In offset printing, the phenomenon of inaccurate color printing is frequently encountered. This manifests on printed materials as spots or blurring in solid areas, incomplete images or lines, poor tonal reproduction, or missing dots. Many factors contribute to inaccurate ink printing, including paper properties, ink properties, printing plate quality, printing blanket quality, printing pressure, printing press performance, and printing processes.
If the paper and ink properties are stable, the printing plate quality meets printing requirements, and the printing press performance is relatively stable, but inaccurate ink printing still occurs, the primary cause is likely printing pressure. In offset printing, the magnitude of printing pressure directly determines ink transfer. Uneven pressure in certain areas will directly lead to inaccurate ink printing.
In offset printing, the compression and deformation of the printing blanket is the fundamental guarantee for generating printing pressure; ink transfer is the result of this compression and deformation. Therefore, the magnitude and uniformity of printing pressure are directly related to the blanket used in offset printing.
The blanket, wrapped around the cylinder, is simultaneously subjected to its own tension and printing pressure. With each rolling pass, the accumulated stress from compression deformation causes the blanket to stiffen, losing its original softness and high elasticity. It may even develop shine or cracks on its surface, leading to uneven printing pressure in certain areas during the printing process.
Furthermore, because the blanket cylinder is installed between the plate cylinder and the impression cylinder, each tiny unit on the blanket contacts both cylinders once per revolution. Therefore, the stress on the blanket changes periodically. This periodic stress causes internal friction, leading to blanket aging, affecting the uniformity of printing pressure, and ultimately potentially rendering the blanket unusable. Especially on modern high-speed offset printing presses, the frequency of the blanket's stress cycle is higher, resulting in greater internal friction, a shorter blanket lifespan, and a greater impact on printing pressure.
Besides providing printing pressure for offset printing, the printing blanket should also possess excellent ink transfer capabilities under the pressure of the printing plate to the substrate surface, i.e., ink absorption and release capabilities. However, aging of the printing blanket alters this ink transfer capability, leading to uneven ink coverage.
Therefore, one of the main causes of uneven ink coverage in printing is the decline in the performance of the printing blanket. Addressing uneven ink coverage in offset printing should begin with improving the printing blanket itself. When selecting a printing blanket, the following technical indicators should be considered:
1. Suitable hardness: Suitable hardness ensures clear dots and uniform ink coverage in the printed product. Generally, a blanket with a hardness of 70-80 degrees (Shore) is selected.
2. Low compression deformation: To prevent the printing blanket from reducing its thickness, elasticity, and hardness during printing, a blanket with low compression deformation is preferred.
3. Excellent ink transfer performance: As the intermediate medium for dot transfer, the printing blanket must possess good ink absorption and transfer capabilities, as well as strong hydrophobicity. Otherwise, it cannot transfer sufficient ink to achieve the desired tonal effect in the printed product. Meanwhile, the surface adhesive layer of the blanket should have a certain roughness, and the surface should be fine, clean, and free of impurities;
4. Good oil and solvent resistance: The blanket should not swell due to contact with chemicals during the printing process, thus compromising its performance;
5. Uniform thickness: The flatness error of the blanket should be within ±0.04mm; otherwise, the printing pressure will be uneven, resulting in noticeably uneven ink color and severely affecting the quality of the printed product;
6. Appropriate elongation: The lower the elongation of the blanket, the better, as this ensures accurate registration, complete dots, and clear images during printing.
