Some Discussions on Improving Print Resistance of Printing Plate

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Under the premise of guaranteeing the quality of printing, the highest printing quantity that the printing plate can bear is called the printing plate's printing durability. The level of printing durability of the printing plate directly affects the level of printing costs, so for printing companies, it is of practical significance to study how to improve the printing durability of the printing plate.
Before discussing how to improve the printing plate durability, we must first understand the following questions: What structures on the printing plate determine the printing plate durability? What are the factors that affect the durability of the printing plate? As the saying goes, cows need to hold their noses. Only by grasping the main factors that affect the printing rate of printing plates can we be more effective in improving the process by improving the printing rate of printing plates. Below we discuss separately:
1. The structure and nature of the printing plate that determines the printing durability of the printing plate:
1. The base of the PS version is aluminum. Aluminum plates have the advantages of light weight (density of 2.7g /?), Easy processing, and good flexibility, so they are widely used. However, aluminum is a relatively active metal, which is easily oxidized by oxygen in the air and many substances such as acids, alkalis, salts, etc., thus changing the printability; in addition, aluminum has high ductility (tensile strength of 14kg / m? 3 to 4%), poor mechanical resistance (Rockwell hardness of 15 to 19), the above can also be said to be the chemical and physical defects of the aluminum plate as the base.
2. The thickness of the aluminum plate base is between 0.2 and 0.4? This further determines the low tolerance of the aluminum plate to mechanical forces (bending, stretching, knocking, etc.). If the mechanical force on the aluminum plate exceeds the range of the aluminum plate, it will definitely affect the normal use of the plate material or even be discarded.
3. Micropore structure on the surface of PS plate

This microporous structure can effectively improve the hydrophilic performance of the blank part of the printing plate during the printing process, but it is relatively easy to be damaged under the action of friction. As shown in the figure, the micropore structure on the printing plate can be divided into four grades of ABCDE. The closer to the top (AB), the more vulnerable to wear. When the degree of wear reaches E, the micropore structure on the printing plate is basically After smoothing, it loses the ability to contain moisture, and it cannot effectively resist the ink, which means that the printing plate at this time is basically scrapped.
4. The hydrophilic salt layer on the surface of the printing plate The PS plate has a relatively dense hydrophilic salt layer (sodium aluminosilicate). This hydrophilic salt layer itself is porous, high in hardness (similar to chromium), and hydrophilic Strong characteristics, so it has strong water storage and wear resistance. The hydrophilic salt layer on the surface of the printing plate is equivalent to a protective layer on the surface of the printing plate. The protection and timely repair of the hydrophilic salt layer will directly affect the printing durability and quality of the printing plate.
5. Photosensitive resin layer on the surface of the printing plate This layer of photosensitive resin is the graphic basis of the surface of the printing plate. However, due to the influence of chemical components (diazoquinone compounds), alkali resistance is relatively poor. And because the surface film layer is only 1.5 ~ 2.0μm, slight scratches will cause damage to the layout graphics, which is difficult to repair. Therefore, the protection of the graphic part (photosensitive resin layer) is more important.
Second, we can summarize the factors that affect the printing durability of the printing plate into several aspects: the choice of raw materials; the plate making and printing process; the damage of the mechanical plate to the printing plate; the electrochemical corrosion and chemical corrosion of the printing plate. Below we discuss separately.
1. Selection of raw materials:
i. Choosing a good PS original is the primary prerequisite for the life of the printing plate during the printing process. A good PS must have the characteristics of uniform surface sand mesh, smooth surface of the photosensitive layer on the printing plate, and good dot reproduction.
ii. Paper: It requires high surface strength, light powder removal and fluffing, and less surface degumming. Ink: The pigment particles are required to be fine. The choice of paper and ink directly affects the abrasion of the printing plate during the printing process.
