Talk about the main factors affecting the printing durability of the PS version
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The PS version is currently the most widely used printing plate in the printing industry in China. When we went to the market for technical services, many manufacturers reported that the PS version was not resistant to printing.
Offset printing, there are quite a lot of printed products in China, a large amount of printing. For example, the national central newspaper and the provincial newspapers and life newspapers of the provincial offices issue hundreds of thousands or even millions of copies every day; textbooks, especially the printing of textbooks for primary and secondary schools, due to the large population of China, many school-age children, the provinces The printing volume is in the millions of copies; there are also some commercial printing, such as cigarette, wine, medicine packaging, the amount of printing is quite large. Printers want a plate to print as much as possible. The more a version is printed, the lower the printing cost is. In addition, the number of times of changing the plate is reduced. The replacement of the version is time-consuming, labor-intensive, wasteful of paper, waste of ink, and waste of various consumables.
The printing durability of the PS version is actually a comprehensive assessment of the various factors of the entire process of printing. First of all, it is the state of the printing machine. The gap between the roller and the roller during printing is too small. If the gap is too small, the pressure on the printing plate is too high. In this case, the printing durability of the printing plate is obviously Be greatly discounted.
Offset printing relies on ink and water balance to achieve printing. The composition of the dampening solution tends to have different effects on the resin layer of the graphic portion of the printing plate, especially the commonly used alcohol dampening solution, which has a certain dissolution effect on the resin layer.
Secondly, it is the influence of paper on the printing durability of the printing plate. The quality of the paper has a great influence on the printing resistance. After a positive version of the PS version is made into a plate, if printed on coated paper, it can print 10 million impressions. If you use the printed textbook to print, if you don't use the baked version, you can only print about 50,000 printing. If you print on the printed paper, a printing plate can usually print 7 to 80,000 impressions. The rougher the surface of the paper, the fewer prints the plate has.
There is also the quality of the ink, the same plate, the use of high-quality ink is more than the use of ordinary ink.
Therefore, to talk about the printing resistance of the PS version, the actual use of the PS version of the printing plate to resist printing, is the comprehensive embodiment of the entire printing process.
For the PS version, there are positive PS and negative PS, and the negative PS is made of photopolymerizable resin. When the resin is crosslinked, it forms a larger molecule after cross-linking. Printed to 20 to 300,000 impressions. After a positive version of the PS version is made into a printing plate, it can only print about 7 to 80,000 impressions, and can print up to 11 to 120,000 impressions. Because it uses a photodegradable resin. Of course, if the baking plate is used, the plate coated with the baking glue is baked at a temperature of about 250 ° C for about 8 minutes, and the printing durability of the printing plate can be multiplied. Of course, if it is not baked, the printing durability of the printing plate will not increase much; if it is baked, not only can it not increase the printing durability of the printing plate, it may be counterproductive.
The following is a discussion and analysis of the influence of the PS version and its various factors in the plate making process on the printing durability of the plate.
First, the impact of the printing strength of the PS version on the quality of printed products
The quality of offset prints is a comprehensive reflection of the resolution and print durability of the plates. The unity of resolution must be guaranteed by the printing durability of the plate. The relationship between the quality of printed matter and the printing durability of the printing plate is very close.
1. Impact on outlets
The dot is the smallest unit of reflection that reflects resolution and resistance to printing. During the printing process, the dots on the original plate were full and round, and the layers were clear and clear. With the gradual change of printing time, the failure of the dot will gradually occur, printing to a certain time, the high-profile small dots of the layout will start to be lost, and the low-key dots will merge; compared with the printed samples, the tone will be too narrow and the original will be lost. The true reproducibility, the quality of printed products will gradually decline.
2. Effect on ink color
In the printing, due to the potential change of the dot, the ink color on the printed sheet also changes correspondingly, resulting in uneven printing ink color.
3. Impact on overprinting
The plate printing durability is too low, resulting in an increase in the number of plate changes. Each time the plate is changed, factors such as the lining error, the plate printing error, and the positioning of the rule may cause the image size to be unstable, which makes the overprinting of the next color difficult.
