Flexo Printing Answers Collection (16-26)

16. What role does the main exposure play?
The main exposure of the flexible photosensitive resin plate is performed by a negative or calendering layer, the role of which is to form a graphic for printing.

17. How to make the main exposure?
First, place the plate face up on the vacuum platen, peel off the protective film on the plate, and place a negative image for plate making at a predetermined position. Note that the film emulsion surface must be attached to the plate surface. Then use an anti-static brush to clean the layout and negative film, cut the polyester film or cellulose acetate film into strips, cover the parts of the plate that are not covered by the negative image, and then use a plastic sheet to press the plate. , Film, polyester strips are all covered, wiped with a soft cloth, the film between the gas discharge, vacuum (vacuum pressure above 0.8Pa), open the light source, you can carry out the main exposure.

18. In the main exposure, what is the function of the polyester film strip or acetate film covering the area of ​​the plate that is not covered by the negative image?
The function of the polyester film strip or acetate film strip is to prevent the uppermost cover film from sticking to the sticky plate material and create a venting condition for vacuuming.

19. Why is it necessary to set up a high-light part (such as a 2% dot) to generate more energy than to create a field?
To understand this problem, readers should first understand how the photosensitive polymerization process is spatially controlled. First, the solid part is formed by irradiating UVA light through a large transparent area on the film, whereas 2% of the dots are created by exposure of UVA light through a portion of the film that transmits only 2% of the UVA light. In this way, only 1/50 of the UVA energy is used for the polymeric relief at the 2% dot, and the height of the polymerization is from 0.5 to 0.7 mm from the top to the bottom. It is not difficult for us to conclude that in order to reach the level of undissolved polymerization, the exposure time required for the 2% halftone part is much longer than the solid part. Exposing spatially with UVA light of a two-dimensional plane influences the shape of the three-dimensional three-dimensional plate material to varying degrees, ie, controls the formation of relief.

20. How to set the main exposure energy?
The photosensitive resin plate is exposed through a negative film or Other barrier layer. Setting the main exposure energy is particularly important for producing high-quality printing plates. In order to accurately set the energy of the main exposure, first of all, it should be clear what happens to the fine anti-yin part when the highlight part is exposed for a long time. High-quality prints often involve the creation of high-light dots and small negative parts on the same plate, that is, the problem of challenging the polymerization limit of the photosensitive resin plate. As shown in FIG. 2 , a large amount of light energy is required to establish a support structure to form a high light dot. However, at the same time, in the small negative part, the UVA light is diffused and reflected at a part that does not require exposure. This energy impinges on the photosensitive resin at the part and starts the polymerization cross-linking to form a so-called “anti-yin”. Fill in the results. There are a variety of ways that a plate maker can deal with this issue. But the most important thing is to know in advance the standards of the plates used. This information can be obtained from suppliers or through exposure tests. These tests can simultaneously obtain the minimum exposure to establish a 2% dot and the maximum exposure to ensure that the anti-female lines (eg, 0.7 mm) are not filled. These limits indicate the photosensitivity of this type of plate. When some designs exceed the limit, the platemaker can selectively block the portion of the main exposure that has been completed, and continue exposure to obtain a fine dot. This is an example of comprehensive use of space and time technology. In addition, when we are paying attention to the exposure of UV imaging, we should not ignore the development or washing stage, some factors will affect the latitude of the plate. For example, if the solvent in the processor is not properly replenished or if the brush is improperly set, it will cause blocking of the negative part, which may be incorrectly interpreted as the problem of exposure latitude. All the links need to be combined effectively, and UV exposure imaging is only one of them.

21. What is the specific process of photopolymerization?
In the plate-making process, photopolymerization started when UVA light with a peak value of 265 mm was subjected to main exposure through a negative film or back exposure through a substrate. Step Ia shows that the photoinitiator absorbs UVA light and causes the reactive free radicals (broken egg pattern represents reactive groups) to break. Free radicals react with monomer molecules as they bind together. Step 1b is the chain reaction initialization process. The reactable monomers interact to form a criss-cross network structure. As a result, the physical properties of the raw material also change, and this part of the chain reaction forms a lattice structure called the proliferation stage. Step 2 shows that when two reactive molecules (a monomer or a radical interact with each other) form a part of a chain of an unreactive lattice structure. At this point, the reaction is over. As long as the UVA light is always Irradiation, with the presence of photoinitiators and monomers, which govern the proliferative response. Once the light source is turned off, the reaction is over due to the withdrawal of dominance. Therefore, we can control the exposure by controlling the switch of the UVA light source. Time.The flexo plate can be used to determine where the polymerization will take place with a negative or opaque layer.Figure 3 shows that the energy of the UVA light determines the photopolymerization process.

22. What is development?
After the plate is exposed, the process of removing the unhardened portion on the plate by the solvent is called development. The development time is determined by the thickness of the plate and the area of ​​the graphic, and different media are used for different plates.

23. What is the theoretical basis for plate material development?
In the graphic part formed by the interlaced grid structure, the photopolymerization changes the physical properties of the photosensitive resin. This is the theoretical basis for media development. The non-crosslinked structure can be dissolved in the solvent of the wash, while the crosslinked structure of the polymeric portion is insoluble in the wash solvent. This change in the solubility of the photosensitive resin determines the mode of development of the plate. Similarly, for liquid and thermal plates (FAST), changes in fluidity and heating performance can also be the basis and means of development.

24. Why dry?
Since the solvent penetrates into the plate after development, the plate is usually inflated and sticky, and the original line in the picture looks like a wavy line, and the text is also twisted. This is normal. This phenomenon is eliminated when the excess solvent is removed by drying.

25. How to dry?
The developed plate was placed in an oven, and the plate was dried with warm air at a temperature of 50 to 70C for several minutes to 30 minutes.

26. What is the viscosity?
Since the surface of the printing plate after drying has a viscous property, it needs to be removed with a UV-C (wavelength of 254 mm) light source to enhance the strength of the plate material and complete the plate making process.

(to be continued)