A method of producing a sandwich panel core of mineral wool fibers from a cured mineral fiber product includes the steps of providing a cured mineral fiber product with a dual density having a top layer with mineral fibers with a first density and a base layer having mineral fibers with a second density, the second density is at least two times lower than said first density. The top layer and the base layer have different fiber orientations. Further steps including cutting the mineral fiber product into a plurality of lamellae, rotating the lamellae 90 degrees, and re-joining the lamellae so that the top layers with the first density are vertically oriented.