For a long time, the plywood industry in Germany was a traditional user of large-dimension timber. This has, however, almost disappeared, despite the fact that there is still a high demand for plywood in Germany.
A joint project, funded by the German Federal Ministry for Education and Research (BMBF) via the Project Management Jülich (PTJ), aims at creating the possibility of utilising large-dimension timber in the manufacture of rotary-cut veneer products. The work programme for the project addresses questions concerning forestry, technology and economics.
A significant point therein is an increase in the wood yield. This could be effected by, for example, new techniques in the processing methods for the pared veneer panel. The usual procedure is for the panel to be cut (clipped) in fixed widths, transverse to the rolling direction. Flawed sections are clipped out in strips and completely discarded, thereby causing considerable losses of around 20%. A reduction in these losses would appear to be possible, provided the following measures, developed in co-operation with the Buddenberg Agency, are put into effect:
- The clip length in the rolling direction is variable; the longest length between two flaws must always be used.
- The flaw-free sections of the veneer panel are combined to form a continuous panel.
- The good parts of the flawed veneer sheets are re-utilised by combining them and adding them to the continuous veneer panel.
- The continuous veneer panel is cut to the required dimensions.
In this way, clip losses could be reduced to around 5% - at a cost, however, of the additional work measures. In order to analyse the economic efficiency of new clipping processes, the characteristics of the veneer flaws, such as their spatial distribution over the veneer panel, flaw size, type of flaw, etc., must be determined. As such data is only available in limited form from plywood manufacturers, peeling tests were carried out within the framework of this project in order to experimentally determine the flaw statistics for spruce and beech.
Using a line scan camera positioned above the production line, the surfaces of the veneer panels were recorded at around 80m/min with an exposure of one pixel per mm – a total of around 170 peel rolls from beech and spruce with a length of up to 100m. The raw data for one peel roll can be up to 200 MByte. In order to calculate flaw statistics therefrom, additional image processing steps are necessary:
- location-dependent contrast and brightness correction
- combination of the image series to form one large image
- detection of “darker“ image regions as flaws and measurement.
The illustration shows an interim finding of the analysis of an image section. Flaws (large branches, holes, cracks) are marked with rectangles whose length and width is sufficient for the description of the flaw size. The resulting data is then used for virtual simulation of the various clip strategies.