Discussion on improving the printing durability of printing plates
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Under the premise of ensuring the quality of printed matter, the highest printing quantity that the printing plate can withstand is called the printing durability of the printing plate. The printing plate printing rate has a direct impact on the printing cost, so it is very practical for the printing company to study how to improve the printing durability of the printing plate.
Before discussing how to improve the printing durability of the printing plate, we must first understand the following question: Which structure on the printing plate determines the printing durability of the printing plate? What factors are affected by the printing durability of the printing plate? As the saying goes, the morning glory is going to hold the nose. Only by grasping the main factors affecting the printing durability of the printing plate, can we improve the process more effectively by increasing the printing durability of the printing plate. Below we discuss separately:
1. The structure of the printing plate and the nature of the printing plate that determine the printing durability of the printing plate:
1. The PS version is based on aluminum
The aluminum plate is widely used because of its light weight (density of 2.7 g/cm), ease of processing, and good flexibility. However, aluminum is a relatively active metal, which is easily oxidized by oxygen in the air and many substances such as acid, alkali, salt, etc., thereby changing the printability. In addition, aluminum has high ductility (tensile strength is 14kg/mm2, and the elongation is 3 to 4%), the mechanical resistance is poor (Rockwell hardness is 15 to 19), and the above can also be said to be a chemical and physical defect of the aluminum plate as a plate base.
2. The thickness of the aluminum plate base is between 0.2 and 0.4 mm.
This further determines that the aluminum plate has low tolerance to mechanical forces (bending, stretching, knocking, etc.). If the mechanical force of the aluminum plate exceeds the tolerance of the aluminum plate, it will affect the normal use or even the disposal of the plate.
3. Microporous structure on the surface of PS plate
The microporous structure can effectively improve the hydrophilic property of the blank portion of the printing plate during the printing process, but is more susceptible to damage under the action of friction. As shown in the figure, the microporous structure on the printing plate can be divided into four grades of ABCDE. The closer to the top (AB), the more susceptible it is to wear. When the degree of wear is to E, the microporous structure on the printing plate is basically Smoothed, lost the ability to contain water, it can not effectively resist the ink, which means that the plate at this time is basically scrapped.
4. The hydrophilic salt layer on the surface of the plate
The surface of the PS plate has a relatively dense hydrophilic salt layer (sodium aluminosilicate). This layer of hydrophilic salt is porous, high in hardness (similar to chromium), and strong in hydrophilicity, so it has a strong storage. Water and abrasion resistance. The hydrophilic salt layer on the surface of the printing plate corresponds 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 basis of the graphic surface of the printing plate, which is oleophilic and water-repellent, and the abrasion resistance is better than that of the blank portion. However, due to the influence of chemical components (diazonium compounds), alkali resistance is relatively poor. Moreover, since the surface film layer is only 1.5 to 2.0 μm, slight scratches may cause damage to the layout image and it is difficult to repair. Therefore, the protection of the graphic portion (photosensitive resin layer) is more important.
Second, we can summarize the factors affecting the printing durability of the printing plate into several aspects: the choice of raw materials; plate making and printing process; the damage of mechanical damage to the printing plate; the galvanic 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 longevity of the printing plate during the printing process. A good PS must have a uniform surface sand, a smooth surface on the surface of the printing plate, and a good reproduction of the dots.
Ii. Paper: It is required to have high surface strength, light powder removal and less surface degumming. Ink: The pigment particles are required to be fine. The choice of paper and ink directly affects the wear of the plates during the printing process.
2. Plate making and printing process
i. Improper storage or handling, running before exposure or development; overexposure; overexposure. The result of the three is the same, all of which affect the photosensitive resin 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 making the PS version, it is necessary to strictly implement the process rules to meet the printing needs, follow the principle of “strong exposure, weak development” in the printing process, diligently test the light source, test the developer concentration and record frequently, and achieve accurate exposure time and development. The speed is right.
