Problems You May Encounter During Doctor Blade Use

Whenever we encounter paper defects, our initial response is typically to inspect the pulp and papermaking clothing, often overlooking the impact that doctor blade and their operational status have on the papermaking process.

In reality, however, doctor blades are critical components in the papermaking process, playing pivotal roles in functions such as roll cleaning, pulp distribution control, sheet peeling, and creping. Consequently, their operational status is a significant factor directly influencing the paper's quality and condition. The occurrence of various issues, including streaking, rewetting, uneven coating, roll surface contamination, and sheet breaks, can all be attributed to the performance of these blades.

Keep the Doctor Blade in Good Condition

• Prevent pulp accumulation on the guide rolls and breast roll.

• The first outer roll is critical for preventing damage to the wire caused by pulp lumps.

• The doctor blade angle (approximately 25 to 30 degrees) is essential for ensuring proper dewatering without generating scraping noises or blade chatter.

  
  

• The doctor blade load requires iterative adjustment through trial and error, but overloading must be avoided.

• For any blades, oscillation serves as an excellent early warning indicator.

• The breast roll doctor blade should be inspected frequently to prevent the formation of wire marks or excessive water carryover on the breast roll.

  

  
  

Doctor Blade Instability

Doctor blade instability occurs when the blade cannot maintain stable, uniform, and continuous contact with the roller surface. This is primarily manifested by vibrations, blade skipping, and misalignment during operation.

This may be caused by:

Excessive pressure: When the load on the blade is too heavy, the blade tip bends and presses against the surface of the roll, preventing it from sliding normally.

Mechanical structural issues: If the doctor blade is not securely installed, the blade holder lacks sufficient rigidity, the blade edge is worn or deformed, or the pneumatic system malfunctions, these factors may cause the blade to lose stability due to external influences during operation.

Substrate issues: The presence of rotten pulp or loose fibers in the base paper, as well as impurities on the paper or the accumulation of lint, powder, or fibers at the blade edge, along with the temperature of the paper before it enters the coater, can all affect the operation of the doctor blade.

Problems Caused by Doctor Blade Instability

Blade instability can cause a range of paper defects. The most immediate issue is the appearance of streaks on the paper surface due to uneven scraping. Furthermore, if the doctor blade undergoes high-frequency vibration, it will also cause chatter marks, thereby compromising the aesthetic quality of the paper.

Doctor blade marks typically manifest as continuous or intermittent linear defects on the paper surface and coating. These marks are particularly noticeable on high-grade printing paper. They significantly reduce the paper’s smoothness, coating uniformity, and print quality.

Chatter marks typically appear as repetitive transverse patterns or vibrational stripes across the paper web or roller surface. This not only affects the paper’s appearance but also its subsequent processing performance, compromising the hand feel and even causing paper breaks.

Doctor blade instability can also lead to localized overheating, scratching, or uneven wear on the roller surface. Once the roller surface is damaged, the periodic impact between the blade and the rotating roller surface will further exacerbate vibration and shuddering. This not only severely affects the quality of the produced paper but also leads to further wear of the blade and increased damage to the roller coating.

Solutions

Judicious Selection

Select the appropriate doctor blade thickness and material. Simultaneously, replace worn blades in a timely manner to minimize the impact of the blade on cross-directional coating variations.

Oscillation

Doctor blade oscillation can be employed to prevent the accumulation of particles within the nip between the blade and the roll. This action distributes wear and pressure across the entire length of the blade edge, thereby preventing issues such as uneven wear caused by prolonged contact between a specific blade area and the roll surface.

Establishing appropriate parameters for the doctor blade's oscillation frequency, stroke length, and speed is essential; this helps resolve issues such as blade vibration, blade chatter, localized wear, and the resulting paper defects.

Regular Maintenance

Regular maintenance is necessary.

Conduct daily inspections of both the doctor blade and the machinery.

Verify the cleanliness of the roll surface and check for any vibration in the doctor blade.

Inspect the area behind the blade, particularly the interior of the roll, to ensure it remains clean.

Paper Web Management

During the base paper manufacturing process, enhance the surface strength of the base paper through measures ranging from pulp formulation and preparation techniques to internal sizing and surface sizing.

Ensure uniform basis weight and moisture content across the paper web's cross-direction to minimize doctor blade deflection caused by poor paper surface quality.