The flexographic printing plate material has undergone a long process from the rubber plate to the photosensitive resin plate. In the field of modern flexo printing, photosensitive resin plates have many advantages over rubber plates, such as significantly reduced plate shrinkage, better stability, simpler plate-making processes, and higher accuracy in reproducing originals. However, no matter what kind of material is used in the flexible version, there is a problem of how to extend the service life of the printing plate. The following is a detailed analysis of the factors that affect the life of the flexographic printing plate.
1. Platemaking process
The production process of the photosensitive resin flexographic plate is:
Negative picture preparation → cropping → back exposure → front exposure → rinsing (development) → drying → debonding → post exposure
â‘´Exposure
The first is back exposure, that is, uniform exposure of the plate from the back. Its main function is to establish the embossed depth of the printing plate and 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 plate base. Moderate backside exposure can make the monomer of the photosensitive flexographic plate fully cross-linking, and improve the printing plate's durability. Plates generally have an effective exposure time range. In order to avoid underexposure on the back side, it is best not to set the minimum exposure time for back exposure.
The second is the main exposure, which is the front exposure. The main exposure time mainly depends on the plate model and the size of the graphic on the negative picture, which is generally proportional to the area of ​​the graphic. If the minimum value is set for the main exposure time, there is also a risk of underexposure. For example, for a sheet with a thickness of 2 to 4 mm, the exposure time is generally a few minutes to a dozen minutes. If the main exposure time is too short, it will cause shallower graphics and lower plate hardness, shortening the life of the plate.
The last is the post-exposure, that is, comprehensive exposure of the dried printing plate. After the post-exposure, the entire resin plate can be completely photopolymerized, and the plate surface resin can be fully cured to achieve the required hardness, improve the printing plate durability, and improve the solvent resistance of the printing plate, thereby extending the life of the printing plate.
⑵Flush
Rinse and develop, that is, the exposed part of the plate is hardened after exposure to light, and the unexposed part is removed with a brush under the action of a solvent. The depth of the brush is the height of the graphic relief. Generally, the development time is about a few minutes to 20 minutes. If the development time is too long, it is easy to cause graphic damage, bulging surface or unequal height of the layout, which shortens the printing plate due to excessive friction during the printing process. life.
⑶Dry
After being rinsed, the printing plate swells due to the absorption of the solvent, and the absorbed solvent is discharged by hot air drying to restore the printing plate to its original thickness. This process is called drying. The drying temperature is generally 50 ° C to 70 ° C, and the drying time is 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 the force will be uneven during the printing process, shortening the life of the plate.
â‘·Light to remove
The purpose of detackification is to remove the tackiness of the surface of the plate and enhance the inking ability. The methods for debonding the plate are light method and chemical method, among which light method is more used for debonding. The length of the light debonding time depends on the development and drying time. If the debonding time is too long, the printing plate will become brittle and crack. In addition, excessive light may also lead to poor ink transfer, which looks like the lack of pressure between the printing plate and the substrate on the final printed product. Therefore, the printing operator often just adjusts the printing pressure to make the printing pressure too large, which leads to increased abrasion of the printing plate.
2. Printing process
â‘´Ink
After the production of the flexible printing plate is completed, the next process is to paste and install the plate and prepare for printing on the machine. At this time, it should be noted that the photosensitive resin flexographic printing plate only matches a certain type of ink and cleaning liquid phase. 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 flexographic water-based inks with high pigment concentrations, if the ink dries on the surface of the plate cylinder or anilox roller, it will also accelerate plate wear. The pH value of water-based ink is generally controlled at around 9.0, slightly alkaline. If the pH value floats too low, it will cause the ink thixotropy to change and become thick, easy to dry the conjunctiva and difficult to re-dissolve. The dried ink film peels off the friction printing plate, accelerating the wear of the printing plate.
In addition, the type of defoamer used will also affect the life of the printing plate. For example, water-based defoamers are easier to shorten the life of flexographic plates than silicone-based defoamers. Because the water-based defoamer is more likely to cause the printing plate to swell or crack.
⑵Solvent
Many people may have noticed that flexographic printing plates are more likely to crack in summer than in winter. Changes in the summer environment have led to many factors that accelerate the cracking of the printing plate. First, the increase in temperature exacerbates the expansion of printing plates coated with large amounts of ink. Secondly, the humidity in summer increases, in order to ensure the good fluidity of the ink, it is necessary to increase the content of acetate in the ink, thereby intensifying the corrosion of the printing plate and shortening the life of the printing plate. In addition, there are many thunderstorms and lightning in summer, which will generate a large amount of ozone. The photosensitive resin flexographic plate is very sensitive to ozone, and the plate will easily produce some small cracks when it comes into contact with ozone, making it impossible to print. However, when the ozone concentration is very low, the main factor that accelerates the cracking of the printing plate is that the concentration of acetate contained in the solvent of the ink and the printing plate cleaning agent is too high. In actual printing, the amount of inks and cleaning agents actually used by many printing operators far exceeds the recommended values ​​of platemaking experts. If it is in winter, there may not be any problems, but in summer, the problem is much more serious.
⑶ substrate
The rougher the surface of the substrate, the greater the friction with the plate, and the faster the plate will wear.
â‘·Printing pressure
Printing pressure is also one of the important factors to accelerate plate wear. In the actual printing process, if the pressure between the anilox roller-plate cylinder and the plate cylinder-impression cylinder is too large, it will cause the edge of the plate to wear or the plate surface to crack. In addition, the web and anilox should be evenly pressed.
⑸Drum
The performance of the printing cylinder is one of the important factors affecting the printing quality. Normally, the tolerance of the overall radial runout value of the drum is 0.00025 inches. If the error value is too large, it will cause uneven force on 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, which affects both printing Quality will accelerate plate wear.
3. Plate preservation
After printing, the printing plate should be cleaned with an appropriate solvent and soft brush immediately. Because the ink is difficult to remove after the printing plate surface is dry, it will cause the printing plate to crack when the printing plate is bent again.
The plate must avoid the irradiation of ultraviolet rays, incandescent lamps, fluorescent lamps and sunlight, and away from heat sources and ozone sources (such as motors of printing presses, corona discharge treatment devices, etc.). For non-adhesive printing plates, each piece of printing plates should be lined with paper or white plastic foam, and then lay flat in a box specially equipped with printing plates. If the plate is installed on the plate cylinder, it should be wrapped and sealed with black polyethylene film when stored with the plate cylinder. In addition, you can also spray the protective liquid on the surface of the printing plate to protect, but it should be noted that these protective liquids cannot be sprayed on the back of the board, because this will cause the board to tilt.
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