Flexographic plate making research (on)
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In recent years, flexographic printing has made great progress in the global field with its unique advantages. In many applications, flexographic printing has already challenged offset printing and gravure printing, not only in terms of efficiency, but also in terms of quality. .
Today, in order to meet the growing demand for high-quality prints in the market, most of the North American and European countries have adopted relatively thin plates. Depending on the end application, the thickness of the thin plate can be as thin as 0.76mm and the thickest can reach 2.84mm. The industry expects that the thin plate process technology can bring greater development advantages to product quality and performance, including lowering the cost of raw materials, shortening the plate making cycle, making use of higher hardness plates, shallower relief depth, and improved image quality on the plate. Good ink transfer control, reduced plate wear, reduced press consumption, and reduced distortion.
At present, the application of a 1.14 mm thick plate in the printing of folding cartons (such as cigarette cases, wine boxes, medicine boxes, health care products boxes, cosmetic boxes, etc.) is common. This is mainly due to the rough surface of the cardboard used for folding cartons and the uneven thickness (error of 0.02mm). If printing is made with a thinner plate (such as 0.76mm), the printing process and the quality of the printed products are very good. Hard to control. In the study, we used a typical 1.14mm thick plate for a specific plate, in order to obtain high-quality, high-performance prints, the plate-making process conditions are very important. Because of the protection of the company's property rights, there are very few technical articles about this aspect. This article will detail the plate making research and related issues regarding various plates with a thickness of 1.14mm.
Experimental introduction
1. Experimental Materials
(1) Yin picture
The experimental image uses the image information on the negative image to contain the three primary colors of the field, lines and mesh. The mesh image information includes 100 lines/inch, 120 lines/inch, and 150%/inch of 1%-90% of the dot information. The width of the negative and positive lines on the negative image ranges from 0.05 to 0.50 mm. The size of the positive and negative characters are 6, 8, 10 and 12Point. The size of the independent male and female points is 0.15, 0.25, 0.50 and 1.00 mm.
(2) Plate
The thickness of the plates used in the experiment was 1.14 mm.
2. Plate making process
The plate making equipment used in the experiment was a UV exposure unit (76 inches x 102 inches, 193 cm x 259 cm), a rinsing unit (90 inches x 120 inches, 29 cm x 305 cm), and a drying unit (76 inches x 102 inches, 193 cm x 259 cm). The rear exposure unit (76 inches x 102 inches, 193 cm x 259 cm) and the UV light source plate refurbishing unit (76 inches x 102 inches, 193 cm x 259).
The first step in the plate making process is to cut the size according to the size of the negative image, and then use a UV-a light source with a wavelength of 365 nm to perform back exposure (also called pre-exposure) under a semi-vacuum (360 mbar) state. The energy reached was 254 mW/cm2. The function of the back exposure is to determine the height of the relief on the printing plate, that is, the depth of corrosion, to enhance the plate base, to ensure the depth of the relief, and to improve the printing durability of the printing plate. The back exposure time is determined by the required substrate thickness. The exposure time is too long, the printing is easy to get dirty; the exposure time is too short, the substrate is too thin, and the support ability for small characters and the like is lowered.
The second step in the plate making process is to place the negative image on the plate and perform a frontal exposure (also called main exposure) under full vacuum (700 mbar) to form the printed portion of the plate and cure it ( The front exposure is the same as the light source used for the back exposure). It is the key to ensuring the quality of the plate, directly affecting the clarity of the plate image. In order to avoid halo phenomenon in the photosensitive underlayer of the negative image, the emulsion layer should be brought into contact with the plate during exposure. The length of the front exposure time is related to the type of the plate, the intensity of the light, the thickness of the image on the negative image, and the complexity. If the exposure time is too short, the resulting embossed slope will not be enough, the lines will be wavy and the small dots will be washed off; if the exposure time is too long, the paste will be blurred and the writing will be blurred. If there are large and small characters, thick and thin lines on the same plate, the black film can be used to cover the exposure separately, and the small part will not be lost due to the flushing, thus ensuring the quality of the plate.
The third step in the plate making process is washing. After the exposure, there is a latent image of the graphic on the plate, the plate is still flat, and can not be used for printing. At this time, the non-photosensitive portion is removed by washing with an organic solvent to harden the resin of the exposed portion, and the photopolymerized relief is retained. The length of the plate-washing time is determined by the thickness of the plate and the depth of the embossing. The plate-washing time is too short. The plate will leave unsensitive resin and affect the plate-making depth. If the plate-washing time is too long, the plate will swell, resulting in fine deformation or falling off. . At the same time, the washing time is directly related to the temperature, concentration, concentration of the liquid, the pressure of the brush, and the level of the liquid.
