Guides / Buying

Before You Spec a Casting, Check What You Are Giving Up

September 27, 2026 by The Alu CNC Parts editorial desk / how this is checked

The two processes get compared as if they were interchangeable options for the same part. They are not. Die casting and CNC machining start from different material, hit different tolerances, and produce parts with different mechanical properties even when the drawing looks the same. Picking wrong costs more than a bad quote, it costs a redesign after tooling is already cut.

They do not even use the same alloy family

This is the detail that gets skipped most often. Die casting uses aluminum-silicon casting alloys, commonly A380 or ADC12, formulated with high silicon content so the molten metal flows into a thin-walled die before it freezes. CNC machining works from wrought alloys, 6061, 6063, 5052, 7075, rolled or extruded material with a very different microstructure.

That difference is not cosmetic. Casting alloys are chosen for flow, not for strength, weldability or anodize response, and they cannot be swapped for a wrought alloy just by changing the process. If your drawing specifies 6061, it was never headed for a die casting quote in the first place.

What die casting is actually good at

High pressure die casting injects molten aluminum into a hardened steel die under several hundred to a few thousand bar of pressure. Once the die exists, cycle times run in seconds and unit cost drops to a fraction of a machined equivalent. It is the right process for complex, thin-walled shapes made in large volume: housings, brackets with cast-in bosses and ribs, anything where the geometry would take many setups to machine from solid.

The cost is tooling. A production die runs from the low tens of thousands of dollars upward depending on size and complexity, and it takes weeks to build. That cost has to be paid before the first good part comes off the line, and it only makes sense once volume is high enough to spread it thin.

What die casting gives up

Porosity. Trapped gas during the injection shot leaves microscopic voids through the casting. That limits fatigue strength, makes the part unsuitable for pressure-tight applications without a secondary sealing step, and is the reason a cast surface cannot take a clean anodize.

No T6 heat treatment, usually. The porosity that is safe to leave alone at room temperature can blister during a solution heat treatment cycle, so most die castings are used as-cast or with only a light stress relief, not brought up to the full strength a wrought T6 temper reaches.

No good weld. The same trapped gas boils during welding and porosity shows up in the weld itself. A cast part that will need welding in assembly is usually the wrong call from the start.

Draft angles and uniform wall thickness are mandatory, not a preference. Every surface parallel to the die pull direction needs a draft angle to release from the tool, and wall sections need to stay close to uniform to avoid sink marks and voids where thick and thin sections meet. A machined part has none of these constraints; it can hold a sharp shoulder and a variable wall thickness because there is no die to pull away from.

What CNC machining gives up in return

No tooling cost and a lead time measured in days, not weeks, but the unit price does not fall nearly as fast with volume. A shape with many pockets, a lot of stock to remove, and multiple setups stays expensive per piece even at a few hundred units, because you are still paying for cycle time and setup on every single part.

Complex internal geometry that a die casts in one shot, ribs, bosses, thin webs woven through a housing, can take a machined part several operations and fixtures to reproduce, if it is even practical to reach with a cutting tool at all.

The actual decision

Reach for CNC machining when the part needs full wrought strength, a clean weld, an even anodized or hardcoat finish, tight tolerances straight off the machine, or when volume is low enough that tooling cost cannot be justified, prototypes through the first few thousand units in most cases.

Reach for die casting when volume is high enough to amortize a die, the geometry is genuinely complex and thin-walled, the part does not need welding or a cosmetic anodized finish, and porosity is not a functional problem.

Some parts end up hybrid: cast to a near-net shape, then CNC-finished on the faces, bores and threads that actually carry a tolerance. That is a legitimate path, but it is a different job than either process alone, and it is worth stating plainly on the RFQ so the shop quoting the machining step knows it is finishing a casting rather than starting from bar stock.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• Die casting
• Aluminium alloy, casting alloys
• Porosity

Frequently asked questions

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