Die Casting gating strategy preventing turbulence and material oxidation during fill phase

Content Menu

● Introduction

● Flow Behavior in the Fill Phase

● Oxidation Mechanisms Specific to Die Casting

● Core Design Rules for Low-Turbulence Gating

● Gating System Types Compared

● Simulation-Driven Optimization

● Production Case Studies

● Practical Checklist Before Tool Release

● Conclusion

● Q&A

Introduction

In high-pressure die casting, the fill phase lasts only a few milliseconds, yet it determines whether a part will be sound or rejected. Molten metal enters the die at velocities that can exceed 50 m/s and under pressures up to 150 MPa. Under these conditions, any mistake in the gating system immediately translates into trapped air, oxide films, cold shuts, or porosity. These defects are not random; they follow directly from how the metal stream behaves as it leaves the runner and enters the cavity.

The gating system is the only part of the tooling that can be adjusted to control flow pattern, velocity distribution, and surface exposure time. A well-designed gating system delivers metal in a progressive, wave-like front that pushes air ahead of it toward the vents. A poorly designed one breaks the stream into jets and droplets, entrains air, and folds fresh oxide layers into the casting. The difference between the two often decides whether yield is 92 % or 62 % on the same machine and alloy.

This article focuses on practical gating solutions that have been proven in production and backed by published research. The goal is to give manufacturing engineers and tooling designers clear rules and examples they can apply tomorrow morning.

Flow Behavior in the Fill Phase

When molten metal accelerates through a constriction, its velocity increases inversely with the cross-sectional area. At the ingate, velocities commonly reach 30–60 m/s for aluminum and even higher for magnesium. Above approximately 25–30 m/s, the flow regime shifts from laminar to turbulent in most gating channels. The Reynolds number in the gate typically exceeds 100 000, far into the turbulent region.

Turbulence is not inherently bad, but uncontrolled turbulence is. Jets that shoot across the cavity impinge on opposite walls, fragment, and fall back as droplets. Each droplet carries a new oxide skin that becomes a bifilm defect. The entrained air pockets collapse during intensification and leave gas porosity or blunt cracks.

Real production experience with a 2 mm thick magnesium electronics housing showed exactly this mechanism. Four narrow ingates produced velocities around 48 m/s. High-speed video revealed distinct jets that crossed the cavity and broke into spray. Porosity levels reached 8–12 %. After widening the gates and changing to tangential entry, gate velocity dropped to 26 m/s, the jets disappeared, and porosity fell below 1 %.

Oxidation Mechanisms Specific to Die Casting

Aluminum and magnesium oxidize instantly in air at casting temperature. The oxide film on aluminum is coherent and protective when undisturbed, but turbulence tears it into fragments. John Campbell’s bifilm theory explains why these folded films act as cracks: each film has two oxide surfaces in contact with virtually no bonding across the interface.

In die casting, the main sources of oxide are:

the wave in the shot sleeve (plunger turbulence)
free surface turbulence in runners and at the gate
jetting and droplet formation inside the cavity

The shorter the time the metal is exposed to air, the lower the oxide content. Bottom-filled or submerged gating systems keep the metal stream under its own liquid surface after the first 5–10 mm of travel, dramatically reducing new oxide generation.

A telecom heat-sink casting in A360 alloy originally used top gating. Oxide inclusions were visible as dark networks on machined surfaces. Changing to a single wide bottom gate reduced oxide content from roughly 4 vol% to less than 0.8 vol%, measured by density comparison and SEM analysis.

Core Design Rules for Low-Turbulence Gating

Gate velocity target: 25–35 m/s for aluminum, 20–30 m/s for magnesium and zinc.
Sprue–runner–gate area ratio: start with 1 : 1.8–2.2 : 1.4–1.8 and adjust after simulation.
Runner cross-section: trapezoidal or rounded, never sharp corners.
Ingate location: place below the midpoint of the cavity height whenever geometry allows.
Ingate direction: tangential or slightly upward (8–15°) to promote rotational filling.
Taper: 2–5° on runners to maintain constant or slightly decreasing velocity.
Filters or flow breakers: ceramic filters or expanded metal sections in the runner to calm the stream when high plunger speeds are unavoidable.

