The gating system — the network of channels through which liquid aluminium flows from the ladle into the die cavity — is one of the most consequential design decisions in gravity die casting. A well-designed gating system fills the die cavity smoothly, without turbulence, at a controlled velocity that allows air and gas to escape ahead of the metal front, produces directional solidification from the casting extremities toward the riser, and minimises the oxide film defects that form wherever the metal surface is disturbed and aluminium reacts with the oxygen in the cavity atmosphere. A poorly designed gating system does the opposite: it creates turbulent flow that entrains the oxide skin from the metal stream surface into the casting body, generates cold shuts where two metal streams meet with insufficient superheat to fuse cleanly, produces misruns in thin sections that the metal front reaches too cool to fill completely, and places the last-to-solidify region in a location remote from the riser where it cannot be fed. Understanding the hydraulic and solidification engineering principles behind gating design — and applying them systematically rather than empirically — is the difference between a die that works consistently from the first trial and one that takes dozens of modifications to produce acceptable castings.
Sprue Design: Preventing Aspiration
The sprue is the vertical channel through which metal descends from the pouring cup into the runner system. Its critical design requirement is that it must be tapered — wider at the top (at the pouring cup) and narrower at the bottom (where it connects to the runner) — to prevent aspiration. The physics of this requirement is straightforward: as metal accelerates down the sprue under gravity, its velocity increases and its cross-sectional area must decrease proportionally (by continuity) to maintain a full, air-free stream. If the sprue has a parallel or diverging cross-section, the accelerating metal stream separates from the sprue wall, creating a vacuum at the sprue wall surface that draws air into the metal stream — the aspiration that generates oxide films and gas porosity in the casting. The correct sprue taper follows the velocity-area relationship: if the sprue entrance has cross-sectional area A₁ at height h₁ and the exit has area A₂ at height h₂, the taper should satisfy A₁/A₂ = √(h₂/h₁), where heights are measured from the metal head in the pouring cup. In practice, a taper of approximately 10–15% reduction in diameter per 100mm of sprue height gives a good approximation for most GDC applications, though the exact calculation should be performed for each specific die geometry and pouring height.
Runner Sizing: The Choke and Flow Control
The runner is the horizontal channel at the base of the sprue that distributes metal to the ingates. Its sizing determines the flow rate into the cavity — too large and metal enters the cavity too fast and turbulently; too small and the cavity fills too slowly, allowing metal to cool before the cavity is complete, causing misruns and cold shuts. The choke — the minimum cross-sectional area anywhere in the gating system, typically located at the ingate rather than the runner — controls the fill rate and is the primary flow control element. Runner cross-sectional area should be larger than the total choke area (the sum of ingate areas) by a factor of 1.0–1.5× for a pressurised gating system (where the runner runs full and flow is controlled at the ingate) or 3–5× for an unpressurised system (where the runner does not run full and the sprue exit is the choke). For aluminium GDC, a pressurised gating system — where the ingates are the minimum area and the runner runs full — is generally preferred because it maintains a continuous, air-free metal stream through the gating system and reduces oxide entrainment in the runner. Runner cross-sections should be trapezoidal rather than circular where possible (easier to machine in the die parting surface) and should have smooth transitions at junctions to avoid flow separation and turbulence.
Ingate Design: Velocity, Placement, and Directional Solidification
The ingate is where metal enters the die cavity, and its design controls three critical outcomes: metal entry velocity (which determines whether entry is turbulent or laminar), entry location (which determines how the metal front progresses through the cavity), and the relationship of the ingate to the solidification sequence (which determines whether the gating system can feed the casting or will freeze before feeding is complete). Gate velocity for aluminium GDC should be in the range of 0.3–0.6 m/s — fast enough to fill the cavity before the metal cools excessively, but slow enough to avoid the turbulence that generates oxide inclusions. Gate velocity below 0.3 m/s risks cold shuts and misruns; gate velocity above 0.6 m/s generates turbulent entry that entrains oxides into the casting body. Gate area is calculated from the target fill time, the casting volume, and the target gate velocity: ingate area = casting volume / (target fill time × gate velocity). Gate placement should direct metal into the thickest section of the casting first, with flow progressing toward thinner sections — this produces the directional solidification (from thin to thick) that allows risers placed at the thickest section to feed shrinkage as the casting solidifies. Ingates placed at thin sections that fill first may create premature freezing at the gate location that cuts off feeding from the riser before the thick section has fully solidified — a common cause of shrinkage porosity in poorly gated castings.