Discussion on standardization, datalization and st

2022-08-19
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Discussion on the standardization, digitization and standardization of positive PS plate printing

printing plays a connecting role in the whole plate making process. The quality of printing plate directly affects the quality of printing products. For many years, printing has been operated by masters with apprentices or by experience. Because the quality of printing directly affects the overall product quality, it must be regulated by objective means to make the operation of PS version standardized, digitized and standardized, so as to improve work efficiency and product quality

I. quality control of print film

print film can be divided into three types: text, image and pictures, of which image is divided into point and field. The two important indicators of its quality control are point density and point virtual halo

1. point density photochemical reaction occurs in the photosensitive layer of the printing plate during the exposure process. During development, the visible part is dissolved by the alkaline developer, showing the unexposed image part (point composition). Therefore, in order to prevent the spot covered part from passing through the light, it is required that the spot has a sufficient density. According to fo, due to the high degree of automation, the spot density on the plate should not be less than 2.30, that is, the light transmittance of the spot part should be less than 1/200

2. The density of dots on the print plate should be consistent from the center to the edge, so that the dots can be truly restored and transferred to the print plate after exposure and development. However, in practical work, there are often some points with high central density, low surrounding density and some virtual edges. When the exposure and development change slightly, the virtual halo points will change the size and shape of the points when they are transferred to the printing plate, resulting in the loss of small points. The virtual halo of the original text is mostly manifested in the pen edge and fine strokes of the text, resulting in the reduction of the font of the printing plate, lack of pens and broken strokes. According to Fogra standard, the width of the virtual halo edge of the original version cannot be greater than 4m M. only in this way can we ensure the true restoration and transfer of graphics and texts

II. Exposure control

1. Plate printing light source. There are two clear indicators of the light source for plate printing. One is the spectral characteristics of the light source; The second is the uniformity of illumination. The wavelength range of the photosensitive spectrum of Yanggu PS plate photosensitizer is between nm, and the most sensitive spectral region is 370-400nm. At present, the spectral energy emitted by metal halogen lamps (such as gallium iodide lamps) is concentrated between nm (see Figure), and the illumination is uniform, with no light scattering, less heating, and no harmful gas precipitation. It is an ideal light source for drying PS plates

the uniformity of light source will directly affect the quality of printing plate after printing. Due to the uneven illumination, the light decomposition of the photosensitive coating on the printing plate is uneven and incomplete, resulting in the reduction of some parts of the points and graphics, while some parts are underexposed or overexposed, so that the image gradients and points on the original plate cannot be completely transferred to the printing plate. Therefore, be sure to use a photometer to test the four corners and five points at the center of the exposure frame of the plate printing machine, and calculate the attenuation rate of each detection point of the plate printing machine a=emax-e1/emax '100% (E1 - illuminance of the measurement point, Emax - Maximum illuminance of the measurement point) through the formula. The attenuation rate of 5%~10% is ideal, and it can still be used when the attenuation rate reaches 20%. If the attenuation rate exceeds 20%, printing with this plate printing machine will have a significant impact on the point transfer, Therefore, it must be improved or replaced

2. Exposure time the exposure efficiency of the photosensitive layer decreases gradually from the outside to the inside with the irradiation of light. The length of exposure time plays an important role in controlling the dot transfer and improving the resolution. The exposure time of the positive PS version is too long. Due to the diffraction of light, the photosensitive adhesive layer that should not see light is also decomposed by light, resulting in point deformation. It has the sharpest real exhibition hall and exhibition hall in Poland, with the loss of dots, the thinning of text lines and short broken strokes. Reduce the printing quality and printing plate resistance; The exposure time is insufficient, the light of the photosensitive agent in the photosensitive adhesive layer is not completely decomposed, the dot is expanded, and the dark part is easy to paste the plate. After normal development, the printing plate will appear as dirty, which is very easy to get dirty when printed on the machine. Therefore, we must use Bruner test strip or gray scale to test the exposure of the printing plate to find the ideal exposure time. Usually, the proofing plate is controlled at 3.5~4.5 levels of whiteness, and the machine printing plate is controlled at 4.5~5.5 levels of whiteness and non inking

