Do You Know How to Use The Doctor Blade Properly?

In the paper production process, the doctor blade, as a key component, directly affects the surface quality of the paper, fiber distribution, and equipment operating efficiency. The correct use of doctor blade requires comprehensive adjustment based on material characteristics, installation angle, pressure parameters, and process conditions to ensure even contact between the blade and the roller surface, and to avoid the occurrence of stripes, scratches, or fiber accumulation.

What Should be Noted before Installing Doctor Blade?

Before doctor blade installation, check if the blade is straight and not bent, if there are no gaps or burrs on the blade edge, and if the contact surface between the blade body and the doctor blade holder is clean and free of foreign objects. Place the blade vertically into the blade slot, gradually tighten the fixing bolts with a special tool, and follow the diagonal sequence to tighten multiple times to ensure that the blade is evenly stressed. After installation, use a feeler gauge to check the gap between the blade and the backing roller, with a deviation of no more than 0.05 millimeters across the entire width range. For high-speed paper machines, it is recommended to use pneumatic pressurization systems instead of traditional mechanical pressurization. Real time monitoring of contact pressure is achieved through pressure sensors, and the working pressure is usually controlled within the range of 0.15-0.35 MPa. Specific values can be adjusted based on roller diameter, line speed, and paper quantity.

Doctor Blade Operation Precautions

Start the roller at low speed during operation and observe whether the contact line between the doctor blade and the roller surface is continuous and uniform. If there are intermittent bright lines, the machine needs to be stopped and the installation angle needs to be readjusted. It is recommended to set the doctor blade installation angle in the range of 15-25 degrees. Excessive contact pressure can accelerate blade wear and increase driving load, while insufficient pressure can lead to incomplete scraping. The appropriate pressure can be determined by observing the fiber accumulation behind the doctor blade. Regularly use an infrared thermometer to detect the working temperature of the doctor blade. Under normal operating conditions, the temperature of the blade should not exceed 30 ℃ above the ambient temperature.

Doctor Blade Cleaning Cycle and Precautions

The cleaning cycle is determined according to the changes in the production paper type. When producing coated paper, the machine needs to be stopped for cleaning every 8 hours, and for cultural paper, it can be extended to 24 hours. When cleaning, use a copper doctor blade specifically designed for cleaning. Gently scrape off the adhesive along the 45 degree angle of the blade. It is strictly prohibited to use metal hard objects to directly strike the blade. The lubrication system should ensure that 3-5 drops of specialized lubricating oil are dripped per minute, and the oil selection should comply with the HM hydraulic oil in ISO6743-4 standard. When the blade wear reaches 20% of the original thickness, it should be replaced in a timely manner, and a 30 minute no-load running in operation should be carried out after each replacement.

Troubleshooting Common Problems With Doctor Blade

In the handling of common problems, if periodic horizontal stripes appear, priority should be given to checking whether the ovality of the roller surface exceeds the standard. When the doctor blade vibrates abnormally, it is necessary to check the clearance of the transmission system bearings and the alignment of the coupling. If the position of fiber accumulation deviates by more than 200 millimeters, the distribution of the lip opening of the headbox should be adjusted. When it comes to safety operations, GB8958<<Safety Requirements for Paper Machinery>>must be strictly followed. When dealing with paper breakage, the pressure of the doctor blade must be released first. Before maintenance operations, energy isolation procedures must be implemented, and mechanical blocking devices must be set up to prevent roller rotation.

In terms of process parameter optimization, for every 100m/min increase in speed, the contact pressure of the doctor blade needs to be correspondingly increased by 5% -8%. When producing low basis weight paper, it is recommended to use a silicon nitride ceramic coated doctor blade with a surface roughness controlled below Ra0.2um. For paper with a filler content exceeding 18%, a high pressure spray system should be used to maintain water pressure within the range of 1.2-1.8MPa and nozzle spacing not exceeding 150mm. After each process adjustment, samples should be taken to detect changes in paper smoothness. When measuring with a Parker Print Surf instrument, the fluctuation range of the results should be controlled within 3ml/min.

Maintenance and Replacement of the Doctor Blade

The doctor blade maintenance records should detail the time, pressure value, angle parameter, and corresponding paper type of each adjustment, and establish a doctor blade service life prediction model. Through statistics, it has been found that when the cumulative operating mileage of the doctor blade reaches 8000-12000 kilometers, significant plastic deformation will occur at the micro blade edge. At this time, even if the blade thickness meets the requirements, it is recommended to replace it. When renovating equipment, modular quick change tool holder system is preferred, which can shorten the replacement operation time from traditional 4 hours to 45 minutes, effectively improving equipment operation rate.