The mandrel has a cantilever-type mounting and includes a rotatable longitudinally slit tubular steel shell providing a molding surface on which thermosetting resin and glass fiber reinforcing material may be deposited to form tank bodies or pipe. A row of pneumatic actuators mounted inside the shell and spaced axially thereof includes piston rods attached to the shell along the slit on one side thereof. Retraction of the piston rods collapses the shell, which is flexible over the greater portion of its periphery and supported in the flexible portion by a plurality of rows of pivotable turnbuckle connectors or support rods, for removal of a cured tank body or pipe.