2. Plate-making and printing process i. Improper storage or operation, run out before exposure or development; overexposure; overdevelopment. The results of these three are the same, and all have an effect on the photosensitive resin glue on the surface of the PS plate (the direct result is that the photosensitive film is thinned and the ability to withstand friction is reduced). When printing PS plates, strictly follow the process regulations to meet the printing needs, follow the principle of "strong exposure and weak development" in the printing process, test the light source diligently, test the developer concentration and record diligently, make the exposure time accurate and develop The speed is right.
ii. Baking the developed PS plate is a very effective method to improve the printing plate durability. Let's make a more detailed introduction: after the printing plate is baked, the photosensitive group in the photosensitive layer disappears, and the phenolic resin undergoes thermal crosslinking reaction when heated, changing from linear molecules to large network molecules. At this time, the solvent resistance, corrosion resistance, abrasion resistance of the printing plate and the adsorption of the photosensitive layer to the plate surface are greatly improved. However, during baking, in addition to the change in the photosensitive layer of the graphic part, the surface of the anodized aluminum plate substrate is sealed with pores to seal the micropores, which may increase the water flow due to water loss, causing the plate surface to become dirty. Therefore, before baking the plate, it is necessary to apply a layer of protective agent to the plate surface uniformly with absorbent cotton to protect the blank part from being dirty. The general coating amount is 20ml-30ml for one pair of openings. The baking plate temperature is controlled at 230 ℃ -250 ℃. The time is 5-8 minutes. It must be noted that the special baking machine used for baking plates must have a constant temperature control device, the temperature difference range is between ± 2 ℃, too high baking temperature or too long time will cause the carbonization of the photosensitive layer, aluminum plate deformation and mechanical strength The reduction will affect the printing plate durability and the life of the plate base; the temperature is too low or the time is too short, because the photosensitive layer is not cured enough by heat, it will also affect the printing resistance. After baking, it must be cooled naturally, and it is not advisable to use water for forced cooling to avoid deformation of the base. After cooling, the protective agent is washed away with waste developer, and then neutralized with 3% phosphoric acid, washed with water and dried, and coated with gum arabic for protection.
! Note: During the baking process, the baking temperature and baking time must be strictly controlled. Exceeding the specified temperature and time will not only improve the printing durability of the printing plate, but will also reduce the service life of the printing plate.
3. Mechanical force:
i. Static (the printing plate is installed on the printing machine, when the machine is stopped, or the printing plate is under the machine, it is called static). At this stage, due to artificial bending of the printing plate; scratches caused by hard or sharp objects on the surface of the printing plate; scratches on the surface of the printing plate during storage, etc., it is easy to affect the service life of the printing plate, that is, durability Force, causing great impact.
ii. Dynamic (when the printing plate is printed on the press, it is called dynamic)
During the printing process, the printing plate continuously contacts the ink roller, water roller, and rubber cylinder. This contact is made under pressure. The greater the pressure, the greater the frictional force that can be generated; in addition to the ink roller, water Rollers, blankets themselves, as well as ink, paper wool, paper powder and other substances brought by them, these substances will also wear on the surface of the printing plate under the effect of pressure and speed difference; in addition, due to the rubber roller, blanket, The speed difference caused by the deformation of the water roller and the printing plate pad will also cause friction on the surface of the printing plate, which will then wear the printing plate. Every kind of friction starts to destroy the surface structure of the printing plate from the beginning of printing.
4. Electrochemical corrosion and photochemical reaction.
i. Static. The printing plate easily reacts with oxygen and other gases in the air during storage to reduce the hydrophilicity and abrasion resistance of the blank part; the graphic part of the printing plate decomposes when exposed to light, reducing the intensity of the graphic part.
ii. Dynamic. The printing plate reacts with the acid in the fountain solution during the printing process, which will cause corrosion of the graphic parts and reduce the printing durability.
After discussing the printing plate structure and external factors that affect the printing plate durability, it is easier to discuss the protection of the printing plate in actual production. Below we introduce some protection methods used in printing production.
Due to physical and chemical constraints, and also affected by process conditions and process operations, a series of protective measures should be taken to maintain the normal printing of the printing plate and extend its service life. In daily printing production, plate protection can generally be divided into: static protection and dynamic protection.
1. The static protection of the printing plate The general principle of the static protection of the printing plate is: to wipe the hydrophilic colloid (such as gum arabic) uniformly on the plate surface and keep the plate surface dry. But the printing plate should also pay attention to the following points in static protection:
A. Prevent oxidation. If the surface of the printing plate is not covered with a protective layer, it is directly covered in the air and can be oxidized. The resulting oxide has lipophilic and lipophilic properties, making the printing plate dirty, destroying the printing plate and reducing the printing resistance.
