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Rapid Prototyping Processes: CNC Machining vs 3D Printing vs Injection Molding
A vendor-neutral engineering guide to the three dominant rapid prototyping processes — CNC machining, 3D printing, and bridge injection molding — written from the perspective of a U.S.-based, ITAR-registered precision CNC machining shop in Hanover, Pennsylvania. Use it to pick the right process for lead time, cost, material, and tolerance before sending a quote.
TL;DR Selection Criteria
- Functional metal prototype, 1–500 pieces: CNC machining.
- Early form-and-fit check, 1–5 pieces, plastic acceptable: FDM or SLA 3D printing.
- Production-grade plastic prototype: SLS or MJF 3D printing, or CNC machined Delrin/PEEK.
- Bridge production, 500–10,000 plastic parts: Aluminum-tool injection molding.
- Tight-tolerance, end-use, load-bearing: CNC machining — every time.
Process Comparison Table
| Criterion |
CNC Machining |
3D Printing |
Bridge Injection Molding |
| Lead time (1 part) |
3–10 business days |
1–3 business days |
3–5 weeks (with tool) |
| Tooling cost |
None |
None |
$3,000–$25,000 (aluminum) |
| Per-part cost @ qty 10 |
$$ (moderate) |
$ (low) |
$$$$ (tool-dominated) |
| Per-part cost @ qty 1,000 |
$$$ (linear) |
$$$ (linear) |
$ (tool amortized) |
| Typical tolerance |
±0.001–0.005″ (tighter on request) |
±0.005–0.015″ (process-dependent) |
±0.003–0.005″ |
| Surface finish |
32–125 µin Ra; can be polished to mirror |
Layer lines visible; post-processing required |
As-tool; SPI A1–D3 achievable |
| Material range |
Aluminum, stainless, titanium, brass, copper, Delrin, PEEK, UHMW, Inconel |
PLA, ABS, nylon, TPU, resins, limited metals (DMLS) |
All commodity and engineering thermoplastics |
| Mechanical strength |
100% of wrought stock |
40–80% of injection molded equivalent (anisotropic) |
100% of resin spec (isotropic) |
| Geometry constraints |
Tool access required; internal radii ≥ ⅓ cutter dia |
Near-unlimited (overhangs need support) |
Draft angles, uniform wall, no undercuts |
| Best for quantity |
1–1,000+ (metal); 1–500 (plastic) |
1–50 (functional); 1–500 (cosmetic SLS/MJF) |
500–100,000+ |
| Certifications available |
AS9102 FAI, material certs, ITAR, CMMC L1 |
Limited (varies by vendor) |
Material certs; mold-flow reports |
Costs and lead times are industry-typical ranges as of 2026 and vary by geometry, material, and vendor capability.
CNC Machining for Prototypes
CNC machining starts from solid bar, plate, or billet and removes material to reach the final geometry. It is the only rapid prototyping process that delivers a part made from the production material with production mechanical properties — there is no anisotropy from layer adhesion and no shrinkage from a polymer melt.
Strengths
- Full strength of wrought stock — load testing on the prototype predicts production performance.
- Tolerances to ±0.0005″ achievable on multi-axis machines with proper fixturing.
- All wrought metals and engineering plastics — aluminum 6061/7075, stainless 304/316, titanium Ti-6Al-4V, Inconel 718, Delrin, PEEK.
- Surface finish controllable from 125 µin as-machined to mirror polished.
- Documentation: AS9102 First Article Inspection, CMM reports, material certifications, Certificates of Conformance.
Limitations
- • Internal geometry must be tool-accessible — deep narrow pockets, internal lattices, and enclosed cavities cannot be machined in a single piece.
- • Internal corners cannot be sharper than the cutter radius (typically ⅛″ minimum for milled pockets).
- • Per-part cost is linear with quantity — at quantities over a few thousand plastic parts, injection molding becomes cheaper.
See precision CNC machining for our shop capability detail, or prototype-to-production for how we scale the same setup from a one-off prototype into a documented production run.
3D Printing (Additive Manufacturing)
Additive processes build parts layer by layer from a digital model. The dominant industrial processes are FDM (fused deposition modeling), SLA / DLP (resin), SLS (selective laser sintering of nylon), MJF (HP Multi Jet Fusion), and DMLS / SLM (direct metal laser sintering).
Strengths
- Geometry is essentially unconstrained — internal channels, lattices, generative-design topologies, and conformal cooling are routine.
- Lead time of 1–3 days for a single part with no setup or fixturing cost.
- Per-part cost is geometry-driven, not feature-count driven — a complex shape costs about the same as a simple one of equal volume.
Limitations
- • Mechanical properties are anisotropic — Z-direction strength is typically 40–80% of XY strength for polymer processes.
- • Tolerances are looser (±0.005–0.015″ typical) and process-dependent. Tight-tolerance bores and threaded features almost always require post-machining.
- • Material selection is narrower than CNC. Production-grade thermoplastics like glass-filled nylon, ASA, and Ultem are available but cost considerably more than CNC plastics.
