To prolong the life of flexographic plates during production and use

The development of flexographic printing plates from the rubber plate to the photosensitive resin plate has undergone a long process. In the field of modern flexo printing technology, the photosensitive resin plate has many advantages compared to the rubber plate, such as a significant reduction in the shrinkage of the plate, a better stability, a simpler plate making process, and a higher reproduction accuracy of the manuscript. However, regardless of the type of material used, there is a problem of how to extend the life of the printing plate. In the following, we will analyze the factors that affect the life of flexo plates.

1. Platemaking process

Photosensitive resin flexographic production process flow is:

Yin picture preparation → Cropping → Back exposure → Front exposure → Flush (development) → Drying → Detacking → Post-exposure

(1) Exposure

The first is the back exposure, that is, the uniform exposure of the media from the back. Its main role is to establish the emboss depth of the printing plate and to strengthen the adhesion of the polyester support film and the photosensitive resin layer. Generally, the back exposure time is short, and the length of the back exposure time determines the thickness of the substrate. Moderate back exposure allows the photosensitive flexo monomer to fully cross-link and increase the plate's resistance to printing. Plates generally have a valid exposure time range. To avoid underexposure on the back, it is best not to set a minimum exposure time for back exposure.

Followed by the main exposure, the frontal exposure. The main exposure time mainly depends on the plate type and the size of the graphic on the negative picture, which is generally proportional to the area of ​​the graphic. If the main exposure time is set to the minimum value, the risk of underexposure also occurs. For example, for a plate with a thickness of 2 to 4 mm, the exposure time is generally several minutes to ten minutes. If the main exposure time is too short, it will result in lighter graphics and lower plate hardness, shortening the plate life.

The last is the post-exposure, ie full exposure of the dried plate. After exposure, the entire resin plate can completely undergo photopolymerization reaction, and the plate resin can be completely hardened to achieve the required hardness, improve the printing plate resistance to printing, and improve the solvent resistance of the printing plate, thereby prolonging the life of the printing plate.

(2) Rinse and rinse development, that is, the plate material is hardened by exposure to light after exposure, and the part with no light is removed by a brush under the action of a solvent. The depth of the brush is the height of the graphic relief. Usually the development time is from a few minutes to twenty minutes. If the development time is too long, it is easy to cause graphic damage, surface bulging or unequal layout issues, so that the printing plate will be shortened due to excessive friction during the printing process. life.

(3) Drying

The washed plate is swollen due to the absorption of solvent, and the sucked solvent is discharged by hot air drying to restore the original plate thickness. This process is called drying. The drying temperature is generally 50°C to 70°C and the drying time is from several minutes to thirty minutes. When drying, the temperature and time should be strictly controlled. If the drying is not sufficient, the layout will not be evenly expanded, so the stress will be uneven during the printing process, shortening the life of the printing plate.

(4) Light to remove

The purpose of de-bonding is to remove the tackiness of the surface of the plate and enhance the ink repellency. The method of de-bonding the plate surface includes light and chemical methods, in which light is used for detackification. The length of time the light is removed depends on the development and drying time. Excessive time for debonding can easily cause the plate to become brittle and crack.

In addition, excessive light may also cause poor ink transfer, which appears to be due to insufficient pressure between the plate and the substrate on the final print. Therefore, the printing operator tends to adjust the printing pressure only so that the printing pressure is too high, resulting in increased wear of the printing plate.

2. Printing process

(1) Ink

After the flexible printing plate is produced, the next process is to be pasted, loaded, and ready for printing on the machine.

At this time, it should be noted that the photosensitive resin flexo plate is only matched with a certain type of ink and cleaning liquid. If the model is wrong or the type is correct but the solvent content is too high, it will cause the printing plate to expand, contract or crack.

When using a flexo-based water-based ink with a high pigment concentration, if the ink is dried on the surface of the plate cylinder or the anilox roller, it will also accelerate the plate wear. The pH of water-based ink is generally controlled at about 9.0, slightly alkaline. If the pH is too high, it can cause the thixotropy of the ink to change and thicken, making the conjunctiva dry and difficult to redissolve. The dry ink film peels off the plate to accelerate the plate wear.

In addition, the type of defoamer used will also affect the life of the plate. For example, water-based defoamers tend to shorten flex-plate life more easily than silicone-based defoamers. Water-based defoamers are more likely to cause plate swelling or cracking.

(2) Solvents

Many people may have noticed that flexographic plates are more prone to cracking in summer than in winter. Changes in the summer environment have led to many factors that have accelerated the cracking of the printing plate. First, the increase in temperature exacerbates the swelling of the printing plate coated with a large amount of ink. Secondly, the humidity increases in summer. In order to ensure good ink flowability, it is necessary to increase the acetate content in the ink, thereby increasing the corrosion of the printing plate and shortening the life of the printing plate. In addition, there are more lightning flashes in summer, which will produce a lot of ozone. The photosensitive resin flexo is very sensitive to ozone. The exposure of the plate to ozone can easily produce some small cracks, resulting in the inability to print. However, when the ozone concentration is very low, the main factor for accelerating the cracking of the printing plate is the high concentration of acetate contained in the solvent of the ink and the plate cleaning agent. In practical printing, the actual amount of ink and cleaning agent used by many printing operators far exceeds the recommendations of the platemaking experts. If it is in the winter, there may be no problem, but in the summer, the problem is much more serious.

(3) Substrates

The rougher the surface of the substrate, the greater the friction with the printing plate, the faster the printing plate wears.

(4) Printing pressure

Printing pressure is also one of the important factors in accelerating plate wear. In the actual printing process, if the pressure between the anilox roller, the plate cylinder, the plate cylinder and the impression cylinder is too large, the edge of the plate may be worn or the plate may be cracked. In addition, the web and the anilox roller should be evenly compressed.

(5) Roller

The quality of the printing cylinder is one of the important factors that affect the printing quality. In general, the overall radial runout of the drum allows an error of 0.00025 inches. If the error value is too large, it will lead to uneven force of the printing plate, the pressure between some areas and the impression cylinder is too large, and the pressure between other areas and the anilox roller is too large, thus affecting the printing. The quality will also accelerate the wear of the plate.

3. Printing plate preservation

Printed plates should be cleaned immediately with a suitable solvent and a soft-bristle brush. Because the ink is difficult to remove after the surface of the printing plate is dry, cracking of the printing plate occurs when the printing plate bends again.

Plates must be kept away from ultraviolet, incandescent, fluorescent, and sunlight, and away from sources of heat and ozone (such as printer motors, corona discharge devices, etc.). For non-adhesive plates, between each two plates should be lined with paper or white plastic foam sheet, and then lay flat in the special printing plate box.

If the plate is loaded on the plate cylinder and it is stored with the plate cylinder, the plate should be wrapped and sealed with a black polyethylene film. In addition, it is also possible to protect the surface of the printing plate by spraying with a protective liquid, but it should be noted that these protective liquids cannot be sprinkled on the back of the plate since this will cause the plate to tilt.