The tube is at least two inches long and has successive sections of progressively smaller diameters to produce a correspondingly shaped sprue passage communicating with a pattern cavity in a casting investment, so that fluid casting material passes with increasingly fluid velocity between an inlet and outlet of the sprue passage during the casting process. A bulbous portion of the tube is spaced from its free end, to form a reservoir chamber in the sprue passage, spaced sufficiently from the pattern cavity, so that casting material flowing through the sprue passage attains maximum velocity at the outlet of the passage entering the pattern cavity.
