PURPOSE: To improve distribution of a reaction mixture to achieve a mixture film of uniform thickness having a short residence time spectrum by performing the distribution by means of a stream of air produced by ventilation above a substrate.

CONSTITUTION: A device 1 consists of two symmetrically positioned ventilator rolls 7, 9 which are in mirror image arrangement and have smooth surfaces. The rolls 7, 8 have drives 9, 10 connected thereto. These ventilator rolls 7, 8 are replaceable, and have their greatest diameter downstream of the mixture feed point 11 at a location which corresponds to an extension thereof. The diameter of ventilator rolls reduces like a truncated cone towards the limits for a foam width. The drives 9, 10 are mounted to swivel around their axes in the vertical plane at right angles to a conveyor belt 2, so that the opening angle of gap between the ventilator rolls 7, 8 and the top layer of paper 15 conveyed on the conveyor belt 2 can be adjusted according to requirement. The ventilator rolls 7, 8 are also mounted such that they are adjustable in height, by positioning the drives 9, 10 on supports 16 of adjustable height. These are mounted on blocks 18 which can be pushed longitudinally on tracks 17, so that the distance between the ventilator rolls 7, 8 and the feed point 11 can be selected.