How the two processes actually work
CNC machining is subtractive: a computer-controlled mill or lathe cuts away material from a solid block of metal or plastic until the finished part remains. There is no tool to design or qualify, so the first part can ship as soon as programming is done.
Injection molding is formative: molten thermoplastic is injected under high pressure into a hardened steel or aluminum cavity, cooled, and ejected. The cavity - the mold - is itself a precision tool that can take weeks to cut and cost thousands of dollars, but once built, parts form in seconds.
When CNC machining is the better choice
Choose CNC from a single prototype up to a few hundred parts, especially while the design is still changing. With no mold to pay for, you can revise the CAD model between batches at zero tooling cost, making CNC ideal for validation, fit checks, and bridge production.
CNC also wins for metal parts and for components that must carry load, hold tight tolerances, or resist heat and chemicals. It works directly in aluminum, stainless steel, titanium, and engineering plastics without the material limits that thermoplastic molding imposes.
When injection molding wins
Once the design is locked and volume is genuinely high - typically several thousand identical parts per year - injection molding becomes the lowest-cost route. The expensive mold is amortized across the whole run, so per-part cost falls to a fraction of what CNC can achieve.
Molding also delivers unmatched consistency. Every shot from a cavity is dimensionally identical, with excellent surface replication and the ability to form living hinges, snap fits, threads, and textures in one cycle. For consumer products at scale, this repeatability is decisive.
Cost and the break-even point
For one part, CNC is far cheaper because you pay only for programming, machine time, and material. Injection molding forces you to fund the mold before any part exists, so the first article is by far the most expensive unit you will ever buy.
The crossover sits in the low thousands of units, usually between 1,000 and 10,000 parts depending on size, material, and mold complexity. Below it, CNC wins on total cost and lead time; above it, molding pulls ahead. CNC the early batches, tool up only when demand is proven.
Tolerances, accuracy and materials
CNC holds tight, repeatable tolerances - about plus/minus 0.02 mm on metal and plus/minus 0.05 mm on plastic - because the machine references a calibrated coordinate system. It also taps the full range of engineering metals and plastics, including grades that simply cannot be molded.
Injection molding holds roughly plus/minus 0.05 to 0.2 mm, but real precision depends on material shrinkage, mold temperature control, and features like draft angles and wall thickness. Materials are limited to thermoplastics and some thermosets - ABS, PP, PC, PA, POM, PEEK, TPE - yet the catalog is broad.
Surface finish and post-processing
A machined part can be delivered near-spec straight off the machine: mill finish, bead blast, brushed, anodized, or polished, often in one setup. Cosmetic metal surfaces look finished without extra tooling or a mold.
Molded parts inherit the cavity's polished or textured surface directly, so they can look production-ready with no secondary work. Still, gates, ejector marks, and parting lines often need trimming, and the mold itself must be surfaced to achieve the look - an upfront cost CNC avoids.
Lead time and iteration speed
From a standing start, CNC is fast: a prototype can ship in days because no mold must be built. Programming and setup add some time, but the first good part never waits on tooling lead time.
Injection molding carries a long front end. Designing, cutting, and qualifying the mold can take four to twelve weeks before the first part is made, which makes molding wrong for urgent prototypes or designs still in motion. Afterward, cycle times drop to seconds.
Strength, material properties and end-use parts
CNC parts are essentially isotropic in their parent material - you shape existing stock, so grain structure and mechanical properties are retained. That makes CNC the conservative choice for load-bearing, pressurized, or safety-critical components.
Molded parts use engineered polymers whose behavior is well understood but distinct from metals; strong in their design envelope yet sensitive to creep, UV, and heat. For plastic housings, clips, and consumer parts molding is excellent, but it does not replace metal where real strength is needed.
Typical applications by industry
Aerospace, medical, and automotive teams use CNC for flight-critical brackets, surgical instruments, and functional prototypes where certification and tight tolerances matter. Molding serves them for high-volume consumables, device housings, and sterilizable plastic parts once volumes justify the tool.
Electronics and appliance makers lean on CNC for pre-production validation and low-volume custom parts, then move to molding for enclosures, buttons, and lenses made by the millions. The two are rarely competitors at one stage - they are consecutive steps in a product's life.
A practical decision checklist
Ask four questions: is volume below a few thousand units, is the design still changing, is the part metal or load-bearing, and do you need it in days not weeks? If you answered yes to most, CNC is the right call.
If instead the count is high and stable, the geometry frozen, the material a thermoplastic, and unit cost dominant, the mold is worth it. When in doubt, start with CNC to prove the product, then commit to a mold only when orders are repeatable.
How FabProto supports both processes
FabProto runs CNC machining and injection molding support under one roof, so you never guess alone. Send your CAD model and we advise the right route for your stage - CNC for proof and bridge volume, molding once the numbers make sense.
Our team returns a clear cost comparison showing the exact break-even quantity where a mold begins to pay off, plus material and tolerance guidance for your part. That visibility is why many teams keep CNC early and only move to molding when the market has spoken.