12 an elongate supply duct 60 is secured to the boss 46 by means of a cap 50 that traps an integral shoulder 62 between the end 48 of boss 46 and the underside of cap 50 the duct 60 extends through the ejector plate assembly 24 and core 14 to project into the cavity 22 and is formed with a tubular wall 63 that defines an internal bore 64 gas from the passage 42 may flow through chamber 44 and along the bore 64 into cavity 22 a valve member 66 is slidably mounted within the bore 64 and as can best be seen in fig3 and 4 includes a cylindrical head 68 carried on an elongate needle 70 the head 68 extends radially over an end face 65 of the duct 60 to seal the internal conduit 64 the inner surface of wall 63 includes a cylindrical portion 67 adjacent the end face 65 and a diametrically enlarged portion 69 on the balance of its length as shown in fig3 the needle 70 is generally cylindrical to conform to the inner surface of the cylindrical portion of wall 63 and has a pair of flats 72 formed to allow passage of gas along the bore 64 the needle 70 is radially.