and inexpensive tape without degrading the quality of the print.

Third, gearless direct drive technology

A gearless direct drive system (electronic control) with a separate motor is used for the press or other workbench. For flexographic printing, there are several advantages:

â–  printing length can be changed steplessly;

â–  can choose the best drum speed for different material thickness;

â–  The tolerance of the mounting plate can be adjusted;

â–  There is no limit to the registration adjustment in the printing direction;

â–  No need to store gears of different specifications, so no need to change gears;

In general, the tooth pitch of the plate cylinder gear determines the minimum unit of the change of the printing surface. The value is approximately 5-10mm. If it is necessary to print the job produced by other printing methods in a ratio of 1:1, the loss of the substrate is required. May be larger. A manufacturer of flexographic printing presses calculated this loss, with 1%-2% loss in 50% of jobs.

In the case of using a plastic film with a thickness of 60 μm and a paper jam of 1 mm, the thickness of the substrate is different, which is equivalent to a change in the diameter of the substrate roll. In the gear drive technology, the rotational motion between the gears and the rotational movement between the printing plate and the substrate roller rarely match. With the gearless direct drive technology, we can adjust the roller surface speed according to the needs of the substrate. .

The diameter of the plate cylinder on which the plate has been mounted will change with the size of the cylinder, sleeve, tape, and printed body. When the printing plate has just passed the developing process, its thickness will be larger due to the expansion; after the printing plate is left for 2-4 hours, its thickness will become slightly smaller. That is, if the plate is not placed long enough after development, the thickness of the plate will change after it is mounted on the plate cylinder. As a result, the printing length will also change.

In general, the registration in the printing direction is accomplished by the axial movement of the helical gear on the plate cylinder shaft in opposition to the helical shaft gear of the impression cylinder. The width of the plate cylinder limits the amount of correction and uses a gearless drive. After the technology, this can be completely avoided and there are more choices in controlling the length of the printing.

Fourth, the anilox roller and printing plate cleaning

There are only a few components in the printing section of the flexographic printing press. The correct order of these components is a prerequisite for good print quality. This means that the anilox roller must be cleaned according to the rules in order to maintain its ink supply quality. For various reasons, all ink systems can cause the anilox roller to slowly decrease the amount of ink. Therefore, anilox rollers require cleaning systems, and there are many such systems on the market. They can be divided into internal cleaning and external cleaning (using friction, chemical, temperature, and supersonic principles, respectively).

In addition to the factors of the printing plate, some substrates may also lead to a reduction in printing quality. In particular, non-coated paper or cotton paper may bring some powder to the printing press. The non-printing area of ​​the plate may accumulate some fibers and ink and eventually result in extra prints, which makes it necessary to clean the plate at work. There are only a few online cleaning systems on the market - so it is often necessary to clean the plates when they are down.

Fifth, the characteristics of flexographic printing

advantage:

â–  Can be printed on various absorbent and non-absorbent substrates;

â–  Variable width in the printing direction;

â–  can achieve a higher printing speed;

â–  Since the doctor blade is completely filled with new ink, there is no residual graphic information on the anilox roller;

â–  printing plate ink uniform;

â–  The structure of the ink supply system is simple;

â–  Plates are cheaper than gravure printing engraving rollers;

â–  can choose a variety of inks;

â–  The printing effect on various printing materials is good;

â–  Substrate consumption is minimal because the ink path is short. After a simple ink distribution, the ink film is stable; and the substrate travels a short distance between different printing units.

Disadvantages:

â–  The pictures to be printed may have an impact on the smooth operation of the machine, leading to a reduction in print quality;

â–  The printing quality is closely related to the distance between the plate cylinder and the impression cylinder, which depends to a great extent on the thickness of the plate and the compressibility of the substrate. In the halftone printing, dot gain rate is higher.

â–  The print quality depends on the printed content (color blocks, halftone images, and line images) and the compressibility of the plate liner;

â–  Expansion occurs due to the use of solvents during the development process of the photosensitive resin plate. To eliminate swelling, the plate must be stored for many hours after development;

â–  Because of the elasticity of the printing plate and the tape, and the rebound speed of the printing plate after pressing, there is a need to adjust the distance between the printing plate and the impression cylinder according to the rotation speed of the printing press;

â–  When printing on loose substrates such as tissue paper, the plates and anilox rollers will be contaminated; the plate needs to be cleaned halfway, and the anilox roller needs to be cleaned more frequently.

Sixth, concluding remarks

CTP technology, sleeve technology and gearless direct drive technology are important factors in ensuring product quality, high competitiveness and efficiency. With the support of these technologies, the printing quality and economic feasibility of flexographic printing are guaranteed. In the past, people thought that the flexographic printing process was a low-quality process, but now, in most cases, it is difficult to distinguish between flexographic and offset prints even with a magnifying glass.

After years of printing production, it is proved that flexographic printing is a printing method that is in line with China's national conditions. Many printing companies have achieved good economic benefits after adopting a flexographic printing process. In some printing companies with rapid technological progress, the use of flexographic printing has also reached a very high level of quality.

With the development of technology, especially the technical progress of flexo printing plates and flexo printing inks, the quality of flexographic printing has been able to meet higher requirements.

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