III. Development

development is one of the important links that determine the quality of PS version. The printing plate quality problems caused by improper control of developer concentration, temperature and time are irreversible and difficult to remedy. The purpose of development is to remove the photolytic part together with the sulfur soluble resin under the action of alkaline developer to expose the sand mesh of aluminum plate base and become the hydrophilic part of the printing plate

the unexposed part remains because it is insoluble in the developer and becomes the image part of the printing plate

ps version developing solution should be prepared according to the alkali resistance and chemical stability of the photosensitive film provided by the manufacturer and the recommended formula. Its main developing agent is generally sodium hydroxide. Because sodium hydroxide is easy to react with carbon dioxide in the air to produce sodium carbonate, the alkali force in the developing solution is weakened and the pH value is unstable, so add 2% of sodium phosphate or potassium nitride and other inhibitors that do not support any power and activities that split China, Make its pH value stable in a certain range. The pH value of the developing solution should be controlled between 7.5~9, the temperature should be (25 ± 2) ℃, and the development time should be calculated by the 3~4 levels of whiteness of the gray scale of the printing plate. Only in this way can we ensure that the oxide film of the blank part of the printing plate is not corroded, the photosensitive layer of the graphic part is not thinned, and the dots are not lost, so as to ensure the reproduction of the fine layers of the printing plate and the printing resistance

IV. cleaning and baking plate

the developed printing plate shall be thoroughly cleaned with clean water to remove the dissolved photosensitive adhesive layer and residual developer on the plate, and then the dirty spots such as tape or frame marks shall be removed with cleaning agent. The time is about 30s. Rinse quickly after dirt removal to prevent the dirt remover from damaging the graphic part. When the dirt remover is not in use, it should be sealed in time to prevent the solvent from volatilizing and reduce the dirt removal capacity

baking plate is an effective way to improve the printing resistance of positive PS plate. Under the action of thermal radiation, the printing plate crosslinks the thermoplastic phenolic resin in the photosensitive film into large molecules. The photosensitive horizontal tensile testing machine adopts the mature universal testing machine technology, and the optical group disappears. The photosensitive layer changes from green to red brown, and its wear resistance, corrosion resistance, solvent resistance and adhesion are significantly improved. The printing plate before baking should be cleaned, and the dirt remover, dissolved photosensitive adhesive layer, dust and moisture should be completely removed. A layer of baking protection solution should be evenly coated with absorbent cotton to protect the pores of the sealed oxide layer from reopening and restore its strong adsorption, so that the plate is not polluted. Then it should be baked in an oven with a constant temperature of 230 ℃ ~250 ℃ for 5~8min. After taking it out, the printing plate should be naturally cooled before secondary development. Baking temperature and time must be well controlled. Too high temperature or too long will cause carbonization and deterioration of the photosensitive layer, deformation of the plate base and non inking of graphics and texts; If the temperature is too low or the time is too short, the desired purpose will not be achieved due to insufficient curing degree

v. glue stabilizing, drying and storage

the sun dried printing plate must be dried in time after applying a layer of protective glue (Arabic gum) with uniform thickness. If the protective glue is applied unevenly, the printing plate is very easy to oxidize in case of environmental temperature and humidity changes. When printing on the machine, there will be strips of glue bars, which can be removed in light cases and scrapped in serious cases

in order to ensure the good effect of machine printing on the printing plate, the printing plate must be placed in a cool, dry and dark place (except the baked printing plate), that is, the temperature is (20 ± 2) ℃, the humidity is 60%~65%, and a dry backing paper is added between the printing plate and the printing plate

the standardization, digitization and standardization of Yangtu PS version can be said to be a systematic project, involving a wide range of areas. Each factory should summarize and improve a set of process and technical data and standards suitable for itself according to the specific situation, standardize the operation, stabilize and improve the plate making quality, and bring good economic and social benefits to the enterprise

source: printing technology

Author: Qian Jin

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