Xi'an University of Technology

In recent years, flexographic printing has made great progress in the global field due to its unique advantages. In many application fields, flexographic printing has challenged offset and gravure printing, not only in terms of efficiency, but also in quality. .

Nowadays, in order to meet the growing demand for high-quality prints in the market, most of North American and European countries have adopted relatively thin plates. Depending on the terminal application, the thinnest version can reach a thickness of 0.76mm and the thickest can reach 2.84mm. People in the industry expect that the thin-plate process technology can bring greater development advantages to product quality and performance, including reducing raw material costs and shortening the plate-making cycle, the use of higher-hardness plate materials, shallow relief depth, and improved image quality on the plate. Good ink transfer control, reduce plate wear, reduce printing press consumption, reduce distortion, etc.

At present, the plate material with a thickness of 1.14mm is widely used in the printing of folding cartons (such as cigarette boxes, wine boxes, medicine boxes, health care products boxes, cosmetic boxes, etc.). This is mainly due to the rough surface of the cardboard used for folding cartons and the uneven thickness (the error can reach 0.02mm). If you use a thinner plate (such as 0.76mm) for printing, the printing process and print quality are very high. Difficult to control. In the research, the typical plates of 1.14mm thickness we used are: APR series from Asahi company (made of liquid polybutadiene resin and other raw materials), Nylonflex series of BASF company, Gold series of Chase company, DuPont The company's Cyrel series and MacDermid's Flex-light series. For a specific plate material, in order to obtain high-quality, high-performance printed products, plate-making process conditions are very important. In order to protect the company's property rights and interests, very few technical articles in this area have been published. This article will introduce in detail the plate-making research and related problems of various plates with a thickness of 1.14mm.

Experiment introduction

1. Experimental Materials

(1) Yin pictures

The negative picture used in the experiment was provided by Asahi (UK) Ltd. The image information on the negative picture contains three primary colors of solid, line and mesh tone. The screen tone image information includes 1% to 90% of the dot information of 100 lines / inch, 120 lines / inch, and 150 lines / inch. The width of the negative and positive lines on the negative picture is 0.05 ~ 0.50mm. The size of Yang and Yin characters are 6, 8, 10 and 12Point. The sizes of independent male and female dots are 0.15, 0.25, 0.50 and 1.00mm.

(2) Plate material

The thickness of the plates used in the experiment are 1.14mm. Among them, AFP HD photosensitive resin plates are provided by Asahi (UK) Ltd. EPIC plate material is provided by Huyton Truck, now MacDermid's UK sales department.

2. Plate making process

The plate-making equipment used in the experiment is the 920 Quick series produced by Asahi Chemical Co., Ltd. (Japan). The equipment consists of a UV exposure unit (76 inches × 102 inches, 193 cm × 259 cm), a rinse unit (90 inches × 120 inches, 29 cm × 305 cm), a drying unit (76 inches × 102 inches, 193 cm × 259 cm), and a post exposure unit ( 76 inches × 102 inches, 193 cm × 259 cm) and UV light source plate finishing unit (76 inches × 102 inches, 193 cm × 259).

The first step of the plate-making process is to cut the plate according to the size of the negative picture, and then use a UV-a light source with a wavelength of 365nm to perform back exposure (also known as pre-exposure) under a semi-vacuum (360mbar) state. The energy received is 254mW / cm2. The role of back exposure is to determine the height of the relief on the printing plate, that is, the depth of corrosion, which plays a role in strengthening the base, ensuring the depth of the relief, and improving the printing durability of the printing plate. The back exposure time is determined according to the required substrate thickness. If the exposure time is too long, the printing is easy to get dirty; the exposure time is too short, the base is too thin, and the ability to support small characters and the like is reduced.

The second step of the plate-making process is to place the negative picture on the plate and perform front exposure (also called main exposure) under a full vacuum (700 mbar) to form the printed part of the graphic on the plate and cure it ( The same light source used for front exposure and back exposure). It is the key to ensure the quality of plate-making, which directly affects the clarity of the plate image. In order to avoid the halo phenomenon in the photosensitive bottom layer of the negative picture, the photosensitive emulsion layer should be in contact with the plate during exposure. The length of the front exposure time is related to the type of plate, the strength of the light, the thickness of the image on the negative picture, and the complexity. If the exposure time is too short, the embossed slope of the produced plate will be insufficient, the lines will be wavy, and the small dots will be washed away; if the exposure time is too long, it will lead to paste and blurry writing. If there are large and small characters, thick and thin lines on the same printing plate at the same time, if necessary, cover them with black film to expose separately, the small parts will not be lost due to washing, ensuring the quality of the printing plate.

The third step of the plate-making process is to wash the plate. After exposure, there is a latent image on the printing plate, and the printing plate is still flat and cannot be used for printing. At this time, it is necessary to wash away the non-photosensitive part with organic solvent, harden the resin of the exposed part, and retain the photopolymerized relief. The length of the plate-washing time is determined by the thickness of the printing plate and the depth of the relief. The plate-washing time is too short, leaving unsensitized resin on the plate and affecting the depth of the plate-making. If the plate-washing time is too long, the plate will swell, causing the fine parts to deform or fall . At the same time, the washing time is directly related to the temperature, concentration of the solution, the concentration of the chemical liquid, the size of the brush pressure, and the level of the liquid water level.

