In offset printing , the plates and rollers are often shut down due to paper falling off the powder. There are two main effects on the printing of paper lint off powder: one is paste, and the other is to stack ink on a blanket or plate.
When the paper is browned, the paper powder or filler will be mixed with the ink and gradually accumulated on the blanket to form a stack of ink. When the stacking ink starts to form, the printed image becomes rough. This is because the paper powder and the ink are mixed, and a bulge is formed on the rubber roller, which causes difficulty in ink transfer, that is, the ink stack hinders the ink transfer, and some of the ink is not transferred to the paper. , making the image color dull and less expressive. If the area of ​​the ink is small, small white spots will appear on the printed image; if the area of ​​the ink is large, large white spots will appear on the printed image, and it is necessary to stop the cleaning.
The author uses a pair of two-color offset printing machines for printing production, and selects 70g/m2A grade double-adhesive paper, 70g/m2B grade double-adhesive paper and 70g/m2 light-weight paper as the substrate to find the products that affect the offset paper. factor. The relevant production conditions are: printing speed of 6,000 sheets/hour, printing workshop temperature of 23 ° C, humidity of 50% to 60%, and ink viscosity of 30 Pa·s. When printing the above three types of paper, the hair loss on the printing plate is as follows.
(1) A grade double-adhesive paper has little hair loss and no need to stop cleaning the blanket in the middle, suitable for high-quality offset printing.
(2) Class B double-adhesive paper has a slight hair loss, but within reasonable limits, occasionally it is necessary to stop cleaning the blanket.
(3) The phenomenon of falling off the powder on the light paper is more serious, resulting in some obvious white spots on the surface of the printed matter. It is necessary to frequently wipe the blanket when printing, so it is suitable for general offset printing.
The author speculates that the main reasons for the paper falling off the powder are the two factors of paper moisture content and paper tightness.
Paper moisture content
The moisture content of the three papers was measured by the wet weight method, and then the conditions of the three different water content papers were compared under constant printing conditions.
The quality of the three types of paper before and after drying in the blast drying oven is shown in Table 1, where: water content = (paper quality before drying - paper quality after drying) çº¸å¼ paper quality before drying × 100%.
Under normal circumstances, as long as the water content of the paper is within the normal range (about 8%), the higher the water content, the more favorable the swelling of the paper fibers and the interweaving between the paper fibers, thereby improving the surface strength of the paper and reducing the lint during printing. The probability of falling off the powder.
2. Paper tightness
The thickness of each of the three paper samples was measured, and the thickness of the single sheet of paper was calculated. Then, according to the formula: paper tightness = paper basis weight, paper thickness, the tightness of the three paper samples was calculated, as shown in Table 2 above.
The tightness relationship of the three paper samples was found to be: Grade A double-adhesive paper > Grade B double-adhesive paper > Lightweight paper. It can be seen that the smaller the tightness of the paper, the more serious the phenomenon of falling off the powder. Therefore, we preliminarily judge: to a certain extent, the greater the tightness of the paper, the higher the surface strength of the paper, and the more the falling hair is lost. less.
Under the normal printing conditions (printing speed, ink viscosity, workshop temperature and humidity), the effect of the paper itself on the hair loss of the paper was verified by experiments, and it is hoped that it can provide a certain reference for the majority of printing practitioners.
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