By Nick Mike, NGM Tissue Consulting, LLC
The limitations of tissue production at the doctor blade are determined by the weakest point of the sheet blade interface. This is the point where the sheet will break, corrugate, pick or tear causing the machine slow down to run. To optimize your tissue machine, periodic doctor blade alignment is usually made, however, very few machines have also performed a Deflection Analysis.
John Holton, a long-time US paper engineer, developed this technique as a step above and beyond doctor alignment. Holton felt that even though the tip of the doctor blade may be uniform in the CD, the pressure at the tip may not be uniform. The bending or "deflection" of the doctor blade represents energy at the blade tip. Variations in doctor blade pressure across points in the cross direction can causes several operational issues at the blade-coating-sheet interfaces, such as:
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Too much pressure at the blade tip could be stripping vital adhesion by taking off too much yankee coating. Or, too little pressure could cause the coating to heavily build.
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The position of the doctor blade in the Z direction of the coating could be modified causing picking from the blade riding too high on the coating surface or the sheet not being fractured at the blade if the blade is riding too low in the coating resulting in no crepe (corrugation).
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Yankee surface contact with the doctor blade could occur if the blade tip is exerting too much pressure from excessive deflection. Yankee damage, premature blade life, and sparking (fires) could also occur.
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Overall, a non-uniform, cross directional impact of the doctor blade could cause speed reduction, or sheet quality variation.
Deflection Analysis involves a cross directional measurement of doctor blade pressure measured by variation in doctor blade tip pressure. The tip angle is measured and the doctor blade base angle is measured. The difference is deflection or how much the doctor blade is bending. This is accomplished using an electronic leveling device with an attached small leveling foot, typically used to measure doctor blade tip angle. This procedure can be done on a hot or cold yankee with expected deflection results manifesting in different shaped graphs.
Holton recommends this be done every time the holder gets aligned and a record kept to look for trends or concurrent issues. Machine speed up, fewer breaks, and longer doctor blade life usually result.
Nick Mike of NGM Tissue Consulting, LLC can be reached by calling 603-616-8931, or by email at nmike03580@yahoo.com.
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