Offset Print Quality Control of PS Plate Making

In offset printing, platemaking technology is always the guarantee of printing quality. It plays a role in the prepress and printing. Only the quality of PS plate-making can be ensured to ensure the quality of printed matter.

1 Original and Plate Quality

1) Original quality problem

The original refers to the film or sulfate paper produced by prepress production, and its quality directly affects the PS platemaking effect. The original quality has two important indicators:

(1) Dot density

In order to achieve an effective exposure during the printing of the positive print, the original negative film net point is completely transferred to the printing plate so that the amount of light passing through the dot does not pass. Therefore, the dots must have sufficient density, the density of a single dot should reach 3.0 or more, and the density in the field should reach 3.5 (preferably 4.0). The density of the original film should be the same, the transmission density of the blank area on the film should be less than 0.05, otherwise it will affect the density contrast on the printing plate. The original density value is too low, or there are soft dots with low density and active dots with low density in the middle density. The printing stability of these two types of dots is poor, exposure and development change slightly, and the depth of the printing plate does not change. One, which directly affects the printing plate resistance to printing force.

(2) The outlets are bright and clean, with no halo

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 the plate will be dirty. The original is preferably a complete film without a knife edge and collage tape. This will ensure that the film is tightly integrated with the PS plate, and that dots that are caused by scattering of light will be avoided.

2) Plate quality

(1) Edition base

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 grains (Rz) is 5 to 7.5 μm, and the average roughness (Ra) is about 0.6 μm.

(2) Photoresist layer

The photosensitive coating should be uniform, with a moderate thickness, and it should not only completely cover the peaks of the grit, but also prevent the blank part from becoming dirty and not too thick, which will affect the resolution of the printing plate. The thickness of the general PS plate coating layer is about 1.8 g/m2.

3) Problems caused by the quality of media

(1) Partial pattern, paste version

Because some domestic PS plates have a certain gap between the thickness and flatness and international standards, such as too large thickness and uneven flatness, this affects the quality of the PS plate, causing local rapid wear and tear. Edition, 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.

In addition, since the photosensitive film is too thin, the alkaline developing solution can easily damage the oxide film during the development, and when the printing is performed on the upper surface, the printing plate and the space can be dirty.

2 printing

Drying out of the PS plate requires firm and smooth dots, so that the edge of the dot can withstand the long-term erosion of water and ink, and is not easy to produce shrinkage or expansion, in order to ensure the stability of the plate dot size during printing. At the same time, the blank part of the printing plate is required to be clean and free from dirt, and the graphic and text part is better in terms of oleophilicity. This kind of PS plate is resistant to printing force.

1) Exposure time

The exposure time of the printing plate is related to the depth of the printing plate. If the exposure is excessive (the time is too long), the dot area will be reduced, which will affect the restoration of the image tone level. Insufficient exposure (time is too short), the printing plate is relatively deep, not only affects the restoration of image tone levels, but also non-graphical parts are easy to get dirty. The exposure time is also very much related to the original and media. The original outlets are small, but the relative density is low. The relative time is short: the field, coarse net, and yellow are relatively long. The exposure time required for different PS plates is different. Therefore, each purchased PS plate should be subjected to an exposure test to obtain the best exposure time.

2) The dot is not true and the blank is not clean

First, the platen glass should be kept clean. The transparency of the glass is not enough and the light penetration will be affected. If the lamp is used for too long, its power will decrease and the exposure will be insufficient, so that the printed version will be blurred. Careful attention should be paid in the printing to the original version and the PS version. The unevenness of the printing glass and blanket, the dust pad, the padding of the adhesive tape and dirt on the film, and the insufficient vacuum can all result in poor adhesion between the film and the photosensitive layer of the plate. When the light passes through, Diffraction phenomenon will occur, so that the dots of the printing plate will be imaginary or reduced or there will be no ink-absorbing white spots, which will affect the reproducibility of the image reproduction, and the quality of the printing plate can not be ignored. In addition, the overlap of film during imposition is also not practical. When the blanket ages, there will be a small amount of air leaks at some edge areas, and the PS version will not stick to the film, and the printed text will also become empty.

