A model for gap formation between dry preforms and curved tool surfaces in closed-mould composites manufacturing processes

The formation of a gap between the dry reinforcement and the tool surface at a bend, i.e. a geometrical feature with single curvature, in a tool for closed-mould manufacturing processes is caused by tensile forces in the reinforcement. The gap shape was analysed based on the mechanical model of an arched beam supported by an elastic foundation. The compaction stiffness of the reinforcement was derived from linearisation of the power-law approximating the reinforcement compression response. Fifth-order differential equations were derived and solved with the appropriate boundary conditions to find expressions for the local gap height. Applying a minimum-energy principle, the gap was found to typically extend into the flat sections of the geometry on both sides of the bend. Its size increases with increasing bend angle and applied force. It decreases with increasing bend radius, cavity height, level of pre-compaction, and stiffness of the reinforcement in compaction and in bending. If the gap height is expressed as a function of the fibre volume fraction, the correlation between the gap size and the fibre volume fraction was found to be negative if the tensile force is constant, and positive if the force increases with increasing fibre volume fraction. The solutions derived here allow the geometry of the gap and the bent preform to be described in more detail than in previous studies, which allows further analysis of the manufacturing process, e.g. predicting the effect of racetracking in the formed gap during a Resin Transfer Moulding process.