How much do you know about package offset printing (187-197)

187. What are the factors that affect the drying speed of paper?
1 paper structure. The paper with rough surface, loose structure and small sizing degree has strong absorption of the binder and the imprinting is fast drying. Therefore, offset paper printing is easier to dry than coated paper, and it is less prone to back-scratching.
2 paper pH. The pH of the paper is too low, and the acidic substances in it react with the driers in the ink, slowing the drying of the ink.
3 paper moisture content. With high water content, the paper fibers are in a relaxed state, the capillary action is weakened, and at the same time some capillary tubes are blocked by water molecules, the absorption of the web by the web is weakened, the absorption capacity for oxygen is also weakened, and the drying speed is slow.

188. How does the ink layer thickness affect the dryness of the ink?
The thicker the ink layer, the more difficult it is to dry. The binder on the lower part of the ink film seeps into the paper, but the amount of material that can be allowed to penetrate in the paper has a certain limit. When the ink layer is too thick, the linking material in the upper part of the ink film can no longer penetrate into the paper, and the ink cannot rely on the infiltration gelation to fasten firmly and can easily cause the backside dirtiness.

189. How does the pH of the wetting fluid affect the dryness of the ink?
When the pH of the wetting fluid is too low, the wetting fluid reacts with the driers in the ink. Therefore, the stronger the acidity of the wetting fluid, the slower the drying of the ink.

190. How does the ambient temperature and humidity affect the dryness of the ink?
As the temperature of the environment increases, the chemical reaction speeds up, so the oxidative polymerization drying is accelerated. As the humidity of the environment increases, the activity of oxygen molecules in the air decreases, and the amount of dry vegetable oil absorbed by oxygen decreases, delaying the drying rate of oxidative polymerization. At the same time, as the humidity of the environment increases, the moisture content of the paper also increases, which is also not conducive to the drying of the ink.

191. How does offset printing process affect ink drying?
1 ink emulsification degree. The more serious the emulsification is, the more water is contained in the ink film. Since the presence of water molecules in the ink can block and inhibit the oxidative polymerization of the ink into a film, the drying is slower.
2 printing pressure size. Appropriate increase in printing pressure, can promote the ink pressure penetration, is conducive to ink drying.
3 sheet stacking method. After printing the finished product into a stack, the molecular oxygen needed for the drying of the printed sheet can only be obtained from the air infiltrating the edge of the printed sheet. If the area of ​​the printed sheet is large, the surface of the sheet is smooth, and the sheet stacking is relatively high, the air is difficult to penetrate into the lower and middle portions of the sheet bundle. Due to insufficient oxygen supply, the printed sheet is difficult to dry.

192. What is the back dirty?
In offset printing, the ink on the previous sheet is not dry, and the next sheet is superimposed on it. The ink on the previous sheet may be partially transferred to the back of the next sheet, which is called back-scratching. The backside smudging not only makes ink on the back of the printed product, but also causes the front image to be patchy, which seriously affects the quality of the printed product.

193. How to prevent back scumming?
1 speed up the ink drying speed. For the factors mentioned above that affect the ink drying, take appropriate measures to speed up the ink drying, accelerate the cohesion of the ink film surface molecules.
2 spray powder. The special powder is sprayed on the print sheet. Using the support function of the powder, the distance between the ink film surface of the previous print sheet and the back surface of the next print sheet is increased, and the ink is not easily transferred to the back of the next print sheet. Dusting also raises the distance between the sheets, which is conducive to the infiltration of air.
3 Reduce the stack height of printed products. The higher the stacking number, the lower the distance between the surface of the ink film on the printed sheet and the back of the next sheet, in addition to the above-mentioned drying delay, the greater their secondary intermolecular binding force, and the more likely the backside is dirty. Therefore, when printing a thicker ink layer with coated paper, it is not appropriate to stack the printed sheets too high. Conditional printing plants can use dedicated drying racks separated by layers to reduce the height of printed sheets.

194. What are the rollers that make up the stamping section of the offset press?
It consists of three cylinders, a plate cylinder (including cylinder, pad and plate), blanket cylinder (including cylinder, pad and blanket), and impression cylinder (including cylinder and substrate).

195. What is imprinting surface?
After the offset press is pressed together, the blanket lining of the blanket cylinder is pressed by the impression cylinder and the impression cylinder, and compression deformation occurs. The contact surface of the impression cylinder and the impression cylinder are respectively called an embossing surface. Since the elastic modulus of the plate cylinder and the impression cylinder is much larger than that of the blanket cylinder, the main cause of deformation is the blanket cylinder, so the embossing surface is an arc surface.

196. What is printing pressure? What are the methods for expressing the printing pressure?
Printing pressure refers to the pressure on the embossing surface after the offset press is pressed, that is, the force on the embossing surface along the normal direction of the embossing surface. The methods for expressing the printing pressure include point pressure, line pressure, average pressure, total pressure and maximum compression, and the width of the embossing surface.

197. Where does the offset printing pressure come from?
The offset printing pressure is derived from the elastic force generated by the compression of the blanket cylinder liner (including liners and blankets).

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