The Application of Ink Doctor Blade in Gravure Printing

The selection, installation, and adjustment of the ink doctor blade in gravure printing have a significant impact on the quality of printed products. Below are some insights on the application of the doctor blade in gravure printing and related issues.

1. Selection and Installation of the Doctor Blade For high-speed gravure printing machines, imported, highly elastic steel doctor blades with a thickness of 0.15–0.2mm are commonly used. When installing the doctor blade, it is important that the blade is longer than the printing plate by 10–20mm to prevent ink from splashing onto the rubber roller. The steps and key points for installation are as follows:

2. The distance between the backing and the blade should be approximately 10mm. The distance between the backing and the blade should be 5–7mm. However, depending on specific circumstances, the distance can be appropriately increased (i.e., using a soft blade) or decreased (using a hard blade).

3. Place the new blade behind the backing and insert it into the blade slot. Tighten the backing screws, starting from the middle of the blade and gradually working outward. Alternate tightening from both ends. Here is a tip to avoid blade warping: when tightening the screws, it should be done in two or three rounds, not all at once. While tightening the screws, use a piece of cloth to clamp the blade and backing and pull it toward one side with force. This ensures that the blade is installed more evenly. A flat blade ensures a uniform ink layer on the printing plate.

4. Select appropriate tools and methods for blade sharpening.

When sharpening a blade with a sharpening stone, if the stone is too coarse, it is not suitable for new blades. Based on practical experience, using 1-grit diamond sandpaper is suitable for sharpening both new and old blades with significant damage. If 1-grit sandpaper feels too coarse for sharpening a new blade, you can finish sharpening with finer 0-grit sandpaper. There are two main sharpening methods: one is folding the sandpaper into a “V” shape, ensuring several layers to prevent the blade from cutting through the paper and hurting the user. This method is efficient and easier to control the blade’s edge angle, but it is not very safe. The other method is to use both hands to pull the sandpaper at both ends while sharpening, which is safer but less precise for controlling the angle of the blade. Both too blunt and too sharp blades can negatively affect printing quality.

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2. Proper Adjustment of the Doctor Blade The pressure, sharpness, and the contact position of the blade with the printing plate all influence printing quality. By properly adjusting these factors according to the actual needs, many printing problems can be resolved.

3. Pressure Factors that affect the pressure of the doctor blade include the cylinder pressure, the choice of soft or hard blades, and the angle at which the doctor blade crosses the printing plate. The greater the angle, the higher the pressure. Excessive pressure can reduce ink transfer efficiency and cause greater wear on both the printing plate and blade. Insufficient pressure can lead to dirty plates or visible blade lines.

4. Blade Sharpness The sharpness of the blade depends on whether it is new, the sharpening angle, and the type of abrasives (such as sharpening stones) used. The optimal sharpness should effectively clean the ink without creating blade marks. Overly sharp blades tend to produce blade marks, and they also wear out the printing plate more quickly.

5. Contact Distance between the Blade and the Printing Plate Reducing the contact distance between the doctor blade and the printing plate can improve ink transfer efficiency, but it can also cause dirty plates. Conversely, increasing the distance can reduce the occurrence of dirty plates and avoid fine line defects, but it may lower ink transfer efficiency.

6. Blade Flatness The flatness of the blade and whether it warps primarily depends on the installation method, though it may also be influenced by foreign matter on the blade or backing. If the blade is not flat, it can cause issues such as water drop-like marks on the print edges or intermittent lines. Some may think that rough ink or poor ink flow is the cause of such issues, but in reality, an uneven blade can also lead to these problems.

7. Movement of the Doctor Blade The lateral movement of the doctor blade is important for reducing blade lines, increasing blade utilization, and minimizing wear on the printing plate. If the blade movement is irregular, it should be adjusted to improve performance.