A mandrel device (50) for manufacturing a hollow spar beam (30) for a segmented wind turbine blade (10) in a mould (100), the mandrel device comprising: a plurality of mandrel elements (52, 53, 54, 55, 56, 57, 58, 59) each comprising a substantially rigid material, and a plurality of attachment devices (60) configured for detachably attaching adjacent mandrel elements, wherein the mandrel device has an assembled configuration and a detached configuration, wherein: in the assembled configuration, the plurality of attachment devices is engaged so that the plurality of mandrel elements is detachably attached to each other, and the plurality of mandrel elements extends along a longitudinal axis of the mandrel device so that the mandrel device has a substantially box shaped cross-section that varies along the longitudinal axis, and in the detached configuration, the plurality of attachment devices is disengaged so that the plurality of mandrel elements is detached from each other, and the plurality of mandrel elements is inwardly collapsible.