During lithographic offset printing, we often encounter the phenomenon of unrealistic color printing. The main parts of the printed matter are the spots on the solid part of the printed matter or the fuzziness, the incomplete graphics or lines, the poor reproduction of the tone of the image or the loss of the dots. There are many reasons for the false ink color during the printing process, and there are many factors involved, including paper performance, ink performance, printing plate quality, printing blanket quality, printing pressure, printing machine performance and printing process, etc. .
If during the printing process, the performance of the paper and ink is stable, the quality of the printing plate meets the printing requirements, and the working performance of the printing machine is relatively stable, the ink color printing is not realized. The main reason is the problem of printing pressure. . In offset printing, the size of the printing pressure directly determines the transfer of ink during the printing process. If local pressure unevenness occurs during the printing process, it will directly cause the color of the printing ink to be untrue.
In offset printing, the compression deformation of the blanket is the fundamental guarantee for generating printing pressure, and the transfer of printing ink is the result of the compression deformation of the blanket. Therefore, the size and uniformity of the printing pressure are directly related to the blanket used for offset printing.
The blanket coated on the cylinder is simultaneously affected by its own tension and printing pressure. With the continuous increase of the number of rolling, the stress of the compression deformation of the blanket accumulates, and the blanket will become stiff and lose its original softness. And high elasticity, even on the surface of the blanket, there will be bright or crack marks, resulting in partial unevenness of the printing pressure during the printing process.
On the other hand, since the rubber cylinder is installed between the plate cylinder and the impression cylinder, each tiny unit on the blanket needs to be in contact with each of the two cylinders once every revolution. Therefore, its force changes periodically. It is precisely due to the periodic change in the force of the blanket that will cause the internal friction of the blanket, lead to the aging of the blanket, affect the uniformity of the printing pressure, and may eventually lead to the scrap of the blanket . Especially on modern high-speed offset presses, the frequency of the stress cycle of the blanket is higher, the internal friction generated by the blanket is greater, the life of the blanket will be shorter, and the impact on the printing pressure will be greater. [next]
In addition to providing printing pressure for offset printing, the blanket should have a good ability to transfer the ink of the printing plate to the surface of the substrate under the action of the embossing force, that is, the ability to receive and release ink. But the aging of the blanket will change the ability of this ink transmission, and it will also lead to the phenomenon of unrealistic ink color.
Therefore, one of the main reasons for the printing ink color is false is the performance of the printing blanket is reduced, the processing method to reduce the printing ink color in the offset printing process should also start from the blanket. The following technical indicators should be considered when choosing a blanket:
1. Appropriate hardness: If the appropriate hardness is provided, the dots are clear, and the ink color of the printed product is uniform. Generally, a 70-80 degree (shaw) rubber cloth is used;
2. Small compression deformation: In order to prevent the thickness of the blanket from decreasing during printing, reduce the elasticity and increase the hardness, it is better to choose a blanket with small compression deformation;
3. Excellent transfer performance: As an intermediate medium for transferring graphics and text dots, the blanket must have good ink absorption capacity, ink transfer capacity and strong hydrophobicity, otherwise it will not be able to transfer enough ink for the printed matter, and it will not reach The desired effect of the print tone. At the same time, the surface adhesive layer of the blanket should have a certain roughness, and the surface is fine and clean without impurities;
4. Good oil resistance and solvent resistance on the surface: the rubber cloth can not expand due to contact with the chemical substances in the printing process, and destroy its application performance;
5. The thickness must be uniform: the flatness error of the blanket should be within ± 0.04mm, otherwise the printing pressure will be uneven, and the ink color of the product will be significantly uneven, which will seriously affect the quality of the printed matter;
6. Proper elongation: The smaller the elongation of the rubber cloth, the better. This will ensure accurate overprinting, complete dots and clear graphics in the printing process.
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