Ii. Bake processing of the developed PS plate is a very effective means of increasing the printing durability of the plate. In the following, we will introduce in more detail: after the printing plate is baked, the photosensitive groups in the photosensitive layer disappear, and the phenolic resin undergoes thermal crosslinking reaction by heat, and the linear molecules become large mesh molecules. At this time, the solvent resistance, corrosion resistance, abrasion resistance of the printing plate and the adsorption property of the photosensitive layer to the plate surface are greatly improved. However, at the time of baking, in addition to the change of the photosensitive layer in the image portion, the surface of the anodized aluminum plate base is sealed by the pore sealing, and the water flow may be enhanced due to water loss, causing the plate to be dirty. Therefore, before baking, you must use a cotton wool to evenly coat the surface with a protective agent to protect the blanks from dirt. Generally, the coating amount is 20 ml to 30 ml for one open coating. The baking temperature is controlled at 230 ° C - 250 ° C. The time is 5-8 minutes. It must be noted that the special baking machine used for the baking plate should have a constant temperature control device with a temperature difference between ±2 °C. If the baking temperature is too high or the time is too long, the photosensitive layer will be carbonized and deteriorated, and the deformation and mechanical strength of the aluminum plate will be affected. Decrease, affect the printing durability of the printing plate and the life of the plate; if the temperature is too low or the time is too short, the photosensitive layer will not be cured by heat, which will also affect the printing durability. After baking, it must be naturally cooled. It is not advisable to use forced cooling with water to avoid deformation of the plate. After cooling, the protective agent was washed away with a waste developer, neutralized with 3% phosphoric acid, washed with water, and coated with gum arabic to protect it.
Note: During the baking process, the baking temperature and baking time should be strictly controlled. Exceeding the specified temperature and time will not only improve the printing durability of the printing plate, but will reduce the service life of the printing plate.
3. Mechanical force:
i. Static (the plate is mounted on the press, when the machine is stopped, or the plate is under the machine, called static). At this stage, due to the artificial bending of the printing plate; the scratch caused by the hard object or sharp object on the surface of the printing plate; the scratching of the surface of the printing plate during the preservation process, etc., the life of the printing plate is easy to be printed, that is, the printing durability Force has a great impact.
Ii. Dynamic (when the plate is printed on the press, it is called dynamic)
In the process of printing, the printing plate continuously contacts the ink roller, the water roller and the rubber roller. The contact is carried out under the pressure. The greater the pressure, the greater the friction force can be generated. At the same time, except for the ink roller and water. Rolls, blankets themselves, as well as inks, paper wool, paper powder, etc. brought by them, these materials will also wear on the surface of the printing plate under the pressure and speed difference; in addition, due to rubber rollers, blankets, The speed difference caused by the deformation of the water roller and the plate liner also causes friction on the surface of the printing plate, which in turn wears the printing plate. Each type of friction begins to dampen the surface structure of the printing plate from the beginning of printing.
4. Electrochemical corrosion and photochemical reactions.
i. Static. During the preservation process, the printing plate easily reacts with gas such as oxygen in the air to reduce the hydrophilicity and wear resistance of the blank portion; the image portion of the printing plate is decomposed by light, and the intensity of the graphic portion is reduced.
Ii. Dynamic. The printing plate reacts with the acid in the fountain solution during the printing process, which will cause corrosion of the graphic part and reduce the printing durability.
After discussing the plate structure and external factors that affect the printing durability of the printing plate, it is easier to discuss the protection of the printing plate in actual production. Below we introduce some of the protection methods used in printing production.
Due to physical and chemical constraints, as well as process conditions and process operations, a series of protective measures should be taken to maintain the normal printing of the plate and extend its useful life. In daily printing production, plate protection can generally be divided into; static protection and dynamic protection.
1. Static protection of the plate
The general principle of plate static protection is to evenly rub the hydrophilic colloid (such as gum arabic) and keep the layout dry. However, 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 can be directly covered in the air and can be oxidized. The resulting oxide has lipophilic and lipophilic properties, which makes the printing plate dirty, destroys the printing plate and reduces the printing durability.
B. Prevent moisture. The plate is placed in humid air, especially when the relative humidity of the air is relatively large, which is the premise of accelerating the oxidation of the plate (electrochemical corrosion occurs). At this time, the oxidation can cause the metal plate to be corroded, destroying the blank portion and changing the hydrophilicity. Oily, making the layout dirty.
