Plate making process of positive image PS plate
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The PS plate of the photosensitive layer is pre-coated, and after printing, developing, removing dirt, and coating, a printing plate for printing is prepared. In the positive-working PS plate, the exposed photosensitive layer undergoes a photodecomposition reaction, and is dissolved by the developer to be removed from the plate surface, and after coating the adhesive layer, a stable hydrophilic blank portion is formed; and the unexposed photosensitive layer remains On the plate, it is lipophilic and forms part of the image.
First, exposure
The original image of the positive image is closely adhered to the photosensitive layer of the PS plate, placed in a special printing machine, and after vacuum pumping, the light source of the printing machine is turned on, and the photosensitive layer of the non-image portion is lighted under the illumination of light. Decomposition reaction, this process is called exposure, also known as the printing plate.
1. Original site quality
The PS version has fine sands, high resolution, good tone reproducibility and dot reduction. 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 plate for printing.
The original version of the PS version is usually made by electronic scanning, photography or laser photolithography. 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 a small edge density value and a large central density value will be formed. In this original printing plate, the slight increase or decrease of the exposure amount will cause a large decrease or increase in the area of the printing plate dot point, thus affecting the reduction of the PS plate to the dot. Therefore, the dot on the original plate for printing can be a hard dot with a high density value, uniform edge and center density, and a clean peripheral edge. Generally, the dot density value is required to be 3.0 or higher (the density value of the solid version should be higher). The exposure density of the hard dots is large, and even if the exposure is somewhat changed, the dot area on the printing plate will not change too much, and the quality of the printing plate is relatively stable.
For the original plate for printing, the surface is also required to be clean, free of scratches, pinholes, and glue marks that spliced the image.
2. Exposure principle
The side of the original image with the emulsion layer is attached to the photosensitive layer of the PS plate, vacuumed to make it close, and then the light source in the printer is exposed 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, releasing nitrogen gas to form an anthrone compound, and forming a phthalic acid compound in contact with water.
Exposure time of several photosensitive resins
Plate photosensitive resin model Exposure time (s) light source
Type 215 15~20 Iodine lamp
Type 214 20~30 Iodine lamp
Type 124 40～75 Iodine lamp
3. Printing light source
The photosensitive layer of the PS plate has a certain spectral sensitivity, and only the light having a wavelength within a certain range can cause a change in the molecular structure of the photosensitive layer, that is, only under the condition that the luminescence spectrum of the light source matches the spectral sensitivity of the photosensitive layer. The photochemical layer may undergo a photochemical reaction. According to the measurement, the wavelength of the light wave which can cause the photochemical reaction of the photosensitive layer is in the range of 250 to 420 urn, and in view of the absorption of light by the printing machine glass and the original printing film, the range is actually 300 to 450 nm. The wavelength of the light wave in the above range is mainly high-energy ultraviolet light and blue-violet light. Therefore, in order to quickly sensitize the photosensitive layer of the PS plate so that the tone of the image is well reproduced on the printing plate, it is necessary to select a light source capable of generating ultraviolet light of sufficient intensity or blue-violet light.
The light source used for printing is not only matched with the spectral sensitivity of the photosensitive layer, but also has the following conditions: the light energy distribution is uniform and stable, and the data is standardized and standardized in the printing process; The printing plate has high efficiency; the structure is simple, the operation is convenient, and the maintenance is easy; the factors that pollute the environment (such as the released heat, ozone, smoke, etc.) are minimized.
According to the above requirements, at present, as the PS version of the light source, there are mainly high-pressure mercury lamps and iodine gallium lamps. The high-pressure mercury lamp is made by sealing 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 PS plate photosensitive layer, and the printing efficiency is very high. The iodine gallium lamp is made by adding bromide of bismuth and gallium metal to the tube of the high pressure mercury lamp. The amount of light in the spectrum of the emission near the wavelength of 400 nm is significantly increased, which is more consistent with the spectral sensitivity of the photosensitive layer of the PS plate. The printing plate is more efficient. Using a iodine gallium lamp as a light source is about 0.5 to 1 times more efficient than using a high pressure mercury lamp.
The quality of the plate is not only related to the spectrum and light intensity of the light source, but also to the distance between the plate 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 portion of the plate that is vertically illuminated by the light is more exposed than the surrounding portion of the plate that is obliquely illuminated, which causes the portion of the plate that is directly exposed to light to be overexposed and partially exposed to the light to be underexposed. Phenomenon, in order to avoid this phenomenon, the distance between the layout and the light source is preferably equal to or slightly larger than the length of the diagonal line of the plate.