Processes for the formation and further treatment of monofilaments are fundamentally known. The known process steps are described in detail in the Handbuch der Kunststoff-Extrusionstechnik [Manual of Plastics Extrusion Technology] II, Carl Hanser Verlag Munich, Vienna, 1986, pp. 295 to 319. According to this source, thermoplastic monofilaments (having a diameter greater than 60 .mu.m) may be produced by spinning, for example in water, at a maximum delivery speed of 600 m/min.
Monofilaments of substantially smaller cross section and multifilaments are spun directly in air at a markedly greater delivery speed using different processes. Thus, German Offenlegungsschrift DE 41 29 521 A1 describes an apparatus for high-speed spinning of multifilaments at winding speeds of at least 2000 m/min., in particular at least 5000 m/min.
By contrast with the process according to the invention, multifilaments are in this case spun in air and wound directly. A particular feature of this patent is the cooling device. It comprises a porous tube which is open in the direction of spinning and is disposed concentrically to the spinning line. The high winding speeds obviate the need for active supply of a cooling medium. The process described therein relates to filament yarns having single filament titres of from 0.1 to 6 dtex, and is not applicable to monofilaments of a diameter greater than 50 .mu.m (approx. 20 dtex).
International Application WO 91/11547 discloses a process and an apparatus for high-speed spinning of monofilaments having a single titre of from 1 to 30 dtex (corresponding to approx. 10 to 55 .mu.m). The melt-spun monofilaments are in this case cooled with blown air, drawn over a friction element, provided with a spin finish and wound onto bobbins at a speed of up to 6000 m/min. This process differs from the process according to DE 41 29 521 A1 only in terms of the active cooling of the monofilaments by blown air and in the friction element by way of which the filament tension is influenced.
Both direct spinning-stretching processes (DE 41 29 521 A1 and WO 91/11547) are as a matter of principle restricted to small monofilament diameters (O&lt;55 .mu.m) by the unfavourable nature of heat removal resulting from air cooling and poor internal thermal conduction in the wire.
The object of the present invention is to provide a continuous process for melt-spinning monofilaments having a diameter of from 60 to 500 .mu.m from filament-forming polymers, in particularly polyamide, in which spinning takes place in a cooling bath and which, despite the high delivery speed of from 600 to 4000 m/min, is still controllable, in particular while passing through the cooling bath, and which affords a filament quality at least comparable to that of the production processes known hitherto (at a delivery speed of from 200 to 400 m/min; see Handbuch der Kunststoffextrusionstechnik II, Hanser-Verlag (1989) Patents, Knapp, Hensen Chapter 10).