Efficient Heat Sinks are better made in Rheocasting

Electronics are becoming more powerful, but the circuit boards aren’t growing with them. And more power means more heat. This means that more and more heat must be dissipated over the same projected area.

That heat must be transferred away from the electronic component quickly and reliably to prevent reduced performance, shorter service life, or failure.

The obvious solution is to increase the heat sink’s cooling surface area. But simply making the heat sink larger is often not possible. The available package is fixed, so more active surface area must be created within the same space. This requires more fins, thinner fins, lower draft angles, and more efficient fin arrangements.

Straight, parallel fins are not always the best thermal solution. They create a stable layer of warm air directly on the aluminium surface. This boundary layer reduces heat exchange.

Angled, offset, or interrupted fins disturb the airflow and break up this standing layer. They increase mixing between the fins and improve heat dissipation without increasing the overall size of the heat sink. But these geometries are also much more difficult to cast, especially when you’re casting in the liquid state and under turbulent conditions.

Turbulent Filling traps Air

With fully liquid metal or a slurry with a low solid fraction, the filling behaviour is turbulent. The metal follows the path of least resistance and quickly fills the thick base. It then reaches the overflow. Only after the overflow is filled does it begin backfilling the narrow rips with cold metal. This backfilling traps air inside the fin cavities. The problem is particularly visible near the gate, where the air is pushed. The results are incomplete fins, cold shuts, and porosity. And as you can imagine, this lowers OEE and the heat sink’s effectiveness.

High-Solid-Fraction Rheocasting changes the Flow

At a solid fraction of 35% or more, the slurry gains thixotropic properties. Instead of rushing through the base like a liquid, the semi-solid slurry advances with a controlled, laminar or near-laminar flow front. The base and fins fill at the same time.

As the slurry moves from the gate towards the overflow, it enters the fins progressively and pushes the air ahead of it. The ribs are part of the main filling sequence rather than being backfilled. The air-removal direction and the metal-flow direction remain aligned. Air is pushed towards the overflow instead of being enclosed between unstable metal fronts.

Semi-Solid Slurry enables better Venting

High-solid-fraction slurry also allows different venting concepts. Fully liquid aluminium can escape through very small gaps, so conventional die-casting tools need a perfect seal to prevent flash buildup outside the part.

Semi-solid slurry has a higher apparent viscosity and does not enter extremely fine gaps as easily. This makes it possible to vent through clearances below a tenth of a millimeter at ejector pins and other small openings, similar to plastic injection moulding.

Therefore, the air has more ways to leave the cavity. For heat sinks, this is a major advantage. Every narrow fin cavity contains air that must be removed. Progressive filling combined with distributed venting creates the conditions for complete and connected fin structures.

Better Filling creates finer Geometry

Improved filling behaviour gives the designer more freedom. In conventional casting, fins often need thicker bases, shorter heights, and wider spacing to reduce production risk. Draft angles are increased, and complex fin structures are simplified. These compromises make the component easier to cast but reduce its thermal performance.

Rheocasting supports thinner ribs, taller fins, and lower draft angles because the fins are filled from the beginning of the shot.

Lower draft angles keep the fins straighter. This allows more ribs to be placed within the same available space while keeping the airflow channels open. More fins create more active surface area. Thinner fins also reduce the component’s weight.

Fin Orientation improves Heat Dissipation

Straight, parallel fins are not always the most efficient thermal solution. When air flows smoothly between parallel fins, a stable layer of warm air can remain on the aluminium surface. This boundary layer reduces further heat transfer.

Angled, offset, or interrupted fins disturb the airflow. They mix the air between the ribs and destroy the stationary hot boundary layer with cooler air. This improves heat dissipation without increasing the overall size of the heat sink.

Such structures are difficult to cast with turbulent liquid filling. With thixotropic properties from high solid fraction Rheocasting, they are proven in series production.

Low-Silicon Alloys increase Thermal Performance

Rheocasting also creates more freedom in alloy selection. Conventional die-casting alloys typically contain relatively high levels of silicon because it improves the castability of liquid metal. However, it also reduces thermal conductivity.

High-solid-fraction Rheocasting can process low-silicon aluminium alloys, since silicon content affects castability. Thermal conductivities of up to 198 W/mK are possible with the Rheocool, an AlSi2Fe alloy designed for Rheocasting and sustainability.

This allows a cast heat sink to compete with components machined from wrought aluminium blocks while still retaining the cost advantages of high-pressure die casting.

Conclusion

The performance of a heat sink begins with its filling behaviour. Turbulent liquid filling tends to fill the base first and backfill the thin fins afterwards. This traps air and limits the geometry that can be produced reliably.

High-solid-fraction Rheocasting fills the base and fins together. The thixotropic slurry flow pushes air towards the overflow and through fine local vents.

This enables thinner fins, lower draft angles, tighter spacing, more complex airflow structures, and lighter components. Combined with low-silicon alloys offering thermal conductivity of up to approximately 198 W/mK, Rheocasting creates heat sinks that are not only easier to manufacture. They are smarter, lighter, and more efficient!

To learn more about Rheocasting heat sinks, schedule a Free Consultation Call via the tool below this article or sign up for the Rheocasting Masterclass.