The development of flexographic printing plates from the rubber plate to the photosensitive resin plate has gone through a long process. In the field of modern flexographic printing, the photosensitive resin plate has many advantages compared with the rubber plate, such as the plate shrinkage is significantly reduced, the stability is better, the plate making process is simpler, and the reproduction accuracy of the original is higher. However, regardless of the flexographic version of the material used, there is a problem of how to extend the life of the printing plate. The following is a detailed analysis of the factors affecting the life of flexographic printing plates.
1. Plate making process
The manufacturing process of the photosensitive resin flexo is:
Negative picture preparation → dicing → back exposure → front exposure → rinsing (development) → drying → de-sticking → post exposure
(1) Exposure
The first is the backside exposure, which is a uniform exposure of the media from the back. Its main function is to establish the relief 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 enables the photosensitive flexographic plate to fully crosslink the monomer and improve the printing durability of the plate. Plates generally have an effective exposure time range. To avoid underexposure, it is best not to set the minimum exposure time for back exposure.
Followed by the main exposure, that is, the front exposure. The main exposure time depends mainly on the type of the plate and the size of the image on the negative image, which is generally proportional to the area of ​​the graphic. If the minimum exposure value is set for the main exposure time, there is also a risk of underexposure. For example, for a sheet having a thickness of 2 to 4 mm, the exposure time is generally from a few minutes to ten minutes. If the main exposure time is too short, the image is lighter and the hardness of the printing plate is lowered, and the life of the printing plate is shortened.
Finally, the post-exposure is a full exposure of the dried plate. After the post exposure, the entire resin plate can be completely photopolymerized, and the surface resin is fully hardened to achieve the desired hardness, improve the printing durability of the printing plate, and improve the solvent resistance of the printing plate, thereby prolonging the life of the printing plate.
(2) Rinse
Rinse and develop, that is, the plate is partially cured after exposure, and the portion where no light is seen is removed by a solvent under the action of a solvent. The depth of the brush is the height of the graphic relief. Usually, the development time is from several minutes to twenty minutes. If the development time is too long, it is prone to image breakage, surface bulging or unevenness of the layout, which shortens the printing plate due to excessive friction during printing. life.
(3) Drying
The washed plate is expanded by absorbing the solvent, and the solvent is sucked by hot air drying to restore the printing plate to the original thickness. This process is called drying. The drying temperature is generally from 50 ° C to 70 ° C, and the drying time is from several minutes to thirty minutes. The temperature and time should be strictly controlled during drying. If the drying is insufficient, the degree of expansion of the plate will be uneven, so that the printing process will be uneven and the plate life will be shortened.
(4) Light to stick
The purpose of de-bonding is to remove the stickiness of the surface of the plate and enhance the ink-filling ability. There are illumination methods and chemical methods for de-bonding the layout, and the illumination method is more suitable for sticking. The length of time during which the light is removed depends on the development and drying time. If the debonding time is too long, the plate may become brittle and cracked.
In addition, excessive illumination may also result in poor ink transfer, which appears to be caused by insufficient pressure between the printing plate and the substrate on the final print. Therefore, the printing operator often only adjusts the printing pressure, causing the printing pressure to be too large, resulting in an increase in printing plate wear.
2. Printing process
(1) Ink
After the flexible printing plate is finished, the next process is to be plated, loaded, and ready to be printed on the machine. At this point, it should be noted that the photosensitive resin flexographic printing plate is only matched with a certain type of ink and cleaning liquid. If the model is wrong or the model is correct, but the solvent content is too high, it will cause the printing plate to expand, shrink or crack.
When using a flexo water-based ink with a high pigment concentration, if the ink is dried on the surface of the plate cylinder or anilox roll, the plate wear is also accelerated. The pH of the water-based ink is generally controlled at about 9.0, which is slightly alkaline. If the pH is too large, it will cause the thixotropy of the ink to change and become thicker, and the conjunctiva is dry and difficult to dissolve. The dried ink film peels off the rubbing plate to accelerate plate wear.
In addition, the type of defoamer used will also affect the life of the plate. For example, aqueous defoamers are more likely to shorten flex life than silicone defoamers. Because aqueous defoamers are more likely to cause the plate to swell or crack.
(2) Solvent
Many people may have noticed that flexographic plates are more susceptible to cracking in summer than in winter. Changes in the summer environment have led to many factors that accelerate plate cracking. First, the increase in temperature exacerbates the expansion of the plate coated with a large amount of ink. Secondly, in summer, the humidity is increased. In order to ensure good fluidity of the ink, it is necessary to increase the content of acetate in the ink, thereby increasing the corrosion of the printing plate and shortening the life of the printing plate. In addition, there are many lightning lightnings in summer, which will generate a lot of ozone. The photosensitive resin flexographic plate is very sensitive to ozone. When the plate is exposed to ozone, it is easy to produce some small cracks, which makes it impossible to print. However, at low ozone concentrations, the main factor in accelerating plate cracking is the high concentration of acetate in the ink and plate cleaner solvent. In actual printing, the amount of ink and cleaning agent actually used by many printing operators far exceeds the recommendations of the plate making experts. If it is in the winter, there may not be any problem, but in the summer, the problem is much more serious.
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