A new method for the characterisation of micro-scale stress variations of press felts

A new method for the measurement of the local stress variation between a felt and a smooth surface has been developed. The method is based on a thin plastic film that is coated with a stress-sensitive layer (Cronapress Conversion film). The film is compressed between a felt and a smooth surface and the initially opaque layer becomes locally transparent with the degree of transparency being a function of the locally applied stress. The film is scanned and the transparency values are transformed into stress values. The minimum spatial resolution of the method is presently 25 mum, which means that even details of the order of the size of a batt fibre diameter could be identified. The method was compared, at a lower resolution, with an established method based on a Tekscan stress sensor. Good agreement was found. The applicability of the method was tested in laboratory wet pressing experiments using 50 g/m(2) sheets made from bleached softwood pulp. 12 differently designed felts were used. A strong correlation between the standard deviation of the applied stress and the achieved dryness was observed. The batt fibre diameter was found to be the dominating felt parameter in respect to dewatering.