Flexible plate making process
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The surface of the flexographic plate must be elastic and should be bent without deformation. The materials that meet the performance requirements of flexible printing plates are generally in the two categories of rubber plates and photosensitive resin plates, and their manufacturing processes are completely different.
1 rubber version
Due to different manufacturing processes, it can be divided into the following three methods:
1) Hand engraving. It is often used for corrugated box printing. The screen is generally composed of thick lines, large and small color blocks and thick texts. The content is simple and the printing requirements are not too high.
Production process:
Depicting the original image → anti-image sticking on the rubber plate → engraved version
The transparent original image is reversely attached to a certain standard, flat rubber surface, and the text image must be clear. The operator uses the engraving knives of different sizes and shapes, and removes the rubber of the non-printing part according to the contour of the text image to form a depression, and the text image is retained as a printing plate.
2) Molding rubber plate (this process is more complicated. First make a metal plate (copper plate or zinc plate), then press the phenolic resin plate (similar to the letterpress paper version), also known as the master plate. Then cover the master plate with a rubber plate, After special high-temperature and high-pressure treatment, the rubber is vulcanized and cooled in the master mold, and after being smoothed, it becomes a rubber plate.
The production process of the mold rubber plate:
Original → negative negative film → dry metal plate → pressed phenolic resin plate → high temperature and high pressure rubber plate → thickness measurement → grinding rubber plate back.
3) The laser engraving method uses photoelectric scanning, analog-to-digital conversion, and laser engraving to automatically engrave the rubber plate. When engraving, according to the graphic data, the layout is carved into a different depth on the rubber plate to become a printing plate. This method can make a screen version, especially for seamless sleeve printing, which is more suitable.
2 photosensitive resin plate
It can be divided into two types: solid photosensitive resin plate and liquid photosensitive resin plate. The two types of plates are different in composition, the production process is different, and the performance is somewhat different, but the application range is the same.
l) Liquid photosensitive resin plate. The photosensitive resin is pre-coated on a specific exposure device during plate making. The exposure device consists of two upper and lower precision glass plates and upper and lower ultraviolet light sources, so that the plates can be exposed to the front and back.
* Apply liquid resin and perform main exposure and back exposure.
As shown in Fig. 1, when applying the liquid resin, the emulsion of the negative image is placed face up, placed on the lower glass, covered with a film, and protected by vacuuming to make the film in close contact with the negative image (coated on the cover film). The photosensitive resin liquid is placed on the resin layer, and the upper layer is placed on the resin layer. The photosensitive resin layer is sandwiched between the two glass plates. Figure 2 is a back exposure, main exposure device. Back exposure causes the substrate and resin to bond. Firm; in the main exposure, the light is transmitted through the negative image onto the photosensitive resin layer, and the resin in the light receiving portion is hardened to become solid, and the photosensitive resin in the unexposed portion is still in a liquid state. Therefore, the exposed printing plate can be tilted and placed. The unexposed excess resin on the plate is recovered in a recovery tank.
※ Flush (developing).
For the already exposed plate, use a special brush to brush the residual photosensitive resin solution in the groove, as shown in Figure 3. The rinsed printing plate is dried in an oven and then subjected to a post-exposure treatment to completely harden the resin on the printing plate.
The domestic flexo industry uses a small number of liquid plates and is only used in corrugated box making. It has problems such as limited format, low resolution, and poor hand feeling, and it is not suitable for making fine image plates.
2) Solid photosensitive resin plate. This is a pre-coated version, just like the PS version used in lithography. It should be stored in a light-proof cardboard box. When using, it is very convenient to take out the cutting from the box according to the size of the plate making.
At present, the commonly used flexible solid photosensitive resin plates in China include the CYREL version of DuPont of the United States, the BASF version of Germany, the GS version of China, the Poly version of the United States, and the co-prosperous version of Japan. These plates are available in a range of sizes, thicknesses and hardnesses. Their commonalities are: a. the same structure of the plate; b. the same exposure machine and plate washer; c. the same washing solution, all of which are organic solvents such as trichloroethylene and tetrachloroethylene; d. Go through the same process.
The following is a description of the operation of the solid resin plate for reference.
