Abstract: An apparatus for converting thermoplastic molten material stream into fibers to produce fibers, particularly to produce mineral wool from molten mineral raw material. The device combines a slinger cup wheel with a pair of fiberizing rotors. The rotors have each a concavely rounded peripheral groove. The cup wheel is virtually completely surrounded by the two grooves of the rotors so that there is no random spreading of the molten material as all of discrete, fine streams produced by the cup are attenuated by the rotors. As there is no sudden "bouncing" of the attenuated material, the invention produces an improved uniformity of the diameter of final fibres and substantially reduces the shot content in the final product. The stress to which the machinery is subjected is also limited.