Die Casting

The short answer

Start with the material, not the process

Die casting and injection moulding look similar on a factory floor: a steel tool, a clamp unit, a shot of molten material, a part ejected every cycle. The difference is what is being shot. Die casting runs aluminium, zinc or magnesium at 620 to 700 degrees Celsius; injection moulding runs thermoplastics at 180 to 350 degrees Celsius. That single fact fixes almost every downstream property. A die-cast housing is electrically conductive, thermally conductive, fire resistant, dimensionally stable at high temperature and roughly three times stiffer than the same wall in glass-filled nylon. A moulded housing is an insulator, weighs a third to a half as much, can be moulded in colour with a textured surface, and can integrate living hinges and snap fits that metal cannot produce in one shot.

So the first question is not which process is cheaper; it is which set of properties the housing has to deliver. A motor housing that must sink 40 watts and survive a short circuit is a metal part, and no amount of cost engineering will make a moulded cover do that job. A consumer device cover that must be light, warm to the touch, radio-transparent and available in five colours is a moulded part, and casting it is an expensive way to make it heavier.

Tooling, tool life and the volume where each pays

The tooling difference is larger than the material difference. A single-cavity aluminium die casting tool typically costs 15,000 to 60,000 dollars and takes 8 to 20 weeks; a comparable single-cavity plastic mould costs 3,000 to 25,000 dollars and takes 6 to 10 weeks. Die casting tooling carries thicker plates, hot-work steel and slides that survive thermal cycling, which is where the money goes. Moulds can use pre-hardened steel and simpler cooling, because the melt is far cooler.

Tool life runs the other way. An aluminium die is commonly rated 100,000 to 150,000 shots before heat-check refurbishment, with premium steel and good thermal management reaching 300,000 or more; zinc dies last considerably longer because the melt is cooler. A hardened plastic mould commonly lasts several times longer than an aluminium die, and its wear parts are cheaper to replace. The practical effect is that die casting carries a higher tooling cost over a shorter tool life, which is only recovered by the per-part economics at volume: plastic housing programmes usually justify tooling from about 1,000 to 10,000 parts, die casting from about 5,000 to 50,000, depending on size and complexity.

Wall thickness and the geometry each process allows

Housing walls sit in a narrow band in both processes, for opposite reasons. Die cast aluminium is practical from about 1.0 mm to 3.0 mm, with 1.5 to 2.5 mm the comfortable middle for a structural housing; thinner sections lose fill and thicker ones create isolated mass that cannot be fed, producing shrinkage. Moulded ABS runs 1.1 to 3.6 mm, polycarbonate 1.0 to 3.8 mm and glass-filled nylon 0.8 to 2.9 mm, with the same rule that walls should be uniform: a thick section cools last, pulls material from the skin and leaves a sink mark on the far face.

Geometry follows the material. Die casting needs draft on every vertical face, generous fillets at internal corners and ribs that are thinner than the wall they stiffen, with slides for undercuts; it can produce very fine detail, sharp logos and textured surfaces straight from the die, and it can cast in bosses and mounting pads that would otherwise be separate parts. Injection moulding allows thinner ribs, deeper draws, snap fits, living hinges, threaded inserts moulded in place and multi-shot colour, and it can hold a Class A cosmetic surface with a texture that would need secondary work on a casting.

Cycle time, tolerance and what happens after the shot

A housing-sized aluminium part cycles in roughly 20 to 90 seconds, a plastic housing in 15 to 60 seconds, so cycle time rarely decides between them; the difference is smaller than the difference in material cost. Tolerance does differ, but less than buyers expect, and it is governed by standards rather than by slogans: castings are toleranced under the ISO 8062 system and moulded plastics under ISO 20457, and in both cases the tolerance you get depends on the feature, the tool and the material. As a working rule, a die casting holds around plus or minus 0.1 to 0.3 mm on as-cast features and a moulded part plus or minus 0.05 to 0.2 mm, with machined or reamed features on either reaching 0.05 mm or better.

Post-processing is where the two diverge most in cost. A casting usually needs deburring, trimming, shot blasting, and often machining on mating faces, seal grooves and threads, plus a conversion coating or powder coat; a moulded part usually needs gate trimming and nothing else, with colour and texture built into the tool. That is why a die-cast housing is rarely the cheapest part at the same wall thickness: the secondary operations are part of the process, not an option.

The hybrid housing is often the right answer

Many production housings are both. A die-cast aluminium chassis carries the load, spreads heat from the power stage and provides the electromagnetic shield; a moulded cover closes it, provides the visible surface, the colour and the user interface, and keeps hands away from a hot or live surface. The joint is designed once, with a gasket groove in the casting and a matching lip in the cover, and the assembly is usually cheaper and lighter than an all-metal box and more capable than an all-plastic one.

The design rules for the hybrid are worth stating. Put the tight tolerances and the sealing groove in the metal part, because it is more dimensionally stable and can be machined. Use the plastic part for features that need to flex, clip or insulate. Keep the thermal path in the metal and the user-contact surfaces in plastic. Decide the split before either tool is cut, because the parting line between the two parts is the one interface that cannot be fixed later without changing both tools.

A decision sequence for a new housing

Ask whether the part must be metal: structural load, heat spreading, electromagnetic shielding, fire rating or a grounded chassis. If yes, die casting is the default at volume, and machining at low volume. If no, ask whether the annual volume clears the plastic tooling threshold, which for most consumer housings is a few thousand parts. Then ask whether the surface must be cosmetic in colour, in which case moulding wins on both cost and finish. Finally, check the tool: draft, wall uniformity, gate position, ejector marks and, for a casting, the thermal balance of the die. Most housing failures at first article are one of those five, not a material problem.

See die casting for the metal route and its alloys, injection moulding for the plastic route and its design rules, and surface finishing for the coating and anodising steps that follow either process.