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Continuous densification of wood with a belt press: how knot features impact the densification outcome
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.ORCID iD: 0000-0002-1489-0839
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.ORCID iD: 0000-0003-0869-5858
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.ORCID iD: 0000-0002-4526-9391
2023 (English)In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 18, no 4, p. 1587-1596Article in journal (Refereed) Published
Abstract [en]

Densification, i.e. the transverse compression of sawn timber has been studied and commercialised for well over 100 years but remains an expensive niche product with low annual production volumes. One reason for this is the reliance on time-consuming batch processes in a hot press. To solve this, a continuous densification process using a belt press, capable of densifying full-sized sideboards was developed. However, there is insufficient knowledge about the effect of knots on the densification outcome. The objective of this study was to assess how different knot parameters affect the densified wood in terms of damage and deformation to the knot itself and the surrounding wood material. Multivariate data analysis methods were applied to a dataset of 171 knots, described by 23 variables. The data showed that it is possible to densify knots in a continuous process without causing damage. Especially sound knots are often unproblematic, even at relatively large sizes, while densifying dead knots often resulted in unacceptable damage to the knot or the surrounding wood. From a material selection standpoint, any knots bleeding into the board edge and dead knots greater than 20 mm in diameter should be avoided altogether.

Place, publisher, year, edition, pages
Taylor and Francis Ltd. , 2023. Vol. 18, no 4, p. 1587-1596
Keywords [en]
Compression of wood, full-size samples, surface densification, wood defects, wood industry
National Category
Wood Science Other Mechanical Engineering
Research subject
Wood Science and Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-99295DOI: 10.1080/17480272.2023.2228278ISI: 001012945900001Scopus ID: 2-s2.0-85163005843OAI: oai:DiVA.org:ltu-99295DiVA, id: diva2:1786360
Funder
The Kempe FoundationsLuleå University of Technology
Note

Validerad;2023;Nivå 2;2023-11-08 (joosat);

Full text license: CC BY-NC-ND

Available from: 2023-08-08 Created: 2023-08-08 Last updated: 2025-01-28Bibliographically approved
In thesis
1. Advancing Continuous Surface Densification of Wood: Process Development, Material Characterisation, and Digital Integration
Open this publication in new window or tab >>Advancing Continuous Surface Densification of Wood: Process Development, Material Characterisation, and Digital Integration
2025 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Framsteg inom Kontinuerlig Ytkomprimering av Trä : Processutveckling, Materialkarakterisering och Digital Integrering
Abstract [en]

Surface densification of wood enhances the density and mechanical properties of the surface layer, increasing the value of underutilised wood species and expanding their suitability for a broader range of applications. By improving attributes such as strength and hardness, this process positions densified wood as a sustainable alternative to conventional materials such as plastics, metals and tropical hardwoods. However, despite decades of research, widespread commercial adoption remains limited, due to high production costs, variability in product performance, and the complexities of scaling densification methods for industrial use. 

This thesis addresses key barriers to industrial-scale adoption of surface-densified solid wood intended for flooring, with a focus on continuous densification processes and the optimisation of material properties. Specifically, the research aimed to: (1) evaluate the feasibility of continuous densification using a custom-designed belt press, (2) investigate the influence of growth ring orientation and knots on the densification process and resulting material quality, and (3) establish methods for characterising and optimising density profiles to improve mechanical performance and reliability of product properties.

The belt press studies demonstrated the potential for high-throughput continuous densification, achieving targeted density profiles while minimising non-value-adding steps. Pre-heating and increased heat transfer during compression were identified as critical for mitigating undesired densification and ensuring efficient plasticisation. The influence of growth ring orientation revealed significant variability in strain distribution and spring-back behaviour, underscoring the need for customised material selection and process parameters. These findings are particularly relevant for utilising low-quality or fast-growing wood species as they exhibit high variation in those features.

Mechanical characterisation of surface-densified wood revealed a strong correlation between density profiles and hardness, with the indentation depth during hardness tests shown to be significantly influenced by the density distribution beneath the surface. This relationship shows limitations in traditional hardness testing methods, which may fail to accurately assess performance due to variability in density profiles. To address these challenges, the study proposed an application-focused approach to testing the hardness of surface-densified wood, where the densification process and hardness test parameters are tailored to meet specific end-use requirements rather than relying on arbitrary mechanical metrics. Additionally, a method for predicting density profiles from hardness test data was developed, offering a practical alternative to X-ray densitometry for process control and design.

The development of machine learning models for predicting density profiles from photographic images of the materials’ cross-section offers a method for real-time quality assessment which is industry-suited. By combining this approach with the established correlation between density profiles and hardness, it may become possible to predict hardness solely from images. This integration of digital tools advances data-driven manufacturing in wood densification, providing scalable solutions.

To gain further insights into the densification process, an innovative in-situ measurement technique was developed, combining digital image correlation and computed tomography scanning (CT). This method enabled real-time analysis of density distribution changes during thermo-mechanical densification by aligning 2D strain fields with CT data and employing a constitutive model to estimate density profiles. The method was validated against experimental results and can be used in research, process optimisation, and to provide local density data to refine numerical models for precise and application-specific simulations.

This work bridges scientific principles with industrial practice, providing a foundation for the commercialisation of surface-densified wood products. By addressing critical challenges in process efficiency, material variability, and characterisation, this thesis lays the groundwork for large-scale industrial production of surface-densified flooring boards. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2025
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
densification, density profile, continuous process, hardness
National Category
Wood Science
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-111448 (URN)978-91-8048-744-3 (ISBN)978-91-8048-745-0 (ISBN)
Public defence
2025-03-27, A193, Luleå University of Technology, Skellefteå, 09:00 (English)
Opponent
Supervisors
Available from: 2025-01-29 Created: 2025-01-28 Last updated: 2025-01-29Bibliographically approved

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Neyses, BenediktScharf, AlexanderSandberg, Dick

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