The fourth step in the plate making process is to dry the layout. The freshly washed version contains a lot of solvent and must be placed in the dryer to allow the solvent absorbed in the plate to evaporate as quickly as possible, returning the plate thickness to the standard value. The baking temperature is generally between 50 and 60 °C. The baking time is determined by the length of the plate thickness and the time of the plate washing. Generally, the thick plate is 2 hours and the thin plate is 1 hour. When the baking temperature is too high, the baking time is shortened, the printing plate becomes brittle and deformed, and the printing plate will be rusted. When the baking temperature is too low, the drying time will be prolonged, and the resin will be hardened by aging. After the plate is dried, a small amount of solvent remains, and if these solvents are not completely removed, the plate thickness uniformity will be affected. Therefore, the plate at this time is allowed to stand at room temperature for a period of time during which the residual solvent can be completely evaporated from the plate to ensure good curing of the plate.
The fifth step of the plate making process is post exposure. The printing plate is irradiated with ultraviolet rays (wavelength 365 nm) to maximize the photopolymerization of the printing plate. The energy received on the surface of the printing plate is 10.2mW/cm2, which makes the photosensitive insufficient, and the partially cured part is completely cured, so that the printing plate reaches the hardness index, and the resistance of the plate to the ink and solvent is increased, and the printing is improved. The wear resistance of the plate.
The final step in the plate making process is de-bonding. In order to eliminate the stickiness of the printing plate, the plate finish was increased to facilitate the ink transfer, and the printing plate was exposed to a UV-c light source of 245 nm, and the plate received an energy of 2.9 mW/cm2.
3. Measuring method and instrument used
(1) Dot area measurement
The Viipex 330 is specifically designed for dot area measurement of transparent flexographic and negative images. The operation steps are as follows: first select the correct version (if there is a cover plate and a non-covering plate, it is required to set different imaging functions), then put the flexographic plate on the light-transparent table to ensure that the part to be measured can be lasered accurately. It is detected that the image is previewed, the image is captured, the image is captured and the dot is clearly observed, and the analysis of the image is completed. Usually, the sample image size required to be measured should be not less than 1.0 cm × 1.0 cm. A lot of useful information can be obtained in image analysis, such as the dot area of the plate (or negative image) and the screen specification parameters.
In this analysis phase, some outlets are automatically measured (of course, can also be obtained by manual measurement). These recorded information are statistically significant and can also be used to detect the lack of negative images or plate dot images. And to check whether the size of the dot is uniform, whether there is deviation in the process of transferring the graphic information from the negative image to the printing plate, and optimizing the plate making conditions.
(2) Image contour analysis
The food used was a Joel JSM-820 scanning electron microscope (the Oxford CXL type X-ray spectrometer was used with it). According to the selected detection position (such as the sample surface or the intersection of the sample edge) and the product requirements, the excitation voltage value varies between 10~25kV, and the magnification is 65~1000 times. The representative area of each sample to be tested is Can be reflected by electron micrographs. Next, the developed photo is analyzed, and the following image outline information, the structure of the surface of the printing plate, the information of the negative line, and the information of the shoulder angle of the dot are obtained.
(3) Plate relief and plate thickness measurement
The food used in this test was a Kennedy tester with a resolution of up to 0.0001 inches (manufactured by Kennedy Measurer, 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 plate has a direct impact on the quality of the plate. An important relationship between the selected exposure light source energy and the plate relief depth can be obtained by the back exposure test. In general, the length of exposure time (in seconds) can be used to indicate the desired relief depth.
However, it is worth noting that when the exposure device is used for a period of time, the light intensity of the UV lamp will be reduced, and the relief depth indicated by the above time value is less 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. The Asahi 920 Quick Series platesetter uses advanced integrated exposure technology to represent the ideal relief depth with different energy units, effectively controlling the back quality.
We can select one type of plate from each company for back exposure experiments and analyze and evaluate the results.
A 2 cm wide strip was taken from the edge of the experimental plate and exposed to the front for 7 minutes. The rest of the media (used in the back exposure experiment) was partially covered with a cover strip. The purpose of this operation is to obtain a 2 cm wide hard ground on the plate sample. The parameters of these solid areas can be used as a reference when measuring the depth of the embossing. As shown in Figure 1 (flushing 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 plate, with the back exposure time Increase, the depth of the relief is correspondingly shallower. In contrast, under the same exposure conditions, the Asahi HD plate has a deeper relief than the Polyfibron EPIC plate, indicating that the two types of plates have different sensitivities to the UV source. Repeating the experiment leads to the conclusion that, under the same time and conditions, the back exposure of the two different types of photosensitive resin plates results in a relief depth of approximately 0.025 mm.