- • Surface finish shows layer lines and usually requires bead blasting, vapor smoothing, or machining for cosmetic or sealing surfaces.
- • DMLS metal parts require heat treatment, support removal, and often a finish-machining pass on critical surfaces — by which point the total cost frequently exceeds straight CNC.
Bridge Injection Molding
Bridge tooling uses a quick-turn aluminum mold (rather than hardened production steel) to produce 500 to 10,000 plastic parts at production-equivalent quality. It bridges the gap between prototyping and full production tooling.
Strengths
- Isotropic, fully dense parts in any commodity or engineering thermoplastic — ABS, PC, PC/ABS, glass-filled nylon, PEEK.
- Per-part cost drops to a few dollars or less at quantities above a few hundred.
- SPI surface finishes from textured to mirror-polished available straight out of the mold.
Limitations
- • Tooling lead time of 2–4 weeks before the first part — too slow for pure prototyping iterations.
- • Tooling cost of $3,000–$25,000 is sunk before any parts ship.
- • Geometry must follow molding rules — uniform wall thickness, draft angles, no undercuts without side actions.
- • Design changes after the tool is cut are expensive; significant geometry changes usually require a new tool.
Selection Decision Matrix
| Scenario |
Recommended Process |
Why |
| Aerospace structural bracket, 5 pieces for fit check + load testing |
CNC machining (7075-T6) |
Production material, full strength, AS9102 FAI available |
| Consumer electronics enclosure concept, 3 pieces for ergonomics review |
SLA 3D printing |
Cosmetic appearance, 1-day lead time, no functional load |
| Robotics gripper finger, 50 pieces for integrator trials |
CNC machined Delrin or MJF nylon |
Below molding break-even; needs production-equivalent strength |
| Medical instrument handle, 2,000 pieces for first clinical batch |
Bridge injection molding (glass-filled nylon) |
Tool amortizes, autoclavable thermoplastic, sterilizable surface finish |
| Suppressor baffle, 10 pieces, Inconel 718 |
CNC machining |
ITAR-controlled, requires CAGE-coded shop, tight tolerances on baffle stack |
| Internal cooling-channel manifold with topology-optimized geometry |
DMLS + CNC finish pass |
Internal channels can't be machined; sealing faces still need CNC |
Cost vs Quantity: The Crossover Points
Three useful rules of thumb when costing a prototype program:
- 1 to 10 pieces: CNC and 3D printing are competitive. 3D printing wins on lead time when material and tolerance permit; CNC wins on mechanical performance and surface finish.
- 10 to 500 pieces (plastic): CNC machined Delrin/PEEK or MJF/SLS dominate. Injection molding tooling has not yet amortized.
- 500 to 10,000 pieces (plastic): Bridge injection molding becomes the lowest unit cost.
- All quantities (metal, tight tolerance, ITAR-controlled, AS9102 required): CNC machining is the only practical path.
How Olympus Supports Rapid Prototyping
Olympus Machining is an ITAR-registered (CAGE 9V9P0), CMMC Level 1 precision CNC shop in Hanover, Pennsylvania. We do not run 3D printing or injection molding in-house — we focus on what we do best, which is multi-axis CNC machining of aluminum, stainless, titanium, Inconel, and engineering plastics with tolerances to ±0.0005″ and AS9102 Rev C First Article Inspection in-house.
For programs that need additive or injection-molded parts in addition to CNC, we work alongside the customer's chosen additive or molding supplier and own the machined components, CMM verification, and final assembly documentation. Material certifications with heat-lot traceability ship with every order.
Related capabilities: CNC milling, CNC turning, prototype to production, materials machined, quality assurance, and AS9102 First Article Inspection.
Frequently Asked Questions
Is 3D printing always cheaper than CNC for a single prototype?
No. For a small, simple metal part — a bracket, spacer, or fitting in aluminum — CNC is often comparable in cost and arrives as production-grade material with a measurement report. 3D printing wins on cost for large-volume plastic shells, complex internal geometry, and concept models where appearance is the deliverable.
Can a CNC prototype become the production part?
Yes. The same machining program, fixturing, and inspection plan used for a CNC prototype can be scaled directly into a production run with documented setups. This is the standard path for aerospace, defense, and medical components where the prototype must match the production article one-to-one.
What is "bridge tooling" and when does it make sense?
Bridge tooling is an aluminum injection-mold tool built quickly (2–4 weeks) and rated for 500–10,000 shots. It bridges the gap between CNC/3D-printed prototypes and a hardened steel production tool. It makes sense when you need production-equivalent plastic parts in mid quantities while the steel tool is being designed in parallel.
Which process supports the tightest tolerances?
CNC machining. Multi-axis CNC routinely holds ±0.001–0.005″ and can reach ±0.0005″ on critical features with appropriate fixturing and inspection. 3D printing tolerances are process-dependent and typically 3–5× looser. Injection molded tolerances depend on shrinkage and tool precision and typically fall in the ±0.003–0.005″ range.
Need a CNC Prototype Quoted?
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Last reviewed: June 9, 2026