Choose a material for functional prototypes

A visual prototype only needs to look right. A functional prototype has to work: snap fits must snap, hinges must flex, housings must survive a drop and brackets must carry load. The goal is a material that behaves close enough to the final production material that your test results mean something.

What fails

• Brittle snap fits. Many standard resins and PLA crack the first time a clip flexes.
• Misleading results. A printed part that is much weaker, or much stiffer, than the final moulded part gives false passes or false failures.
• Layer weakness. FFF parts often break along layer lines, which a moulded part would not.
• Tolerance stack-up. Shrinkage and warping make mating parts fit badly.

Which properties matter

Match the property that controls your test.

• For snap fits and clips, look at elongation at break and flexural modulus. You want enough stretch to deflect and enough stiffness to hold.
• For drop tests, look at impact strength, ideally the notched value.
• For load-bearing parts, look at tensile strength and Young’s modulus.

Compare the printed material with the datasheet of your target production material. Our article on comparing brands explains how to compare values fairly, and typical vs minimum values explains what the numbers promise.

Material families that usually work

• Tough and durable resins are designed to imitate ABS or PP. They give fine detail and good snap-fit behaviour. Our engineering resins show elongation at break of 1–180 % (median 15 %, 63 materials).
• PETG, ABS and PC filaments are cheap, tough and close to common moulded plastics.
• Nylon, as filament or powder, is a strong stand-in for moulded nylon and many tough plastics.
• PA12 and PA11 powders give isotropic, tough parts that test well. Our SLS materials show tensile strength of 8–84 MPa (median 47 MPa, 17 materials).

Process considerations

FFF is the fastest and cheapest way to iterate, but properties change with direction. Resin gives smooth, accurate, nearly isotropic parts, which is ideal for fit and snap tests. Powder gives the closest thing to production-grade toughness, at a higher cost per part. Many teams use FFF early, then move to resin or powder for the final rounds. See the process guide.

Design and post-processing tips

• Orient for the test. Print clips so they flex along layers, not across them. See print orientation.
• Add fillets at the root of clips and bosses.
• Allow clearance. Start with a fit test coupon on your printer before printing the full assembly.
• Post-cure resins fully. Under-cured parts are weaker and more flexible than the datasheet.
• Condition nylon before testing. Dry nylon is stiffer and more brittle than nylon that has absorbed moisture from the air.

A shortlist from our data

For drop and impact testing, these engineering filaments have the highest notched impact strength:

1. PLA 172.4–38.9
2. PETG 92.6–11.6
3. PC 53–21.28
4. ABS 106.1–38.6
5. PA6 38.9–35.6
6. PC/ABS 313.3–53

Bar = lowest to highest, dot = median. 47 materials, live from our database.

Common mistakes

• Testing snap fits in a brittle standard resin.
• Treating a printed pass as proof the moulded part will pass.
• Ignoring direction in FFF parts.
• Comparing printed values from one test standard with moulded values from another. See ASTM vs ISO.

See all engineering materials 229 materials