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Wood is as valuable an engineering material as ever, and in many cases, technological advances have made it even more useful. Forming of veneers, in particular roll forming of veneers to produce profiles is an innovative idea to add value to the end product. This thesis describes the roll forming of plywood using heated rolls; the continuous nature of the process coupled with its versatility and speed makes it an extremely attractive technique for producing light weight, Structurally efficient components from sheet materials. Many advances in wood technology have been arrived at empirically, but, having discovered some potential for improvement, it is usually necessary to rationalise and optimise the process by which this is achieved. To do this, knowledge of the wood Structure and understanding of the complex processes of wood formation are essential. In view of this a detailed literature survey on the material has been included. The formability characteristics of extruded thick woodfibre-polypropylene composites have been understood by conducting single-curvature vee-bending tests as a prelude to thermoforming or profile manufacturing by roll forming process. Proper forming temperature window has been established by conducting differential scanning calorimetric heating and cooling scans. A suitable bend radius to sheet thickness ratio has also been established to aid the design of forming tools. Furthermore, the mechanical properties of the extruded woodfibre-polypropylene composite sheets have been studied in pre-bending and post-bending conditions, and compared with the properties of neat polypropylene. The findings of the mechanical test results have been supplemented by extensive optical and scanning electron microscopy of the fractured samples. The surface strains experienced by the samples due to vee-bending have been understood by carrying out grid strain analysis. Wood veneer is highly anisotropic natural composite consisting of fibres interconnected by a predominantly lignin binder. Thermoforming of softened veneers can result in undesirable distortion owing to large differences between in-plane and out-of plane dimensional changes resulting from variations in temperature and moisture content. Therefore, to manufacture a product of desired geometry, it is essential to know the formability characteristics in terms of spring-back and spring-forward properties, and the ratio of minimum bend radius to veneer thickness so that a compensatory tool can be designed. In view of this, vee-bending of softened single, two ply veneer, and three ply veneer sheets (plywood) using heated dies has been carried out. The information obtained from vee-bending experiments has been extended to thermoforming of veneers into profiles by means of matched-dies. In addition, the grid strain analysis of the formed samples has been included. The mechanical properties of plywood and sandwich panels have been obtained by performing tests. Having understood the formability characteristics of veneers, the profile manufacturing of three ply veneer sheets (plywood) by roll forming has been studied. Prior to the actual roll forming process, the roll schedule and pitch length have been decided based on the deformation length study. This thesis describes an energy minimisation technique to predict the deformation length and the modified equation agrees well with the deformation length measurements from the static roll bend tests. The results suggest that the deformation lengths, Iike in metals and polymeric composite sheets, depend essentially on the geometric parameters of the profiles produced and are practically independent of the mechanical properties of the work material. Further, the macroscopic deformation of veneer sheets due to roll forming has been analysed kinematically using grid strain analysis and the geometric conformance of the product has also been verified. In addition, the real-time strain measurements by strain gauges bonded on the roll formed specimens give the real-time strain distributions which compare well with the trends obtained for metallic and continuous fibre-reinforced thermoplastic sheets. |
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