As we all know, in the lithography process, the printing plate is between the plate making and the printing, and the original printing plate (divided color) after proofreading and signing is attached to and printed on the lithographic plate coated with special photosensitive material. Process, the quality of the printing plate directly affects the printing quality. This article makes a preliminary analysis of what factors in the production process will affect the quality of printing and its solutions. (This article refers to the positive version of the PS version of the print version)

First, the influence of the density of the original text and non-graphics of the printing plate

Stencil operators will encounter such a situation: no matter how kind strictly in accordance with the printing plate operation rules printing, drying out version of the feeling of graphic is always not strong or non-graphic (blank part) is not clean and printed stains, in fact This is mostly caused by too much fog in the non-graphical parts of the original text graphics. PS version of the original version of its graphic textual density values ​​generally require more than 3.0 (in the field of higher density values). If the density is too low, part of the strong printing light will penetrate the original image when performing the printing at the normal exposure, so that the photo-sensitive layer of the PS plate under the picture will undergo photochemical reaction, but due to this part of the light The light energy is so small that it is far from the critical exposure amount that makes the entire photosensitive layer completely photochemically reactive under the graphic. The photochemical reaction is only the surface portion of the photosensitive layer, and this portion of the photosensitive layer is dissolved by the developer during development. Therefore, the printed version feels light and not strong.

By the same token, if the non-graphic part of the original printing plate has fog, when the printing is performed at normal exposure, part of the light energy is blocked and absorbed by the black metal bank that forms fog, and the light energy reaching the photosensitive layer of the PS plate is absorbed. It is not possible to completely photochemically react the entire photosensitive layer underneath the non-graphic portion. After development, a portion of the photosensitive layer that has failed to undergo photochemical reaction remains on the surface of the PS plate to form a so-called print stain. These prints formed on the non-graphical parts will attract ink when they are officially printed, that is, dirty and affect the print quality. In order to make the PS plate-making plate to achieve accurate copying effect and to make the plate clean, reduce the dirty time, there must also be strict requirements for the printing plate original. If the density of graphics and texts does not reach the required or non-graphical parts of the fog is too large, it must be returned to the previous process to redo.

Second, the influence of spectrum of light source

The photosensitive layer of the PS plate is different from other photosensitive materials in that it is composed of a photodecomposition diazo compound as a sensitizer, a film-forming substance, a dye, and the like, and has a spectral spectral sensitivity, only the wavelength of light in a limited range. Can cause changes in the molecular structure of the photosensitive layer. That is, the photochemical reaction of the PS plate can take place only under the condition that the light emission spectrum of the light source matches the spectrum sensitivity of the photosensitive layer. Therefore, the correct choice of printing quality is very important, and it is also very beneficial to shorten the exposure time and accurately duplicate the original dot level.

According to the measurement, the spectral sensitivity of most PS plates is in the wavelength range of 210-460 nm. However, considering that the plate base of the printing plate, the glass of the printing plate, etc. have a strong absorption effect on the light below 320 nm, in fact, this range is within the range of 320 to 460 nm. Therefore, if the photosensitive layer of the PS plate is to be rapidly sensitized so that the gradation level of the image is well reproduced on the plate, it is necessary to select the luminaire having a higher luminous intensity in the range of the luminescence spectrum as the serigraph light source. However, in practical operation, it is preferable to specifically select the printing plate light source according to the dosage form of the photosensitive agent used in the PS plate photosensitive layer. For example, the photosensitive layer is a PS plate with an ortho-nine azide-type sensitizer (spectral spectral sensitivity range of 320-460 nm). When iodine-negative lamps are used for printing, the exposure time is only about 20 seconds, and the use of high-pressure mercury lamps is 30 seconds. Above, this is because: Although the spectral components of both lamps are in the range of 300-460 nm, the distribution of light energy near the 400-nm wavelength band in the spectrum emitted by the sulfonium xenon lamp and the light energy are much more uniform than the high-pressure mercury lamp. Many, and more consistent with the spectral sensitivity of this sensitizer type, luminous efficiency is also higher.

Third, the impact of exposure time

The requirement of the PS plate printing plate is that the photosensitive plate of the PS plate corresponding to the non-graphic part of the printing plate is completely dissolved by the developing solution during development, and the photosensitive plate of the PS plate corresponding to the graphic part must be left after development. On the layout. To meet this requirement, the exposure time must be controlled correctly. If it is overexposed, the line strokes on the printing plate will be thinner than those of the original printing plate, and the dot area will be reduced, which will affect the reduction of the image price level, and will affect the printing quality. Insufficient exposure will not only affect the restoration of image tone levels, but also the non-graphical parts will be dirty. Therefore, it is very important to determine the correct exposure time before printing.

