How Does Launder Design Affect Molten Aluminum Flow?

What Is a Launder in Aluminum Casting?

In aluminum casting, a runner is a refractory U-shaped channel used to transport molten aluminum at high temperatures; it serves as the “molten aluminum pipeline” in an aluminum plant, allowing the molten aluminum to flow from one piece of equipment to another by gravity.

How Does Molten Aluminum Flow Through a Launder?

Core Principle: Relying on gravity-fed flow without the need for a pump to pressurize the system, the flow channel is designed with a slight downward slope, allowing the molten aluminum to flow forward along the inclined channel.

Complete Flow Process：

1. Aluminum flows out of the outlet

The outlet of the holding furnace or melting furnace is opened, and molten aluminum at 720–780°C flows directly into the initial section of the flow channel.

2. Gravity-Driven Downhill Flow

The entire flow channel is inclined downward, typically at a slope of **0.5° to 1.5°**, which translates to a drop of approximately 8–25 mm per meter of length.

The slope should not be too steep: if the flow rate is too high, it will cause turbulence, entrained air, and a large amount of alumina scum; if the slope is too gentle, the flow will be slow, making it prone to cooling, crusting, and solidification.

 Flow Rate:

The casting process requires a stable flow rate, so a liquid level-plug system is used on-site to maintain a consistent liquid level and ensure a constant flow rate to the casting machine;

Flow Velocity：

The flow velocity is determined by a combination of  slope, liquid level, channel cross-section, and molten aluminum viscosity.

- Excessively high flow velocity: Turbulence causes the molten aluminum to churn and tumble, entraining air and generating Al₂O₃ oxide slag and hydrogen absorption, resulting in porosity and inclusions in the castings;

- Excessively low flow velocity: The molten aluminum remains in the channel for too long, causing a significant temperature drop, surface crusting, and a gradual narrowing of the flow cross-section, which further impairs flow;

Gravity：

Molten aluminum flows by gravity through a channel with a slight downward slope, keeping the liquid level at the inlet higher than at the outlet.

- Gravity serves as the driving force for flow.

- At a given liquid level, a steeper slope increases the flow rate; however, excessive steepness can cause the molten aluminum to churn, entrain air, and generate a significant amount of aluminum oxide slag.

Key Launder Design Factors That Affect Molten Aluminum Flow:

Key design factors affecting molten aluminum flow in launders include slope, flow cross-section, refractory lining surface, weir and baffle layout, inlet-outlet drop height, thermal insulation, and liquid level control. A gentle slope, rounded corners, and minimal free-fall height help maintain laminar flow, reduce turbulence, and limit dross formation and hydrogen pickup. A smooth, non-wetting refractory lining prevents aluminum sticking and channel blockage.

Launder slope and launder length：
- Short launder（＜5m）：2‰‑4‰
- Medium‑long launder（5‑15m）：1.5‰‑2.5‰
- Long distance launder（＞15m）：1‰‑2‰
-The longer the flow channel, the greater the temperature drop and the more joints there are, which increases the risk of aluminum leakage; the layout should prioritize minimizing the total length of the flow channel.


Bends and Flow Transitions:

Bends-design rules
1. Prefer large‑radius rounded bends, avoid 90° sharp corners.
2. Bend radius ≥ 1.5× launder width (minimum); better: R ≥ 2‑3 × internal channel width.
3. Outer wall of bend can be slightly heightened to prevent overflow during liquid‑surface surging.
4. Where sharp turn cannot be avoided: install flow‑guide baffle inside to smooth flow pattern, suppress vortex.

Flow Transitions：

Includes: widening / narrowing cross‑section, inlet‑outlet connection, connection to degassing box / filter box, step changes.

Two typical transition scenarios

1. Contraction:

Flow velocity rises locally. Abrupt contraction triggers jet flow, surface agitation.
Design: use gradual tapered transition, avoid sudden step‑in shrinkage.

2. Expansion：

Velocity drops, residence time increases. Dead zones easily appear at corners of expanded section, dross settling and crusting.
Design: slow taper expansion, eliminate sharp inner corners.

 Thermal Design and Heat Loss：

In open‑top molten‑aluminum launders, heat loss mainly occurs via surface radiation‑convection, refractory conduction and joint leakage. Launder length and flow velocity determine melt residence time and temperature drop. Multi‑layer refractory‑insulation structure, pre‑bake treatment and insulated covers mitigate heat loss. Auxiliary heating is optional for long transfer lines. Excessive heat loss increases melt viscosity, causes crusting and risks channel blockage.

How Launder Design Affects Casting Quality:

Launder design governs molten‑aluminum flow pattern, turbulence level, heat loss and dross entrapment. Poor launder geometry introduces oxides, hydrogen and inclusions into melt, directly triggering casting defects including porosity, blisters, slag inclusions and surface blemishes.

Every detail of launder design — slope, length, bends, transitions, thermal insulation, weir‑baffle layout and refractory lining — influences melt cleanliness and temperature stability. Turbulence induces oxidation and hydrogen pickup; excessive heat loss produces crust‑related inclusions. Optimized launder suppresses dross generation, maintains steady laminar flow and stable melt temperature, which is the prerequisite for low‑defect, repeatable casting quality.

How to Optimize a Molten Aluminum Launder Design？

-Define Flow Requirements
-Optimize Geometry
-Control Temperature
-Minimize Turbulence
-Validate with CFD and Testing

Common Launder Design Problems and Solutions：

Improper launder design creates turbulence, excessive heat loss, sticking‑aluminum crust, dross entrainment and unstable flow, resulting in various casting defects. Most issues can be solved by adjusting slope, geometry, thermal structure and internal baffle layout.