UNION lines depend on the desired results provided by customers. If the productivity is below 500 kg/h, a twin-screw extruder with slow unidirectional rotation can be used; if higher, up to 2000 kg/h, then a tandem system can be used. This is due to the fact that it is necessary to separate the first part of the process (melting, mixing and injection of foaming gas) in the first extruder with a rapidly rotating screw from the second part (cooling of the molten material) in the second extruder with a slowly rotating screw.