between adjacent horizontal surfaces without interfering with the desired mating engagement between surfaces 40 and 16 the vertical dimension of spacers 26 and 38 are selected to locate these surfaces in engagement the upper member 46 has an aperture 70 which meets a corresponding aperture 72 in the lower member 50 and the support rod 23 passes through the apertures 70 and 72 to support the chamber member 39 against the spacer 38 the inlets 60 62 to each plenum chamber are connected by flexible air supply tubes 74 to air control cylinders 76 76 another cylinder 76 not clearly shown also support a manifold 78 which is also connected to a neck ring cooling head 80 which directs cooling air onto a neck ring mechanism 82 of the parison forming mechanism only shown diagrammatically the mechanism according to the invention offers improved control over mold cooling a separate adjustable air control in each air control cylinder 76 controls the supply of cooling air to the neck ring cooling and to each of the plenum chambers independently in general in a blank mold mechanism the molds near the mold pivot become hotter when the machine is in use than those further away from the pivot and in the present construction the supply of cooling air to the two mold members can be adjusted to allow for this if it is desired to change the height of the mold member in the machine the position of the mold member can be adjusted by choosing spacers 26 38 of appropriate size such spacers being effectively simple collars are much less expensive than the mold hangers generally in use further it is possible to ensure that the clamping forces exerted on the mold members by the mold arms 4 6 are located near.