Apparatus for forming fibers or flakes of material comprises means (1) for producing a heated stream of molten material (9), means for feeding the stream in a substantially vertically downward direction and means (7) for receiving the downwardly directed stream and for forming fibers or flakes therefrom. The apparatus includes means (11, 13, 15, 17) for effecting a change in the temperature of the stream subsequent to the production thereof whereby fibers or flakes of a desired thickness are obtained. Instead of, or in addition to, the temperature changing means, there may be provided means for controlling the mass or volume flow of the stream. The invention also provides methods corresponding to the apparatus.