2. Front exposure
In this experimental study, a 1.14 mm thick plate with a relief depth of 0.52 mm was used for the Asahi AFP HD. The negative images used in the experiments have also been carefully checked to ensure that there are no obvious defects, otherwise they will be reflected on the plates, which will inevitably affect the experimental results.
First, measure the dot area of some typical screened areas on the negative image; the measurement results are shown in Table 1. Obviously, for the negative image studied, the area of the dot of the actual measurement area is smaller than the corresponding theoretical value. One of the reasons is limited by the software algorithms used by the measuring instrument. The representation of Figure 2 is 10% of the dot information on this negative picture.
Dot area 100 lines / inch 120 lines / inch 150 lines / inch
2% 1.5 1.6 0.7
5% 4.6 4.3 3.5
10% 9.3 9.1 8.8
15% 14.9 14.2 13.4
20% 19.4 19.3 18.3
Front exposure affects plate embossing depth, dot size, dot shoulder angle, and negative line information.
(1) The effect of frontal exposure on the relief depth of the printing plate
It is generally believed that there are only two factors that affect the depth of the relief of the plate, namely the back exposure time (which is also related to the energy of the light source) and the flushing conditions. The front exposure should have no direct effect on the relief depth. In fact, when the front exposure is correct and reasonable This idea is undoubtedly correct, and when the frontal exposure is insufficient, the idea is untenable.
As shown in Figure 3 (reverse exposure of 160 units; rinsing speed; 200 mm / min; flushing pressure; 0.04 mm; drying temperature; 60 ° C; drying time; 2 hours), when the front exposure time is only two minutes, it will result 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 having photopolymerization properties are not completely crosslinked and are washed away during the rinsing process, resulting in a relatively thin plate portion and a shallow relief depth.
(2) The effect of frontal exposure on the surface structure of the printing plate
The reduction is a normal phenomenon that the photosensitive resin coating appears under the action of a UV light source. In the flexographic plate making process, it is necessary to meet the most basic requirements of the surface of the printing plate without cracks. The front exposure is the first time the surface of the plate receives UV light. If the frontal exposure is excessive, all subsequent processing steps are meaningless, because it is easy to cause paste, which causes waste when printing. In order to avoid this phenomenon, the surface structure of the solid area on the printing plate should be carefully monitored because the solid area absorbs more UV light than the screened area. In this experimental study, there are both small dots and solid blocks on the layout. In order to make 1% of the small dots not lost, the main exposure time must be extended, but this may cause overexposure in the field. Scanning electron microscopy analysis showed that even if the main exposure time was extended to 25 minutes, no cracks appeared on the surface of the Asahi HD plate.
(3) The effect of frontal exposure on the area of the printing plate
After 2 minutes of front exposure of the plates, 1% and 3% of the various screen lines used in the study were washed away. This shows that the above exposure conditions simply cannot guarantee a subtle level of reproduction. For the 3% of the dots on the plate, we selected some areas to give a positive exposure of 7 minutes, 10 minutes, 15 minutes, 20 minutes and 25 minutes, so that we get the dot area of 3.9%, 4.5%, 5.8%, respectively. 5.9% and 6.1%. The results show that the idea that the dots on the plate can be consistent with the dots on the negative image is wrong. For the printing plate, appropriately increasing the front exposure time can significantly improve the quality of the mesh adjustment point. From Figure 4 (back exposure; 160 units; rinsing speed; 200 mm/min; scouring pressure 0.04 mm; drying temperature; 60 ° C; drying time; 2 hours), it can be seen that the front exposure time can be extended by 1%. Not lost, but will cause the increase of outlets in other areas.
(4) The effect of frontal exposure on the shoulder angle of the dot
As can be seen from Table 2, the length of the front exposure time largely affects the sharpness (sharpness) of the dots. As the front exposure time increases from 2 minutes to 15 minutes, the shoulder angle of the dot increases significantly from 22° to 33°. Continue to increase the exposure time, the corresponding angle changes tend to be flat, ranging from 33 to 35 °. For a particular type of plate, the change in the shoulder angle of the dot has a double effect on the quality of the plate; on the one hand, the dot is sharp, the contact area with the substrate material is small during the printing process, and the dot gain is small; The base of such a dot is not very stable, and under certain printing pressure, a large degree of deformation will inevitably occur, resulting in a more serious dot increase. In order to balance the effects of both sides, it is very important to choose the appropriate shoulder angle.
Table 2 The shoulder angle of 5% of the dots at 120 lines/inch (to be continued)
Front exposure time (minutes) dot shoulder angle (°)
2 22
5 24
7 26
10 30
15 33
20 34
25 3