The invention provides an injector having a suitably formed head which can partially penetrate through a lateral aperture in a spindle into the vicinity of a chamber containing an expansible capsule.Penetrating into the chamber, the injector laterally displaces the capsule and by injecting through a beak a fluid under pressure creates in the axial passage a negative-positive pressure necessary for transporting the filament.In the vicinity of the chamber there can advantageously be provided a mobile section which is circumferentially displaceable by a projection which is included on the head of the injector, when the injector penetrates or is penetrating the spindle.The injector is further provided with an arm which may be pivoted in the vicinity of the balloon container or on an auxiliary bench.