the core of the bolt which fluid conduit 21 extends the entire length of the bolt 20 fitted into the injection fluid outlet end 22 of the fluid conduit 21 is a length of microtubing 23 through which the injection fluid is injected into the interior of the extruded polymeric fiber fig1 b is a partial cross sectional view of a capillary exit suitable for use in injecting a gaseous or liquid fluid into the interior of an extruded fiber as shown the at least one wall 11 which is disposed toward the bottom of polymer chamber 14 forms an opening 30 into which the injection fluid outlet end 22 of the fluid conduit 21 extends opening 30 is sized to accommodate injection fluid outlet end 22 and microtubing 23 extending therefrom so as to form an annular region 32 between the outer surface of the injection fluid outlet end 22 of fluid conduit 21 and the wall of opening 30 and between the outer surface of microtubing 23 and the wall of opening 30 which in the area surrounding microtubing 23 has a reduced diameter compared to the portion of opening 30 surrounding injection fluid outlet end 22 of fluid conduit 22 annular region 32 provides fluid communication between polymer chamber 14 and the capillary exit 33 enabling.