Transparent polymer elements, containing both 3D-positioning structures and planar optical elements made by surface structuring, open the way for the mutual passive alignment of optical elements with respect to fibers, detectors and light sources in micro-optical benches with sub-micron precision. A fabrication process is presented for polymer inserts in micro-optical benches, which combines the mechanical precision of the LIGA-process with the wide variety of optical functions offered by diffractive optical elements. For this purpose, metal masters with lens elements made by surface structuring, and frame structures made with deep X-ray lithography and electroplating were used in a combined molding tool, and precision micro-optical elements were replicated by injection molding. The fabrication of the different parts of the mold insert and the alignment and fixing schemes for metal plates forming the micro cavity is described in detail. Injection molding experiments have been carried out using polycarbonate, a polymer known for its good optical properties. We discuss the different designs of mold inserts and injection geometries used for the mold, which were chosen in order to control the shrinkage of the molded element, to restrict damages during demolding, and to avoid inhomogeneities in the area of the lenses due to flow anisotropies and seam lines. We report on the characterization of the molded lens components. Injection molded lens structures are compared with hot embossed replicas, and used for the purpose of collimation applications. The imaging properties of these optical elements from single mode fibers onto single mode fibers is discussed. The miniature optical elements are arranged in arrays with 250 micrometers pitch which make them well suited for applications with fiber ribbons. Various positioning schemes and bench arrangements are under development.