The fourth step of the plate-making process is to dry the plate. The freshly processed plate contains a lot of solvent, so it must be placed in a dryer to evaporate the solvent absorbed in the printing plate as soon as possible to restore the thickness of the printing plate to the standard value. The baking temperature is generally between 50 ~ 60 ℃. The baking time is determined by the thickness of the plate material and the length of the plate washing time. Generally, the thick version is 2 hours and the thin version is 1 hour. If the baking temperature is too high, the baking time will be shortened, making the printing plate brittle and deformed, and the printing plate will appear rotten. When the baking temperature is too low, the drying time will be extended and the resin will age and harden. After the printing plate is dried, a small amount of solvent will remain. If these solvents are not completely removed, it will affect the uniformity of the thickness of the printing plate. Therefore, the printing plate at this time should be left at room temperature for a period of time. During this time, the residual solvent can be completely evaporated from the printing plate to ensure that the printing plate is cured well.

The fifth step of the plate-making process is post-exposure. By ultraviolet (wavelength 365nm) irradiation, the printing plate will produce the greatest degree of photopolymerization. The energy received on the surface of the printing plate is 10.2mW / cm2, which makes the photo-sensitivity insufficient, and the incompletely cured parts are all cured, so that the printing plate reaches the required hardness index, and increases the resistance of the plate to ink and solvents, and improves the printing. Version of the wear resistance.

The final step in the plate-making process is debonding. In order to eliminate the viscosity of the printing plate and increase the printing plate finish to facilitate ink transfer, the printing plate is exposed to a UV-c light source at 245 nm, and the energy received by the plate surface is 2.9 mW / cm2.

3. Measuring methods and instruments used

(1) Network area measurement

Lipu Vipflex 330 is specially used to measure the dot area of ​​transparent flexographic and negative images. The operation steps are as follows: first select the correct plate type (if the covered plate and the uncovered plate require different imaging functions), then place the flexographic plate on the light-transmitting table to ensure that the part to be measured can be accurately lasered Detect, preview the image, focus and capture the image and ensure that the dots can be clearly observed, and finally complete the analysis of the image. Normally, the size of the sample image to be measured should not be less than 1.0cm × 1.0cm. Many useful information can be obtained in image analysis, such as the dot area of ​​the printing plate (or negative picture) and screen specifications.

In this analysis stage, some dots are automatically measured (of course, it can also be obtained by manual measurement). The information retained and recorded is very statistically significant. It can also be used to detect the defects of negative pictures or printed dot images. And check whether the dot size is uniform, detect whether there is deviation in the process of transferring the graphic information from the negative picture to the printing plate, and optimize the plate making conditions.

(2) Image contour analysis

The food used is a Joel JSM-820 scanning electron microscope (with the use of an Oxford CXL type X-ray spectrometer). Depending on the selected detection position (such as the sample surface or sample edge intersection) and product requirements, the excitation voltage value varies from 10 to 25 kV, and the magnification is 65 to 1000 times. Each representative area of ​​the tested sample is Can be reflected by electron micrographs. Next, analyze the developed photo, and you can get the following image outline information, the structure of the printing plate surface, negative line information and dot shoulder angle information.

(3) Plate relief and plate thickness measurement

The food used in this test is a Kennedy tester with a resolution of up to 0.0001 inches (product of Kennedy Calibration & Calibration Instruments, UK). The measurement is performed on a horizontal plane.

Experimental results and analysis

1. Back results and analysis

In flexographic printing, the relief depth of the printing plate has a direct impact on the quality of the printing plate. Through the back exposure test, an important relationship between the energy level of the selected exposure light source and the depth of the plate relief can be obtained. Generally, the length of exposure time (in seconds) can be used to indicate the depth of relief to be obtained.

However, it is worth noting that when the exposure equipment is used for a period of time, the light intensity of the ultraviolet lamp will be reduced, and the relief depth indicated by the above time value is not so accurate. If the original exposure time is still used, the desired relief effect cannot be obtained, and the back exposure time must be adjusted accordingly. Asahi 920 Quick series plate-making machine adopts advanced integrated exposure technology, and represents various ideal relief depths with different energy units, which can effectively control the back quality.

We can select a type of printing plate from each company for back exposure experiment, and analyze and evaluate the results.

