NOVELTY - Forming composite shell structure having reinforced fiber material embedded in cured resin, involves arranging first material comprising number of fiber layers on preform forming surface (13) to form preform; bringing first mold part (10) and preform forming part (12) together to assembled position so that first mold part molding surface (11) faces preform forming surface and a cavity is defined between the surfaces and accommodates fiber material arranged on preform forming surface; and transferring preform to first mold part by releasing fiber material from preform forming surface.    USE - For producing a composite shell structure such as a shell part of a wind turbine blade or a wind turbine blade (claimed).    ADVANTAGE - By forming a preform of the fiber material in a separate preform forming part and then transferring the preform to the first mold part, the so-called form time, i.e. the time during which the mold is occupied for producing a single composite shell structure, is reduced considerably compared to known methods, where the laying-up of the fiber material also takes place in the mold. The method produces large shell structures, as the time for arranging the fiber material is longer than molding time, i.e. the time for injecting/infusing the resin and curing the resin when using the resin transfer molding (RTM) or by vacuum-assisted resin transfer molding (VARTM) and the time for heating and thus liquefying the resin and curing the resin when using prepreg.    DETAILED DESCRIPTION - Producing a composite shell structure comprising a reinforced fiber material embedded in a cured resin, involves providing a first mold part (10) having a concave mold part molding surface (11) with a contour defining an outer surface of the composite shell structure; providing a perform forming part (12) having a convex and upwardly facing preform forming surface (13) being shaped essentially complementary to at least a part of the first mold part molding surface; forming a preform by arranging a first material comprising a number of fiber layers on the preform forming surface; bringing the first mold part and the preform forming part together to an assembled position so that the first mold part molding surface faces the preform forming surface and a cavity is defined between the surfaces and accommodates the fiber material arranged on the preform forming surface of the preform forming part; transferring the preform to the first mold part by releasing the fiber material from the preform forming surface so that it is received on the molding surface of the first mold part; removing the preform forming part from the first mold part; forming a mold cavity by arranging a second mold part on top of the fiber material; and curing and/or allowing the resin to cure, where the resin is either pre-supplied to or supplied to the fiber material when the latter is arranged on the preform forming surface in the preform forming step or supplied to the mold cavity following the mold cavity forming step. The fiber material comprises a resin-impregnated material, i.e. a so-called prepreg material. The fiber material only comprises a prepreg material. The resin is supplied to the mold cavity which is evacuated. The second mold part is a flexible mold part formed of a flexible material, preferably a polymer film. In the preform forming step, cores, such as balsa wood or foam cores, are added to the fiber material and then to the preform. Subsequent to the preform forming step the method involves arranging polymer foils on top of the preform.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of a step, in which first part mold has been turned upside down and brought together with the preform forming part into an assembled position.    First mold part (10)    First mold part molding surface (11)    Preform forming part (12)    Preform forming surface (13)    Lower fiber layer (15)    Main laminate (16)    Leading edge fiber reinforcement (17)    Trailing edge fiber reinforcement (18)    First core material (19)    Second core material (20)    Upper fiber layer (21)