Analysis of Flow of Molten Metal, a foundation in the Gating System Design on Castings

Gating System Design on Castings is dictated by the flow of molten metal inside mold.

During my training in Japan in the Field of Foundry Engineering, I have learned that the flow of liquid can either be laminar or turbulent. In the laminar flow, the particles of fluid all proceed smoothly, parallel to the direction of flow. Turbulent flow is characterized by irregular movement of the particles of fluid. In the turbulent flow, air and mold sand are likely to be included and the dross inclusions does not float up.Therefore, it is of utmost importance that turbulence should be minimized if not prevented in the pursuit of attaining a sound casting free of defects. Generally, turbulent flow will occur when the velocity or direction of flow is changed, specially a sudden change of flow creates turbulence. In the principles of fluid mechanics, the flow of all fluids in ducts can be related by their Reynolds Number(N) and is equated to the fluid density(d) multiplied by the average flow Velocity(V) further multiplied by the diameter(d) of the duct and the whole divided by the viscosity(v) of the fluid (N=dVD/v).

Reynolds Number less than 2000 is laminar; 2000 to 4000 is a little bit turbulent but not harmful; greater than 4000 is turbulent. Usually Reynolds Number of about 2000 to 20000 are obtained in Foundry Practice, so nearly always turbulence appears in the classical sense. But its degree must be reduced to eliminate its harmful effect to metal quality with well-designed gating system. For light metal and bronze, the tendency of turbulence is high; for cast steel and nodular iron, the tendency is medium; and for cast iron and brass, the tendency is low.

The velocity of flow depends on the pouring rate of molten metal, the height of the ladle, and the gating system of the mold. In the gating system, the velocity of flow will change according to the law of continuity which states that for a system with impermeable walls and filled with incompressible fluid, the Rate of Flow(Q) is equal to the Cross Sectional Area(A) of the gate times the Velocity(V) of flow: Q=AV.

The effect of the height of the flow on the velocity follows the principle of the conservation of energy, known to be the Bernoulli Equation, which expresses that if no frictional losses are involved, the sum of potential energy, the kinetic energy, and the pressure energy terms for any point in the same system is equal to that for any other point.

Scientific Basis for Risering Design for Steel Castings

In one consultation with my foundry clients regarding their casting defect problems and upon evaluating their methoding practice, I have discovered that they were confused in applying density values in the risering design calculations for steel casting that is why the problem on shrinkage cavity. Wrong density choice may result to rejection due to either shrinkage or over design resulting to non-economical yield. Although mild shrinkages in steel castings might be repairable through build-up by welding, still some additional cost will be incurred, an issue on economics.

Three interesting density values of steel must be taken into mind:
1) Just above melting point for liquid low carbon steel is, 0.115 kg per cu in, as basis in calculating weight of riser in its liquid form from riser dimensions;
2) Just below the freezing point of solid steel is, 0.1195 kg per cu in, as basis in calculating casting weight from pattern dimensions; and
3) To calculate weight of casting from casting dimensions (not mold or pattern dimensions), use 0.129 kg per cu in

Erroneous density application may cause a difference in weight or volume exceeding 10%.

Shape Factor is also a major element in the design of riser but most prevalently applied in cast iron castings. It is equal to the Length plus Width of the casting which the riser must cover divided by the Main Thickness of the part where the riser is effective [(L+W)/T].