The blanket is one of the main consumables of the offset printing process, and it is also the medium for offset printing transfer. Its performance has a great influence on the printing quality of the product. Therefore, correctly understanding and mastering the structure and performance of the blanket, scientifically and rationally using and maintaining the blanket can not only improve the performance and life of the blanket, but also effectively improve the printing efficiency and product quality.
First, the difference between ordinary blanket and air cushion blanket structure
The blanket is made of 3 to 4 layers of plain cloth bonded with a rubber adhesive. Its surface is coated with a thick rubber layer. This rubber layer is further vulcanized to meet the printing requirements. The elasticity and hardness are then calendered to obtain a uniform and accurate thickness. Since the elongation and strength of the blanket mainly depend on the nature of the base fabric, the fibers of the cloth are pre-stretched, and the thick line is removed to make it uniform in texture, and then the synthetic rubber roller is used on the coater. The surface of the cloth is coated with a thin layer of viscous rubber solution - a layer of glue. It has good elasticity and small compression deformation. Therefore, natural rubber is generally used, and it contains a vulcanizing agent and an antioxidant. The excess solvent is scraped off with a rubber knife. The coated fabric is then fed into a steam heated roll such that a layer of dried vulcanized rubber adheres to the surface of the cloth. Thereafter, a layer of the bonding compound was uniformly applied over the entire surface, and a three- or four-layered blanket substrate with an additional back side of the linen was laminated. Since the rubber layer on the surface of the blanket is constantly in contact with the printing plate, the ink, the fountain solution, the gasoline, etc., and is continuously compressed, the surface rubber layer has good ink transfer and acid resistance. In addition to solvent resistance and other properties, it should also have certain mechanical properties. The surface rubber layer is generally made of a rubber having good oil resistance, such as nitrile rubber or neoprene.
Offset printing also has a blanket called air cushion blanket, which is mainly different from the above-mentioned ordinary blanket in that, in addition to various fabric layers and rubber layers, there is also a gas-filled or micro-bubble-like aeration layer in between. Between the aerated layer and the surface layer is a strong carrier layer to strengthen the bonding of the upper and lower layers. During the embossing process, the air cushion blanket is reduced in volume due to the pressure of the air cushion layer, and the volume reduction is achieved by compression of the air groove or microbubbles. When the ordinary blanket is squeezed, the volume cannot be reduced. Therefore, a convex hull is formed before and after the embossing line. The convex hull cannot be quickly taken down in an instant, and thus the deformation of the embossing, the dot increase, and the graphic are easily caused. Blurring, ghosting, high-profile partial reproducibility and increased wear and tear on the surface of the printing plate. The air-cushion blanket does not form a convex hull, and thus does not cause this series of ills. Therefore, it is best to use air-cushion blankets to print the fine products to improve the printing quality of the products.
Second, the technical requirements and characteristics of the blanket
1. Thickness. The thickness of the three-layer blanket is 1.6 to 1.7 cm; the thickness of the four-layer blanket is 1.8 to 1.9 cm. The thickness of the blanket should be uniform and the thickness difference should be less than 0.04cm. The thickness of the blanket should be reasonably selected based on the characteristics of the equipment or printing. When the blanket is used, it can be properly padded on the back side according to the pressure condition to meet the printing quality requirements.
2. Hardness. Hardness refers to the ability of a blanket to resist external force. Generally, the hardness of the blanket is between 60 and 70 (Shore). The high-hardness blanket is clear and easy to obtain a uniform ink color, but it is easy to wear the plate. The rubber with low hardness can compress a large amount, the receiving amount of the ink is correspondingly large, and the printing is easy to expand and deform, so it is suitable for printing materials with poor gloss and general products.
3. Elongation. The smaller the elongation of the blanket, the smaller the coefficient of deformation (such as elongation and thinning), and the elongation depends mainly on the number of layers of the base fabric, material properties and strength. A blanket with a small elongation has the advantages of accurate overprinting and clear graphic imprint. Conversely, the print quality effect is poor.
4. Elastic and compressive deformation values. When the blanket is pressed, it will sink down. Under the action of elasticity after pressing, its sinking surface should be quickly restored and reset. This requires the elasticity of the blanket to be good and the compressive deformation value to be small to prevent the blanket. The shrinkage is thinned to ensure the stable printing quality of the product.
5. Smoothness and flatness. The smoothness of a blanket refers to the smoothness of its surface, while the flatness refers to the uniformity of its thickness. The surface of the blanket is smooth, and the dot and graphic contours are printed clearly. However, the blanket with good smoothness has poor ink absorption on the surface, which will not only affect the smooth and uniform acceptance and transfer of the ink, but also easily adsorb. Impurities such as paper dust. Generally, there are numerous fine sands on the surface of the blanket. When the blanket is used for a long time, the surface is smoother. The cleaning agent can be used to destroy the shiny layer to enhance the ink absorption of the surface. The better the flatness of the blanket, the more uniform the printing pressure, ink absorption and ink transfer performance.
6. Oil and solvent resistance. During the use of the blanket, its surface needs to be constantly in contact with the ink and the cleaning agent, and the oil or solvent easily causes the rubber to swell, thereby affecting the printing quality of the product. Therefore, the blanket must have good oil and solvent resistance to ensure stable printing quality.
7. Transmissibility and hydrophobicity. The blanket is the medium for the transfer of the printing ink layer. Therefore, the blanket must have good ink adsorption and strong hydrophobicity to ensure the printing quality of the product.
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