Zinc Die Casting vs CNC Machining: Porosity, Strength, and When to Cut

Ask a buyer why they are considering zinc die casting instead of CNC machining and the answer is almost always the same: the casting is cheaper per part once the mould is paid for. That is true, and it is also only half the story. A die casting and a machined part are not two ways to make the same thing — they are two different materials with different limits, and the part that comes out of the die is not simply a less accurate version of the one that comes off the mill. The cast part has porosity inside it, a chilled skin on the outside, and a microstructure set by how fast the metal froze. The machined part starts as solid wrought stock and keeps that solidity through every cut. Whether the savings survive contact with your tolerance and strength requirements depends on which of those differences your design can live with.

Why the two processes make different materials, not just different shapes

The core difference is in how the metal solidifies, and it decides everything downstream. In die casting, molten zinc alloy is injected into a steel mould under pressure and freezes in seconds, so the part is full of the things that fast solidification produces: a fine-grained skin where the metal chilled against the die wall, and a porous, slightly coarser interior where gas and shrinkage got trapped as the metal set. Porosity is not a defect you can tune away entirely; it is a property of the process, and every die cast part carries some of it, concentrated near the thick sections and the gates.

A machined part starts from wrought stock — bar or plate that was cast into an ingot, then worked, which squeezes the porosity shut and aligns the grain. When you mill or turn that stock, you are cutting into solid, dense metal, and the finished part keeps that density. That is why a machined part has a consistent strength through its whole cross-section, while a die cast part has a hard skin over a softer, porous core. The moment you machine a cast part — to hit a tolerance or add a flat sealing face — you cut through that skin and expose the porous interior, which is exactly the surface you were hoping to avoid. The two processes do not just cost different amounts; they give you a different thing to work with.

Strength follows the same split. Zinc die casting alloys are engineered for castability and low melt temperature, not for strength, and a typical cast zinc part is weaker in tension than a wrought aluminum like 6061-T6 that a machinist would reach for first. The published numbers are not even close once you account for the porosity, because a porous section fails at a lower load than the same section of solid metal. If your part carries a real structural load, or if it gets tapped and torqued repeatedly, the cast zinc is the one that will let you down first.

Porosity is the spec nobody puts on the drawing

Porosity does not appear on most drawings, which is exactly why it causes so much trouble. It shows up at the worst possible moments: a casting that passes a dimensional check in the inspection room leaks on the test rig, because the gas voids connect through a wall the print never flagged. A threaded hole strips on assembly because the tapped region was porous and the threads only had two-thirds of the metal they were supposed to have. A plated part blisters in the oven because trapped gas under the skin expanded when it got hot.

This is the practical difference between cast and machined for anyone who has to guarantee a part works. A machined part from wrought stock is effectively porosity-free, so the numbers on the material datasheet are the numbers you get. A die cast part has a porosity level you have to manage, and managing it means limiting wall thicknesses, adding feed paths, and accepting that some fraction of the batch will fail a leak or pressure test. If the function is "hold pressure" or "seal a fluid," the machined part removes an entire failure mode that the cast part leaves on the table.

The skin effect cuts the other way too. Because the cast skin is fine-grained and dense while the interior is porous, the strength and the finish of a cast part are not uniform — they are best right at the surface and fall off inside. That is fine for a decorative housing that only ever gets touched on the outside, and it is a liability for a load-bearing bracket where the stress runs through the middle. Once you understand that the skin is doing the work, a lot of cast-part failures stop looking mysterious and start looking predictable.

Where die casting actually wins, and what it costs you

None of this means die casting is the wrong choice — it means it is the right choice for a specific, narrow job, and you should know what that job is before you commit. Die casting wins on volume economics. Once the steel mould exists, each additional part costs very little, so at thousands of parts the unit price can drop well below what any machined part can reach. It also wins on complex net shapes: thin walls, ribs, and bosses that would take a mill several operations and a lot of scrap can come out of the die in one shot, which is why die casting owns things like zinc alloy connectors, small gear housings, and decorative hardware.

The cost is paid up front, in the mould, and paid again in the constraints. A die cast part has draft angles on every wall so it can eject, minimum wall thicknesses so the metal fills before it freezes, and gate and ejector pin marks that have to be designed around. Changing the geometry means changing steel, so the process is hostile to iteration, and a design that is still moving should not be in a die. You also inherit the porosity, the skin dependence, and the lower strength we already covered, plus a finishing step, because raw cast zinc looks like what it is and usually needs plating, painting, or powder coat before it will pass as a finished product.

The break-even is the number to nail down before you pick either route, and it is driven by how many parts you need against the cost of the mould. For a run of a few hundred parts, the mould cost alone usually makes casting more expensive than machining, even before you count the porosity risk and the finishing. For tens of thousands of a simple, low-stress part, the casting wins decisively. The middle ground — a few thousand parts of something that has to hold tolerance and strength — is where the two are genuinely close and where you have to do the arithmetic instead of reaching for the default.