B. Prevent moisture. The printing plate is placed in humid air, especially when the relative humidity of the air is relatively large, which is the prerequisite for accelerating the oxidation of the plate surface (electrochemical corrosion occurs). At this time, the oxidation can cause the metal plate base to be corroded, destroying the blank part and changing the hydrophilicity. Oily, makes the board dirty.
C. Prevent the acid and alkali corrosion of the plate surface. The base of the PS version is aluminum, which is located before the hydrogen in the metal activity sequence. It is a relatively active metal element and can be reflected with acids and alkalis. The products can destroy the printing plate. The printing plate must be strictly prevented during printing. Acid and alkali (especially strong acid and alkali) contact.
D. Prevent friction and knocking on the layout. When the printing plate is placed on the desktop, the plate should be facing up, and the plate cannot be dragged down to prevent the plate from being scratched. If it is placed down, it should be separated with paper. If two printing plates are stacked, the two printing plates face each other and are separated by paper. The layout should strictly prohibit hitting with hard objects or placing heavy and hard objects (such as wrenches, machine parts, etc.).
E. Prevent horseshoe marks. The method of holding the plate should be proper, and it should not be bent arbitrarily, to avoid the local horseshoe mark when the plate is held, which is uneven, causing partial damage to the graphics.
F. Prevent the glue channel from the layout. The gum arabic rubbed on the plate should have a moderate concentration, to avoid too thick thick glue channel and corrosion of the printing plate and text. It does not absorb ink, nor can it be too thin to lose the effect of glue to protect the printing plate. Especially the glue that has deteriorated must not be used.
2. Dynamic protection of the printing plate.
During the operation of the printing plate on the machine, the measures to eliminate or reduce the interference caused by external factors are called dynamic protection.
In most cases, the damage of the printing plate is caused by the dynamic imprinting. There are two factors: First, the printing plate and the blanket cylinder, the ink roller and the water roller are formed in the rolling dynamic Another point is the rolling corrosion effect of the acidic fountain solution in the water roller in the presence of various friction factors.
In view of the damage to the printing plate from the above two points, it is very important how to correctly assemble the machine, adjust the working accuracy of each component, and maintain the accuracy of the machine. Normally, the dynamic protection of printing plates must do the following:
A. During the contact between the printing plate and the rubber cylinder, ink roller and water roller, the printing pressure should be as small as possible to reduce the friction on the plate surface of the printing plate.
B. In the process of machine adjustment, try to make the lining of each roller reasonable, and keep the linear speed between the rollers and the surface of the ink roller and water roller equal to reduce the speed difference.
C. Try to eliminate the axial movement of the water roller and ink roller and cause the abrasion of the plate surface.
D. Correctly control the concentration of the fountain solution in the water bucket. The PH value of the solution is controlled within a reasonable and appropriate range (this range requires that the acid value of the fountain solution must first be met so that the salt layer on the surface of the printing plate can be It can be repaired in time when damaged; in addition, the acid value should not be too large, so as not to cause vicious corrosion to the printing plate).
E. Control a small amount of water on the layout and maintain normal water supply.
F. Control the amount of dry oil in the ink to avoid excessive use of the dirty plate phenomenon and damage the printing plate.
H. Try to shake off the sand and other debris of the paper as much as possible. If there is a lot of sand and the powder is seriously depilated, the blanket should be washed frequently to prevent the accumulation from grinding the printing plate. Always pay attention to observe the accumulation on the printing plate or blanket, and clean up in time, especially when printing rough paper. Experience has shown that after printing 3-4 thousand sheets of general paper, the accumulation thickness caused by paper, ink, powder spraying, etc. can reach more than 0.1 mm. If it is not cleaned in time, the accumulation thickness will continue to increase, increasing the friction, and the dots will be lost due to excessive friction.
I. When the blanket is damaged, it should be accurate and thin when patching the blanket to save money.
J. Adjust the precision of the machine structure, and try to eliminate the influence of the roller bar and ink roller bar on the printing resistance of the printing plate.
K. During the printing process, try to avoid using strong plate cleaner or cylinder cleaner to clean the plate and cylinder. After cleaning, it must be rinsed with clean water immediately.
L. Handle or replace the aging rubber roller or blanket in time to avoid vicious friction on the printing plate.
Above, we discussed the influencing factors of the printing plate printing durability from several aspects, and some ways to improve the printing plate printing durability and some protection methods in daily production. I hope to be of some help to everyone.

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