Photographic Process of Positive PS Plate (1)

The PS plate that has been coated with a photosensitive layer in advance is subjected to processes such as exposure, development, removal of dirt, and gluing to form a printing plate for printing. In the positive PS version, the exposed photosensitive layer undergoes a photodecomposition reaction and is dissolved by the developer solution to be removed from the plate surface. After the adhesive layer is coated, a stable hydrophilic blank portion is formed; and the unexposed photosensitive layer is left in the The plate has lipophilic properties and forms part of the image (Figure 1).

Fig. 1 Positive Pattern PS Process

One, exposure

The photosensitive positive layer of the positive and PS plates is sealed and placed in a special printing machine. After vacuum extraction, the light source of the printing machine is turned on, and the photosensitive layer of the non-image part is lighted under light irradiation. Decomposition reaction, this process is called exposure, also known as printing.

1. Original outlet quality
PS version of the sand fine, high resolution, with a good reproduction of the tone and reproduction of the network. In order to make full use of this advantage of the PS version, there are certain requirements for the dot density and dot finish of the original used in the printing.

The original version of the PS plate was usually made by electronic scanning, photography, or laser photography. In the original production process, since the gamma value of the photographic film is low, when the exposure, development or thinning is excessive, a soft dot having a low density value or an activity having a small edge density value and a large central density value will be formed. With such an original printing plate, a slight increase or decrease in the amount of exposure will cause a large reduction or increase in the area of ​​the printing plate's Internet site, thus affecting the reproducibility of the PS plate to the dot (see FIG. 2 ). Therefore, the dots on the original plate used for printing should be hard dots with high density values, uniform edges and center density, and smooth peripheral edges. Generally, the dot density value is required to be 3.0 or higher (the density value of the field version should be higher). The exposure latitude of rigid outlets is relatively large. Even if there is a slight change in exposure, the dot area on the printing plate will not change too much. The quality of the printing plate is relatively stable.

For the original printing plate, the surface is also required to be clean, no scratches, pinholes, and glue marks of the stitched image.

Fig. 2 Rendering effect of soft and hard dots

2. Exposure principle
One side of the original positive emulsion layer is attached to the photosensitive layer of the PS plate, and vacuum suction is used to close it. Then the light source in the printing machine is turned on to expose the plate. The light passes through the non-image portion of the original plate to reach the photosensitive layer, and the light-receiving portion of the photosensitive layer undergoes a decomposition reaction to release nitrogen gas to form an fluorenone compound, which reacts with water to form a tannic acid compound.

Several photosensitive resin exposure time

Plate photosensitive resin model

Exposure time (s)

light source

Model 215

15 to 20

Iodine lamp

Model 214

20 to 30

Iodine lamp

Model 124

40 to 75

Iodine lamp

3. Shaiban light source

The photosensitive layer of the PS plate has a certain degree of spectroscopic sensitivity, and only the light with a wavelength within a certain limited range can cause a change in the molecular structure of the photosensitive layer, that is, only when the light emission spectrum of the light source matches the spectral sensitivity of the photosensitive layer. Photosensitized layer may happen photochemical reaction. According to the measurement, the wavelength of the light wave that can cause the photosensitive reaction of the photosensitive layer is in the range of 250 to 420 nm. Considering the absorption of light by the printing plate glass and the original printing plate at the time of printing, this range is actually 300 to 450 nm. The wavelength of the light wave in the above range is mainly ultraviolet and blue violet with higher energy. Therefore, in order to rapidly photosensitize the photosensitive layer of the PS plate so that the gradation of the image is well reproduced on the plate, it is necessary to select a light source that can generate ultraviolet light or blue-violet light of sufficient intensity.

In addition to matching the light emission spectrum with the spectral sensitivity of the photosensitive layer, the light source for the printing plate should have the following conditions: the light energy distribution is uniform and stable, and “the data and standardization of the printing process will be reduced; the power consumption is small. The printing efficiency is high; the structure is simple, the operation is convenient, and the maintenance is easy; the factors that pollute the environment during use (such as heat released, ozone, smoke, dust, etc.) are reduced as much as possible.

In accordance with the above requirements, the current high-pressure mercury lamps and iodine lamps are mainly used as the PS version. 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 from 300 to 450 nm. It matches the spectral sensitivity of the photosensitive layer of the PS plate and the printing efficiency is high. The iodine deuterium lamp is made by adding cesium and bismuth metal bromide to the lamp of the high-pressure mercury lamp. The amount of light near the wavelength of 400 nm in the emitted spectrum is significantly increased, which is more consistent with the sensitivity of the PS plate photosensitive layer. The printing efficiency is higher. The use of iodine lamps as a light source for printing is approximately 0.5 to 1 times more efficient than the use of high-pressure mercury lamps.

The quality of the printing plate is not only related to the light source's spectrum and light intensity, but also affected by the distance between the layout and the light source. The shorter the distance, the less exposure time is required. However, the light source is close to the plate surface, and the portion of the plate surface that is vertically irradiated with light has a larger amount of light than the peripheral portion of the plate surface that is obliquely illuminated by light. This will cause the portion of the plate surface that is directly illuminated to be overexposed and the portion that is exposed to light to be underexposed to be underexposed. Phenomenon, in order to avoid this phenomenon, the distance between the layout and the light source should be equal to or slightly larger than the length of the plate face.

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