Machining after casting: the hybrid that hides the problems

A lot of zinc die cast parts are not finished as they come out of the die; they are machined afterward to hit a tolerance the casting cannot hold. This "cast then machine" hybrid looks like the best of both worlds — cheap near-net shape, then a light cut to clean it up — but it is often where the real problems surface. The moment the cutter passes through the dense cast skin and into the porous core, it opens up the voids, and you are now machining a material with inconsistent hardness and random holes in it, on a part you chose specifically to avoid paying for solid stock.

There are parts where the hybrid makes sense: a casting that needs one flat sealing face or a few reamed bores, where the machining is shallow and stays inside the sound skin. The trouble starts when the tolerance or the flatness requirement runs deep, because then the cut reaches the porous interior and the surface you produce is pitted and unreliable. Tapping into cast zinc is the classic failure — the tapped hole lands in a porous region and the threads simply do not have the metal to hold, so the fastener pulls out at a fraction of its rated load. If a cast part needs threaded holes that will actually be loaded, the practical answer is often an insert, and now you are stacking process on process to patch a problem the machined part never had.

The honest way to think about the hybrid is that casting buys you the rough shape and machining buys you the accuracy, but the two do not combine cleanly the way they are advertised to. Every feature the casting leaves loose is one more machining operation on a porous, skin-dependent blank, and the cost you saved on stock comes back as tool wear, scrap, and rework. When the machined features are shallow and cosmetic, the hybrid pays; when they are structural or precision, it usually does not.

How to decide, and where machining wins outright

Strip the choice down to three questions and the answer falls out. First, what volume are you actually committing to, and is the design stable enough to pay for a mould that locks the geometry in? Second, what is the part doing — is it carrying a load, holding pressure, or sealing a fluid, or is it a low-stress housing? Third, what tolerance and finish does the print demand, and can you hit them as-cast or will you be machining the casting anyway? Volume points toward casting, and function and tolerance point toward machining, and it is rare for all three to point the same way.

Machining wins outright in a set of situations that are easy to recognize. Low to medium volume, where the mould cannot amortize. Anything that must be structurally sound, tapped and torqued, or pressure-tight, where porosity is a deal-breaker rather than a nuisance. Anything with tight tolerances or a machined surface finish called out, because you would be machining the casting to get there anyway and paying for the privilege. Anything still in development, where the geometry is moving and you cannot afford to re-cut steel with every revision. And anything in a wrought alloy like 6061-T6, where the strength and the machinability both come from the material itself.

Zinc die casting keeps its own clear territory: high volume, low stress, geometry that exploits thin walls and ribs, and a part that will be plated or painted anyway. The mistake is treating the two as interchangeable, and the fix is to pick based on what the part has to do rather than what the quote says. If the part has to hold tolerance, carry a load, or stay sealed, the cheaper casting is usually the more expensive part in the end.

• Check the volume first — below a few thousand parts, the mould usually makes casting the pricier route.
• Ask if the part carries load — porous cast zinc is weaker than solid wrought stock like 6061-T6.
• Watch for pressure and sealing — porosity is a leak path that no drawing flags.
• Look for threaded holes that get torqued — tapped cast zinc strips early; plan inserts or machine from solid.
• Account for the cast skin — strength and finish live at the surface; deep cuts expose the porous core.
• Count the finishing step — raw cast zinc needs plating or coating before it looks finished.
• Know the tolerance the cast holds as-cast — tight features mean machining the casting, which is the worst of both.
• Do not die-cast a moving design — moulds lock geometry in; iterations are cheap on a mill, expensive in steel.

"Die casting and machining are not two prices for the same part; they are two materials with different failure modes. The casting is cheap because the mould does the shaping and the porosity comes free. The machined part costs more per unit because you are paying for solid stock and cut time — and buying back the strength, the tolerance, and the pressure-tightness the casting gave away. Decide by what the part has to do, not by what the quote says."

Choose the process by the function, not the unit price. If the part is high-volume, low-stress, and headed for a plating line, die casting earns its keep. If it has to hold a tolerance, carry a load, seal a fluid, or survive a tapped and torqued joint, machine it from solid stock and get the material the drawing actually assumes. Both processes have a real job; they just are not the same job, and confusing the two is how a part that looked cheap on the quote ends up leaking, stripping, and failing in the field.

If you are weighing a die-cast design against machining a part from solid 6061-T6 or another wrought alloy, send the drawing over and we will give you the machining side of the comparison — cost, tolerance, and where the porosity risk lands. See how we handle CNC milling for solid stock and wrought aluminum and zinc alloys, or get a quote on your machined part.