This question comes up on almost every plastic part quote under a few thousand units, and the answer is not fixed. It moves depending on geometry, material, and how many revisions the part is likely to go through before it is finished.
Injection molding needs a mold before it makes a single part. A simple single-cavity aluminum mold for a small plastic bracket might run a few thousand dollars. A production-grade steel mold with side actions or a multi-cavity layout can run tens of thousands. That cost gets amortized across the run, which is why molding gets cheaper per part as volume climbs and stays expensive per part at low volume.
CNC machining has no tooling cost in that sense. Every part is cut from stock, so the first part and the hundredth part cost roughly the same per unit (setup time aside). That is why CNC wins on small batches and molding wins once volume is high enough to spread the mold cost thin.
Where exactly that crossover sits depends on the part. A simple geometry with no undercuts might break even around a few hundred units. A complex part with tight tolerances that would need multiple CNC setups but molds cleanly in one shot can shift that number lower, sometimes into the low hundreds.
A molded part and a machined part in the same nominal material do not always perform the same way. Molded plastic parts have a grain structure and internal stress pattern that comes from how the material flowed and cooled in the mold. Machined plastic parts are cut from solid stock, so they do not carry that flow-line stress, but they can have different surface characteristics and, depending on the material, different moisture absorption behavior at a cut edge versus a molded skin.
For parts with tight dimensional requirements, machined plastic often holds tolerance more consistently across a production run, because there is no mold wear or shrinkage variation to manage. For parts where flow characteristics or a molded skin finish matter functionally, like a snap-fit feature relying on material flex, molding is usually the better answer regardless of volume.
At low volume, CNC is almost always faster to get parts in hand. There is no mold design, no mold build, no first-shot trial and adjustment cycle. A machined part can often ship in the same window a mold alone would still be in fabrication.
This matters most during early production or when a design is still likely to change. Cutting a mold for a part that gets revised twice more before it is finalized means paying for tooling changes on top of the original mold cost. Machining the same revisions is just a new program, no new tooling.
If the part is likely to change in the next few revisions, machine it, even if the eventual production volume will be high enough to justify a mold later. Lock the design first.
If volume is firmly in the thousands and the design is stable, start pricing mold amortization against machining cost per part, the crossover is usually somewhere in that range depending on part complexity.
If the part needs a molded-only feature, like a living hinge or a snap-fit that depends on how the plastic flowed, mold it regardless of volume, machining cannot replicate that behavior.
If you are not sure which side of that line your part sits on, send us the drawing and expected volume. We quote both ways when it is genuinely close, so you are pricing the actual tradeoff instead of guessing at it.
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