3 Development flush

After development, develop and rinse. 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. The effect of the development is to remove the photosensitive layer of the blank portion of the PS plate, so that the surface of the PS plate substrate remains hydrophilic. Insufficient or excessive development will render the PS plate obsolete. In addition, the lack of flushing can also affect the quality of the plate.

1) Control developer concentration

PS version of the developer is mainly composed of developer, inhibitor and wetting agent. The developer is the main component in the developer. The higher the content thereof, that is, the higher the concentration of the developer, the faster the development speed. When the concentration of the developer is too high, the photosensitive layer on which no photochemical reaction takes place on the PS plate will also be dissolved, so that the sensory properties of the plate surface and the printing plate's resistance to printing will be reduced, and the "off plate" phenomenon will occur during printing. When the developer concentration is too low, the development is insufficient, the dots of the layout image portion are enlarged, and the non-graphic portion is dirty. Therefore, different developer solutions should be selected according to different types of plates. In order to accurately control the concentration of the developer, a test strip test can be used. After the printing plate is developed for 30-60 seconds, it is observed whether the hollow point of the Bruner test strip is 98% to 99% and the small dot is 1% to 2%. If it is firm, if the point is clear and strong and the small dot is firm, the developer concentration is standard. If the loss of small outlets is too high, it means that the concentration is too high, such as the hollow paste, then the concentration is too low. Should be based on this increase or decrease the developer content of the main agent. For the accurate development effect, visually, there is no change in the green color of the graphic text on the layout, while the greenness of the blank part completely disappears, and the base is light gray. If developed, the blank part still has a blue-green color. It shows that the developer is weak or invalid and underexposure.

2) The temperature of the developer

In the case of other relatively stable development conditions, the higher the development temperature, the faster the development speed, and vice versa. The developer should be circulated and stirred so that the liquid can be kept even and constant. General development temperature should be controlled at 20 ~ 25 °C is appropriate, if the temperature exceeds 30 °C, the photosensitive layer and the film-forming material in the photosensitive layer of the binding force between the molecules will weaken, anti-alkaline will decline, no light part of the The photosensitive layer and the oxide layer of the aluminum plate will be dissolved, resulting in the dissolution of dot on the plate, the image becomes lighter, the percentage of the dot area is reduced, the reproducibility of the layout tone level is destroyed, and when the temperature is lower than 5°C, the development Almost at a standstill.

3) developer fatigue

It depends on the number of developing plates and developer storage conditions. Such as the use of hand-developed development, due to exposure to the air exposed in the air, the absorption of water and carbon dioxide, accelerated developer degeneration, affecting the development performance. Another cause of the fatigue of the developing solution is that the photodegradable substance is dissolved therein, and the developing ability is significantly reduced. Therefore, the currently used alkaline developer should minimize contact with air during storage and use. Reduce the impact of moisture and carbon dioxide on it, and regularly replace and quantify supplemental medicine.

4) Control pH and development time

The developer is a weakly alkaline solution. The general pH value should be controlled at about 9 to 11, and the development time should be controlled at 60 to 100 seconds. 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. The development time is affected by various factors such as the exposure amount of the printing plate, the performance of the photosensitive agent, the development method, and the status of the developer. Therefore, to control the degree of development, the above-mentioned variable factors should be grasped and stabilized first, and then the test should be performed with a gray scale. , to determine the development conditions and the correct development time.

5) In addition to dirty

The stencil machine should be repaired in time, replace the blanket cylinder, and regularly clean the crystals in the tank and on the wall of the tube. In addition to the dirty process, the operation is not careful, in addition to the lack of dirty fluid, the use of liquid for a short time, in addition to dirty liquid failure, in addition to dirty liquid used in the layout will cause the printing plate dirty. (to be continued)

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