Inflatable dorn

1. A mandrel for producing a fibre-reinforced composite part, which is a blade for a wind turbine, wherein the mandrel comprises:

an expandable body with an outer surface onto which a fibre material of the fibre-reinforced composite part is layable out,

wherein the expandable body is expandable in an expanded state, wherein in the expanded state the outer surface forms an interior shape of the fibre-reinforced composite part to be produced,

wherein the expandable body is collapsible in a collapsed state,

wherein the expandable body comprises a hull,

wherein the expandable body is expandable by pumping air into the hull,

a plurality of spacer elements installed in the hull,

wherein each spacer element is independently connected to two separate regions of the inner surfaces of the hull for defining a predetermined distance between the two separate regions of the inner surfaces in the expanded state, and

wherein the spacer elements are not in contact with each other in the expanded state of the expandable body.

2. The mandrel of claim 1 , wherein the expandable body ( 100 ) is collapsible by exhausting air outside the hull.

3. The mandrel of claim 1 , wherein the hull comprises at least an air inlet adapted for being connected to an air pump.

4. The mandrel of claim 1 ,

wherein the hull comprises a plurality of air-tight chambers , and

wherein each of the plurality of air-tight chambers comprises an air inlet adapted for being connected to an air pump.

5. The mandrel of claim 1 ,

wherein the expandable body comprises the hull and a foldable framework,

wherein the foldable framework is located in the hull in such a way, that the expandable body is expandable by unfolding the foldable framework and the expandable body is collapsible by folding the foldable framework.