A mould is provided for the injection molding of a plastic piece that includes at least two adjacent parts of two compatible plastic materials, assembled to one another by welding or gluing, of different types and/or colors. The mould includes a mould cavity defined by at least two non-planar walls and at least one retractable wall movable between extended and retracted positions, dividing the mould cavity into at least two compartments supplied separately with plastic. When extended, a non-linear edge of the retractable wall cooperates with a wall of the mould cavity in a play-free manner. The retractable wall includes panels joined in at least the extended or retracted position, movable in relation to one another so they can modify the shape of the non-linear edge of the retractable wall to adapt it to the shape of a second wall of the cavity in the retracted position.