80 prevents heating of the cool mould inserts 50 as illustrated in fig1 the centring body 80 which is all in all rotationally symmetrical with respect to the longitudinal axis l comprises a broad flanged edge 81 and a narrow cylindrical collar portion 82 the latter is inserted into the melt channel 30 and in the nozzle mouthpiece 40 respectively from the bottom side and reciprocates in them in longitudinal direction at its end facing the mould cavity the centring body 80 forms a cylindrical end section 84 which engages in a longitudinally displaceable manner into a cylindrical seat 54 said centring seat 54 formed by the mould inserts 50 is concentrical with respect to the gate 51 and may have a slightly conical form in an upper section 55 in order to facilitate the introduction of the centring body 80 with its end section 84 during assembly its outer diameter is selected so as to provide a pressure tight connection and to prevent the melt from entering the air chamber 22 simultaneously longitudinal movements of the centring body 80 in the seat 54 are possible so that thermally induced changes in length of the needle valve nozzle 10 can be compensated if necessary the end face 85 of the end section 84 is preferably flat just as the bottom surface not specified of the centring seat 54 so as to provide an accurate fit it will be noted that the melt channel 30 is continued through the.