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Sometimes the already described MD and CD variation are called systematic errors and they could be eliminated on principle or reduced in reality by elimination of the corresponding defects in the papermachine.
If all systematic variation is eliminated, there are still the non-systematic (residual or stochastic) variations. In the view of the author, this residual variation is a measurement to characterize the performance of a paper production unit. But this is only true if the systematic CD and MD variations are eliminated in fact.
There are other statistical and number screening tools available to analyze systematic variations in CD and MD direction and to find the reasons. Some of this methods are looking on the (time) series of basis weight figures and try to find out variations, which could be constructed by overlapping variations of special frequencies and amplitudes. For example it is possible to get from such an analyze a traceable variation of mass distribution (significant amplitudes) with a special frequency and the problem is not far beyond to be solved. The most frequent and significant basis weight variation with frequency about 20 Hertz is caused nearly always from the rotor of the screens in front of the headbox. The rotating wings of the rotor are covering the main outlet of the screen periodically and this generate variations in pressure and flow to the headbox which could be found as periodic basis weight variations in the paper.
That is the reason, why B&H-Systems are carefully designed in aspect of mechanical and hydraulic vibrations. The B&H distribution tank with a perforated plate and suitable design is one of the most performing unit to reduce hydraulic variation from the constant part and the special perforated plate converts the non desired vibrations to micro turbulence, just right to provide flocculation of the suspension.
Within the B&H-System the forming unit takes care of the uniform distribution of the fiber containing suspension in cross machine direction. If the headbox
generate systematic CD variation, this could be reduced on conventional headboxes by bending the slice or lip and on dilution headboxes with zone controlled consistency by dilution water, well known as profile control. The B&H - System is based on simple hydraulic basics and it works stable without any (sophisticated) control. Contrary to other systems the additional feeded dilution water for basis weight profile control does not effect the amount of flow to the zones of the headbox or forming unit. If there were an alteration of flow, due to profile control, into the feeding zones of the headbox, this would cause CD flow inside the headbox immediately. This ends up with the well known but non-desired result in paper quality, very similar to results from headboxes with slice control. Very often the defects are non flat paper and bump effects on paper with faster speeds of PM. Faster speed needs a more stronger drying, and drying amplifies this well known defect caused by different shrinking and shrinking rates, originally initiated by non uniform mass distribution and non uniform fiber orientation in the paper. On cut sheets or copy paper you may see non-flat paper, on printing papers there are wrinkles or prints out of register very often.
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