How to correctly use the test strip to control the exposure of the PS plate

Normally, we used a 21-step gray scale (density difference of 0.15) to measure the exposure of the PS plate. According to different printing platemaking requirements, the gray ladder rule is generally controlled at about level 3 (also known as “no ink, blank or whitening”), and the so-called “clean” is the photosensitive layer corresponding to the three steps of the plate and the gray step. See all light decomposition. However, in practice, test strips often cannot be placed in the printing area of ​​the PS plate, that is, the graphic part; they can only be placed on the edge of the plate material. At this time, if you press the gray ladder ruler level 3 to control the exposure As a result, the graphic image part in the center of the plate-making machine may be exposed to sunlight, resulting in the loss of the dot and the decline in the printing rate. The positive plate version of the PS plate is a photosensitive layer corresponding to the transparent part of the original plate (ie, blank part), and is completely removed from the plate after development; and the photosensitive layer corresponding to the original blackened part (image part) is left after development. On the plate. The principle of this process is that the photosensitive layer of the PS plate is decomposed into substances that are soluble in an alkali solution after being irradiated with light of a certain energy. The standard exposure after exposure to radiation is decomposed into substances dissolved in alkali solution. The standard exposure during printing is the exposure of the specified dot percentage and yin and yang lines. That is, the standard exposure time is the exposure time of the screen reproduction and the yin and yang line width values. Which underexposure, the printing plate part of the screen line thicker, expanded dot area, the blank part may be dirty; if overexposed, the plate image part of the line becomes thinner, the screen area is reduced and directly affect the image tone. PS version printing is done with a printing machine. There are two types of structures that are commonly used in the printing machine, one is a light source upper setting plate printing machine, and the other is a light source lower setting box type rotary printing plate printing machine. Whether it is the upper light source upper and lower plate-making machine, the light source is a point light source. The illuminance on the layout when printing is determined by the intensity of the light source and the distance and angle between the light source and the illuminated surface. According to the law of inverse square of optics, the illuminance is proportional to the intensity of the light source and is inversely proportional to the distance from the light source to the illuminated surface. Illuminance: E=(F(luminous flux))/S=dF/dS=1/r2COSa According to the law of illuminance cosine, the angle of the light source perpendicular to the illuminated surface is 0, and the illuminance is the maximum; when the illumined surface and the light source parallel rays are at an angle of 90 degrees The illuminance is 0; the illuminance of the illuminated surface changes with the incident angle of the light source. The size of the illumination is affected by the distance and angle. To reduce this effect, the light source of the plate printing machine is generally added with a convergent lampshade, so that the illumination of the illumination range is as uniform as possible, but there are still differences. The standard of the printing machine stipulates that the illuminance uniformity greater than 88% is a superior product. Therefore, the illuminance at each position of the stencil machine being illuminated will not be consistent. To this end, we experimented with three types of stencils: Table 1 Incidence angle (° degrees) 01020304560708090 Illuminance (relative value) 10.890.940.870.710.500.340.170 I. Dainippon mesh prints Machine structure: Light source under-mounted rotary printing machine Model: P422-G Light source power 3300W Layout size 1140×940mm Light distance 900mm Maximum incident angle 32° Test 1: The middle position of the layout is clean, and the layout is left and right. The front, rear, and middle plus test strips each exposed a PS plate (the same amount of exposure) data as shown in Table 2: Table 2 Location Left Front Center Right Rear Exposure (S) 20 Steps (Polar) 1.01.531.51. 0 network reproduction (%) 2 to 972 to 972 to 982 to 972 to 97 network lines (μ) 12 to 612 to 620 to 612 to 612 to 6 2. According to the cleanliness of the front edge of the layout, the left, right, front, back, and middle test strips of the layout are each exposed to a PS plate. The data is shown in Table 3: Table 3 Position Left Front Center Right Rear Exposure (S) 40 steps Scale (pole) 3.03.0533.5 Dot reproduction (%) 2 to 982 to 982 to 982 to 972 to 98 Network cable (μ) 12 to 612 to 620 to 612 to 612 to 6 Second, Thai instrument light source upper pair Stencil machine model: SBY920 light source power iodine lamp 2000W layout size 1200mm maximum incident angle 26° Test 3: the center of the layout of the 3 position clean as a benchmark, the layout of the left, right, front, rear, plus the test strip Each exposure of a PS version, exposure time: 70S data, such as Table 4: Table 4 position left front center and back right ladder feet (level) 22.532.52 III, Da Nippon mesh light source on the plate machine: Model: P832-G Light source power 4000W Layout 1200×900mm Lamp distance 1100mm Maximum incident angle 28° Exposure unit: 12S Table 5 Position Left Front Center Rear Rigid Scale (Level) 2+2.5+3+2.5+2+ From the above four groups of tests Look. The light source is set on the printing machine, and the light on the screen is evener than the light source. Of course, in general, the light source on the light source is large. The exposure between the middle and the two edges of the cabinet printing machine under the Da Nippon mesh light source is twice as big (or the printer may not work properly if we try this). Regardless of the structure of the stencil, the difference in illuminance at each part of the plate is obvious. Therefore, we must pay attention to the location of the test strips during the plate making process, and find out the correspondences among them to properly control the exposure. For the PS version, the higher the sensitivity, the smaller the absolute value of the exposure window width, and the greater the difference in the amount of exposure of each portion of the PS plate at the time of printing. The determination of the amount of exposure during the printing should be taken into account before, after, left, right, and in the middle of the line. It should not be based on the cleanliness of the edge 3, so that the middle part should be overexposed; nor can the middle 3 levels be clean, so Exposure to the edge is clearly insufficient. The exposure of the PS version should also take into consideration the reproducibility of the formal print and the reproducibility of the proofs. In theory, the size of the sample Internet sites is basically the same as that of the original, and the reproducibility of the formal prints should also be consistent with the reproducibility of the proofs. But the official printing offset press is a round press rotary press, and the proof press is a round flat press. The printing speed, printing pressure, and ink transfer method are different for each other. The level of small proofs is generally used to increase the exposure, so that the network points are slightly reduced, so that the printed official print can be basically consistent with the proofs. The criteria for dot gain values ​​relate to printing presses and paper. The dot enlargement value of coated paper is obviously smaller than that of offset paper. The expanded value standards of various countries are also different, and the benchmarks are not the same. Our country's network expansion value is set on the basis of 50% outlets. Network expansion values: Fine products ≤12%, general products ≤18%. Of course, the illumination uniformity of the different printing machines is not the same, and it is best for the user to test an optimal exposure condition himself. In general, when making a plate, you should pay attention to the position of the test strip, explore the correspondence, and properly control the exposure. For plates, the PS plate with higher sensitivity and smaller exposure latitude should pay special attention to this issue when making plates. (Author: Henan Second Film Factory)