Snap-Fit Strain Limits for 3D Printing

How much strain can a printed snap-fit arm safely take?

Bayer and DuPont publish permissible strain limits for snap-fit arms in named plastics. Both figures are molded, isotropic data, so an FDM arm printed with its layers running across the bending direction fails at a strain far below either table, and neither manual measured a printed part.

What strain does Bayer publish for its own resins?

Bayer's Snap-Fit Joints for Plastics manual lists permissible short-term strain at 23 C for six named resins, from 2.0 percent for its 20 percent glass-filled Makrolon up to 4.0 percent for unfilled Apec and Makrolon PC. Bayer recommends derating to about 60 percent of these figures when a joint separates frequently rather than once.

What do Bayer's deflection equations say about beam shape?

Bayer's formula for a constant-section snap arm is deflection equal to 0.67 times allowable strain times arm length squared, divided by arm thickness, and the company gives 1.09 as the same coefficient for a beam tapered in thickness or 0.86 for one tapered in width. Bayer's own optimum root fillet radius is 0.6 times arm thickness, and the manual states in its own words that a thickness taper increases the permissible deflection by more than 60%.

Bayer's own coefficients, 0.67 for a constant section and 1.09 for one tapered in thickness, are printed exactly as Bayer published them. A claim sometimes repeated without a source, that a tapered beam allows twice the deflection of a constant-section one, is not what Bayer's own coefficients or its own wording of more than 60% support.

What strain does DuPont publish for its own resins?

DuPont's Table 9.04 lists permissible strain for a snap arm flexed once versus one flexed repeatedly, across eight named resin grades. The used-once figures run from 0.8 to 1.2 percent for glass-filled Zytel nylon up to 20 percent for Hytrel, and every used-frequently figure in the table is lower than its own used-once figure for the same resin.

DuPont assigns a stress concentration factor, C equal to 1.0 for a filleted corner and 2.0 for a poorly rounded one, the same corner-radius effect Bayer's own root fillet guidance addresses separately.

Are any of these resins something we actually print?

No: every resin named above, polycarbonate, polycarbonate blends, acetal, nylon, polyester, and thermoplastic elastomer grades, is an injection molding material, not one we print. The tables are useful here for the beam mechanics behind a snap-fit design, not as a strain allowance for a PLA, PETG, or ABS part made on an FDM printer.

Does layer orientation change how a printed snap fit actually fails?

A cantilever snap that bends across its own layer lines relies on interlayer strength, which our orientation page shows printing weaker and less consistent than in-plane strength across every published FDM study. No published figure states a safe strain for a printed snap arm at any orientation, so the deflection figures above describe molded plastic mechanics only.