Residual stresses and their implications in medium density fibreboard

ResearchSpace/Manakin Repository

Show simple item record Van Houts, Jeroen H. en 2009-08-19T05:40:35Z en 2009-08-19T05:40:35Z en 2000 en
dc.identifier.citation Thesis (PhD--Mechanical Engineering)--University of Auckland, 2000. en
dc.identifier.uri en
dc.description Full text is available to authenticated members of The University of Auckland only. en
dc.description.abstract The hot pressing process is one of the most critical steps in making medium density fibreboard (MDF) of acceptable quality. MDF is manufactured by hot pressing a mat of resinated wood fibres in either a continuous or a multi-opening press. The same processes occur within a fibre mat during hot pressing regardless of the type of press. As mat compaction begins, heat is conducted from the press to the outer surfaces of the fibre mat. Moisture in the mat surfaces vaporises and becomes the medium for convective heat transfer to the core of the mat. The mat of fibres is pressed for a period sufficient to allow resin cure to occur through the entire thickness. Different heat and moisture conditions through the mat thickness during hot pressing lead to a panel density profile where the outer regions are of a considerably higher density than the core. After a panel leaves the hot press, the temperature and moisture content profile through the thickness gradually equilibrates to surrounding conditions. These changes in temperature and moisture content within the panel lead to the development of residual stresses. The density profile of a panel and its viscoelastic behaviour are both important factors in relation to the development of these residual stresses. Two techniques, namely the dissection and hole drilling methods have been successfully adapted for measuring residual stresses in directions parallel to the surface of MDF. A theoretical model that considers a number of parameters including changes in temperature and moisture content after pressing, density profile, and viscoelastic behaviour has been developed to predict the profile and magnitude of these residual stresses in MDF. Results from the theoretical model closely match measurements made by both the dissection and hole drilling methods. A strong link between residual stresses and thickness swell due to water soak has been identified. Post-hot pressing treatments have been developed to significantly reduce the magnitude of residual stresses present in MDF. Taguchi analysis was applied to interpret the effect of these different treatments on panel properties such as internal bond strength, thickness swell, tensile strength and tensile modulus. In most cases, it was found that the applied treatments reduced panel thickness swell due to water soak up to 20%. The mechanism by which these treatments relieve residual stresses can partially be explained by the viscoelastic behaviour of MDF under different temperature and humidity conditions. en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof PhD Thesis - University of Auckland en
dc.relation.isreferencedby UoA9994500514002091 en
dc.rights Restricted Item. Available to authenticated members of The University of Auckland. en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. en
dc.rights.uri en
dc.title Residual stresses and their implications in medium density fibreboard en
dc.type Thesis en The University of Auckland en Doctoral en PhD en
dc.rights.holder Copyright: The author en

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