The way molten metal is introduced into a mold directly influences casting soundness, defect rates, and yield. Among the available methods, bottom pouring has proven to be highly effective for minimizing turbulence, improving filling patterns, and enhancing casting quality. The velocity of molten metal at the ingates, which dictates whether the flow is laminar, transitional, or turbulent — and therefore whether the casting emerges clean or defect-ridden.
Why Bottom Pouring Matters
Laminar Filling
In top pouring, molten metal falls freely through the sprue and impinges onto the mold cavity surface, often entraining gas and oxide films in the process. Bottom pouring introduces the metal from the lowest part of the mold, allowing it to rise uniformly and displace air upward through vents and risers. This results in laminar filling conditions and significantly less turbulence.
Reduced Oxide Entrapment
The upward rise of liquid metal reduces the folding of oxide skins into the casting, which is a common defect in alloys like aluminum. With proper gating, oxide films remain on the advancing front and are directed toward risers rather than trapped in the body of the casting.
Understanding Velocity
1. What Determines Velocity
The velocity of molten metal at the ingates is governed by fundamental fluid mechanics:
Head height (h): The vertical distance between the molten metal surface in the mold and the ladle controls potential energy. In ideal flow, velocity (V) at the base of the sprue is calculated using V=√2gh where g = acceleration due to gravity and h is the vertical height to the pouring ladle.
Gating geometry: The cross-sectional area of the choke, runners, and ingates determines flow rate (continuity principle).
In practice, energy losses due to friction, turbulence, sharp turns, and filters reduce actual velocity, so foundry engineers apply coefficients to calculate real flow.
2. Effects of Excessive Velocity
Turbulence: High velocity generates turbulent flow, which entraps air and fold oxides into the melt.
Mold erosion: Sand or investment mold surfaces may wash away, contaminating the casting.
3. Effects of Insufficient Velocity
Misruns: Metal may solidify before filling the cavity if momentum is too low.
Cold shuts: Weak streams may fail to fuse when fronts meet, leaving discontinuities.
4. Recommended Velocity
<0.5 m/s at ingates
Integrating Bottom Pouring with Velocity Control
Bottom pouring achieves its greatest benefit when combined with precise velocity control through a well-designed gating system. Features such as correctly tapered sprues, properly sized runner and ingates, and incorporation of filters result in controlled flow and minimize turbulence.
The synergy between bottom pouring and correct velocity leads to:
Cleaner, inclusion-free castings
Improved surface finish
Enhanced mechanical properties
Higher yield and reduced scrap rates
Conclusion
Bottom pouring is more than a convenience — it is a fundamental requirement for achieving high-quality gravity poured castings. By introducing metal from the bottom and allowing it to rise steadily, turbulence and oxide entrapment are minimized. However, the process succeeds only if the velocity at the ingates is carefully controlled. Too high, and turbulence and erosion occur; too low, and filling defects appear.
Casting quality therefore hinges on both pouring method and gating design. Bottom pouring provides the framework for laminar filling, while proper control of velocity ensures the mold fills efficiently and defect-free. Together, they form the cornerstone of modern foundry practice.
Success begins with calculating the velocity correctly and knowing how to reduce velocity from the peak velocity at the base of the sprue to the ingate velocity as molten metal enters the casting. If you are interested in Beta testing V1.4 you can download it below.
