Plate making process I of the most popular PS plat

2022-08-26
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Plate making process of positive PS plate (I)

the PS plate with a pre coated photosensitive layer is made into a printing plate for printing through the process of exposure, development, dirt removal, gelatinization and so on. In the positive PS plate, the exposed photosensitive layer undergoes photodegradation reaction and is dissolved by the developer. It is removed from the plate. After the adhesive layer is coated, a stable hydrophilic blank is formed; The unexposed photosensitive layer is left on the printing plate, which is lipophilic and forms the image part (Fig. 1)

figure 1 positive PS plate making process

I. exposure

close the photosensitive layer of the positive original and PS plate, place it in a special plate printer, and turn on the light source of the plate printer after vacuum pumping. The photosensitive layer of the non image part will undergo photodegradation reaction under the irradiation of light. This process is called exposure, also known as plate printing

1. The original point quality

ps version has fine sand mesh, high resolution, good gradient reproducibility and point reduction. In order to make full use of this advantage of PS version, there are certain requirements for the point density and point finish of the original version used for printing

the original used for printing PS version is usually made by electronic scanning, photography or laser phototypesetting. In the process of making the original, due to the contrast coefficient of photographic film γ Low value, when overexposure, development or thinning, it will form soft spots with low density value or active spots with small edge density value and large center density value; When printing with such a master, a slight increase or decrease in exposure will cause a large reduction or increase in the dot area on the printing plate, thus affecting the point reduction of the PS version (see Figure 2). Therefore, the points on the original plate used for printing should be hard points with high density value, uniform density at the edge and center, and bright and clean surroundings. Generally, the point density value is required to be above 3.0 (the density value of the field version should be higher). The exposure tolerance of hard spots is large. Even if the exposure changes, the dot area on the printing plate will not change much, and the printing quality is relatively stable

for the original used for printing, the surface is also required to be clean, without scratches, pinholes, and no glue marks of stitched images

figure 2. Reproduction effect of soft and hard points

2 Exposure principle

stick the emulsion layer of the original positive image together with the photosensitive layer of the PS version, vacuum pump to make it close, and then turn on the light source in the plate printer to expose the printing plate. The light passes through the non image part of the original to the photosensitive layer, and the light seeing part of the photosensitive layer undergoes a decomposition reaction, releasing nitrogen to produce indene ketone compounds, and in case of water, indene acid compounds

exposure time of several photosensitive resins

plate photosensitive resin model

exposure time (s)

light source

type 215

15 ~ 20

gallium iodide lamp

type 214

20 ~ 30

gallium iodide lamp

type 124

40 ~ 75

gallium iodide lamp

3 The photosensitive layer of the printing light source

PS plate has a certain spectral sensitivity. Only light with a certain wavelength within a limited range can cause changes in the molecular structure of the photosensitive layer, that is, only when the luminous spectrum of the light source matches the spectral sensitivity of the photosensitive layer, the photosensitive layer can have a photochemical reaction. According to the measurement, the wavelength range of the light wave that can cause the photochemical reaction of the photosensitive layer is 250 ~ 420nm. Considering the absorption of light by the glass of the printing machine and the original film of the printing plate, this range is actually 300 ~ 450nm. Ultraviolet light and blue violet light with high energy are the main light wavelengths in the above range to avoid the interaction between the chemical components of plastic packaging and beer. Therefore, in order to make the photosensitive layer of PS plate photosensitive quickly, so that the image tone can be well reproduced on the printing plate, we must choose a light source that can produce sufficient intensity of ultraviolet light or blue violet light

the light source used for printing, in addition to its luminous spectrum should match the spectral sensitivity of the photosensitive layer, it should also have the following conditions: the light energy distribution should be uniform and stable, It is applicable to the digitization and standardization of the printing process; low power consumption, high printing efficiency; simple structure, convenient operation, easy maintenance; environmental pollution factors in use (such as heat, ozone, smoke, etc.) Minimize

according to the new method, it is obviously necessary to produce C-type frame first through other processes. At present, as the light source of PS plate printing, there are mainly high-pressure mercury lamps and gallium iodide lamps. The high-pressure mercury lamp is made by sealing the mercury vapor into the lamp tube. The emission spectrum is continuous, and the wavelength range is 300 ~ 450nm. It matches the spectral sensitivity of the photosensitive layer of PS plate very well, and the printing efficiency is very high. Gallium iodide lamp is made by adding bromide of gallium and gallium metal into the lamp tube of high-pressure mercury lamp. The amount of light near the wavelength of 400nm in the emitted spectrum has increased significantly, which is more consistent with the spectral sensitivity of the photosensitive layer of PS plate, and the printing efficiency is higher. The efficiency of using gallium iodide lamp as the light source for printing is about 0.5 ~ 1 times higher than that of using high-pressure mercury lamp

the quality of printing is not only related to the spectrum and light intensity of the light source, but also affected by the distance between the layout and the light source. The shorter the distance, the less exposure time is required. However, when the light source is close to the layout, the part of the layout that is vertically illuminated by light receives more light than the surrounding part of the layout that is slanted by light, which will cause the overexposure of the part of the layout that is directly illuminated and the underexposure of the part of the career "administrative assistant" that is slanted by light. In order to avoid the occurrence of this phenomenon, the distance between the layout and the light source is still strong, It is better to be equal to or slightly larger than the diagonal length of the layout

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