14 HEIMBACH Hydraulic marks in paper


need no longer be an issue Specialised felts with longer flow paths for water have the capability of minimising shadow marks on paper. PPL reports on new developments with Heinbach’s Atrojet felt


I


f there is too little void volume for the water pressed out in the nip, disturbing hydraulic marks


may occur in the paper. Due to the periodic structure of base fabrics, grooves and drilled holes, the resulting marks are periodic as well and can therefore be investigated and identified relatively easy by means of an FFT Analysis (Fast Fourier Transformation). Hydraulic groove marks can also occur if the groove pattern of the roll is unfavourably designed. As soon as the pressed-out water starts to flow into the grooves the hydraulic pressure in the press nip decreases. The greater the width of the land area the higher is the hydraulic pressure above the land area, as shown in Figure 1. The reason for this is the longer flow path of the water. If the hydraulic pressure is too high hydraulic


Pulp Paper & Logistics


Atrojet


groove marks may occur in the paper. Therefore the width of the land area should be as narrow as possible.


In the same way shadow marks


can occur in the case of blind- drilled rolls and suction press rolls. Shadow marks, as illustrated


in Photo 1, occur if the hydraulic pressure between the drilled holes of a roll cover is much higher than above the holes. As a result, the water flows at a high speed towards the holes and can disturb the structure of the paper web. Thus opacity and penetration


properties of the paper are affected. In the worst case the disturbance of the opacity results in shadow marks. Often shadow marks are not visible until the paper gets coated or printed. Larger hole diameters result in higher


Photo 1: Paper sample with disturbing shadow marks (transmitted light image )


July/August 2019


Figure 1: Grooved roll: The wider the land area, the higher the pressure above (longer flow path of the water)


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