Design Limits for Die-Cast Heat Sinks: Fin Thickness, Height, Pitch and Draft

01Four dimensions that constrain each other

Four dimensions box in a die-cast fin: thickness, height, pitch and draft. They cannot be chosen independently — thinner and taller fins fill harder and drag more on ejection, while a smaller pitch leaves a thinner core between fins, which hurts both cooling and strength. In our experience the normal envelope is 1.5–2.5 mm thick, height no more than eight times thickness, 4–8 mm pitch and 1.5°–3° of draft per side. The limit is 1.2 mm at a 10:1 ratio, and reaching it needs conformal cooling in the fin block and venting at the tips, at a cost in yield and tool life.

Fig. 1 — The working range for fin thickness, height, pitch and draft.

02The area that draft eats

This is the essential difference between a die-cast heat sink and an extruded one. A cast fin needs draft to release, and 2° per side on a 15 mm fin means the root is about 0.52 mm thicker per side than the tip: the section is a trapezoid, not a rectangle. Two consequences follow. The thickened root is a hot spot and a favourite home for shrinkage porosity. And the gap between adjacent fins narrows at the root, restricting airflow. Extrusion has no such constraint and can hold constant thickness or even undercut. In the comparisons we have made, for the same envelope a casting typically gives 15–30% less surface area than an extrusion — a gap that belongs in the thermal calculation at selection time, not afterwards.

03When casting is still the right answer

Despite the area penalty, casting wins in three situations. First, when the thermal structure has to integrate with other functions — mounting flanges, sealing faces, cable channels, threaded bosses — none of which extrusion can produce without secondary work and assembly. Second, when the fins are not straight: annular, radial or variable-height fin patterns are beyond a process that can only push a constant section. Third, at volume: a casting's piece cost falls faster with quantity than extrusion plus machining, and by six figures the advantage is clear. If the part is genuinely a flat plate with a row of straight fins, extrusion is almost always the better buy.

Table 1 — Three routes to a heat sink (our working figures, for orientation only)
Item	Die casting	Extrusion	Skiving
最小齿厚 / Min. fin	1.2–1.5 mm	0.8–1.0 mm	0.3–0.6 mm
最大齿高比 / Max. ratio	8–10 : 1	15–20 : 1	40 : 1+
拔模角 / Draft	1.5°–3°/侧必须	0°	0°
复杂结构集成 / Integration	优 / Excellent	差 / Poor	差 / Poor
非直线齿形 / Non-linear fins	可 / Yes	不可 / No	不可 / No
经济批量 / Economic volume	> 5,000	> 500	任意 / Any
导热（同合金）/ Conductivity	96–120 W/(m·K)	150–200 W/(m·K)	200+ W/(m·K)

← Swipe sideways for all columns →

04Alloy: where AlSi12 earns its place

Alloy selection for a heat sink is not the same exercise as for a structural part. ADC12 conducts at around 96 W/(m·K) and A380 similarly; AlSi12, being near-eutectic with very little copper, reaches roughly 120 W/(m·K) — about 25% more, which on a thermal part is a real performance difference. AlSi12 also flows better, which helps fill thin fins. The price is a soft matrix that is hard on tooling, so heavy machining downstream has to be costed in. One more thing: heat sinks are often powder-coated black to raise radiative dissipation, but the coating is itself a thermal resistance, and film thickness should be specified on the drawing — usually 40–80 μm — rather than left to the coater.

05Tooling and process for the fin block

The fin block is the most fragile region of the tool: long, slender cores that shed heat poorly and carry high loads, which makes them the first place to heat-check and break. We handle it four ways. The fin cores are replaceable inserts, so damage does not condemn the die. The fin block gets conformal cooling that follows the fin profile, which both secures filling and extends life. The fin tips get venting — they fill last, and without a vent the trapped gas leaves the tip short. And the fin cores get nitriding or a PVD coating to resist soldering. With those four in place, volume production at 1.5 mm fins is stable.

Finned die castings — Fig. 2 — Finned castings: the root radius and the draft decide whether it runs stably at volume.

SOURCES AND NOTE

This is a technical introduction and design guide, drawing on the sources listed above and on Jinxiong's many years of production practice. It is for reference only and is not a commitment for any specific project. For a specific project, the technical agreement between both parties governs.

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