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Author: Klaus-B. Graeber(Brandes&Henze)
The main task of a papermachine (PM) headbox is to distribute fibers and fillers from the feeded suspension as uniform as possible on top of a running conveyor (wire) in view of time (machine direction, MD) and in view of localization (cross machine direction, CD).
Even there is a very well performing PM headbox, the basis weight distribution of paper is influenced from other parts of the papermachine as well.
Pressure variations from the constant part of PM could generate flow variations of the jet from the headbox finally. Although headboxes are able to reduce such pressure variations from the constant part very well, - and could be demonstrated with B&H-System - this pressure variations will result in variation of mass distribution on the running wire. It should be mentioned the different principles of headbox design of B&H and others. Conventional headboxes with CD flow spreaders will generate from variation of flow variation of mass distribution in CD and MD in the same time, which is a very insufficient situation.
But the wire itself and all the other dewatering devices will influence the initial mass distribution on the wire. Hopefully a more or less uniform mass distribution could be kept during a uniform filtration process and will not going worse. Non uniform abrasion or insufficient tension and stretching of the wire
or wrong adjusted dewatering elements as foils and suction boxes may influence the mass distribution in cross machine direction.
If there is enough data from measuring basis weight available, statistics could help to carry out the effect of mass distribution and its reasons. The most important tool to rectify the performance of a papermachine as a total system is analysis of variance of basis weight distribution in paper.
To do this, there is the need of about 20 paper samples from the top of the produced paper reel as long as the trim of the papermachine is. Sometimes samples are simply called profiles and from this stripes of paper the basis weight should be measured on a small area.
The two dimensional analysis of variance is able to separate the total variation into three parts. There are systematic variation in machine direction and systematic variation in cross machine direction and non-systematic (residual) variation.
As mentioned above and searching for reasons of such variation, MD variation could be supposed generated by the constant part or poor condition of the wire part. CD variation is mainly generated from the headbox or flow approach system. Generally headboxes with B&H design show significant less CD variation than others do.
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