Thermoforming woodfibre–polypropylene composite sheets

Abstract Thermoforming of woodfibre–polypropylene composite sheets made without any modification of the fibres or the polymer is the focus of this paper, the emphasis being on their formability and the associated issues. Both the degree to which a material conforms to the desired part geometry after deformation and the extent to which a sheet material may be deformed before unacceptable defects occur are considered. Four thermoforming processes such as V-bending, die-match forming, air pressure forming and deep drawing have been utilised to examine both single-curvature and double-curvature deformation conditions. The technique of Grid Strain Analysis (GSA) has been applied to quantify differences in strain distributions during sheet deformation. The effects of thermoforming process parameters and sheet composition on sheet formability are also discussed. Notably, this study considers composite sheets reinforced with wood fibres rather than woodflour, enabling the study of fibre layup and fibre interlocking effects. While the tensile strengths of the composite sheets increase marginally the stiffnesses increase significantly compared to those of unreinforced polypropylene. The key deformation mechanism for layered woodfibre–polypropylene composite sheets is inter-ply shear while intra-ply shear dominates the deformation of homogeneous sheets. Forming temperature and blank size have the most pronounced effects on the formability of these composite sheets.