A mold for manufacturing a turbomachine fan casing made of composite material, includes a main axis mandrel around which a fibrous preform of a fan casing is intended to be wound; a plurality of counter-mold angular sectors assembled on the outer contour of the mandrel which are intended to close the mold and to compact the fibrous preform wound on the mandrel; wherein a flat seal with a main elongation axis directed along the main axis is arranged between each angular sector, the flat seal being compressed between two adjacent angular sectors, a first angular sector including a sealing portion passing below a lower face of the flat seal while a second angular sector including a sealing portion passing above an upper face of the flat seal.