Select a narrow strip 2 cm wide from the edge of the experimental plate and expose it for 7 minutes on the front. The rest of the plate (used in the back exposure experiment) is partially covered with a cover strip. The purpose of this operation is to obtain a hard field of 2 cm width on the plate sample. When we measure the depth of the embossed relief, the parameters of these solid areas can be used as a reference. As shown in Figure 1 (rinsing speed: 200 mm / min; flushing pressure: 0 mm; drying temperature: 60 ° C .; drying time; 2 hours), the length of the back exposure time will affect the thickness of the substrate, with the back exposure The depth of the relief becomes shallower as it increases. Relatively speaking, under the same exposure conditions, the relief depth of Asahi HD printing plate is deeper than that of Polyfibron EPIC printing plate, which shows that these two types of printing plates have different sensitivity coefficients to UV light sources. Repeating the experiment can draw the following conclusions; under the same time and conditions, the back exposure of these two different types of photosensitive resin plates, the resulting relief depth is about 0.025mm.

2. Positive exposure

In this experimental study, the Asahi AFP HD 1.14mm thick relief plate with a depth of 0.52mm was used. The negative pictures used in the experiment are also carefully checked to ensure that there are no obvious defects, otherwise it will be reflected on the printing plate, which will inevitably affect the experimental effect.

First, measure the dot area of ​​some typical screening areas on the negative picture; the measurement results are shown in Table 1. Obviously, for the negative picture studied, the actual dot area of ​​the measured area is smaller than the corresponding theoretical value. One of the reasons is the limitation of the software algorithm used by the measuring instrument. Figure 2 shows 10% of the outlet information on this negative picture.

Front exposure will affect the depth of the relief of the printing plate, the size of the dots, the shoulder angle of the dots, and the information of the negative lines.

(1) The effect of frontal exposure on the depth of plate relief

People generally think that there are only two factors that affect the depth of the relief of the printing plate, that is, the back exposure time (also related to the light source energy) and the processing conditions. The front exposure should have no direct effect on the depth of the relief. This idea is undoubtedly correct, and when the frontal exposure is insufficient, this idea is untenable.

As shown in Figure 3 (160 units of back exposure; rinse speed; 200 mm / min; flush pressure; 0.04 mm; drying temperature; 60 ° C .; drying time; 2 hours), when the front exposure time is only two minutes, it will cause The relief depth is relatively shallow. The main reason for this phenomenon is that the top layer of the printing plate is not completely cured, so that some components with photopolymerization properties have not been cross-linked and are washed away during the rinsing process. As a result, this part of the printing plate is thin and the depth of the relief is shallow.

(2) The effect of frontal exposure on the surface structure of the printing plate

Shrinkage is a normal phenomenon of photosensitive resin coatings under the action of UV light sources. In the flexographic plate-making process, it is necessary to meet the most basic requirement of no cracks on the surface of the printing plate. Front exposure is the first time that the surface of the printing plate receives UV light. If the front exposure is overexposed, all the subsequent processing procedures are meaningless, because it is easy to cause lay-up and produce waste products during printing. In order to avoid this phenomenon, it is necessary to carefully monitor the surface structure of the solid area on the printing plate, because the solid area absorbs more UV light than the screen area. In this experimental study, there are both small dots and solid plots on the layout. In order not to lose 1% of the small dots, the main exposure time must be extended, but this may cause overexposure in the solid area. The analysis results of the scanning electron microscope show that even if the main exposure time is extended to 25 minutes, there will be no cracks on the surface of the Asahi HD printing plate.

(3) The effect of frontal exposure on the dot area of ​​the printing plate

After 2 minutes of front exposure on the printing plate, 1% and 3% of the dots in the various screen lines used in the study will be washed away. This shows that the above exposure conditions simply cannot guarantee the reproduction of subtle levels. For 3% of the dots on the printing plate, we selected some areas to give 7 minutes, 10 minutes, 15 minutes, 20 minutes and 25 minutes of frontal exposure, so we got the dot area of ​​3.9%, 4.5%, 5.8%, 5.9% and 6.1%. The results show that the idea that the dots on the printing plate can be consistent with the dots on the negative picture is wrong. For the printing plate, appropriately increasing the front exposure time can significantly improve the quality of the mesh adjustment dots. It can be seen from Figure 4 (rear exposure; 160 units; rinse speed; 200 mm / min; scouring pressure 0.04mm; drying temperature; 60 ° C; drying time; 2 hours). No loss, but it will cause the increase of other regional network points.

(4) The effect of frontal exposure on the shoulder angle of the outlet

It can be seen from Table 2 that the length of front exposure time greatly affects the sharpness (sharpness) of the dots. As the front exposure time increased from 2 minutes to 15 minutes, the shoulder angle of the dots increased significantly, from 22 ° to 33 °. Continue to increase the exposure time, the corresponding angle changes tend to be gentle, ranging from 33 to 35 °. For certain types of plate materials, the change of the dot shoulder angle has a double impact on the quality of the printing plate; on the one hand, the dots are sharp, the contact area with the substrate material during the printing process is small, and the dots increase degree; The base of this kind of dot is not very stable, under certain printing pressure, it will inevitably produce a large degree of deformation, resulting in a more serious dot increase. In order to balance both effects, it is very important to choose the right shoulder angle of the outlet.

Table 2 The shoulder angle of 5% of the dots at 120 lines / inch (to be continued)

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