1. Three characteristics of offset printing
1. Offset printing uses the middle cylinder of elastomer blanket to form a unique rolling method for indirect printing.
2. Utilizing the selective adsorption law of solid surface with different properties, through technical treatment, the solid surface can meet the needs of flat printing.
3. Taking advantage of the natural law of miscibility of oil and water, both the ink and water are transferred in the offset printing process.
One of the characteristics of offset printing is the use of an elastomer cylinder for indirect printing. As an elastomer, ordinary rubber blankets and air cushion blankets play a decisive role in the transfer of printed graphics. First of all, we have to figure out the transfer process of offset images: the original image-the text, lines, images of sulfuric acid paper before printing or the output film image is the reverse image-the printed PS version is the positive image- —Offset printing transfers from the plate cylinder to the rubber cylinder and shows a reverse image—the blanket transfers to the paper as a positive image.
The blanket plays a role in transferring printed graphics during printing, and the quality of the blanket has a direct relationship with the quality of the transferred graphics.
2. What are the requirements of offset printing on the quality of blankets
In the offset printing process, the role of the blanket is to transfer the graphics and text on the printing plate to the substrate paper completely and clearly. In order to strive for the perfect picture and text transfer, the blanket must have the following characteristics:
1. The thickness of the blanket should be even. The uniform thickness will make the printing pressure evenly distributed, and the color density of the printed ink will be more symmetrical, and the size of the image and text will not change easily during the process of image and text transfer.
2. Has good elasticity. Ordinary solid rubber cloth has good elasticity. The effect of image transfer and ink absorption performance may also be good. During the printing process of ordinary solid rubber blankets, the surface rubber layer often produces a convex hull at the front edge of the impression line. According to the speculation of the impression principle, hard rubber The volume of the cloth will not shrink under pressure, so it can only be squeezed to both sides of the impression line. However, choosing to use an air cushion blanket will not cause the convex hull problem at the leading edge of the impression line.
3. The blanket has a small elongation. The blanket has a dense structure and high mechanical strength, which is conducive to the increase of tensile strength, and thus reduces the amount of deformation of graphic transfer.
4. Good ink absorption performance and transfer performance. It can ensure that the ink color is uniform and the writing is clear during the printing process.
5. The rubber cloth has good oil resistance, abrasion resistance, heat resistance and aging resistance, and has the above various properties, which can extend the service life.
6. With acid, alkali and corrosion resistance. The quality of the blanket is directly related to the quality of graphic transfer. Although there are many factors that affect the quality of graphics, including machinery, plate making, materials, process control, etc., but the quality of the blanket is one of the important factors, because the blanket is penetrated by gasoline, kerosene, and engine oil during the printing process, Under pressure and relative friction between the roller and the roller, periodically stretched, squeezed and deformed, the blanket will be sticky, deformed and lack of good elasticity due to sulfur loss, thus affecting the transfer quality, in this case , You have to replace the old blanket.
3. Problems that should be paid attention to when installing the blanket
1. When installing the blanket, pay special attention to the fact that the liner must not wrinkle. In general, blanket + felt + paper; or blanket + worsted wool + paper. Felt, worsted wool, paper and other lining materials, if you don't pay attention to it a little bit, it will easily wrinkle if it is not flat.
2. When tightening the blanket, the trailing edge and the bite should be carried out at the same time, not one side. Pay attention to safety during operation, avoid inserting tools and screws into the roller, and damaging the blanket.
3. The new blanket has good elasticity and good stretchability. Tightening the blanket should be done gradually. Tighten it once, and then tighten it after the offset press is pressed for 3 ~ 5 minutes, so that the requirement of tightness can be reached by repeating it 2 ~ 3 times. .
4. Whether the blanket and its pads are uniform, if there is ink skin and paper dust in the blanket, bumps will appear, and the blanket should be reinstalled.
5. Before installing the blanket, the thickness of the blanket + felt or worsted wool + paper should be accurately measured with a micrometer, and the four clicks of the micrometer should be heard. Soft lining: blanket + felt + paper, lining thickness: 3.35 ~ 3.45mm. Neutral lining: blanket + worsted wool + paper or blanket + paper, the thickness of the lining is: 3.20 ~ 3.25mm. Rigid lining: blanket + insulating paper or photo paper, lining thickness: 3.18-3.20mm.
6. Blanket cutting requires a rectangular end. Only by selecting the warp direction of the blanket as the direction of cutting, tightening and withstanding tensile force, can the desired effect be obtained. By looking at the barrel shape of the natural roll, we can know its warp and weft directions. The direction of the thread parallel to the core line of the natural roll should be the weft direction, and the direction perpendicular to the weft thread is the warp direction. It is mainly judged according to the thread of the plain (back) plain weave of the blanket.
7. The liner paper in the blanket cover is shorter than the blanket, so as to prevent moisture from seeping into the paper fiber and swell and deform when it meets water, reducing the rust and corrosion of the roller.
8. The blanket is a kind of elastic object, and it must be stretched after being stressed during the whole process of clamping, transmission and embossing. In order to transfer the graphic size on the printing plate to the paper more accurately, it is required that the elongation of the blanket should be small during the entire embossing process and must be uniform. The above elongation is not only related to the quality of the blanket itself, but also depends on the degree of tension of the blanket on the roller. Is the tighter the blanket, the better? Of course, this is not the case in actual work. The blanket should be stretched properly. â‘ The tighter the blanket is stretched, the less the elongation during the printing process and the smaller the change in the graphic size. However, if the leather cloth is stretched too tightly, it will cause an increase in the friction of the plate surface and a decrease in the life of the plate. â‘¡ If the blanket is stretched too loose, the expansion and contraction rate of the blanket will increase, and the amount of friction on the plate will decrease, which may easily cause double printing (ghosting).
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