Framsteg inom Kontinuerlig Ytkomprimering av Trä : Processutveckling, Materialkarakterisering och Digital Integrering

Transverse compression of wood can result in only the surface(s) of the wood material achieving a higher density, a process referred to as surface densification. Increased density generally leads to improved mechanical properties. For low-density wood species, this can increase value and open up new areas of use. By enhancing strength and hardness, surface-densified wood is positioned as a sustainable alternative to conventional materials such as plastic, metal, and tropical hardwoods, which are often selected for their high density. Despite decades of research and industrial development, the commercial spread of densified wood remains limited, which is considered to be due to high production costs, significant variations in product properties, and the complexity of scaling up the process for industrial use. This doctoral thesis addresses several key technical barriers to the industrial production of surface-densified wood, primarily intended for flooring, focusing on a continuous process and the optimisation of material properties. The aims have been to (1) evaluate the feasibility of continuous densification, (2) investigate how knots and the orientation of growth rings in the wood's cross-section affect the process and the resulting product properties, and (3) attempt to establish methods for characterising and optimising the density profile of the compressed wood with regard to the desired mechanical properties of the compressed surface. Achieving repeatability in the process has also been a key focus. Experimental studies have been conducted in a full-scale belt press specially developed for this project, which is also suitable for industrial use. The results from the studies conducted in this project demonstrate the potential of continuous densification, where desired density profiles can be achieved while avoiding non-value-adding production steps. The heat transfer to the wood material before and during compression was identified as crucial for compressing the wood’s cell structure in areas that result in the desired surface properties being obtained. The orientation of the growth rings in the wood’s cross-section was found to significantly impact how deformations vary within the wood during compression and how the wood surface springs back after compression. The results highlight the importance of adjusting material selection and process parameters to achieve the desired final product result. This is particularly relevant when fast-growing wood and so-called "low-quality" wood are used for densification, as such wood often exhibits strong variations in growth ring orientation and knot size. Characterisation of the mechanical properties of densified wood showed a strong correlation between the density profile and the surface hardness of the wood. The indentation depth in traditional mechanical hardness tests is significantly affected by the density profile. This results in limitations of traditional hardness testing methods, such as the Brinell test, which can lead to misleading hardness values for a wood surface depending on the density profile. To avoid misjudging the hardness of a wood surface for surface-densified wood, the proposed approach involves continuously adjusting process parameters and the density profile to meet specific end requirements. The density profile of, for example, conventional wood-based panel materials is usually determined using X-ray densitometry. However, this method can be challenging to implement in a continuous industrial densification process. A practically applicable method for process control and design is proposed, where machine learning models are used to predict density profiles from photographic images of the cross-section of the densified wood. The method is considered applicable for industrial use and can be used in real-time for quality control and process management. This integration of digital tools supports data-driven manufacturing of densified wood. This doctoral work bridges scientific principles and industrial practice, contributing to the potential commercialisation of surface-densified solid wood products. By addressing critical challenges in process efficiency and material characterisation, this work lays some of the foundational stones needed for large-scale industrial production of densified wood, such as for use in flooring.