Method of manufacturing injection molding manifold with plugs

A method of manufacturing an injection molding manifold having a body with first and second parallel opposing surfaces and first and second opposing ends, the manifold having a hot runner passage which extends from a central inlet on said one surface and branches out from an upstream portion at at least one junction to a plurality of spaced outlets on said second surface, the outlets being smaller in diameter than the inlet, comprising the steps of:

• (a) forming the manifold body of a suitable steel to predetermined dimensions;
• (b) drilling a longitudinal duct longitudinally through the manifold body from the first end to the second end, the longitudinal duct having a predetermined diameter and extending parallel to the first and second surfaces;
• (c) drilling a transverse inlet duct to form said upstream portion of the hot runner passage, the transverse duct extending from the inlet to centrally intersect the longitudinal duct to form the junction;
• (d) drilling the longitudinal duct adjacent each end of the manifold body to form enlarged diameter end portions extending from a central portion of the longitudinal duct, each enlarged diameter end portion joining the central portion of the longitudinal duct at a circumferential shoulder;
• (e) forming a pair of cylindrical steel plugs to fit into the end portions of the longitudinal duct, each plug having a cylindrical outer surface with at least one circumferential groove extending therearound, and a concentric well extending partially therethrough, the well tapering inwardly from a mouth at one end to a smaller blind end, the mouth being substantially equal in diameter to the central portion of the longitudinal duct;
• (f) locating brazing material in each of the circumferential grooves, inserting one of the steel plugs into each of the end portions of the longitudinal duct with the said one end abutting against one of the circumferential shoulders and the mouth of the well in alignment with the central portion of the longitudinal duct, and heating the manifold in a vacuum furnace for a sufficient period of time and at a temperature whereby the brazing material melts and flows around the cylindrical outer surface of the plug to securely fix the plug into the respective end portion of the longitudinal duct in order to effect a seal against leakage, during injection molding, of pressurized melt around the plug;
• (g) drilling a pair of diagonal outlet ducts which are smaller in diameter than the central portion of the longitudinal duct, each diagonal duct extending through the manifold body and plug to connect one of the outlets on the second surface of the manifold body to the blind end of the well in a respective one of the plugs;
• and
• (h) smoothly finishing the junction thereby created in each of the plugs between the longitudinal duct and the respective diagonal outlet duct.