4. Impact on print volume
Due to the low printing durability of the printing plate, the number of plate changing times is increased, which not only occupies effective production man-hours, but also seriously affects the printing quantity, and also causes a certain waste of raw materials used.
Second, the main factors affecting the printing strength of the PS version
Usually the plate cannot be used because of the pattern, paste, and dirt. There are many reasons for these phenomena, and there are quality problems of the version. This situation is not much in the modern PS factory with good management. Because it has perfect testing methods, there will still be quality problems in the market. . Of course, the main problem is still to use. For the impact of the PS version of the printing durability, we summarized the following aspects:
1. Impact in PS production
(1) Plate base processing
When producing the PS plate, the surface electrolytic roughening treatment of the aluminum plate is first carried out. The purpose is to form a sand layer on the plate surface, so that the graphic part has a good foundation, and the blank portion is favorable for storing a certain amount of moisture in printing. It can evenly wet the layout. If the sand is too shallow, the water storage capacity is poor, and the ink balance during printing is not easy to control, and it is easy to cause the blank portion to be dirty. If the sand is too deep, the photoresist is not easy to apply evenly. Even if there is enough photosensitive layer to cover it, it will be smoothed during the imprinting process, causing the flower plate.
The metal ion impurities in the electrolyte also have an effect on the printing durability of the PS plate. In the production process of the PS plate, metal ions which are inferior to aluminum in the substance as the electrolyte sometimes precipitate on the sand, which may cause defects which are invisible to the naked eye, and the oxidation process does not cover these defects. These defects are easy to corrode the layout during the printing process, which is an important factor affecting the printing durability of the PS plate. The plate base after electrolytic roughening needs to be oxidized. If the concentration of the electrolyte, the temperature and the amount of the additive are not well controlled, the oxide film formed is thin and loose and brittle, and the wear resistance is poor. Especially the low-grade PS plate production line, due to the simple equipment and no automatic control device, it is easy to cause the electrolytic oxidation state of the plate sand layer is not good.
There are two aspects in the sealing hole caused by the sealing: First, if the calcium salt content in the sealing liquid is too much, the calcium ion reacts with the silicate to form a calcium silicate precipitate, which rapidly changes the composition of the sealing liquid, and the pores of the oxide film It easily absorbs calcium salts, which destroys the hydrophilic properties of the plate and causes the plate to be dirty. Second, the sealing time is short, the amount of colloid deposited on the plate is small, and the micropore sealing is insufficient, so that the adsorption capacity of the plate is increased, so that the photosensitive material remaining in the micropores cannot be sufficiently dissolved during development, thereby destroying the blank portion. The hydrophilicity causes the print to be dirty on the plate.
(2) Photosensitive liquid
The PS version of the photosensitive liquid is generally composed of a sensitizer, a film-forming material, and an additional material. The content of the sensitizer is too high, and the photosensitive layer is strong in alkali resistance, and cannot be completely decomposed during the normal exposure time, and remains in the sand layer after development, so that the blank portion is enhanced in lipophilicity and causes viscera.
2. Impact in plate making
The quality of the printing plate directly affects the printing quality of the printing plate, and also has a great influence on the printing durability of the printing plate.
(1) Exposure
Most of the sensitizers used in the positive-working PS plate are diazo or azide compounds, and the most commonly used diazonium-type sensitizers. After exposure, the visible portion can be removed during development to expose the hydrophilic layer of alumina to form a blank portion of the printing plate; while the portion where no light is visible, the photosensitive layer generally does not change, insoluble in dilute alkali solution, and retained. Form the graphic part of the plate on the layout. If the exposure time is insufficient or the exposure intensity is insufficient during the printing process, the underlying photosensitive material may not be completely decomposed, so that the printing plate is not completely developed, and the bottom surface of the plate may appear.
