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Orient the quality control of printing plate in offset printing plate
In the offset printing of the optical disc, the Printing Plate is an important link between the plate making and the printing, and is an important factor influencing the printing quality. The so-called printing version refers to the process of faithfully transferring the dots on the photographic original (film) to the printing plate (negative pattern PS version) through actinic action. Due to the constraints of processes, materials, equipment and other factors, it is often difficult to achieve the purpose of faithful reproduction. Therefore, in order to ensure the quality of printing, we must control the quality of the printing plate, that is, the transfer of control points. Below, we will analyze how to control the correct transfer of outlets.
First, the factors that affect the PS version network transfer:
Through the analysis of the whole printing process, we can know that the main influencing factors are the following aspects: the quality of the original (film), the reproducibility of the plate, the printing machine and the light source.
1, the original (film) quality
The ideal original quality should be a high dot density (D>3), a very solid idea, and a clean, hard spot. Such outlets have a large degree of exposure and development during printing and are easy to faithfully reproduce. The dot center density and the edge density must be the same, so as to expand the latitude of the exposure time, and completely transfer the dot. The original should be clean and free from dirt, otherwise it will be dirty.
2. Reproducibility of halftone dots
Reproducibility of media outlets means that the photographic prints can reproduce the original web sites to the maximum during the printing process.
The effect of reproducibility is mainly reflected in the resolution and film thickness of the plate. As the PS Plate is a pre-coated photographic plate, its substrate is an Aluminum Plate, and a uniform photosensitive film layer is coated on the substrate. Therefore, the treatment process of aluminum base directly determines the quality of the plate. This part is often invisible to the users of the plates. Therefore, it is particularly important to ensure the quality of the plates through the purchase channels. If the plate is rough and the quality is poor, it will lead to a decrease in resolution, which will seriously affect the printing quality. Some inferior plates increase the thickness of the photo-resist film layer in order to mask the grain on the aluminum plate base. Doing so will result in an increase in the diffraction phenomenon at the time of exposure, which will result in decreased resolution and affect the print quality.
The following are the effects of the quality of the media on the reproduction of outlets and their requirements:
(l) Partial version, paste version
The base of the printing plate should be clean, uniform in thickness, evenly distributed in sand, moderate in thickness, moderate in depth and depth. The photosensitive liquid is evenly coated with no bubbles or scratches. It is generally required that the average depth of the sand (Rz) is 5 to 7.5 μm, and the average roughness (Ra) is about 0.6 μm. This has affected the quality of the PS version, resulting in local rapid wear and tear, "flower pattern", paste version, and then appear off version and dirty.
(2) PS plate surface roughness is not standard
Abundance of coarse sand, so that the water is insufficient, easy to dirty: The sand is too fine, so that the photosensitive film in the microporous is not easy to clean, the base is easy to pollution, easy to dirty when printing.
(3) Partially dropped words, lost version
After the electrolytic roughening, the gray quality of the printing plate base is not completely removed, and the photosensitive layer is not firmly adsorbed. Before the coating of the photosensitive liquid, the plate surface is not rinsed cleanly, leaving traces of acid flow when hanged, which may cause partial plate-off.
(4) not easy to ink
PS version has been beyond the preservation period or improper storage, such as the preservation of the ambient temperature is too high (more than 30 °C), relative humidity more than 65%, too strong light, etc.: there is too low viscosity, photosensitive coating is too thin, causing PS printing on the machine is not easy to ink or not at all.
3, printing machine and light source
When printing, it is required that the suction vacuum degree of the printing machine should be as high as possible to ensure good contact between the original plate and the printing plate, which is beneficial to the transfer of the dot; if the suction is not good, the spacing between the original plate and the printing plate increases. Large, it will lead to diffraction is very serious.
The choice of printing plate light source is also very important. In the printing process, it is expected that the ideal light source should emit absolute parallel light, but the light source used in actual production can hardly meet this requirement. Usually in actual production, the light source is required to maintain a distance of about 1 meter from the printing plate so that the light emitted by the point light source can be as close as possible to the parallel light.
