Heat sink and casting cost drivers: metal, process, yield and finish

The four lines that make up the price

A heat sink or a casting quote is built from four lines: metal, process, yield and finish. Every supplier uses a version of this structure, even when the quote shows a single number. Knowing the lines lets a buyer test the quote instead of accepting it.

Metal is the weight of the part times the price of the alloy, plus the scrap value credit. Process is the machine time or the casting cycle, including the labour and the overhead. Yield is the share of parts that do not make it to the shipping box. Finish is the coating, the machining of interfaces and the inspection.

The lines do not scale together. A part that loses 200 grams of aluminium on a crash diet may save more than a part that moves from one machining setup to two, because the metal line is priced per kilogram and the process line is priced per hour.

Metal: alloy, weight and scrap credit

Aluminium is the default for heat sinks because it is light and conducts heat well. Copper conducts better and costs more per kilogram, and it is much denser, so a copper part of the same volume is heavy. The material choice is a thermal and a commercial decision at the same time.

The alloy matters for both price and process. A common extrusion alloy such as 6063 is widely available and takes a finish well. A casting alloy is chosen for fluidity and strength, not for thermal conductivity alone. A high conductivity casting alloy may cost more and may machine differently.

Scrap credit is often left out of a buyer's model. Machining a heat sink from a block produces chips, and those chips have a value. A supplier who buys a block and machines away 40 per cent of it is not charging for that 40 per cent at the full price, because the chips are sold back. Ask how the scrap credit is treated in the quote.

Process: extrusion, casting or machining

The process choice is the largest lever after weight. An extruded heat sink uses a die to produce a constant section, then cuts and machines the ends. A die cast heat sink forms the fins and the base in one shot, but the fins are thicker than an extrusion can make and the tooling cost is higher. A fully machined heat sink cuts every fin from solid, which is the most flexible route and the most expensive per part.

Volume decides which route wins. At low volume, the extrusion die and the machining setup are cheaper than a casting tool. At high volume, the casting cycle can undercut the machining time, provided the design allows the draft and the wall thickness that the casting process needs.

An illustrative cost split

The chart below shows how a machined aluminium heat sink might split across the four lines. It is an illustrative example, not a market figure. The point is the shape of the split: metal and process dominate, and finish is small until the drawing asks for tight flatness, a full coating or a functional test on every part.

Yield and the cost of a bad part

Yield is the line that buyers underestimate. A casting that leaks, a machined base that bows after the final cut or a fin that breaks during packing all carry the cost of the metal, the machine time and the inspection that went before. If one part in ten is scrapped, the price of the nine good parts has to carry the tenth.

Yield is also a signal. A supplier who quotes a low price and a high reject rate may be cheaper on paper and more expensive per good part. Ask for the process yield and the main defect mode. A supplier who tracks both can explain the number. A supplier who cannot may be hiding it in the price.

Design changes move yield. A sharp internal corner in a casting, a thin fin that vibrates during machining or a deep pocket that needs a long tool all raise the risk. A radius, a thicker fin root or a shallower pocket can reduce scrap without changing the thermal result much.

Finish, flatness and inspection

Finish is priced by area, by type and by masking. Anodising a whole heat sink is cheap per part. Masking the mounting face, plugging threads and leaving a bare earth pad adds labour. A powder coat over fins reduces the heat transfer, so the thermal design may need a larger part to compensate.

Flatness is a machining cost that appears on the drawing as a small symbol. Holding a base flat within a few hundredths across a large face needs a stable fixture, a stress relief step and a finishing cut. Asking for that on a part that does not need it adds cost for no thermal gain.

Inspection is priced by the sample size and the method. A first article with a CMM report costs more than a check with a height gauge. A thermal test on every part is a different order of cost from a thermal test on one sample per batch. Match the inspection to the risk, not to a habit.

What to change first

When a quote is too high, the first question is whether the process route is right. A part that is fully machined at a volume that would justify a casting is a cost problem before it is a price problem. The second question is weight. Removing metal from a non-critical area reduces both the material line and the machining time.

The third question is tolerance. A flatness call, a surface finish call and a coating thickness call each add a step. Keep the tight calls on the interface and relax the rest. The fourth question is volume and release size. A larger release with fewer changeovers usually lowers the unit price, provided the cash and the storage can carry it.

A should cost model built on the four lines is more useful than a comparison of three quotes. It shows which supplier is out of line on which line, and it gives the buyer a basis for a conversation rather than a demand.