Key takeaways
- Fin thickness, spacing and draft are set by the casting process, and a fin that is too thin may never fill.
- A thick base spreads heat sideways; a thin base saves weight but can starve the outer fins.
- The mounting face and the stator bore need machining if the thermal path depends on contact.
- Porosity near a machined face reduces the contact area and can open a leak path in a cooled housing.
Where die cast heat sinks fit
A die cast heat sink makes sense when the part also has to be a structure, a housing or a cover. The casting can carry bosses, mounting feet, a cable entry and a fin field in one piece, which removes assembly steps and fasteners. That is often worth more than the last few percent of thermal resistance.
The process suits aluminium alloys such as AlSi12 or AlSi9Cu3 because they flow well and cast cleanly into thin sections. The trade-off is that a casting is not the best shape for every thermal job. If the part is a plain finned block with no other features, an extrusion usually gives straighter, taller fins at a lower tooling cost.
Die casting works best when the fins are short relative to the base, the airflow is forced or the housing is the product. For a tall natural convection stack, extrusion or a bonded fin assembly is often the better route.
Fin geometry and draft
Every fin needs draft so it can leave the die. A typical draft is around 1 to 2 degrees per side, depending on the depth and the surface finish. Draft reduces the fin thickness from root to tip, so the root is thicker than the drawing may suggest.
Thin fins fill poorly. As a practical starting point, keep the tip thickness at or above roughly 1 mm for a conventional die casting, and increase it as the fin gets taller. A fin that is 0.6 mm at the tip may look efficient on paper but can run cold and leave a short fill.
Fin spacing matters as much as fin thickness. Tight spacing adds surface area but raises the pressure drop in forced air and reduces the boundary layer refresh in natural convection. For natural convection, a wider gap usually beats a dense field.
The fin root needs a fillet. A sharp root concentrates stress and restricts metal flow. A small radius also helps the die fill and reduces the chance of a crack at the base during ejection.
| Parameter | Typical starting range | Why it matters |
|---|---|---|
| Tip thickness | 1.0 mm or more | Fill and die life |
| Draft per side | 1 to 2 degrees | Ejection and tip thickness |
| Fin height to gap | Keep the gap open for the airflow | Convection and pressure drop |
| Root fillet | 0.5 mm or more | Metal flow and stress |
| Base thickness | 3 to 6 mm, part dependent | Spreading and stiffness |
Starting points only. The alloy, die layout, airflow and structural load decide the final values.
Base thickness and heat spreading
Heat enters the base at a die or a component footprint and has to spread to the fins. If the base is too thin, the heat stays under the source and the outer fins run cool. If the base is too thick, the part gains weight and thermal mass without gaining much spreading.
A useful way to think about it is spreading resistance. A small source on a large base needs enough thickness to let the heat fan out at a shallow angle. A source that already covers most of the base needs less thickness.
The base also has to carry the clamp load and the machining stress. A thin base can bow when the mounting screws are tightened, which lifts the contact face away from the heat source. That is a mechanical problem that shows up as a thermal one.
Machining the thermal contact face
A cast surface is not a thermal interface. It has draft, a skin and a waviness that leaves an air gap. If the thermal path depends on contact, machine the face that touches the heat source and specify flatness.
The machined face should be the datum for the fins and the mounting holes. If the face is cut after the holes are drilled, the holes can end up out of position. If the holes are drilled from a cast face, the machined face may not be parallel to the hole pattern.
For a motor housing, the stator bore and the mounting face are the two critical surfaces. The bore carries the stator contact and the mounting face carries the end bracket. Both need a defined relationship, and both are usually machined in the same setup or in a sequence that preserves that relationship.
Motor housings: bore, stator contact and cooling
A motor housing does three jobs: it holds the stator, it carries the bearings and it removes heat. The stator contact is the bottleneck. A thin air gap between the stator lamination and the housing wall adds a contact resistance that the fins cannot fix.
The bore should be machined to a consistent diameter and roundness. A cast bore with draft and ovality gives a contact pattern on two sides and an air gap on the others. If the design uses a shrink fit or a thermal fit, the bore tolerance and the housing stiffness both matter.
Cooling can be built into the casting as a water jacket or as external fins. A water jacket needs a leak tight wall and a clean internal passage. Porosity in the jacket wall is a scrap condition, not a cosmetic one.
Bearing seats need a machined bore with a defined shoulder. The distance between bearing seats sets the shaft end play, so it belongs on the drawing as a critical dimension.
Porosity and the thermal path
Porosity reduces the effective cross section of the metal and adds an air pocket where heat should pass. Under a machined face, a cluster of porosity can show as a blister or a shadow after machining. In a cooled housing, it can connect two passages and create a leak.
Place gates and overflows so that the last metal to solidify is not under the critical machined face. Use a flow simulation to check the fill and the shrinkage pattern. Then machine a sample from each cavity to see what the surface reveals.
If the part must be leak tight, define the test pressure, the test medium and the accept criteria before the die is cut. A leak test added after the first trial is expensive to retrofit.
Finish, assembly and the thermal interface
A cast housing is often painted or powder coated. A coating on the fins adds a thin thermal resistance and can bridge the gap between fins if it is applied heavily. Mask the contact face and any earth bonding point.
The interface between the housing and the heat source needs its own material. A thermal pad or paste fills the micro gaps that machining leaves. The housing flatness and the interface material work together; a perfect pad cannot fix a bowed face.
For outdoor housings, the finish also carries the corrosion duty. A conversion coating plus a powder coat is common, but the coating thickness on the fins should be controlled so it does not close the air gap.
What to put on the drawing
Show the fin thickness at the tip, the draft, the root fillet and the base thickness. Mark the machined contact face with flatness and roughness. Show the stator bore with roundness and cylindricity if the motor depends on it. State the leak test for a cooled housing and the porosity acceptance for the critical faces.
Add the interface material, the clamp load and the mounting hole pattern. If the housing is part of an assembly, give the stack up that sets the bearing position.
A fin drawing without a tip thickness is incomplete
Draft means the fin changes thickness along its height. Specify the tip thickness and the draft, then let the supplier calculate the root thickness from the die geometry.
Frequently asked questions
Can die cast heat sinks match extruded heat sinks?
For short fins and a housing with other features, a die casting can be close enough and avoids assembly. For tall, straight fins in natural convection, an extrusion usually performs better and costs less in tooling. Compare the thermal resistance and the total part cost.
How thin can a cast aluminium fin be?
It depends on fin height, alloy, die layout and fill. A tip thickness around 1 mm is a common starting point, with more thickness as the fin gets taller. Confirm the minimum with a flow simulation and a trial.
Why machine the face of a cast heat sink?
A cast face has draft, waviness and a skin that leave an air gap against the heat source. Machining gives a flat, clean surface that the thermal interface material can bridge. The flatness callout should match the interface material and the clamp load.
How is porosity checked in a motor housing?
It is usually checked by visual inspection after machining, by sectioning a sample, and by a leak test if the housing carries coolant. The acceptance criteria should be agreed before the die is built.
Sources and further reading
- EN 1706, Aluminium and aluminium alloys. Castings. Chemical composition and mechanical properties
- ISO 8062-3, Geometrical product specifications (GPS). Dimensional and geometrical tolerances for moulded parts
- Qumek die cast housing and heat sink DFM notes
Qumek Engineering Desk
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The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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Send the drawing, the heat load and the airflow. The engineering desk will review the fin field, the base and the machined contact faces.
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