No abstract

Wood machining is strongly in¯uenced by the wood texture. Thus, it is a very important ®eld of research in order to achieve the optimum wood machining conditions. This study focusses on MDF, a far more homogeneous wood based material than solid wood. First of all, MDF cutting is described in function of moisture content and density. For constant machining conditions, the behavior of the cutting force components were observed as a function of density changes. Then the surface roughness was analyzed. The same method was further applied for variable cutting conditions. The evolution of cutting power and forces was observed with the aim to forecast it, if the species factor (Ke) and moisture content factor (Kh) are known. After the values of these factors have been adequately determined, the cutting forces and power cutting levels for the different layers of density found in MDF can be easily calculated. Einfluû der Materialeigenschaften bei der maschinellen Bearbeitung von MDF Die Holzbearbeitung wird wesentlich beein¯uût von der Holzstruktur, die daher daher ein bedeutendes Forschungsgebiet ist, um die optimalen Bearbeitungsbedingungen zu ®nden. Diese Arbeit beschra Ènkt sich auf MDFPlatten, deren Materialeigenschaften viel homogener sind als bei festem Holz. Unter konstanten Bearbeitungsbedingungen wurde zuna Èchst der Verlauf der Schnittkra Èfte in Abha Èngigkeit von der Dichte verfolgt. Darauf wurde der Ein¯uû der Ober¯a Èchenrauheit gemessen. Danach wurden dieselben Methoden fu Èr variable Schnittbedingungen angewendet. Der Verlauf der Schittleistung und -Kra Èfte wurde beobachtet, um sie voraussagen zu ko Ènnen, wenn die Ein¯uûfaktoren des Materials (Ke) und der Feuchte (Kh) bekannt sind. Wenn die Werte dieser Faktoren mit ausreichender Genauigkeit bestimmt sind, kann die Schnittkraft und Leistungsaufnahme beim Bearbeiten von MDF verschiedener Dichte leicht berechnet werden.

-The aim of this study is to investigate the influence of the physical and mechanical characteristics on the behaviour of wood during machining (for cutting process 90-0). In order to work on relatively homogeneous wood species, it was decided to use tropical woods. Different characteristics were measured: Physical (specific gravity, shrinkage) and Mechanical (hardness, fracture toughness, shearing, compression parallel to the grain), These characteristics were assessed separately to cutting forces involved during machining. Results obtained showed good correlations, particularly with very good results for fracture toughness parameters. Then, different formulations, based on statistical analysis, using all parameters, allowed to define a new material coefficient K m to predict the general behaviour of wood during machining and the cutting forces involved more precisely. It appeared clearly that the study of wood characteristics is a good means to improve knowledge on cutting condition optimisation, and to predict quality and efficiency of the cutting process.cutting forces / mechanical characteristics / tropical wood / specific gravity / wood machining Résumé -Étude des propriétés de 13 essences de bois tropicaux pour améliorer la prédiction des efforts de coupe en mode B. Le but de ce travail est d'étudier l'effet des caractéristiques physiques et mécaniques du bois sur son comportement durant l'usinage (dans le processus de coupe 90-0). De façon à travailler avec des essences de bois relativement homogènes, il a été décidé d'utiliser des bois tropicaux. Différentes caractéristiques ont été mesurées : physiques (infradensité, retraits) et mécaniques (dureté, ténacité, cisaillement et compression parallèle au fil du bois). Ces caractéristiques furent comparées séparément aux efforts de coupe induits lors de l'usinage. Les résultats obtenus ont montré de bonnes corrélations, avec notamment de très bons résultats pour les paramètres de ténacité. Puis, on a obtenu plusieurs formulations basées sur une analyse statistique, et utilisant l'ensemble des paramètres. Ces formulations, par l'intermédiaire d'un nouveau coefficient de matériau K m , ont permis de définir plus précisément le comportement général du bois au cours de l'usinage et les efforts de coupe induits. Il apparaît clairement que l'étude des caractéristiques du bois est un bon moyen pour améliorer les connaissances concernant l'optimisation des conditions de coupe, et pour prédire la qualité et l'efficacité du processus de coupe. efforts de coupe / propriétés mécaniques / bois tropicaux / infradensité / usinage du bois

Aged and whitened melamine-urea-formaldehyde (MUF) resins in a colloidal state were tested with parallel-plate rheometry to determine the extent of their viscoelastic behavior. Only in advanced colloidal states, and so only when aggregated colloidal clusters occurred, did the resins present clear indications of viscoelastic responses, as illustrated by the crossover of elastic modulus and viscous modulus curves at lower strain percentages. These colloidal clusters were labile microstructures, which, broken by applied shear, justified the known thixotropic behavior of these resins sufficiently advanced by aging or other means. MUF resins already in the colloidal state, but for which colloidal clustering had not yet occurred, behaved exclusively as viscous liquids. Two different cases of physical gelation were observed, reversible physical gelation and irreversible physical gelation, underlying which a true gel situation possibly occurred. Physical gelation due to colloidal superstructures occurred in both, but the difference in the resin average molecular masses revealed if the physical gelation was reversible or irreversible and, therefore, if the liquid/cluster separation was defined as the terminal phase of physical gelation.