(2) Development
Development is an important part of determining the quality of the PS plate. Because the developer concentration is too strong or too weak, it will cause the blank part of the plate to become dirty. Under normal circumstances, the developer concentration is large, the development speed is fast, and vice versa; the developer temperature is high, the developing ability is strong, and vice versa. However, when the concentration of the developing solution is too rich or the developing temperature is too high, the photosensitive layer having no light portion is dissolved, the plate dot is reduced, the ink repellency is lowered, and the plate or the plate is severely damaged. If the developer concentration is too weak or the development temperature is too low, the development time may not be sufficiently developed. The development effect is not satisfactory, and some of the exposed image parts may not be completely dissolved and remain on the plate surface, and the plate surface is prone to dirty phenomenon after being placed on the machine.
In addition, experimental results show that after exposure and development of the PS plate, the developer can pass through the photosensitive layer to the surface of the oxide layer and dissolve part of the oxide layer. Although it is washed with water, it can only wash away the exposed portion of the PS plate. The developing solution of the graphic part, while the developing solution in the photosensitive layer is difficult to be washed out. During the period from development to printing on the machine, a small amount of developer remaining in the photosensitive layer still reacts with the photosensitive layer and the oxide layer, thereby reducing the adhesion of the photosensitive layer to the substrate, resulting in the printing process. The photosensitive layer is easy to fall off. Therefore, the cleaning of the PS plate after development is completely or not, and has a great influence on the printing durability.
(3) drying environment
There are dirty marks on the glass of the printing machine, and dirt on the original negative film will hinder the passage of the light from the printing plate, which will cause the printing plate to become dirty after being on the machine. During the process of making the PS plate (including before the imposition), if the indoor light is too strong, the PS plate is exposed to natural light, which will produce a slow chemical reaction and form a “return to the bottom” phenomenon, so that the blank portion is sensitive to fat, resulting in incomplete development. Make the layout dirty.
The well-made PS version should be protected with the PS version of the protective glue. The protective glue should be “thin and even”. There is no obvious rubber channel on the surface. The items used for the glue should be clean and soft. There should be no “hard” suede in the glue to avoid scratching the surface. If the glue is too thick, the glue on the surface will be too thick. After the protective glue is completely dry, there will be a phenomenon of “peeling off”, that is, the image will be peeled off after the machine is installed, and there will be no ink on the layout or partial ink on the layout. Slow phenomenon occurs.
When storing the prepared plates, do not store them in a high temperature or high humidity environment for a long time, and keep them away from light to prevent the resin layer from continuing to oxidize.
(4) Bake version
In order to improve the printing durability of the PS plate, it is necessary to perform the baking process. The purpose is to enhance the abrasion resistance and ink affinity of the graphic part, but if it is not handled properly, it will cause the blank part to be dirty. There are three main reasons for this: First, the dirtyness is not completely removed before baking, and the dirt is firmly adsorbed on the plate after baking. Second, the items used in the application of the protective glue are not clean; the internal quality of the protective glue changes during the storage process, and the layout is contaminated. Third, the temperature of the baking plate is too high, and the photosensitive layer of the plate is “carbonized”. The ideal baking temperature should be around 250 °C.
The structure of the baking machine is also related to the baking effect. Some manufacturers have installed the lamp longitudinally (that is, the same direction as the running direction) in order to facilitate the replacement of the lamp tube, so that the baked version is easily formed into a wave shape. Therefore, the lamp must be installed horizontally so that the plate is evenly heated during baking.
3. The impact of printing machinery
The ink on the printing plate can be transferred to the substrate under a certain pressure. Due to the printing pressure, the printing plate will inevitably cause wear on the printing plate during the rolling process, and the printing plate surface is worn after the printing plate is worn. Printing resistance will have a direct impact.
First, the blank portion of the plate, which has a stable hydrophilic and oleophobic property, is a sand layer that is completely dependent on the blank portion. The sand layer is gradually worn out during printing, and the specific surface area is reduced, which cannot be compensated for during the printing process. Due to the wear of the sand layer, the water storage capacity of the printing plate is reduced, the stability of the hydrophilic inorganic salt layer and the hydrophilic colloid layer is destroyed, and the blank portion is gradually viscous and dirty, which seriously affects the service life of the printing plate. For the graphic part of the printing plate, in the case of the same friction, the resin layer of the printing plate is also worn, so that the lipophilic hydrophobicity of the graphic portion gradually changes. The dot tone level has a great difference to the printed product, and the pattern is produced in the high-profile part of the printing plate, and the low-key part is pasted. Due to such phenomena, the use of "cleaning agent" to wipe the layout, even more damage to the surface structure of the layout and the adsorption capacity of the layout, making the plate unusable.
