The injection molding apparatus is provided for making highly accurate optical parts having structured reflective or deflecting optical surfaces. The apparatus includes a removable mold insert made of two or more mold insert segments. The mold insert segments are juxtaposed at mating lateral planar surfaces to form a reversed structured surface. The mold insert segments also includes a gas venting circuit having conduits formed when the mold insert segments are juxtaposed. A cooling circuit is provided in the mold insert segments.