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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 59 Figure 4. The runner THE SPRUE The sprue controls the fill rate of the casting and hence is the single most important component of the gating system. Whenever production practices permit, the sprue should be tapered with the smaller controlling area at its base, all subsequent parts of the gating system are determined from the sprue exit area. The sprue is tapered to compensate for the contraction of the falling stream as it accelerates down the sprue ensuring the stream of molten metal remains in contact with the walls of the sprue thus avoiding aspiration of air and mould gases. If the sprue is not tapered, the contracting stream will result in a low pressure gap forming between the metal stream and sprue walls causing air and mould gases to be drawn into the metal stream. The cross section of the sprue can be round, square or rectangular. There is evidence to suggest a rectangular shape is to be preferred due to a reduced tendency to vortex formation and subsequent air aspiration. If for no other reason, a square or rectangular shape may prove easier to construct than a round tapered section. Fig.2. THE SPRUE BASE As stream velocity will be at its maximum at the sprue exit, it is important to cushion the stream and allow the flow to change from vertical to horizontal with a minimum of turbulence. Recommended sizes for the sprue base are: a diameter 2 --3 times the sprue exit diameter and a depth equal to twice the depth of the runner bar. Fig.3. RUNNER AND GATES As previously mentioned, copper-base alloys should always be cast with a non-pressurised gating system with the runner in the drag and ingates in the cope. The junction of the sprue and runner bar is choked to compensate for the vena contracta which is created in the metal stream when it is forced to abruptly change direction. This contraction can cause air and mould gases to be drawn into the stream. The area of the runner bar should be proportioned so that the required maximum stream velocity is not exceeded, and the total area of ingates at least equal to the runner area. This is to ensure the required fill rate is achieved at the lowest possible velocity. Runner cross sections should ideally be rectangular, with a width to depth ratio of at least 2 to1. The wider upper surface is to maximise the potential of the runner bar to collect dross and inclusions. When multiple gates are taken off a runner bar, the area of the runner should be reduced by the area of each gate it passes to ensure flow from each gate is uniform. It is also good practice to incorporate a taper at the end of runners to minimize the risk of the initial metal stream recirculating back along the runner bar. Fig.4. Ingates should enter the mould cavity at the lowest possible level to avoid the turbulence caused by the falling metal stream. Ingates should also be rectangular in cross section rather than square so as not to cause a Figure 3. The sprue base.