Positive Pattern PS Platemaking Process (2)

4. Determination of exposure

The printing requirement of the positive-picture PS plate is that any photosensitive layer corresponding to the transparent part of the original plate (ie, blank part) should be completely removed from the plate after development, and correspond to the original blackened part (this is also the image part). The photosensitive layer must remain on the plate after development. The principle of application to achieve this process requirement is that the photosensitive layer of the PS plate is decomposed into a substance that can be dissolved in a dilute alkaline solution after being irradiated with light having a certain energy, and the positive plate type PS plate can achieve the aforementioned printing process. The required light energy of the irradiated light is called the critical light amount of the PS plate. Under the condition that the developing conditions are fixed, if the exposure amount is lower than the critical light amount, that is, the exposure is insufficient, the lines of the printing plate image portion will be thicker than the original plate, the dot area will be expanded, and the printing plate blank portion will be underexposed. Dirty phenomenon; if the exposure is higher than the critical light value, that is, overexposure, there will be a part of the printing plate image than the original fine lines, dot area reduction phenomenon. Because the change of the dot area directly affects the gradation of the image, the gradation of the image on the plate changes with the exposure amount in the printing plate. In order to have the best reproduction of the print, it is very important to determine the proper exposure during the printing process.

Determine the exposure of the PS version of the printing plate, should take into account the following factors.

First, the reproducibility of the formal print and the tone of the proof should be taken into account. Reproducible relationship. The PS plate is made into a printing plate by the plate-making process such as exposure and development. Before the official printing, the proof plate is printed on the proofer. The proofs are used to check the quality of the plates, and the certified prints (which indicate that the plate quality is also acceptable) are sent to the printing shop as a standard for checking the printed products. The size of the sample Internet site is basically the same as that of the original version. Therefore, the reproducibility of the formal print should be consistent with the reproducibility of the proofs. However, there are many differences between formal printing and proofing. Generally speaking, in formal printing, under the influence of printing pressure, the ink must be spread to a certain extent. Since there are often slight slips between the plate cylinder, blanket cylinder, and impression cylinder, the dots on the printed sheet are relative to the printing plate. The outlets on the network have a certain area of ​​expansion, which is different from the situation on the proofer. For multi-color offset printing presses, the overprinting of ink is performed under wet and wet conditions of the ink, and the overprinting of the ink on the proofer is performed under the condition that the ink is wet and dry; the offset press is a round press. Machines, but most of the proofing machines are flattened flatbed machines, and the printing speed, printing pressure, and ink delivery method are also different. Due to the differences between formal printing and proofing, no other remedial measures are taken, and it is impossible to reduce the amount of dot enlargement on official prints to the proof level. In order to achieve the same dot size for printing and proofing, the following two methods can be used. One method is to adjust the printing performance of the lining of the printing press and the ink to obtain the standard value of the dot printing value for the official printing; the original dot pitch adjustment is adjusted according to this standard; and the original version adjusted by the dot adjustment is used again. Plates are officially printed. This method is very difficult to implement, because to adjust the tone of the original, you must make a proper change from a series of process specifications from photolithography to proofing. These tasks are not only complicated but also very demanding. You must also get customers. Collaboration. Another method is: in the process of drying the printing plate, the exposure of the proof (or proofing) is increased, so that the dots are slightly reduced to offset the inevitable dot gain during the printing of the upper printing. Relatively speaking, this method is more convenient.

Second, if the method of “increasing the exposure and reducing the number of dots” is adopted, the relationship between the value of the dot enlargement standard and the printer and paper used for printing should be taken into consideration. For example, multi-color web offset presses and low-smooth papers allow larger dot gain values. When printing, the amount of exposure to be increased can be correspondingly smaller. The expanded value standards can be found in Table 4-2. The data in the table is based on 75% of outlets (China's outlet expansion standard is based on 50% outlets).

Finally, taking into account the size of the exposure, but also by the type of light source, the distance between the layout and the light source, the photosensitive characteristics of the photosensitive layer, the original quality and other factors, in the printing plate should also be trial exposure to determine the appropriate The amount of exposure (or exposure time).