Since the exposure is a photochemical process that subjects the photosensitive film to light and undergoes a chemical reaction, the process is affected by two factors, namely the illumination E on the image plane and the exposure time T. The energy obtained per unit area of the photosensitive film, ie the exposure (H) equals the product of the illumination E and the time T:
H=E × T
Therefore, to determine the size of the exposure, it is necessary to consider both the illuminance E (light intensity) and the exposure time T.
The light source of the printing machine is generally a metal halide lamp. Such a light source decays after being used for a long time, resulting in a decrease in the illuminance E during exposure. Therefore, in the actual production, after the metal halide lamp is used for a period of time, it is necessary to increase the exposure time T according to the actual situation of the light source, so as to ensure that the exposure amount during the exposure during exposure is maintained.
Correctly grasping the exposure time is very important for the printing process: overexposure will cause some small dots to be sunburned, which will result in the loss of brightness levels; and underexposure will cause the decomposition of the blank (non-textual) part to be incomplete. And easy to stain. Therefore, the following factors need to be taken into account in the exposure process: (1) Choose a light source that matches the spectral sensitivity of the PS platen-short wavelength light source (UV light source). On the one hand, it can shorten the time of printing, on the other hand, it can also reduce the thermal effect of long wave. (2) Minimize the effect of natural light on the PS plate. In general, the use of incandescent lighting is relatively safe, even after the exposure is complete, but also to minimize the light, to prevent the graphic part of the re-decomposition. (3) The exposure time should be coordinated with the development, as stable as possible, which is conducive to print quality. (4) During the period from the development to the printing, if the time permits, the plate should be dried as much as possible in the cool place to prevent the damage of the photosensitive layer caused by strong light or high temperature.
In the printing process, in order to ensure the accurate reproduction of the network points, we must pay full attention to the tight integration of the original version and the PS version. The unevenness of the printing glass and blanket, the dirt of the dust pad, the adhesive tape and dirt on the film, and the insufficient vacuum between the film and the PS plate will cause the adhesion of the film and the photosensitive layer of the plate. Inadequate, resulting in diffraction when the light passes through, causing the dots of the printing plate to appear imaginary, narrow, or non-ink-absorbing white spots, affecting the reproduction of the dot, leading to a decrease in the reproducibility of the image reproduction, and the quality of the printing plate will not be ignored. Impact. In addition, the aging of blanket printing blankets will cause a small amount of air leakage at some edge positions, which will also cause the PS plate and the film to stick tightly, making the drying out of the picture.
Since each optical disc company is equipped with a developing machine at random when the offset printing machine is purchased, and each type of developing machine has a similar function and structure to the Japanese Toray development machine, it can be set according to the developing speed recommended by each device manufacturer. However, it should be noted that the pre-treatment solution must be replenished in time according to the number of plates. Otherwise, the concentration of the pre-treatment solution will be insufficient, and the development speed will not be adjusted, resulting in incomplete decomposition of non-graphical components, resulting in blurred plate images and easy Dirty. Water is the Toray PS version of the developer, so ensuring water quality is critical. It is recommended to clean the filter once a week and change the water. The post-treatment solution is mainly used to color the dots so as to observe the reproduction of the dot after development. Because we can't distinguish the transfer of the dots with the naked eye after the print version of the PS plate, we must use the developed image to determine the reproducibility of the dot, so the control of the developing stage is also very important. In particular, the temperature of the pretreatment liquid (usually set at 38°C or more for the developer to start) and the concentration, water quality, etc., as well as the height of the brush and the hardness of the bristles all affect the development. We can visually compare the developed dots with the dots on the film with a 15x or more magnifying glass. If the size is comparable, it is proven that the printing and development are relatively normal. If they do not match, you must find out the reason and resolve it as soon as possible. Due to the high latitude of the development, it can be detected once a week.