Spinneret

A spinneret core comprising a seat for a capillary insert, a feed port for a center fluid connected to a lateral opening of the seat, a chamber for a center fluid located below the seat, a capillary in fluid communication with the chamber and located at the base of the chamber;

• the seat being in fluid communication with the feed port and the chamber;
• and a capillary insert comprising a threaded section at its lower end arranged in the seat, the outer diameter of the threaded section matching the inner diameter of the seat and the thread of the threaded section and the inner wall of the seat together defining a spiral capillary, the capillary insert being vertically moveable in the seat by an actuator,
• wherein the shaft of the capillary insert comprises a first cylindrical section adjacent to the threaded section and a second cylindrical section adjacent to the first cylindrical section, the second cylindrical section having a diameter which is smaller than the diameter of the first cylindrical section, so that a circumferential ledge is formed on the topside of the first cylindrical section, at the bottom of the second cylindrical section.