The wear and tear on the printing plate during the printing process mainly includes the following aspects:
(1) Friction between rollers
The printing of the printing machine adopts the method of round and round, because it is difficult to make no error due to the diameter between the rollers, and the speed difference and friction must be generated when the machine is running. The blanket cylinder is a friction body that often comes into contact with the plate cylinder. In principle, the linear speed of the contact surfaces of the two rollers is required to be the same, but since the blanket is elastic, when there is a certain pressure during printing, the linear velocity of the surface particles will be The slight difference is that the two rollers produce continuous friction and accumulate when rolling. The greater the pressure, the greater the friction generated. As time goes on, the final plate and blank parts will wear correspondingly. The plate was damaged.
(2) Influence by the ink roll
When the rubber-based ink roll contacts the surface of the printing plate, under the action of pressure, there are two destructive forces of radial rolling friction and axial series friction. In the radial rolling, the plate roller and the ink roller are required to have the same surface linear velocity under a certain pressure. However, the soft ink roller is driven by the contact deformation between the inkjet roller and the printing plate to generate an extrusion deformation. There is an unbalanced relationship between the pressure, the coefficient of friction and the linear velocity, which is one of the reasons for the radial friction and damage to the printing plate.
The axial series friction is a gap between the roller shaft of the ink roller and the bearing, and the ink roller caused sliding friction with the ink roller on the plate surface. This series of sliding friction is the most harmful in printing.
(3) Influence by the plate water roller
The plate water roller is rolled on the plate by relying on the pressure friction between the plate and the traction of the water roller. Therefore, in the work of the plate water roller, in addition to the same radial rolling friction and axial striking friction damage to the printing plate by the plate roller, the wear of the water roller on the printing plate is not only damaged by physical machinery, but also suffered Corrosion damage to chemicals.
4. The impact of the printing process
(1) The ratio of fountain solution
Whether the proportioning of the dampening solution is reasonable or not, it has a lot of influence on improving the printing durability of the printing plate. The acidity of the fountain solution is too weak, and the hydrophilic colloid layer is generated slowly, and the formation of the inorganic salt layer in the blank portion cannot be supplied and replenished in time, so that the hydrophilic and oleophobicity of the blank portion is lowered. The acidity of the dampening solution is too strong, it has a corrosive effect on the sand layer of the printing plate, and the hydrophilic salt layer is destroyed. Under the pressure friction, the printing plate is brightened, the storage moisture is reduced, and the printing plate is dirty and produces flowers. Version phenomenon. If the pH of the dampening solution is too high (weakly alkaline), the image part of the positive PS plate will be partially dissolved, and the common missing plate and pattern will appear, so that the plate cannot be printed. It can be seen that regardless of whether the pH of the dampening solution is too high or too low, it will affect the normal use of the graphic part and the sand part of the printing plate, thereby directly affecting the printing durability of the printing plate. Therefore, be sure to control the pH of the dampening solution used.
(2) The influence of printing pressure
When you look closely at the PS version of the machine under the problem, you can find that the parts that are missing the plate and the pattern are shiny. Generally, the pressure between the plate cylinder and the blanket cylinder is too large, and the sand layer of the layout is compressed and deformed, and the image and the dot are worn, thereby causing the phenomenon of the plate and the plate.
