Characterization of structure properties of OSB

The quality of OSB (oriented strand boards) is mainly determined by three process steps: flaking, mat forming, and pressing. Strand geometry, influenced by the flaking process, strand orientation and homogeneity during mat forming, and the surface evenness are crucial properties which determine the board quality. Fraunhofer WKI in Braunschweig developed off-line and on-line evaluation methods based on image processing, thermography, and optical 3D measurement. Strand orientation during the mat forming process can be monitored using image processing: A camera continuously acquires gray-value images of a part of the mat surface moving from the mat forming station towards the press. After a transformation of the image into the spatial frequency domain (via FFT) and the application of an algorithm suitable for segmentation of the strands in the top layer one can derive indices for the mat quality: The deviation of the orientation angles of the strands from the production direction, an index for the quality of orientation, and an index for the size distribution of the strands. The software using these methods is ready for integration into statistical process control concepts. As a matter of fact the geometry of the strands influences the quality of the OSB, and here in particular the strength properties. Another software for statistic laboratory analyses of the strand geometry was developed allowing the evaluation of size and outline parameters. Thus, for example, the post-disintegration and smoothing of the particle shapes by the drying process could be verified. Thermography transforms the temperature distribution on a surface into gray-value images (thermograms). Thermal images of boards leaving the press reflect the local density distribution of the board. Therefore, thermographic inspection of the boards after leaving the press may be a novel technique for on-line density monitoring. One surface property of OSB is the evenness (or roughness) characterized by the frequency of small gaps (typically some millimeters wide and not deeper than one millimeter) appearing in the surface near crossing strands. For laboratory purposes a measuring technique based on fringe-projection and image processing has been tested to evaluate different surface qualities. As a result, the OSB surface can be visualized as a topographic map together with statistics on amount and size of gaps. Most of the measuring techniques presented here have already been tested in the industry or under comparable conditions. They are ready for implementation to close some gaps in process measurement technology.