However, the type of the PS plate is different, or the sensitizer form used in the PS plate is different, and the exposure time is not the same under the premise of using the same light source for exposure. For example, a PS plate using a condensate of o-quinone diazide (2.1)-5-sulphonyl chloride and a polyphenol resin (referred to as a 2.1.5 type photosensitive agent for short) as a sensitizer has faster photosensitivity than o-azane naphthalene. The sulfonyl chloride (1.2)--4 condensate of the sulfonyl chloride and the resole phenolic resin (abbr. 1.2.4 sensitizers) is used as the sensitizer for the PS plate, and the exposure time for the two plates is different. At present, most tests are used in actual production to determine the correct exposure time of the PS version. Specifically, the standard “Brunner” test strip is used to perform step-by-step exposure with different exposure times on the same printing plate using the actually produced copying light source. After development, it is selected on the printing plate and can be retained. The exposure time of the actual black spot and its corresponding empty spot with the same polarity is the standard exposure time of the batch of PS plates. However, it should be noted that the exposure time of the printing proofing version and the upper printing version is different, that is, the exposure time of the printing plate on the basis of the standard exposure time is slightly more than that of the drying pattern, which is the printing pressure of the offset printing machine. , printing speed and printing conditions and other printing conditions.

Fourth, the development conditions

The development conditions mainly include: the developer temperature, concentration, and development time. The development of the PS plate is actually the process in which the alkaline component of the developer dissolves the photosensitive layer of the produced acid compound (dissolved in a dilute alkaline solution) from the plate surface after the photochemical reaction takes place. It is known from the test that under the same conditions of other development conditions, the higher the temperature of the developing solution, the faster the developing speed. When the temperature exceeds 35 degrees, the photosensitive material and the film forming substance molecules in the photosensitive layer in the photosensitive layer The bond strength between the two will be weakened, and the alkali resistance will be reduced. The photosensitive layer of the unseen portion and the oxide layer of the aluminum plate will be dissolved, causing the image on the plate to become lighter and the reproducibility of the tone level to be destroyed. ; And when the temperature is below 5 degrees, the development is almost at a standstill. Therefore, the developer liquid temperature is preferably controlled within a certain range. The suitable development temperature for the PS plate is around 20 degrees.

PS version of the developer is mainly composed of developer, inhibitor and wetting agent. The developer is the main component of the developer. The higher the content of the developer, the higher the concentration of the developer, the faster the development speed. When the concentration of the developer is too high, the photoreactive layer of the PS plate will be dissolved. , Make the layout of the sense of fat and the printing plate of the printing force decreased, severe print will appear "off version" phenomenon. When the developer concentration is too low, due to insufficient development, the halftone dot ratio of the layout graphic portion is more than normal, and the non-graphic portion is dirty. Therefore, in addition to the preparation of the developer according to the information provided by the manufacturer of the PS plate, the developer should be properly added with a fresh developer according to the use of the developer to maintain the development of the developer and ensure the development quality.

From the foregoing, we can know that the higher the developer temperature or concentration, the shorter the development time (ie, the faster the development speed); the lower the developer temperature or concentration, the longer the development time (ie, the slower the development speed). However, if the development time is too long, the small dots of the image layer on the plate and the fine strokes of the text are easily lost. The development time is too short, the photosensitive layer of the non-graphic portion of the plate cannot be completely dissolved by the developer, and the plate layout will appear dirty. Therefore, accurate control of the development time is an important part of printing. In actual production, it is determined according to the type of PS plate and the conditions of the printing process. For example, for the PS version of the rotary machine, due to the thicker photosensitive layer, the development time can be appropriately extended; while the photosensitive layer of the PS version used on the platform is thinner, the development time can be shortened relatively. However, it is generally believed that the development time should be controlled at about 1 minute.

Fifth, the influence of the adhesion degree of the original printing plate and the PS plate

During the printing process, if the tightness of the original printing plate and the PS plate is insufficient, that is, there is a gap between them, diffraction will occur when the light passes through, and the dots of the printing plate will be imaginary or reduced or there will be no ink absorption. The emergence of dots affects the reproducibility of image reproduction and has a non-negligible impact on print quality. There are several reasons for this situation:

1. The exhaust system has failed. It is common for the vacuum pump's rotary disc to wear out during the process of use, or the aging of the rubber sealing ring of the frame. The printing plate glass cannot be in close contact with it, resulting in insufficient pumping. In this case, it is necessary to replace the rotary sheet or rubber seal to protect enough adhesion between the original printing plate and the PS plate.

2. Due to the effect of adhesive tape, voids are generated. In general, the adhesive tape will not affect the quality of the printing plate when the plate is printed, but if the tape is too close to the graphic (less than 3mm from the picture), the tape will have a certain amount of The thickness (approximately 0.03mm) will create a gap between the original plate and the PS plate. This requires that the tape should not be 5 mm or more in length at the time of imposition, or that the adhesive tape is used for pasting.

3. Effect of dust particles. Sometimes some inconspicuous dust particles stick to the original printing plate, between the original printing plate and the PS plate, so that the printing plate has a blank spot that does not absorb ink. Therefore, the printing room should be cleaned regularly, and air conditioning should be installed wherever possible; before the printing, the original printing plate and the printing glass should be cleaned to reduce the effect of dust on the quality of the printing plate.

In summary, there are indeed more factors that affect the quality of the printing plates. This requires that each copying operator should not only strictly follow the printing plate printing instructions, but also have knowledge of the printing plate theory. Resolving issues that arise in time without affecting the completion of production tasks is beneficial to improving production efficiency.

Source: PACK.CN