Snap-Fit Design Data

The three numbers a snap-fit design actually turns on: how much strain the material will take, how much friction the mating pair has, and what a tapered beam buys you.

Permissible strain

There is no single figure — it depends on the class of plastic and on how many times the joint must come apart. As orders of magnitude, unfilled materials land near 6% and filled ones near 1.5%.

Material class	Referenced to	Fraction	Notes
Semi-crystalline (PP, PE, POM, PA, PBT)	Yield strain	≈ 100%	May be strained almost to the yield point for a single brief assembly.
Amorphous (PC, ABS, PS, PMMA, PVC, SAN)	Yield strain	70%	Amorphous grades are less forgiving of a fast, deep deflection.
Glass-fibre reinforced	Elongation at break	50%	No distinct yield point, so the limit is referenced to break instead.
Any of the above, repeated assembly	— of the value above	60%	Applies whenever the joint is engaged and disengaged more than once.

Friction coefficients

Guide values for plastic on steel. The multiplier applies when both parts are the same plastic — like on like always grips harder, which is exactly the case when a snap joins two mouldings of one material. Friction between two different plastics is equal to or slightly below the steel figure.

Material	µ on steel	Same-material factor	µ like-on-like
PTFE	0.12–0.22	—	—
PE (rigid)	0.20–0.25	× 2.0	0.50
PP	0.25–0.30	× 1.5	0.45
POM	0.20–0.35	× 1.5	0.52
PA	0.30–0.40	× 1.5	0.60
PBT	0.35–0.40	—	—
PS	0.40–0.50	× 1.2	0.60
SAN	0.45–0.55	—	—
PC	0.45–0.55	× 1.2	0.66
PMMA	0.50–0.60	× 1.2	0.72
ABS	0.50–0.65	× 1.2	0.78
PE (flexible)	0.55–0.60	× 1.2	0.72
PVC	0.55–0.60	× 1.0	0.60

Tapered-beam K factor

Multiply the permissible deflection of a parallel beam by K for a beam whose thickness falls linearly from h₀ at the root to h_L at the snap foot. At h_L/h₀ = 0.50, K = 1.636 — the tapered beam takes 64% more deflection for the same root thickness and the same strain.

hL/h₀	K	hL/h₀	K	hL/h₀	K	hL/h₀	K
0.33	2.137	0.50	1.636	0.67	1.338	0.84	1.138
0.34	2.098	0.51	1.614	0.68	1.324	0.85	1.128
0.35	2.060	0.52	1.593	0.69	1.310	0.86	1.118
0.36	2.024	0.53	1.573	0.70	1.297	0.87	1.109
0.37	1.989	0.54	1.553	0.71	1.284	0.88	1.100
0.38	1.956	0.55	1.534	0.72	1.272	0.89	1.091
0.39	1.924	0.56	1.515	0.73	1.259	0.90	1.082
0.40	1.893	0.57	1.497	0.74	1.247	0.91	1.073
0.41	1.863	0.58	1.479	0.75	1.235	0.92	1.064
0.42	1.834	0.59	1.462	0.76	1.223	0.93	1.056
0.43	1.806	0.60	1.445	0.77	1.212	0.94	1.047
0.44	1.780	0.61	1.429	0.78	1.201	0.95	1.039
0.45	1.754	0.62	1.413	0.79	1.190	0.96	1.031
0.46	1.729	0.63	1.399	0.80	1.179	0.97	1.023
0.47	1.704	0.64	1.382	0.81	1.168	0.98	1.015
0.48	1.681	0.65	1.367	0.82	1.158	0.99	1.008
0.49	1.658	0.66	1.352	0.83	1.148	1.00	1.000

Size a joint on the Cantilever Designer, Annular or Torsional tools, and read the reasoning in the interactive snap-fit guide. Sources: Ticona/Celanese "Snap-Fits for Assembly and Disassembly" (K factors) and Bayer "Snap-Fit Joints for Plastics" (strain rules, friction). Always prefer the strain figure your material supplier gives for the specific grade.

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