NOVELTY - Making composite structure involves: a) arranging first fiber material in first forming surface of first outer mold part; b) arranging pre-fabricated reinforcement element on top of material and first and second bonding surfaces; c) arranging second fiber material in second surface of second outer mold part; d) sealing polymer foil above second material; e) closing first and second parts; f) forming mold cavity by arranging first (140) and second vacuum bags (144) between mold parts and element side; g) evacuating mold cavity; h) providing resin in mold cavity; and i) curing resin.    USE - For manufacturing a longitudinally extending composite structure comprising a wind turbine blade (claimed).    ADVANTAGE - The method is simple method of manufacturing a composite structure, such as a wind turbine blade, in one shot. The reinforcement element is used to retain the fiber material against the mold part during layup. The first bonding surface of the reinforcement element, bonds to the first fiber material, and the second bonding surface of the reinforcement element, bonds to the second fiber material. By using a pre-fabricated reinforcement element, it is ensured that the reinforcement element can be arranged with high precision without the risk of the reinforcement element buckling, thus ensuring a mechanical strength of the wind turbine blade. The in-situ polymerizable thermoplastic materials, has the advantage that they obtain a water-like viscosity when heated to approximately 150 degrees C, thereby, it is possible to quickly impregnate the fiber material of very large composite structures to be molded and subsequently curing the resin in very short cycle times. The composite structure made by this method does not contain any bonding lines, which can later lead to hairline fractures.    DETAILED DESCRIPTION - Manufacturing a longitudinally extending composite structure including a shell part comprising a fiber reinforced polymer material including a polymer matrix and fiber material embedded in the polymer material, where the shell part is manufactured in a closed mold comprising at least a first outer mold part having a first forming surface and a second outer mold part having a second forming surface, where the method involves: a) arranging a first fiber material in the first forming surface of the first outer mold part; b) arranging a pre-fabricated longitudinally extending reinforcement element, such as a beam or a web, on top of the first fiber material, the reinforcement element having a first side and a second side, and a first bonding surface and a second bonding surface, the reinforcement element being arranged so that the first bonding surface faces towards the first forming surface; c) arranging a second fiber material in the second forming surface of the second outer mold part; d) sealing a polymer foil above the second fiber material so as to retain the second fiber material against the second forming surface; e) closing the at least first outer mold part and the second outer mold part so as to form the closed mold, and so that the second bonding surface of the reinforcement element faces towards the second forming surface; f) forming a mold cavity by arranging a first vacuum bag (140) between the first outer mold part, the second outer mold part and the first side of the reinforcement element, and arranging a second vacuum bag (142) between the first outer mold part, the second outer mold part and the second side of the reinforcement element; g) evacuating the mold cavity; h) providing a resin in the mold cavity simultaneously with and/or subsequently to steps a) and c); and i) curing the resin in order to form the composite structure. The first fiber material and/or the second fiber material comprise a prepreg material. The resin comprises an in-situ polymerizable thermoplastic material.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross section view of the first and the second mold part in the closed mold configuration.    First vacuum bag (140)    Second vacuum bag (144)