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Analysis on the preparation and use of ordinary wetting solution

ordinary wetting solution is prepared by adding some chemical components into water to prepare a high concentration stock solution, which is diluted with water or made into a solid powder when used, and dissolved in water when used. In lithography, the water film on the blank part of the printing plate should always maintain a certain thickness, that is, it should not be too thin or too thick, and it should be very uniform. If the water content is too large, the printed matter will turn white, and the so-called "water marks" will appear on the field, which will make the imprint weak if the sample is single, and the ink color is uneven; The water content is too small. First, there is ink pollution in the blank part of the printed product, causing the dirty version. With the extension of time, the ink gradually blocks the non inking part, resulting in "paste version". Therefore, the supply of wetting liquid must be moderate. It is generally required that when the wear of mixing blades exceeds the standard, the minimum amount of water shall be used to achieve the balance between water and ink without causing the dirty layout

I. preparation requirements of common wetting solution

1. It can fully wet the blank part of the printing plate. In order to have better wettability, it is necessary to reduce the surface tension of wetting solution. The substance that can significantly reduce the surface tension of solvent is surfactant. Therefore, emulsifier, wetting agent and surfactant must be added

2. Have the ability to clean the oil stain on the blank part of the layout. To remove the dirty spots on the printing plate, you need to add phosphoric acid, nitric acid, citric acid and other acidic substances. Only acidic substances have better cleaning ability

3. It has a very good ability of not feeling fat. You need to add gum arabic. It can supplement the hydrophilic colloid layer lost in the printing plate layout and improve the hydrophilic and oil resistance of the blank part, especially when printing with hot and oily inks in summer, but it should not be used too much, otherwise it is easy to produce the phenomenon of ink roller deinking

4. Hygroscopic agent can be added to adapt to different climate changes

5. The bucket wetting solution can stabilize the pH value and add buffer solvent

6. Acid substances shall be added to the inorganic salt layer that can supplement the loss

II. Use of common wetting solution

it is generally believed that PS version has low corrosion resistance to acid and alkali, and the pH of wetting skull is rigid. The pH value of the wetting solution is related to the dosage of the original wetting solution. If the dosage of the original wetting solution is large, the pH value of the wetting solution is low and the acidity is enhanced. During the printing process, the pH value of the wetting solution shall be controlled in combination with the actual printing conditions, and the dosage of the original wetting solution shall be increased or decreased

b. the printing plate is easy to float and hang dirty.

when the acidity is too weak, the inorganic salt layer of the printing plate can not be continuously supplemented during printing, which makes the hydrophilicity of the blank part weakened and gradually lipophilic, resulting in dirty hanging. Therefore, increasing the dosage of the original solution will slightly reduce the pH value of the wetting solution, which is beneficial to prevent dirt. When the pH value is ≤ 6, the surface tension coefficient of the bucket solution is basically unchanged. When the pH value continues to increase, the surface tension coefficient will decrease significantly, so that the bucket solution is dispersed into the ink to form a water in oil state, causing the floating and dirty phenomenon of the layout

2. Hazards of low pH value (strong acidity)

a. strong corrosion to the printing plate, and the printing plate resistance decreases

when the acidity is too strong, the inorganic salt layer of the layout will begin to dissolve and the graphic part will be corroded and damaged. In serious cases, the sand mesh of the layout will also be damaged. Due to the strong acidity of the water bucket, the sand mesh will be damaged, and the water storage capacity will also be reduced. The blank part is prone to grease and dirt. Due to the strong acid corrosion, the grease sensitivity of the graphic foundation will be weakened, and the dots will become smaller or incomplete, resulting in the stencil

b. floating dirt occurs in ink emulsification

the bucket solution with too strong acidity will often cause the emulsification of the ink. This is because there are many anions such as nitrate ions in the bucket solution, which are excellent surface active substances, which will reduce the surface tension of the bucket solution, and the ink will lose its due adhesion. The separation of the ink pigment and the binder will cause the pigment to be dispersed in the layout, and the blank part will cause floating dirt

c, ink roller deinking

the metal ink roller is easy to react with too strong acidic liquid medicine, resulting in easy deinking of the metal ink roller

d. block the drying of printed matter

the acid is strong, and the acid reacts with salts. When the acid reaches enough to react with the metal salts in the drier, the drying of the ink will be damaged and the drying will be inhibited. Strong acidity will cause the printing ink to dry slowly, which will continuously affect the normal production. At the same time, the back of the product is easy to be sticky and dirty, which will hinder the product quality. When the pH value is less than 3.8, it will inhibit the ink drying

e, dark and matte ink

ordinary wetting liquid has strong acidity, which will destroy the ink structure and emulsify the ink. At the same time, when the acidity is too strong, the color of the ink will be damaged, making the imprinted ink dark and matte


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