The reasons for excessive printing pressure are as follows:
a. During the printing process, due to paper, ink and other factors, deposits formed by paper powder and ink are formed on the blanket, which increases the pressure between the printing plate and the blanket. If it is not removed in time, the thickness of the deposit will continue to increase, the friction of the plate will gradually increase, the layout will be damaged, and the plate-disappearing and patterning will be easily generated.
b. The blanket cylinder is improperly lined. In actual printing, most of the lining is soft. In order to increase the printing pressure, only the thickness of the lining is increased. As a result, the surface linear velocity between the plate cylinder and the blanket cylinder is greatly different, and the friction generated is large. It will also cause the printing durability of the printing plate to decrease.
c. The inks currently used are mostly hydrophobic colloids composed of pigments, solvents and some auxiliary materials. The pigment in the ink has a great influence on the service life of the PS plate. Water (dampening solution) and ink coexist in the printing process. From the printing mechanism, water and ink are incompatible with each other, but in the actual printing process, a small amount of dampening solution penetrates into the ink to emulsify the ink, causing the printing plate to float and the ink is not effectively transferred. , it will cause paste phenomenon. When the ink layer of the printing plate is too thick, the hydrophilic layer of the printing plate is destroyed, so that the dampening solution cannot directly act on the oxide film of the printing plate, and the blank portion is dirty. If the dry oil in the ink is used too much, the ink will dry too fast, which will also cause the paste. The fluidity of the ink is too large, which easily causes the printing dot to expand and deform; the ink fluidity is too small, and the printing plate is prone to the pattern phenomenon.
The pigment particles in the ink are too thick, and during the printing process, the surface of the printing plate is prone to "dark spots", which affects the use.
d. The pH of printed paper is generally neutral, but some papers are too alkaline. During the printing process, the alkaline substance of the paper is neutralized by the acidic dampening solution in the blank of the printing plate, so that the blank part of the hydrophilic adhesive layer is not replenished, and the hydrophilicity is lowered to cause the upper viscera. The paper is rough and has a lot of sand, which will increase the wear on the printing plate, damage the oxide film of the printing plate, and make the printing plate dirty.
Third, how can we ensure that the printed plates have a high print durability?
To ensure that the printing plate made of PS plate has a high printing durability, in summary, we believe that the main points should be noted:
1. To get the correct exposure time (exposure), do not overexpose. Under normal circumstances, four-color film requires 3% of small outlets not to be sunk, 98% of large outlets do not appear paste, lines, text clear, excessively small graphics do not lose the net, full version of the real edge of the neat, hollow version Not paste version.
2. When developing the exposed PS plate, the concentration ratio of the developer must be strictly in accordance with the standard. Do not use inferior developer (powder), and do not rely on the feeling, experience to freely formulate or dilute the developer. Be sure to thoroughly clean the layout after development.
3. Make a good plate or print plate that is interrupted, but need to be reserved for reuse. A uniform PS protective layer should be wiped on the surface of the plate in a timely and proper manner. In the static storage, the plate should be stored in the dark to prevent the plate from continuing to oxidize. Store the place to avoid high temperature, high humidity, but also pay attention to the erosion of acid and alkali, and keep the plate dry. For long periods of downtime, it is best not to face the watering roller with dampening solution.
4. Prevent damage to the plate during the printing process. In most cases, the printing plate's printing durability is caused by the dynamic operation of the machine. The main reasons are: speed difference and friction; second, pressure; third, corrosion of dampening solution; fourth, suitable for ink and paper. Sex and so on.
So also pay attention to:
1 Adjust the pressure of the machine, that is, the pressure between the water roller, the ink roller and the plate cylinder.
2 Reasonably select the lining and select the appropriate roller lining. As far as possible, the angular velocity and linear velocity of the three rollers are basically the same during operation to reduce the friction on the surface of the printing plate.
3 Try to eliminate the axial movement of the water roller and the ink roller to avoid causing wear on the layout.
4 Correctly grasp the concentration and dosage of the dampening solution, so that the pH of the dampening solution is controlled within a weakly acidic range.
5 to control the viscosity, fluidity and other parameters of the ink. During the printing process, efforts are made to ensure that the printed image has enough remaining ink layers to protect the plate.
In short, printing is a process in which multiple factors work together and these factors are interrelated. Therefore, the printing durability of the PS version is determined by many factors, and each factor cannot be ignored. Only by doing all aspects of the work can we ensure that the plates made of the PS plate have high printing durability.