Abstract: The mixing apparatus, which is constructed like a known mixing apparatus with return grooves (18, 19) in the control piston (2), has return grooves (18, 19) of a length which is equal to the distance between the inlet and return openings (4to 7) to be connected by it, the control piston (2) being able to be moved into an intermediate position (B), in which the return grooves (18, 19) connect the inlet openings (4, 5) to the assigned return openings (6, 7), and into a further advanced clearing position (C), in which the return grooves are no longer connected to the return openings. Due to the short return grooves, the length of which is no longer dependent o the length of the mixing chamber or of the outlet pipe of the mixing chamber, the overall length of the mixing apparatus is shortened. It is then only twice the stroke of the control piston. The neck region of the control piston is shortened, as a result of which the area of adhesion is reduced.

Abstract: A method and apparatus for metering and mixing multi-component plastics, in particular polyurethane, in which two or more hydraulic piston and cylinder units (A.sub.1 and A.sub.2) for each component, operate periodically by a balanced procedure. The pressure for the second piston and cylinder unit which takes over metering is equilibrated, before the beginning of the metering procedure, with the pressure of the first piston and cylinder unit which is in the process of metering. Switching from one piston and cylinder unit to the other (e.g. from A.sub.1 to A.sub.2) takes place in such a way that metering is taken over by the second piston and cylinder units at the same time as the first piston and cylinder unit is switched off. This is achieved by providing a pilot valve which has zero overlap and inevitably releases the component stream from the first piston and cylinder unit at the same time as the other piston and cylinder unit is switched off.

Abstract: The present invention relates to a process for the preparation of coatings by reacting polyurethane single component systems, which preferably contain at least one prepolymer containing NCO groups in bonded form and a latent hardener, whereby the polyurethane single component system is mixed with steam at a temperature from 100.degree. C. to 500.degree. C., and fed under a pressure of 0.5 to 10 bars, and finally sprayed using a general spray method. The coatings so formed cure bubble free independent of the moisture content of the surroundings and without exhibiting coating mass runoff.