A structural bracket in AlSi10Mg originally had three center gates feeding from above. Severe jetting caused folded defects along the side walls. The redesign used two wide tangential gates at the bottom edge. Gate area increased 60 %, velocity dropped from 52 m/s to 29 m/s, and the filling pattern changed from jet-dominated to a smooth rotating wave. Mechanical properties improved 18 % in the critical section.

Gating System Types Compared

Fan gates remain the most common for flat or mildly deep parts. They provide uniform front velocity but require careful thickness control to avoid early freezing.

Submerged (submarine) gates work best for deeper cavities. The metal enters below the cavity and rises as a single front. Air entrapment is minimal, but cycle time can increase slightly because of longer flow distance.

Tangential gates excel when rotational filling is desired. They are standard for round or symmetrical parts such as wheels, pulleys, and motor housings.

Overflow + vent blocks at the last-to-fill areas are essential companions to any gating type. They provide a deliberate location for the final air and oxide to collect instead of staying in the casting.

Simulation-Driven Optimization

Modern casting simulation accurately predicts filling patterns when boundary conditions are correct. Mesh size at the gate should be 0.05–0.15 mm to capture velocity gradients. Surface-tension and heat-transfer coefficients must be calibrated against real trials.

A typical workflow:

Build baseline model from existing tooling
Run filling simulation and identify high-velocity jets or wave folding
Modify gate area, angle, or runner taper
Re-run until maximum gate velocity is in target range and air entrapment index is low
Verify with 5–10 physical shots using short-shot technique and X-ray

In one automotive control-arm project, the first simulation showed a 68 m/s jet causing massive air entrapment. Three iterations later, gate area increased 78 %, velocity fell to 31 m/s, and predicted air entrapment dropped 94 %. Production trials confirmed porosity below 0.5 % without vacuum assistance.

Production Case Studies

Case 1 – Magnesium laptop base (1.2 mm wall) Original: four center gates, 45 m/s → 11 % porosity Redesigned: two tangential bottom gates, 27 m/s → 0.8 % porosity Tooling change paid for itself in nine days.

Case 2 – Aluminum transmission case (A380) Original: top center gate → heavy oxide bands in valve body area Redesigned: three submerged side gates + overflow wells → oxide reduced 82 %, leak test yield from 74 % to 99 %.

Case 3 – Zinc connector housing (Zamac 3) Original: pinpoint gates → flash and cold shuts Redesigned: wide fan gate with 1 : 2 : 1.6 ratio → velocity 24 m/s, plating defects eliminated.

Practical Checklist Before Tool Release

Gate velocity confirmed < 35 m/s in simulation
Filling pattern shows progressive front, no jetting across cavity
Vent area ≥ 25 % of total gate area
Overflow volume ≥ 8–12 % of cavity volume
Runner and gate edges radiused ≥ 1 mm
Short-shot trials match simulation within 5 % fill time

Conclusion

The gating system is the single most powerful lever a die caster has to control turbulence and oxidation during the fill phase. Keeping gate velocity in the 25–35 m/s window, feeding from below or tangentially, and using simulation to verify flow patterns before steel is cut will eliminate the majority of air- and oxide-related defects. The examples and rules presented here come from actual production tools that moved from chronic scrap to stable high yield. Apply them systematically, measure the results, and the improvement in part quality and process capability will be immediate and measurable.

Q&A

Q1: What is the safest gate velocity range for A360 aluminum in structural parts?
A: 28–34 m/s gives the best balance between fill time and low turbulence.

Q2: Why do magnesium castings need lower gate velocity than aluminum?
A: Magnesium has lower density and higher oxidation rate; velocities above 30 m/s almost always produce burning and bifilms.

Q3: Is vacuum necessary when a good gating system is used?
A: Not for most commercial parts. Well-designed non-vacuum gating routinely achieves < 1 % porosity in aluminum.

Q4: How much overflow volume should be planned?
A: 8–15 % of cavity volume, placed at the last-to-fill locations identified by simulation.

Q5: Can runner filters replace good gating design?
A: No. Filters calm the stream but add resistance and can cause early freezing if oversized. They are a supplement, not a substitute.