This paper describes an analytical method for characterizing structural non-uniformities, such as formation, that may vary as a function of position across the web. Streaks are spatially persistent non-uniformity defects that can occur in fibrous materials such as paper, paperboard and nonwovens as a result of the complexity of distributing the fibers in a planar web at high forming rates. For paper, streak defects are generated by the hydrodynamics in the headbox and forming zone of a paper machine. Since their occurrence is related to the machine speed, streaks may appear stationary or mobile, they may be intermittent or continuous, and may occur at regular or irregular intervals across the machine width. In this investigation we used spectral analysis method introduced by Kellom?ki et al [1], based on the continuous wavelet transform, to isolate the directionally dependant, static and stochastic components of grammage variability of machine made papers. Several pilot machine samples with streaks generated under controlled conditions and commercially made samples were examined. The spectral analysis of the pilot machine samples showed substantial differences in the static and stochastic spectral energy components for each. Careful examination of the spectra revealed the scale differences of streaks, and of flocs, and variation of such, across the samples. Comprehensive spectral plots were simplified by calculation of the dominant wavelength, which essentially provides the mean feature size as a function of cross machine position. Thus, single profiles for the static component permitted the streak periodicity, and its change with position, to be readily obtained. The results of this study provide the technical specialist in nonwovens with a new method for identifying and characterizing non-uniformities that will negatively impact product quality and end use performance.
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