Causes of poor ink transfer during offset printing

Why is the ink roller powdered, which results in poor ink transfer?

In the offset printing process, water and ink are two very important substances in the ink transfer process. Water is fountain solution, and ink is offset printing ink. Only when the water and ink can be transferred well, the printed product can be beautiful. However, due to the influence of many uncertain factors on ink transfer, ink control becomes the most difficult key problem in offset printing. In the production process of offset printing, there will be a variety of water and ink transfer failures. One of the failures is that the ink roller sometimes has powder coating, which results in poor ink transfer.

The so-called "powder on the ink roller" means that during the offset printing process, after the printer has been running for a period of time, powdery particulate matter appears on the ink roller, and ink deposits on unwanted areas (such as the ink roller) or ink roller deinking The phenomenon of poor ink transfer. This printing failure occurs mainly because the powder particles on the ink roller are oleophilic or hydrophilic. So what are these powdery particulate materials? Where did it come from?

In the offset printing process, the materials that continuously enter the printing machine from the outside are mainly substrates, inks and fountain solution. So these powdery particulate materials must be brought directly or indirectly by these three substances.

In general, if the quality of the surface of the substrate is not good, it is possible to directly bring the powdery particulate material into the printing ink system. For example, when the paper surface itself contains a lot of paper dust or other powders, these dirts will be transferred to the ink transfer system through the printing plate during the imprinting process, resulting in the occurrence of the "ink roller powder" failure, and its solution It is produced by replacing the substrate with a clean surface; in addition, the surface of the paper itself is clean, but due to insufficient coating tightness on the surface of the paper, powder and fluff occur during the imprinting process and are transferred to ink through the printing plate In the transfer system, the "ink roller powder" problem will also occur, which can be solved by reducing the speed of the vehicle, reducing the viscosity of the ink, or replacing a better quality paper.

In addition to the substrate directly bringing the powdery particulate matter into the printing system, the chemical reaction between the fountain solution and the ink may cause calcium precipitation, thereby indirectly bringing the powdery particulate matter into the printing system.

The main component of fountain solution is water, and the hardness of water quality varies from place to place. Generally, the total hardness of soft water is 0 ~ 30ppm, and the total hardness of hard water is above 60ppm. When the water quality is relatively hard (that is, when there is a large amount of Ca +), it usually causes Ca + to contact with the resin in the ink to form calcium saponification. When it comes into contact with the acid in the acid fountain solution, it will produce calcium lemon or calcium phosphate. Calcium salts. Calcium saponification is oleophilic, which will cause the ink to deposit in unwanted areas, causing the surface of the rubber roller to crystallize, resulting in poor water transfer and ink transfer; while the calcium salt is hydrophilic, which will hinder the ink transfer and cause the ink roller to detach ink. Therefore, the solution to the problem is to use soft water with a small Ca + content as much as possible to formulate or dilute the fountain solution. At the same time, those fountain solutions that can effectively reduce calcium deposition should be selected to eliminate the effects of excessive calcium ions.

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