How to Design a Launder System for Molten Aluminum

Molten aluminum launders are critical transfer channels that convey liquid aluminum between melting furnaces, holding furnaces, degassing units, filtration boxes, and casting machines. A poorly designed launder system leads to a series of production problems: metal turbulence, oxide inclusion formation, hydrogen absorption, aluminum dross buildup, heat loss, and even the risk of molten metal leakage. All of these issues degrade final casting quality, increase metal loss, and add to maintenance downtime.  

The core goal of molten aluminum launder design is straightforward: deliver molten aluminum with minimal turbulence, minimal heat loss, low dross formation, stable flow rate, and safe operation. This article breaks down the complete design workflow, key parameters, material selection, structural details, and auxiliary considerations for your launder system.

1. Define Working Conditions and Design Inputs  

Before producing any structural drawings, the basic operating parameters must first be determined. These data define the overall size, slope, cross-section, and refractory selection.  

 

Core design inputs  

1. Molten aluminum flow rate: themaximum and normal throughput (kg/min or t/h). Flow rate directly determines launder width and depth.  
2. Operating temperature: typically720°C–780°C. Certain alloy grades may require higher temperature control.  
3. Total delivery distance: straight sections, elbows, drop heights, and number of branches. Unnecessarily long paths should be avoided.  
4. Alloy type: pure aluminum or 3xxx, 4xxx, 6xxx, and 7xxx series alloys. High-magnesium aluminum alloys oxidize more readily and therefore require additional anti-oxidation design. 
5. Operation mode: continuouscasting or batch transfer. Continuous systems require more stable flow control.  
6. Installation environment: indoor or outdoor layout, ambient temperature, and vibration from adjacent equipment.

A key principle is to minimize the total length and number of turns. Every bend causes flow turbulence and promotes alumina dross formation.

2. Hydraulic Design: Flow Velocity & Launder Slope  

Hydraulic calculation is the core of launder design. Turbulence is the greatest threat to clean molten aluminum.  

Recommended flow velocity  

• Main launder: 0.15–35 m/s
• Branch launder before casting: 0.10–20 m/s

Velocity above 0.5 m/s will violently stir molten aluminum, causing severe oxidation and air entrainment.  

Launder slope  

The launder relies on gravity to transport liquid aluminum. No pumps are used for primary transfer.  

• Normal slope range: 1.5–4 mm per meter.
• Higher flow rate: slightly larger slope; short and low-flow launders use a smaller slope.

Too steep: flow surges and turbulence increases. Too flat: molten metal cannot flow fully, metal stagnates, and dross accumulates inside the channel.

Cross-section selection  

Two common cross-section shapes:  

1. Trapezoidal cross-section: Most widely adopted. Wider topand narrower bottom. Good for dross floating on the surface. Easy for manual skimming.  
2. Rectangular cross-section: Used for compact, narrow branch channels. Better space utilization, but dross skimming is less convenient.

Design the working liquid level to occupy 50%–70% of the internal channel height. Leave enough freeboard to prevent overflow during flow fluctuation.

3. Material Selection: Refractory & Outer Shell  

Molten aluminum does not corrode all refractories equally. The wrong lining material will be wetted by aluminum, leading to penetration, bulging, and cracking.  

 

Refractory lining  

• Main material choice: non-wetting refractory for aluminum. Common options: silicon carbide, high-purity mullite, low-cement castable refractory.
• Avoid materials with high iron oxide content. Iron contaminants will pollute the aluminum melt.
• Lining thickness: determined by launder size and working temperature,usually 60–120 mm.  
• Surface requirement: smooth inner surface. A roughrefractory surface will catch dross and increase metal adhesion.  

Outer steel shell  

• Carbon steel structural shell, with reinforcing ribs welded outside to resist refractory expansion and mechanical impact.
• Add expansion joints on long launder segments to absorb thermal expansion at high temperature.
• For large launder systems, support frames are installed at intervals. Supports must allow thermal elongation of the whole channel.

4. Structural Design of Key Components  

Straight sections  

Standard modular segments are preferred. Modular design helps manufacturing, shipping, on-site assembly, and replacement when damaged. Each segment can be precast with refractory in the factory.  

 

Bends / elbows  

• Use large-radius bends. Radius ≥ 2 × launder internal width. Sharp 90° corners are forbidden.
• At every turn, dross will gather. Reserve space for dross skimming.

Transition sections & drop points  

Sudden height drops cause severe turbulence.  

• If vertical drop is unavoidable, install stilling wells at the drop position.
• Stilling wells stabilize melt flowand trap floating oxides before molten aluminum enters downstream launders.

. Auxiliary & Safety Design  

Heating & thermal insulation  

For long-distance transfer or low-flow systems, external insulation layers are wrapped outside the refractory. In some cases, auxiliary electric heating can be added to compensate for heat loss and prevent melt solidification during standby.  

 

Flow control  

Weirs and dams are installed inside launders to adjust liquid level and separate floating dross.  

• Weir: controls liquid level.
• Dam: retains floating oxides on the melt surface.

Safety design  

1. Leakage containment: ametal catch tray under the launder to collect molten aluminum if the refractory fails.  
2. Thermal expansion compensation: preventsthermal stress cracking.  
3. Anti-spattering guard.
4. Temperature monitoring ports: thermocoupleinstallation holes to track melt temperature in real time.  
5. Avoid water accumulation under the launder. Contact between molten aluminum and water causes explosive hazards.

5. Auxiliary & Safety Design  

Heating & thermal insulation  

For long-distance transfer or low-flow systems, external insulation layers are wrapped outside the refractory. In some cases, auxiliary electric heating can be added to compensate for heat loss and prevent melt solidification during standby.  

 

Flow control  

Weirs and dams are installed inside launders to adjust liquid level and separate floating dross.  

• Weir: controls liquid level.
• Dam: retains floating oxides on the melt surface.

Safety design  

1. Leakage containment: ametal catch tray under the launder to collect molten aluminum if the refractory fails.  
2. Thermal expansion compensation: preventsthermal stress cracking.  
3. Anti-spattering guard.
4. Temperature monitoring ports: thermocoupleinstallation holes to track melt temperature in real time.  
5. Avoid water accumulation under the launder. Contact between molten aluminum and water causes explosive hazards.

6. Pre-commissioning & Maintenance Considerations  

A good design cannot perform well without proper preheating and maintenance. Take these considerations into account during the design phase:  

1. Preheating structure: The launder system must be slowly preheated before first use to remove water inside the Fast heating will cause refractory spalling.
2. Easy disassembly: Modular segments make routine maintenance easier.
3. Internal cleaning access.

7. Common Design Pitfalls to Avoid  

1. Too small launder cross-section → excessive flow velocity.
2. Sharp-angle elbows → turbulence and heavy dross.
3. Wetting refractory material → aluminum penetration intothe  
4. Insufficient freeboard → molten aluminum overflow.
5. No thermal expansion allowance → cracking during heating.
6. Too long launder path → large heat loss and more oxide inclusions.