A mould assembly comprising: - a first mould part with a first mould opening; - a second mould part with a second mould opening where the second mould part is a separate mould part in relation to the first mould part; - a rotating means for rotating at least the second mould part from a position in which the second mould opening faces upwards to a position in which the second mould opening faces downwards; - a crane for moving the second mould part relative to first mould part to a position where the second mould part is located, above the first mould part and for moving the first mould part and the second mould part towards each other with the first and second mould openings (2a, 2b) facing each other until the first and second mould parts (1a, 1b) engage, - a support assembly for supporting at least the second mould part, the support assembly comprising the rotating means and characterised in that the support assembly comprises as well a guiding means for guiding at least the second mould part when it is rotated and/or moved, where the guiding means is prepared to guide the second mould part in a pre-established horizontal and vertical direction during moving the second mould part relative to first mould part by means of the crane such that the second mould part is moved upwards along a linear path and horizontally along a linear path and prepared to allow a 180° rotation of the second mould part, and wherein - the first mould part and the second mould part together constitute a negative form of a wind turbine rotor blade with a blade root section where the first and the second mould part (1a, 1b) each comprise a blade root mould section; - the support assembly is connected to the blade root mould section of at least the second mould part. - a first mould part with a first mould opening; - a second mould part with a second mould opening where the second mould part is a separate mould part in relation to the first mould part; - a rotating means for rotating at least the second mould part from a position in which the second mould opening faces upwards to a position in which the second mould opening faces downwards; - a crane for moving the second mould part relative to first mould part to a position where the second mould part is located, above the first mould part and for moving the first mould part and the second mould part towards each other with the first and second mould openings (2a, 2b) facing each other until the first and second mould parts (1a, 1b) engage, - a support assembly for supporting at least the second mould part, the support assembly comprising the rotating means and characterised in that the support assembly comprises as well a guiding means for guiding at least the second mould part when it is rotated and/or moved, where the guiding means is prepared to guide the second mould part in a pre-established horizontal and vertical direction during moving the second mould part relative to first mould part by means of the crane such that the second mould part is moved upwards along a linear path and horizontally along a linear path and prepared to allow a 180° rotation of the second mould part, and - the first mould part and the second mould part together constitute a negative form of a wind turbine rotor blade with a blade root section where the first and the second mould part (1a, 1b) each comprise a blade root mould section; - the support assembly is connected to the blade root mould section of at least the second mould part.