Main role: 1 The protective layer is a frosted base, which prevents the photosensitive resin layer from being rubbed and blocks the light from illuminating the resin layer to cause photochemical reaction, resulting in waste. 2 The adhesive layer is applied between the protective layer and the photosensitive resin layer, but it cannot be excessively bonded. 3 The photosensitive resin layer is the main body of the plate material, and its photosensitive property should be stable and reliable, uniform coating, uniform thickness and good flatness. 4 The adhesive layer is applied between the bottom of the resin layer and the base, so that the two are firmly bonded and difficult to tear. The 5th plate base is the basis of the solid plate. It is very important to keep the plate size stable and non-stretchable, including the flatness and thickness uniformity of the plate itself.
Solid resin plate production process:
Original → Make a positive image → Cut the material → Pre-exposure → Main exposure → Flush (develop) → Dry → Post-process → Post-exposure → Correct.
Note on each process:
a. First check the quality of the negative film. Check the content: 1 Copy the correct size of the graphic. 2 The graphic must be a positive image, and the film of the film is facing up. 3 Films should have a reading range of 0.06--3.5 on the densitometer. 4 The picture is clear, the layout is free of creases and burrs. 5 outlets are strong, and the high-light dark-adjusted outlets meet the plate-making requirements. In the case where the unit has no measuring instrument, the film can be visually observed towards the sunlight or fluorescent lamp, and the film is qualified if there is no light passing through the opaque film.
Cutting (cutting): The circumference of the layout should be slightly larger than the size of the negative image. It is mainly used to prevent the ink solvent from immersing into the double-sided adhesive when printing, and the printing plate is used as the edge sealing.
Pre-exposure on the back of the plate. Put the cut plate into the exposure device, the photosensitive film faces down, do not use the film, do not vacuum, turn on the light source, and expose.
Flex front back exposure
The length of the pre-exposure is determined by the type of the plate, the strength of the light source, the thickness of the text, and the complexity of the image. Its role is to increase the thickness of the plate base, to ensure that the relief of the plate has a certain depth, and improve the printing durability. The pre-exposure time is proportional to the thickness of the substrate. Long time, thick base; short time, thin base. In fact, the embossing depth of the plates of various models has a certain standard. For example, DuPont Cyrel 2.28mm plate, after removing the protective layer, the standard relief depth is l.lmm, how to achieve it? Can be determined by the following method, as shown in Figure 6, with a long resin plate (length l0cm, width 5cm), first block the face exposure l0s, then divide the layout into five segments, mark it, and then use a piece of black paper, The part from 2 to 5 is covered and exposed for 10 s, at which time the portion of the first section is exposed to light for 20 s. Then, the exposure is sequentially performed from 2 to 5 stages, and each time is exposed for 10 s. Then, the first section receives 50s, and the 2nd, 3rd, 4th, and 5th sections are exposed for 40s, 30s, 20s, and 10s respectively. After being developed and dried, the plate is measured by a thickness gauge, and the thickness of the plate is obviously different. Therefore, the test result can be used as a basis for determining the thickness of the substrate.
Test test of the relationship between pre-exposure time and plate thickness
Front exposure (ie main exposure), as shown in Figure 7. The plate exposed on the back side is turned over, and the protective layer is peeled off to expose the photosensitive resin layer. On the vacuum-printing machine, the emulsion surface of the positive image is closely attached to the photosensitive layer. Cover with a suction film (special frosted transparent material) and vacuum.
Flex front exposure
The determination of the main exposure time should be determined according to the type of the plate, the intensity of the light, the density of the negative film, and the thickness of the text. The length of the main exposure time is related to the depth of the relief of the layout and the slope of the side of the font. If the exposure time is excessive, the depth of the relief will be shallower; while the slope of the side becomes larger, the pores between the words become smaller, which is easy to produce paste and other defects during printing; on the contrary, insufficient exposure time will make the layout embossed too deep, small text The lines are easy to bend, and the small points are difficult to stand on the feet, which is easy to cause a waste version.
It is especially important to master the main exposure time. In general, for each batch of purchased media, the main exposure time needs to be retested as the light source ages. You can use the exposure test or the method of setting the thickness of the yin and yang lines or text on the film to select the appropriate main exposure time.
Different types of plates have different performances, especially in terms of latitude of exposure time. From the experience of the DuPont Seri version, the German BASF version, the domestic GS version, and the American Poly version, I feel that the exposure time of the Seri version and the BASF moderator is easy to master, and its tolerance is large, and it is exposed and exposed. The 15s time has little effect. The United States Poly version is less tolerant, more exposed, less exposure for 15s, the plate will cause problems. Therefore, mastering the main exposure time is the key to ensuring the quality of the plate making.