Dot Expand Value Standard (Japan)
Dot enlargement% Single-color offset press Single-color offset press Multi-color offset press Offset paper 8-12% 10-14% 12-16%
Matte coated paper 10-14% 12-16% 14-18%
Offset paper 14-18% 16-20% 18-22%

Determining the exposure of a printing plate is generally performed according to the following three steps.

1. Prepare a ruler and signal strip for the management of the plate. Commonly used to determine the amount of exposure of the ladder are: transmission ash ladder feet, network ladder step, print version of the ladder and so on.

The density of the transmitted ash ladder differs by 0.15 for each level. Every time the density increases, the exposure increases
The network grayscale ruler is composed of small dots with various percentages of network points, including 5% or less. When used, it is placed together with a ray traverse ruler, and the exposure is performed with different exposures. After the development, the original or proofs can be checked against the change of the percentage of dots.

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Fig. 3 Printability of the print height and plate resolution

Figure 3 shows the Brunner test strips for the print height and resolution of the plate. The print height scale consists of 6 small squares, each of which is uniform. Distributions of 0.5%, 1%, 2%, 3%, 4%, and 5% of dots at 60 lines/cm. When using, first determine the percentage of the dots on the printing plate that must be sun-dried. The exposure amount that is returned by the percentage of this dot in the printing process is the standard exposure.

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Fig. 4 Fogra print control strip

Fig. 4 shows FOGRA control strips, consisting of screen footsteps and different equal-width yin and yang lines. 1%, 2%, 3%, and 4 lines of 60 lines/cm are distributed on the screen ladder. %, 5% of the dots, the width of the lines are respectively 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 30 μm, 40 μm. The method for determining the exposure amount is the same as that for using the Brunshart test strip. The first step is to determine the number of dots to be printed on the printing plate, and the number of series when the yin and yang lines appear at the same time. (i.e., the width of the first line that the continuous lines of the yang are continuous and the lines do not stick to). In the process of printing, the percentage of the specified dot and yin and yang lines are the standard exposure.

The above-mentioned print ruler can be used to determine the standard exposure, and it can also be used to control the exposure of the print.

2. Change exposure time and print. Under the condition that the illumination intensity of the light source is constant, the exposure amount of the printing plate is proportional to the exposure time, and the exposure amount can be adjusted by changing the exposure time. In the actual production, the exposure time is commonly used to express the exposure for convenience.

The gray ladder ruler and the dot ladder ruler (optionally printable ruler) can be overlaid on the same PS plate for exposure and developed. This type of operation is performed separately for the printing plates of various types of printing presses, and the standard exposure time is determined using the dried printing plates.

3. Determine the standard exposure time. The dried printing plate is mounted on a printing press for printing, and the printed product is compared with proofs printed with the same ash ladder or screen ladder (or printing ruler) to obtain the same reproduction on the printed matter. The steps of the step ladder, the exposure time of this step is the standard exposure time. For example, when the grey step on the printing plate is stenciled to a level of 6 blanks, the reproducibility of the printed sheets on the Internet is the same as that of the proofs, and the exposure time of the printing of the 6-level blank is the standard exposure time. The so-called "tank level 6 blank" means that the photosensitive layer of the corresponding portion of the plate on the plate and the gray ladder ruler is completely decomposed by light, and after development, it is completely removed from the plate and no ink is adhered. Therefore, " Level 6 Blank is also called "Level 6 Whitening."

The method of determining the standard exposure time by using a printing degree ruler is relatively simple, and the standard exposure time is the exposure time of the specified dot step and the width of the yin-yang line of the printing plate. At present, most of the production practice uses this method to determine the standard exposure of a PS plate.

When printing, if the distance between the light source and the printing plate changes, the exposure time should be adjusted accordingly according to the rule that the exposure time is proportional to the square of the distance from the light source to the layout. If the illuminance of the light source is non-uniform, or the original plate is poorly adhered to the plate, the printing conditions deteriorate, and the balance between the standard exposure time and the reproducibility of the image tone on the plate is destroyed. When this happens, the exposure time must be adjusted using the print height scale and resolution test strip (resolution check).

The above method for determining the standard exposure time is based on the premise that the printing process is in a stable state. The reproducibility of the printing plate corresponding to the original printing plate is also considered based on the standard dot enlargement value of the stable printing. If the printing conditions are not stable, This method loses its meaning.