Offset printing is an indirect printing method. It utilizes the oleophilic and hydrophobic properties of an elastomer rubber cylinder to transfer the ink on the printing plate to the printing paper. In order to make the ink color of the printed matter uniform, the dots clear, and the gradation rich, the offset blanket must have the characteristics of moderate hardness, small compression deformation, good ink transfer performance, and small expansion and contraction. The quality of blankets mainly depends on the materials used and their structure.
There are two types of offset blankets: air cushion blankets and ordinary blankets. Since the volume of the ordinary rubber does not shrink during the imprinting process, it can only be squeezed in the direction of the force, so that the rubber layer surface rubber layer has a raised convex hull in front of the imprinting line. In the case of an offset printing press working at a high speed, this convex hull cannot be flattened instantly to restore the surface of the blanket, which often causes quality problems such as deformation of the printed dots, blurred graphics, and poor reproducibility of double-printed or high-profile parts.
1 The quality requirements of blankets for offset printing
In the offset printing process, the role of the blanket is to transfer the graphics on the printing plate to the substrate completely and clearly. In order to strive for the ideal picture transfer, the blanket must have the following characteristics:
1) The thickness should be even. The uniform thickness of the blanket can ensure the even distribution of printing pressure, the density of the ink color is relatively symmetrical, and the size of the graphic is not easy to change during the transfer of the graphic.
2) Has good flexibility. The better the elasticity of the blanket, the better the transfer effect and ink absorption performance of the printed image and text, and the better the print quality.
3) Has a smaller elongation. The blanket has a dense structure and good mechanical strength, which is beneficial to increase the tensile strength and reduce the amount of deformation caused by the transfer of graphics and text.
4) Good ink absorption and transfer performance, to ensure that the ink color is uniform and the writing is clear during the printing process.
5) Excellent acid resistance, alkali resistance, oil resistance, wear resistance, heat resistance and aging resistance, which can extend the service life.
2 The difference between ordinary blanket and air cushion blanket
1) Different fabrics and rubber layers. The main difference between an air-cushion blanket and an ordinary blanket is that it not only has different fabric layers and rubber layers, but also has an inflatable layer in the middle to form an elastic layer. The gas-filled layer is mainly divided into a micro-bubble shape and a gas groove shape. There is a solid carrier layer between the surface layer and the microporous layer to strengthen the bonding between the upper and lower layers.
The air cushion blanket uses the principle of air compression. During blanket printing, the blanket is not squeezed after being squeezed. It has compressibility and the surface is not easily damaged.
2) The air cushion blanket has impact resistance, especially the offset printing machine uses a neutral bag lining, and the air cushion blanket will be better. However, do not use air cushions for offset proofers, because the better the elasticity of the air cushion blanket, the better the effect of image transfer and ink absorption performance.
3) The air-cushion blanket has the ability to quickly peel off the paper. The thinner coated paper (65-80g) is more effective and will not peel off the paper.
4) The stretch rate of the air cushion blanket is small and the flatness is high. Because the expansion rate of the air cushion blanket is small, the structure is dense, and the mechanical strength is good, it is beneficial to increase the tensile strength and thereby reduce the amount of deformation of the graphic transfer.
5) Solid ordinary blankets can generally only be used for 3 to 4 months when printed books and magazines work 8 hours a day, and the air cushion blankets made in Tianjin can be used for more than half a year. As long as you do not encounter tattered paper or multiple sheets of rubber blankets, there are fewer traces of air cushion blanket printing. The air cushion rubber cloth has good oil resistance, wear resistance, heat resistance, aging resistance and long service life.
6) The air cushion blanket has good ink absorption performance, ensuring uniform ink color, clear writing, smooth printing dots, and small dot expansion rate during the printing process.
