Method for making an oscillating weight made from composite materials

A method for making a oscillating weight made from composite materials, the oscillating weight being provided to drive an arbor of an automatic winding mechanism of a self-winding watch, and including a central part made from a first composite material and a heavy sector made from a second composite material charged with heavy metal particles, wherein the central part and the heavy sector are joined concentrically to one another by a substantially circular arc-shaped lateral rib belonging to the central part, a rebate being formed in an upper edge of the central part above the lateral rib, the lateral rib including an alternating sequence of first sections that have an undercut part and second sections devoid of undercut parts, the method comprising:

• forming the central part with the lateral rib and the rebate by injecting the first composite material into a mold;
• overmolding the heavy sector onto the central part by injecting the second composite material into a mold containing the central part, so that the second composite material fills the rebate and a gap underneath the lateral rib, so that the lateral rib is embedded in the second composite material and the central part and the heavy sector are interlocked.