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Metal Casting Technologies : December 2007
BacktoBASICS T Gating of Copper-Base Castings he primary function of a gating system is to deliver clean dross-free metal from the pouring ladle to the mould cavity, and to do so in a manner, which will not cause subsequent reoxidation and gas pick-up. All copper-base alloys are subject to dross formation to some extent, some alloys, such as the aluminium bronzes are particularly susceptible. Of paramount importance with copper-base alloys is the need to introduce metal to the mould cavity in a non-turbulent manner, at the lowest possible velocity and to maintain an optimum fill rate for the casting in question. The optimum fill rate for a particular alloy cannot be considered as being a fixed value but depends on many factors including, casting weight, section thickness and casting shape. Optimum stream velocities vary with the alloy in question and can range from as low as 75mm/sec for the aluminium bronzes to more than 500mm/sec for phosphor bronzes. Excessively high stream velocities increase turbulence and the potential for break-up and entrapment of oxides on the metal front, causing at least a reduction in mechanical properties and often rejected castings. The often conflicting requirements of high fill rate and low stream velocity, particularly with short freezing range and strong dross forming alloys, often results in gating systems that may appear to be excessively large, far exceeding generally recommended gating ratios based on surface area. Copper- base alloys should ideally be cast through "non-pressurised" gating systems with ingates taken off the upper surface of runner bars; this is to ensure the runner bar is running full at all times. The distance between the sprue base and first ingate should be maximized to allow time for dross to float out and be trapped against the upper surface of the runner bar. Wherever possible, copper-base castings should be gated into or near the bottom of the casting to minimize turbulence within the mould cavity. THE POURING BUSH The use of a properly designed pouring bush is recommended on all but the smallest of castings. The pouring bush should be designed in such a way so that the pourer can fill the sprue quickly and maintain a near constant head throughout the pour. An offset design incorporating a weir achieves this objective. The pouring bush should be rectangular in shape so that the upward circulation during pouring will assist in dross removal. The exit from the pouring bush should be radiused to match the sprue entrance. A manually lifted pouring stopper is often used in the pouring bush, particularly with alloys such as aluminium bronze; this allows the pourer to fill the bush completely and gives time for dross to float out before the stopper is removed. The practice of pouring directly down the sprue or the use of conical shaped bushes which direct flow straight down the sprue is to be discouraged as not only will air and dross be entrained and carried down into the system, but also the high velocity of the metal stream will result in excessive turbulence in the gating system. Fig. 1. By J. F. Meredith, Casting Solutions Pty Ltd METAL Casting Technologies December 2007 58 Figure 1. The pouring bush Figure 2. The sprue.