C. Prevent corrosion of the acid and alkali of the plate. The PS version is made of aluminum. It is located in front of hydrogen in the metal activity sequence. It is a relatively active metal element. It can be reflected with acid and alkali. The product can destroy the printing plate. The printing must be strictly protected against printing. Acid and alkali (especially strong acid and strong base) are in contact.
D. Prevent plate friction and knocking. When the plate is placed on the table, the plate should face up, and the plate should not be dragged back and forth to prevent the plate from being scratched. If it is placed face down, it should be separated by paper. If two plates are stacked, the two plates are opposite and placed with paper. The layout should strictly prohibit the use of hard objects to strike or place objects that are too heavy and too stiff (eg, wrenches, machine parts, etc.).
E. Prevent horseshoe prints. The method of taking the plate should be proper, and it should not be bent at will, avoiding the local horseshoe print caused by the plate, which is uneven and uneven, resulting in partial damage of the graphic.
F. Prevent the layout from getting glue. The gum arabic rubbed on the plate should be moderate in concentration, avoiding excessive thickening of the rubber road and corroding the printing plate, not absorbing ink, nor too thin to lose the effect of the glue to protect the printing plate. In particular, glue that has deteriorated must not be used.
2. Dynamic protection of the plate.
The protection of the printing plate caused by the interference of various external factors during the operation of the printing plate on the machine is called dynamic protection.
The damage of the printing plate is caused by the dynamic operation of the printing in most cases. There are two factors: one is the plate and the blanket cylinder, the ink roller and the water roller, etc., which are formed in the rolling dynamics. The speed difference friction; another point is that in the presence of various friction factors, the water roller contains the acidic corrosion effect of the rolling solution.
In view of the above two points of damage to the printing plate, how to properly assemble the machine, adjust the working accuracy of each part, and maintain the accuracy of the machine is very important. In general, plate dynamic protection must do the following:
A. During the contact of the plate with the blanket cylinder, the ink roller and the water roller, the printing pressure should be as small as possible to reduce the friction on the plate of the printing plate.
B. In the process of machine adjustment, make the lining of the rollers as reasonable as possible, and keep the line speed between the rollers and the surface of the ink roller and the water roller equal and reduce the speed difference.
C. Try to eliminate the axial sway of the water roller and the ink roller and cause wear on the layout.
D. Correctly grasp the concentration of the dampening 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 dampening solution must first be met in order to be salted on the surface of the printing plate. It can be repaired in time when it is damaged; in addition, the acid value should not be too large to avoid the vicious corrosion of the printing plate).
E. Control a small amount of water on the plate and maintain a normal water supply.
F. Control the amount of dry oil in the ink to avoid excessive amount of the dirty plate and damage the printing plate.
H. Try to shake off the sand and other debris of the paper. If there are many sands, remove the dust and seriously accumulate the blanket, wash the blanket frequently to prevent the deposit from grinding the plate. Always pay attention to the observation of deposits on the printing plate or blanket, and clean them in time, especially when printing rough paper. Experience has shown that after printing 3-4 thousand sheets of paper on a typical paper, the thickness of the stack caused by paper, ink, dusting, etc. can reach 0.1 mm or more. If it is not cleaned in time, the thickness of the pile will continue to increase, causing the friction to increase, and the dots will be lost due to excessive friction.
I. When the blanket is crushed and the blanket is used to save money, it should be thin and thin.
J. Adjust the precision of the machine structure, and try to eliminate the influence of the roller bar and the ink roller on the printing durability of the printing plate.
K. Cleaning during the printing process Avoid using a strong plate cleaner or roller cleaner to clean the plates and rollers. Rinse immediately with clean water after washing.
L. Dispose of or replace the aging rubber roller or blanket in time to avoid the vicious friction on the plate.
In the above, we discussed the factors affecting the printing durability of the printing plate and the ways to improve the printing durability of the printing plate and some protection methods in daily production. I hope I can help you a bit.