Preparation of aligned discontinuous fiber pre‐pregs by deposition from a suspension

In a process for production of aligned short fiber pre‐pregs, fiber suspension passes through a converging channel to emerge as a thin free‐surf ace sheet which is laid down onto a reciprocating horizontal filter surface. Following our previous work on fiber alignment in the channel, detailed studies are reported of the suspension sheet falling under gravity, and of the deposition stage. Close agreement is found between computed and experimental velocity profiles for sheets of fiber‐free liquid. Modification of the velocities resulting from the presence of fibers aligned in the flow direction is taken into account using an increased extensional viscosity. This is evaluated using pre‐existing theory which is tested experimentally and found satisfactory. A simple relationship between velocity and fiber alignment change in the falling sheet is derived and tested experimentally. Alignment is marred by surface disturbances to the sheet. Comments are made on the stability of the free‐surface sheet. An experimental study is made of alignment changes as fibers are deposited on the filter, and the results related qualitatively to operating conditions. In general there is little difference in alignment before and after deposition, except close to the leading edge of the filter where alignment loss occurs. Previous work is reviewed and recommendations made on optimum operating conditions.