The short answer
Choose **PA66 (nylon 66)** CNC prototypes when you need electrical insulation, lower weight, snap-fit-like toughness, or chemical/wear behavior closer to a polymer production intent. Choose **aluminum** (typically 6061-T6) when you need metal stiffness, heat spreading, tight metal fits, threads that will be reused hard, or a stand-in for eventual metal production. Price and speed favor aluminum more often than buyers expect—PA66 machines fast but stock and fixturing quirks matter.
Figures below use **rates.v1.1 estimate** inputs (PA66 bar ~**$5/kg**, CNC time factor ~**×0.8**; Al6061 ~**$6/kg** with baseline machine rates)—planning bands only. Get a live range on [quote-new.php](https://3dwk.com/quote-new.php).
Material behavior that changes the prototype decision
**Aluminum 6061-T6:** stiff, stable, familiar fits, easy to anodize or chem-film later ([6061 vs 7075](https://3dwk.com/resources/6061-vs-7075-aluminum-for-machined-parts), [chem-film vs Type II](https://3dwk.com/resources/chem-film-vs-type-ii-anodize-aluminum)). Threads and inserts behave like production metal ([thread inserts vs tapped holes](https://3dwk.com/resources/thread-inserts-vs-tapped-holes-aluminum)).
**PA66:** tough engineering plastic, moisture absorption can move dimensions, softer threads, excellent for non-conductive fixtures and many housings that will later be molded. See also [engineering plastics PEEK/POM/nylon](https://3dwk.com/resources/choosing-engineering-plastics-peek-pom-nylon). Dry vs conditioned state should be noted if fits are tight.
Cost and lead (illustrative)
Both materials cut quickly relative to stainless or nickel. PA66’s lower machine-time factor can reduce cycle cost, but:
- Soft jaws / careful clamping to avoid crush
- Moisture and burr behavior
- Limited finish options vs anodize on aluminum
Aluminum often wins when the same CAM can later scale to 7075 or production metal. Polymer print (SLS PA12, etc.) is a third path for early form/fit—see [SLA vs SLS vs MJF](https://3dwk.com/resources/sla-vs-sls-vs-mjf-prototypes) and [CNC vs 3D printing](https://3dwk.com/resources/cnc-vs-3d-printing-when-to-choose).
Illustration 1 is a decision matrix: stiffness, insulation, heat, threads, cosmetics, production intent.
Example part
**Example part:** 100×60×12 mm cover with six M3 mounts and a central boss. In aluminum it mimics a die-cast/CNC lid; in PA66 it mimics a molded cover. Label RFQ notes “Example part — material trade study.”
Illustrative **rates.v1.1** one-off bands for a simple cover often sit in a modest two-digit to low three-digit USD region depending on pockets and qty breaks—confirm with upload; public UI shows min/mid/max style ranges.
When PA66 is the better prototype
- Must prove insulation or non-galling contact with mating metal
- Weight targets that aluminum would miss even as a mockup
- Wear pads, guides, or lightly loaded structural plastics
- Design will be injection molded PA66 later—geometry should already avoid metal-only thin walls that crush in plastic ([injection molded ABS cover tips](https://3dwk.com/resources/injection-molded-abs-cover-design) for plastic thinking)
When aluminum is the better prototype
- Stiffness, heat, or EMI shielding stand-ins
- Repeated assembly of threads
- Optical or sealing flatness that plastics struggle to hold ([flatness/parallelism](https://3dwk.com/resources/flatness-parallelism-perpendicularity-machined-parts))
- Path to anodized cosmetics for stakeholder reviews
DFM habits for either choice
- Realistic wall thickness for the material
- Tool radii in pockets ([pockets/thin walls](https://3dwk.com/resources/cnc-pockets-thin-walls-undercuts))
- Tolerances matched to need ([GD&T basics](https://3dwk.com/resources/gdt-basics-for-buyers))
- State moisture condition expectations for PA66 fits
Illustration 2 shows RFQ wording: “Prototype in PA66 for insulation” vs “Prototype in 6061 standing in for production Al.”
What to do next
Pick the property you are actually prototyping (stiffness vs insulation vs mold intent). Upload STEP on [quote-new.php](https://3dwk.com/quote-new.php). Related: [CNC service](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php), cost and lead guides.
Moisture, tolerances and inspection on PA66
Nylon moves with humidity. If a bearing bore must stay put, either open the tolerance, control conditioning, or pick a more stable plastic (POM often machines beautifully for dimensional demos). Write the acceptance condition: “measure after 24 h at 23 °C / 50% RH” or “dry as-machined.” Otherwise vendor and buyer measure different parts.
Aluminum avoids that class of argument. That alone wins many prototype debates when the question is fit to metal mating parts.
Threads, inserts and rework
PA66 tapped holes tolerate fewer aggressive reassemblies than aluminum. For demo units that will be opened daily, specify brass inserts or switch the prototype to aluminum even if production will be plastic. Conversely, if you are proving a molded boss design, CNC PA66 with molded-like draft notes may teach more than a metal stand-in.
Quantity breaks and the prototype lie
Qty 1 aluminum and qty 1 PA66 can land near each other after min charges. At qty 50 the story depends on cycle time and stock. Use the quote page qty breaks rather than extrapolating linearly from a single blog example. **rates.v1.1** includes stepped factors—read the live band when you change qty.
Stakeholder communication tip
Label photos and spreadsheets **Example part** or “material trade study” so nobody files them as qualified production evidence. Prototype purpose statements (“insulation check only”) prevent the wrong finish or cert from being assumed.
Finish and cosmetics on prototypes
Aluminum prototypes can take chem-film or Type II anodize for stakeholder demos that look closer to production covers. PA66 usually stays as-machined or lightly tumbled; paint adhesion and dye stories differ from metal. If the review is really about color and sheen, say so—material choice may matter less than a simple mock finish on either substrate ([bead blast vs tumble](https://3dwk.com/resources/bead-blast-vs-tumble-vs-vibratory-finish), [powder vs paint](https://3dwk.com/resources/powder-coating-vs-painting-metal-parts)).
Hybrid prototype programs
A practical sequence many teams use:
1. Polymer print for envelope and cable routing
2. CNC PA66 or POM for mechanical interfaces that must flex or insulate
3. CNC aluminum for thermal and stiffness risk items
4. Freeze production material intentionally—not by accident from whichever prototype looked nicest in a photo
State that sequence in RFQ notes so quotes are not mixed across intents.
Common questions
Is PA66 always cheaper to CNC than aluminum?
Not guaranteed. Machine-time factor helps PA66, but stock, scrap risk and careful feeds can offset it. Compare ranges on the same model.
Can I switch from PA66 prototype to aluminum without redrawing?
Often with wall and thread reviews. Plastic-safe thickness may be thicker than metal needs—or vice versa.
Should I 3D print PA12 instead of CNC PA66?
For early form/fit, powder-bed PA12 is often faster. For machining-representative edges, threads and strength closer to stock nylon, CNC PA66 fits better.
Does the quote engine know PA66?
**rates.v1.1** includes PA66 bar and ×0.8 time factor as estimates. If a build still returns **needs_manual**, geometry or rules triggered review—upload anyway.