Cupping behaviour of surface densified Scots pine wood: the effect of process parameters and correlation with density profile characteristics

Surface densification is a process by which the mechanical properties of the wood surface can be improved, increasing the attractiveness of low density wood for applications such as flooring. The purpose of this study was to investigate the development of undesired cupping deformations in surface densified solid wood. The effects of process parameters (compression ratio, temperature, holding time, closing time, and initial moisture content) on cupping were studied, and correlation analysis was used to determine the strength of association. Correlation analysis was also performed to explore the potential dependence of cupping on the density profile characteristics of surface densified wood. Correlations of moderate strength were found between cupping and the process parameters, as well as between cupping and the density profile characteristics. Compression ratio was found to be the most influential process parameter: samples with a high compression ratio showed distinctly different cupping behaviour than samples with a low compression ratio. Density profile characteristics were considered a probable contributor to cupping, but the influence of other effects was also considered likely. Interestingly, certain combinations of parameters were found to result in very small cupping deformations. Therefore, the results of this study indicate that cupping can be minimised by optimisation of process parameters.