Two of the most common ways to make a custom part — and picking the wrong one can cost you thousands. Here's a simple framework based on volume, geometry, and material, with the real cost crossover that tells you when to stop machining and cut a mold.
CNC machining and injection molding solve the same problem — turning a design into a physical part — but their economics are opposites. Get the match right and you save money at every unit. Get it wrong and you either overpay per part forever, or you sink five figures into tooling you didn't need.
The one question that decides it: volume
Almost everything comes down to how the cost is structured.
CNC machining has no tooling cost. You pay to cut each part from a solid block, so your cost is roughly flat — the 10th part costs about the same as the 1,000th. That makes it unbeatable for low volumes and prototypes: you can have parts in days with zero upfront investment.
Injection molding is the reverse. You pay a large one-time cost to build a steel or aluminum mold — often several thousand to tens of thousands of dollars — but once it exists, each part costs very little (sometimes cents). All the money is upfront.
So the real question isn't "which is cheaper?" It's "do I make enough parts to pay off the mold?"
Where the cost curves cross
Picture two lines. CNC is a gently rising line starting at zero. Molding is a line that starts high (the tooling) but is nearly flat after. They cross at a break-even quantity — below it, machine; above it, mold.
1 – ~100 parts
CNC almost always wins. No tooling to amortize, fast turnaround, easy to revise the design.
~100 – 1,000 parts
It depends on part size and complexity. Worth quoting both — this is where a DFM review pays for itself.
1,000+ parts
Molding usually wins decisively. The tooling amortizes to near-zero and per-part cost is tiny.
Those bands are a rule of thumb, not a law — a small, simple part pushes the crossover lower (cheap mold), while a large or intricate one pushes it higher (expensive mold, slow machining). The only way to know your exact crossover is to quote the specific part both ways.
When geometry or material forces the choice
Sometimes volume doesn't get a vote — the part itself dictates the process.
| If your part… | Lean toward | Why |
|---|---|---|
| Is metal (aluminum, steel, titanium) | CNC | Injection molding is for plastics; metal means machining, casting, or MIM. |
| Needs tight tolerances or fine surface finish | CNC | Machining holds precision that molded plastic shrinkage can't match. |
| Is a large, simple block or plate | CNC | A big mold is very expensive; machining a simple form is cheap. |
| Is plastic with snap-fits, living hinges, thin walls | Molding | These features are natural in molding and painful to machine. |
| Is small, complex, and high-volume plastic | Molding | Exactly what tooling is built for — pennies per part at scale. |
Watch out for undercuts in molded parts — features that trap the part in the mold. They force side-actions or lifters, which add serious cost to the tool. If you can redesign an undercut away, you often shave thousands off tooling.
The 30-second decision
Ask three questions in order:
1. Is it metal, tight-tolerance, or large-and-simple? → Machine it, whatever the volume.
2. Is it plastic and you need under ~100? → Machine it (or 3D print for a first look).
3. Is it plastic, moldable, and you need thousands? → Cut a mold.
Everything in the middle — plastic parts in the hundreds — is worth quoting both ways. That's exactly the call our engineers make every day, and it's free to ask.
Not sure where your part lands? Send us the model. Every quote includes a founder-reviewed DFM check — we'll tell you which process is cheaper for your volume before you commit a dollar to tooling.