Monday to Saturday - 8:00 -17:30
A PCB Pizotronics Model 309A accelerometer was used to coating doctor blade vibration. This accelerometer has a diameter of approximately 5.84 mm, a height of 7.62 mm, and weighs about 18 grams. The accelerometer was affixed to the tip of the operating doctor blade using high-strength adhesive (acrylonitrile-based). Initially, measurements were taken across the entire 3/4 width of the doctor blade, but no significant differences were observed. Subsequently, measurements were limited to the first 1/4 section (i.e., the area within 1.28 m of the leading edge). The vibration signals were amplified, analyzed using a Fast Fourier Transform (FFT) analyzer, and recorded with a data logger.
- Natural Vibration Frequency
The natural vibration frequency of the coating blade can be calculated using mathematical methods. Using the echo measurement method, the blade was tested without paper, and the coating machine was shut down under static conditions, with the blade pressed against the back roller. The PCB Model 086B80 vibration hammer was used to repeatedly strike the tip of the doctor blade for one minute. The reflected signals were captured by an accelerometer attached to the top of the blade, then amplified and recorded.
The resulting data was processed using an FFT spectrum analyzer. Figure 4 shows the natural vibration frequencies of the coating doctor blade when the hose pressure was 5.25 kN/m (30 Pli). The natural vibration frequency of the straight doctor blade is 875 Hz; the second harmonic frequency is 1750 Hz; and the natural frequency of the curved doctor blade ranges from 875 to 1050 Hz. When cardboard is placed between the doctor blade and the back roller, or when the doctor blade pressure is increased, the natural vibration frequency rises from 875 Hz to a higher value.
- Forced Vibration
Considering the impact of blade tip vibration on K&N printability, we are only interested in high-frequency (short-wavelength) vibrations. A typical blade tip vibration spectrum is shown in Figure 5. There are no high-frequency components on the curved coating blade of the first coater. The low-frequency components are primarily caused by mechanical vibrations of the coating head and vibrations of the back roller.
In contrast, the straight doctor blade on the second coater exhibits a major broad-band peak in the range of approximately 1000–1300 Hz. Table 1 illustrates the vibration characteristics of doctor blades on two other single-wire machines within the same factory. It can be seen that for straight doctor blades, the spectral trends are identical regardless of whether they are on the first or second coater.
Figure 6 shows the results of the impact on the vibration of the tip of the straight doctor blade on the second coater after improving the curing properties of the primer coating. Table 1 indicates that after installing an infrared dryer after the first coater, the blade on the second coater still exhibits broad-band vibration in the range of approximately 1000–1200 Hz, but the amplitude has been significantly reduced to only 0.11–0.22 mm.
In comparison, without the infrared dryer, the amplitude was 0.24–0.62 μm (peak-to-peak displacement). It is assumed that, prior to the installation of the infrared dryer, the base coat on the paper surface passing through the first coater was not sufficiently dried, causing it to reabsorb moisture during coating by the second coater. causing the straight doctor blade on the second coater to directly tear fibers from the high-density flocs on the paper surface. However, after installing an infrared dryer after the first coater, the base coat is completely dry before the second coating application. The doctor blade on the second coater only tears the surface of the base coat, and by this point, the base coat has already reduced the roughness of the paper surface.
Table 1 indicates that when the paper machine operates at speeds ranging from 157 to 288 m/min, the vibration of the straight doctor blade on the second coater within the frequency range of approximately 1,000 to 1,400 Hz is independent of the machine speed. When the pressure applied to the blade changes, the vibration frequency undergoes only a slight variation.
Figure 7 illustrates the results of varying the free protrusion length of the straight doctor blade. When using a backing plate measuring 2.03 mm × 88.9 mm (thickness × length), the free protrusion length of the straight doctor blade can be reduced from 20 mm to 11.1 mm. Consequently, the vibration frequency at the blade tip increases from 1,237.5 Hz to 3,100 Hz, while the amplitude decreases from 0.293 mm to 0.00824 mm (peak-to-peak displacement).
In the experiment, a backing plate was used to improve the surface properties of the coated board and its K&N printability, and its parameters could be adjusted as the doctor blade was modified. However, since it was difficult to adjust the coating formation after changing the doctor blade parameters, this experiment was abandoned.