Abstract: A method for manufacturing a wind turbine blade is provided, including the steps of: a) arranging a fiber lay-up on a mandrel tool, the mandrel tool including a frame and, as seen in cross-section, at least two mandrel portions connected to the frame, and wherein at least a portion of the fiber lay-up is supported by an outer surface of the at least two mandrel portions, b) reducing a cross-section size of the mandrel tool by retracting at least one of the mandrel portions towards the frame, c) arranging the mandrel tool inside adjacent blade sections, d) increasing the cross-section size of the mandrel tool by extending at least one of the mandrel portions away from the frame, and e) infusing at least a portion of the fiber lay-up with a resin and curing the resin to obtain a cured joining portion joining the blade sections inside.