Dirty offset printing is one of the common faults in printing. During the production process, we must conduct a specific analysis according to the signs and parts of the layout with dirt, find the cause of the fault, and take effective measures to prevent and solve. In general, offset printing plates can be classified as dirty, dirty, paste, and stripes.


Dirty refers to a phenomenon in which a non-graphic part with a relatively large range and a relatively low density is dirty. Unlike a dirty part, the dirty part has no fat-sensitive basis, and is not caused by a printing plate, but is mainly caused by Ink emulsion caused. There are two types of ink emulsification, water-in-oil (W/O) type and oil-in-water (O/W) type. Among them, the water-in-oil type ink emulsification is the situation where the ink emulsification is too severe, the viscosity of the ink is reduced, and the ink droplets splash on the surface of the plate to form a buoyancy under the action of the centrifugal force; the oil-in-water type ink emulsification is the phenomenon of “water” of the ink, Make the blank part of the plate lighter ink marks.

In the actual operation, we can judge by the following signs. (1) Contaminated parts can be easily wiped off; (2) Since the hydrophilic salt layer is not destroyed, the dirty parts will often change; (3) The floating dirt or ink residue will fall into the fountain solution, and the flocculent ink particles (water-in-oil) will appear. Type of ink emulsification caused); 4 fountain solution with pigment precipitation or flocculation, printing full version of a color shadow (oil-in-water emulsion caused), that is, the ink "water" phenomenon. Causes: 1 The pH value of the fountain solution and the paper is not suitable. When embossing the printing ink, the ambassador ink is emulsified. At this time, the water-in-oil type ink is mostly emulsified. 2 The layout volume is too large and the quality of the ink binder is poor. , resulting in oil-based ink emulsification, ink is too soft; 3 pigment water resistance, poor acid resistance, poor affinity with the link material; 4 ink pigment dissolved in the fountain solution, the ink was flocculent dispersed in the fountain solution , Caused a large area of ​​dirty, may also be caused by inappropriate printing pressure, improper assembly of the water roller and the ink roller, and insufficient hardness of the plate roller.

Dirty oil

Dirty oil means that the printing plate should not feel fat on the part of the fat, so that the ink adheres to it and forms a dirty plate. The main reasons for causing blank parts of the printing plate: insufficient amount of water in the blank or weak acidity of the fountain solution; the hydrophilic salt layer in the blank part of the printing plate is unstable or even worn due to machine operation, paper, etc., resulting in decreased water retention. The fatty acids in the ink are dissolved in the fountain solution, leaving the blank part of the printing plate feeling fat.

In the actual operation, we can judge by the following signs. 1 The dirty parts are not easy to wipe off; 2 The dirty parts are relatively fixed; 3 The method of adding water, rubbing the syrup and resin can be used, but the dirty oil with serious sand abrasion can not be eradicated. The reasons are as follows: 1 The pressure of the printing plate and the blanket cylinder is too large or the liner is too thick, resulting in increased friction or sliding friction, which accelerates the wear of the printing plate. Through careful observation, we found that the mouth of the printing plate is most likely to lose its ability to hold water because of the sanding hole; 2 the blanket is aging and hard due to long use time, loses its elasticity, and reduces compression deformation. Small, causing insufficient printing pressure. If it is not replaced in time, adding only some paper to the liner can increase the pressure, but the friction will increase accordingly. Therefore, during the transfer process, the water layer on the surface of the printing plate will be used for a long time. Destroyed, causing a large area of ​​the printing plate dirty oil; 3 for the aging of the rubber blanket, hardening phenomenon, we usually first on the aging and damaged parts of the wipe, and then use polypropylene tape for bonding, because polypropylene tape cheap Wumart, this method is very popular; 4 installed version is also very important, if the plate is not tightened, easy to cause the plate and the roller is not real, non-conformable parts will rub against the rubber roller, contact with the water roller too Tight, not only the friction generated is too large, but also will squeeze out the water, so often due to lack of water supply caused by dirty layout. 5 Ink roller reasons: If the ink roller is too heavy, when the plate cylinder is turned to the mouth of the mouth, it will cause a jump and make the oil dirty at the mouth of the printing plate; Rely on heavy plates, wear plates from the dirty; 6 Ink grease and additives used improperly, resulting in oily too strong; 7 wipe the glue, the moisture content of the wet cloth is too large, resulting in a "whole" phenomenon after wiping. Dirty oil; 8 The pH value of the paper is too high, which greatly reduces the acidity of the fountain solution. The water roller is dirty and the water roller assembly is not suitable, resulting in insufficient water supply and dirty oil.

Paste version

The stencil printing refers to the lines and lines of the printing graphic part, between the dot and the dot, and the place where the highlights should not have ink. This is sometimes classified as dirty, and sometimes it belongs to the category of floating.

In the actual operation, we can judge by the following signs. 1 The level of imprinting is unclear, and parts with more dense lines (above 50% of dots) occur; 2 If the oil is dirty, the position with dirty spots is relatively fixed and the ink is dark, and early detection can be eliminated as soon as possible; 3 If it is dirty, With dirty position will change, ink is light, easy to remove. Reasons for the occurrence: 1) Excessive or excessive amount of dry oil will change the ink's adaptability, allowing the ink layer of the graphic part to spread when printing; 2 the ink layer is too thick, the water content is too small, or the ink is too thin and the liquidity is large; 3 Ink emulsification serious; 4 pressure is too large, the graphic part of the ink layer spreading or lining is not suitable; 5 roller partial deformation, water roller and the layout of the poor contact caused by water supply is insufficient or uneven; 6 ink roller surface aging, especially After the two ink rollers have poor adsorption performance, they can't take away floating ink and dirty ink from the layout. 7 When the machine is shut down, the plate dries and oxidizes and even fluffs, leaving a pile of ink on the ink roller and gluing.
There are many signs and reasons for offset faults on offset printing plates. In the actual operation, they should be carefully distinguished and classified, and the phenomena that occur should be analyzed and resolved in a timely manner.

Author: Wu Ying Cui

Reprinted from: Printing Technology

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