NOVELTY - The over molding insert (10) has a base with several openings to insert several hollow projections (20). A pair of parallel side walls are provided to join the base and a pair of end walls (70,80) joined to the base, such that a partially closed space is formed and the ends of projections extend from the space.    USE - For use in manufacturing polymeric heat exchangers useful as solar collectors for heating swimming pools.    ADVANTAGE - Improves the bonds between manifold and conduits due to increased interfacial contact between conduits and insert forming manifold. Improves bond without resorting to higher than normal injection molding temperatures or preheating the inserted conduits. Achieves optimum performance in pumped or thermally induced siphoned flow systems. Withstands thermal and mechanical stress exerted on the heat exchanger, during operation. Efficiency is enhanced and reduces manufacturing cost of solar collectors.    DETAILED DESCRIPTION - The insert has base joined with side and end walls to form partially closed space from which the ends of the projections extend. The insert is made up of polymeric or thermoplastic material preferably a glass fiber reinforced polymeric material. The side and end walls have terminal edges which are not joined to the base and are raised to higher temperature than other portion of insert, when molten polymer is made to contact. The thickness of the base is the same as the thickness of terminal edges of the side and end walls. The corners formed by side walls and base, are chamfered or rounded. A taper is provided adjacent to the front opening of terminal of hollow projection or a step of greater diameter is formed in the cylindrical hollow projection. Several projections are arranged in coplanar and spaced longitudinally in the base.    INDEPENDENT CLAIMS are included for the following:    (1) Heat exchanger which includes manifolds connected to ends of each conduit and joined to over molding insert, such that the conduit is fitted over a hollow projection. The insert is located within manifold wall partially. The conduit is covered by a mass of the manifold material which extends outwards from outer periphery of the manifold. In an alternate arrangement, an enclosed chamber is formed between over molding insert and sub-plenum adapter insert. The enclosed chamber communicates with the internal space of the square or rectangular manifold through several openings and with circular hollow conduits through another openings, so that the manifold side openings have a smaller total area than that of the conduit side openings. The manifold is also formed to circular cross-section. The conduit is made from propylene copolymer, which is joined through a web in a coplanar orientation. Alternately, a mat with conduits, is connected by a web; and    (2) Heat exchanger manufacturing method comprises inserting projections into openings of insert and fitting an end of conduit over the projection. Molten polymeric material is molded over the insert and conduit end to form manifold and the projections are removed. The projection of the insert is connected to the conduit to form a sub-assembly before placing it in a mold.    DESCRIPTION OF DRAWING(S) - The figure shows the top elevation view of the over molding insert.    Over molding insert (10)    Hollow projections (20)    End walls (70,80)