Printing in color printing products darkened part and use spot color printing large area field when, due to the ink layer thick, opposite rubbed against dirty is very easy to attack the problem, ink viscosity, pH value, ink layer thickness and the paper surface rough level and permeability problems will incur negative rubbed against dirty. 

In the process of offset printing ink, before the ink was dry, after a printed sheet of the stacked on it, before a printed sheet of ink have local after transfer to a printed sheet opposite, called negative rubbed against dirty. Negative rubbed against dirty will not only enable printing reverse stained with ink, but also lead to the front picture present piebald shape, seriously affecting product quality, even lead to a product to scrap the whole batch. Especially in the darkened part of the color printing products and use spot color printing large area field when, due to the ink layer thick, opposite rubbed against dirty is very easy to attack the problem and therefore must be careful to prevent. Therefore negative rubbed against dirty is to reduce ink layer thickness, increase the interval between the sheets, improve ink performance and speed up the ink on the paper drying speed following from the discussion in printing, how to prevent printing reverse rubbed against dirty. 

First, the performance of the ink 

Different types, different colors of ink, the convergence, the composition of pigments and the proportion of different, dry way is also different, and the dry speed difference is great. Also thixotropy of printing ink, in the printing process due to the shear effect between rollers, viscosity will fall, and completed the transformation of prepress, viscosity and rapid rise. Because the viscosity is the cohesion of the external performance, increase the viscosity marked increase in cohesion, is conducive to the prevention of printed matter against the dirty dirty. So printing without the background color removal of color printing and a large area of the field should be as far as possible the use of dry faster and thixotropy of printing ink. In order to adjust the printability, sometimes need to add to the adhesive. Viscosity decreases the cohesion of the lower, easy to form a negative rub dirty. It is easy to produce coated paper printing back rubbed against dirty to strictly control the amount of adhesive, control the ink viscosity. Appropriate to add a drying agent can significantly accelerate the ink dry, but the drying agent at the same time is also an emulsifier, will promote the ink emulsification, excess dry agent but will slow down the ink dry. 

Two, fountain solution of pH value and dosage 

In printing easy to attack rubbing against dirty on the opposite of large area field, ink layer thick to incur printing ink to a gap in the local expansion, at this time to reduce the fountain solution of pH value is conducive to inhibit ink expansion. But the pH value is too small, the pH value of the fountain solution needs to be based on the difference of the layout and the color of the page. If the printing ink layer thick spot color, and contains fine lines, or printing to the darkened screen level, due to the easy and paste, fountain solution of pH value demand according to the graphics and text layout and color difference and decide. If the printing ink layer thick spot color, and contains fine lines, or printing to the darkened screen level, due to the easy and paste, pH of dampening solution to low. Printing of red ink, fountain solution of the pH value to be lower; the other color printing ink, pH value should be a little higher. The more serious the ink emulsification, mixed with the ink layer in the more water, the more difficult the ink film. The use of alcohol fountain solution can reduce the amount of fountain solution, reduce the emulsion, is conducive to the ink dry. Using ordinary fountain solution, we must first pay attention to the proportion of the ingredients in the formula, and then to minimize the degree of water to adhere to. At the same time, to control the spread of water on the layout, on the prevention of the opposite is also important to rub dirty. 

Three, the nature of the paper 

The rough level and permeability of the paper surface is an important factor to influence the negative rub. A good penetration of paper, the convergence of materials can be quickly and more infiltrated into the paper, to accelerate the solidification of printing ink. The surface is relatively rough paper is conducive to air infiltration from the gap between the paper, while the surface roughness of the paper, the capillary air content is relatively large, it is conducive to the oxidation of ink. As a result, the negative rub is much more than the time when the ink layer is printed on the coating paper. Offset printing machine printing aluminum paper are particularly vulnerable to attack opposite rubbed against dirty habits, because the surface of the aluminum foil paper complete not soaked to, even if only printed compared a thin layer of ink, also need dusting to avoid negative rubbed against dirty. The acidity of the paper is different, it will also affect the ink drying rate, printed on acidic ink than printed on alkaline paper ink is very slow. If color was once a high number of outlets or field, color overprint, due to the appearance of the original paper has been color ink covered. After one colour and ink soak will be greatly reduced, easy to cause the negative Ceng dirty by wet dry overprint.