The present invention relates to an apparatus for forming air-laid absorbent cores (13,14), comprising a first and second mat-forming wheel (1,2), means for transferring a core element on the first mat-forming wheel onto a core element on the second mat-forming wheel while the latter core element still is maintained in its mould, at least the air-laying means associated with the second mat-forming wheel comprises means for supplying a mixture of air-entrained fibrous material and discrete particles. According to the invention the apparatus comprise means for applying a protective layer to the bottom of each mould (4) of the second mat-forming wheel (2), which protective layer has the function of protecting the mould from discrete particles during air-laying of a mixture of air-entrained fibrous material and discrete particles, means for applying a web (15) of casing material on the peripheral surface of the first mat-forming wheel (1), whereby suction means (6) inside the first mat-forming wheel will draw the material in the web (15) to abutment against the bottom of a mould (3) passing said suction means, means for guiding said web of casing material onto the periphery of the second mat-forming wheel after having passed the nip between the first and second mat-forming wheel (1,2), and means for changing the synchronization of the mat-forming wheels in order to control the time at which the leading edge of a mould (3,4) on one of the mat-forming wheels passes the nip in relation to when the leading edge of a mould on the other mat-forming wheel passes the nip. The invention also relates to a method of producing cores with different sizes without change of moulds.