Metal Casting Technologies : Whos who September 2012
54 www.metals.rala.com.au maintain the choke (the point of maximum velocity) at the correct point in the gating system. This will generally ensure the correct rate of flow in all portions of the gating system, with liquid metal delivered at the required flow rate into the casting cavity. The next piece of information required for gating calculations is the height through which the metal will drop. In the case of horizontal gating, this is the effective height of the sprue (the vertical pipe down which the metal initially flows). For vertical gating, this may be the cumulative height from the top of the mould down to each component. In any case, the velocity of the metal after falling through this height can be calculated from a fairly simple relationship: V = √(2gH) Where: V = velocity, g = acceleration of gravity H = height through which the liquid has fallen This formula is based on basic Newtonian physics, and describes the velocity of any body free-falling in a gravitational field. Now, given the known velocity and the known volumetric flow rate, the cross-sectional area of flow of the liquid metal can be calculated simply from the following equation: Flow Area = Volumetric Flow Rate/Velocity This is the basic calculation which is used in gating design. When calculating flow areas, consideration must also be given to shape efficiencies and friction losses. According to research, for example, a square tapered sprue has an efficiency of around 74%; this means that an area calculated according to the above formula must be increased by a factor of (1/0.74) or 1.351 to account for the energy losses associated with flowing through this type of shape. Also, in flowing through runner systems, the liquid metal loses energy through friction with the channel walls. This friction loss, which is usually expressed as a percentage, must be compensated for by increasing the area of the downstream runner segments. Balancing flow rates on all levels As the flow rate at each level is proportional to the height the metal stream has fallen and the cross sectional area of the choke at that level, it follows that a countermeasure to variable flow rates at different levels in the mould is to proportion the choke Fig 4. Mould Filling sequence at 50% filled Fig 6. Mould Filling sequence at 90% filled Fig 5. Mould Filling sequence at 75% filled Fig 7. Colour representation of Fill Time.
Whos who September 2011