As you know, a variety of techniques are used to evaluate the capability of the yankee coating to shield the cylinder surface, going from on-line monitoring blade vibrations to the determination of the roughness and run-out of the yankee surface in shut down.
The analysis of blade wear is maybe one of the oldest practices used by tissue makers to understand if the attrition between the blade and the cylinder is properly controlled or not. The control of the deterioration is usually more meaningful when done on creping blades made in steel.
When analyzing a steel creping blade after use, the attention of the observer will be mainly concentrated on:
- evenness or unevenness of the blade wear across the whole blade;
- appearance of the blade wear in machine direction;
- extent of the wear;
- eventual signs of overheating of the blade itself.
All this will have to be contextualized on the basis of the duration of the blade life and of the conditions met during it, such as kind of pulp, system pH and cationic demand, refining intensity and many others. In the following case history, the assessment of creping blade wear has been very important to highlight a significant improvement in the protective effect of the yankee coating. A double size, high speed, premium tissue machine suffered from limitations in speed due to instability of the yankee coating.
Picture 1 shows a 60X image of a creping blade used to produce ECF toilet paper. Analyzing the picture, we can see that:
- the total extent of the wear is 0,79 mm; this had been developed on 315 km of revolving yankee surface, coning to 0,26 mm/100 km of specific blade wear;
- the worn beavel shows severe damages, with micro-fragments of metal chipped out of the beavel;
- clear signs of overheating are visible (blue-brownish area). This means most probably the ‘chipped’ missing pieces have been transformed in micro-weldings on the yankee surface.