| name | snap-fits |
| description | Snap-fit design — cantilever snap (deflection, strain, mating force), annular snap (ring snap), torsional snap, permissible strain by polymer (semi-crystalline vs. amorphous), undercut geometry, mold release requirements (draft, side actions), assembly force calculation, retention force, fatigue for multiple-cycle snaps, material selection (POM, PA, PP, ABS, PC), and DuPont/Bayer design guides. |
| metadata | {"priority":7,"promptSignals":{"phrases":["snap fit","snap-fit design","cantilever snap","snap feature","plastic snap","living hinge"],"minScore":3}} |
Snap-Fit Design — Complete Skill
Snap-Fit Types
Cantilever Snap
Most common type: arm (hook) attached at base; deflects during assembly to clear undercut; springs back to lock
Geometry parameters:
- L = arm length [mm]
- h = arm thickness at root [mm]; h_t = at tip (tapered arms)
- b = arm width [mm]
- y = deflection (undercut depth) [mm]
- α = lead angle (mating chamfer) [°]; β = retention angle [°]
Annular (Ring) Snap
Used for circular assemblies (caps, caps over cylinders):
Ring deflects inward/outward over undercut bead; snaps into groove
Example: pen cap, bottle cap, electrical connector backshell
Torsional Snap
Less common: arm twists about its axis during assembly; used for rotational engagement
Cantilever Snap — Mechanical Analysis
Deflection at Tip
Uniform (constant section) beam:
y = (ε_s × L²) / (1.5 × h) [mm; y = deflection = undercut height; ε_s = permissible strain; L = arm length; h = arm thickness]
Tapered beam (h at root, h_t = h/2 at tip):
y = (ε_s × L²) / (1.5 × h) [same formula; tapering does not change deflection if same ε; but reduces stress at root → allows greater deflection at same strain]
Actually for tapered: y_taper = 0.9 × ε_s × L² / (1.5 × h) [correction for taper shape factor ≈ 0.9 for h_t = 0.5h]
Preferred: tapered arms → lower stress for same deflection → less fatigue risk
Maximum strain at root:
ε_root = (3/2) × (h/L²) × y [for uniform beam]
ε_root ≤ ε_s_allow (permissible strain from material data or design guide)
Example:
PA66 GF30: ε_s = 0.02 (2%); L = 20 mm; h = 2 mm; b = 5 mm
y_max = 0.02 × 20² / (1.5 × 2) = 8/3 = 2.67 mm (allowable deflection = undercut limit)
Mating Force and Retention
Assembly force (during mating; arm deflects):
W_mates = F_assembly = (E × b × h³ × y) / (4 × L³) × μ + tan(α) / (1 - μ × tan(α)) [simplified; μ = friction coefficient; α = lead angle]
DuPont simplified formula:
P = (E × b × h³ × ε_s) / (4 × L) × [(μ + tan(α)) / (1 - μ × tan(α))]
Simplification (no friction, α = 30° lead angle):
P ≈ (E × b × h² × y) / (4 × L³) × 1.155 [F in N; for µ ≈ 0.3, α = 30°: factor ≈ 0.87]
Retention force (arm locked; force to disengage):
P_retention = (E × b × h³ × y) / (4 × L³) × [(tan(β) - μ) / (1 + μ × tan(β))]
For β = 90° (perpendicular wall — cannot be removed without special tool): P_retention = ∞ (permanent)
For β = 45°: P_retention > P_assembly (partial retention)
For β < tan^(-1)(μ): P_retention ≤ 0 (no retention — freely releases; pushbutton action)
Permissible Strain by Material
One-time assembly:
Use ε_s = short-term yield strain (from stress-strain curve)
Multiple assembly cycles (snap and release):
Use reduced ε_s (fatigue-based; reduced from single-cycle value)
Generally: multi-cycle ε_s = 0.5 × single-cycle ε_s
Permissible strain by polymer:
| Material | Modulus E [MPa] | Single Assembly ε_s [%] | Multi-Cycle ε_s [%] | Notes |
|---|
| Acetal (POM) | 2,700–3,100 | 4–6 | 2–4 | Excellent snap material; high stiffness, fatigue resistance |
| PA 66 (Nylon 66) | 2,500–3,200 | 3–5 | 1.5–2.5 | Absorbs moisture (E decreases); good for single-cycle |
| PA 66 GF30 | 7,000–9,000 | 1.5–2.5 | 0.8–1.2 | Glass fibers reduce ductility; lower permissible strain |
| Polypropylene (PP) | 1,200–1,700 | 8–12 | 4–6 | Very flexible; high strain; low modulus → long arm needed |
| ABS | 2,000–2,800 | 2–4 | 1–2 | Brittle notch sensitivity; avoid sharp corners |
| PC (Polycarbonate) | 2,300–2,700 | 3–4 | 1.5–2 | High impact; good for moderate cycles |
| POM-C | 2,800–3,200 | 4–7 | 3–5 | Better than POM-H for snap applications |
| PBT | 2,200–3,000 | 2–3.5 | 1–2 | Brittle above Tg; better at elevated temperature than ABS |
| TPU/TPE | 10–100 | 30–60 | 20–40 | Very flexible; integral gasket + snap; low retention |
Environmental correction:
Elevated temperature: E decreases → arm deflects more; ε_s may decrease (creep)
Humidity (PA): E decreases 30–50% at saturation (PA66 dry: 3,200 MPa; wet: 1,500 MPa)
Design for worst-case environmental condition
Annular Snap Fit
Ring Deflection
Permissible undercut (Bayer formula):
y/D_outer = ε_s × (D_outer/wall_t) × 0.7 [approximate for thin-walled circular snap]
Alternative: y/r = 2 × ε_s × (1/(t/D)) [y = diametral deflection; r = radius; t = wall thickness]
Assembly force for annular snap:
F = 2 × P_max × sin(lead_angle) × A_wall_section × μ_factor
Typically lumped into: F_assembly = K × ε_s × E × A_cross [K = geometry constant from FEA or Bayer tables]
Push-off force (retention):
F_retention = P_latch_face × A_snap_face × tan(retention angle - friction angle)
Arm Geometry Design Recommendations
DuPont Design Guidelines
Root radius:
Minimum r_root = h/4 (prevent stress concentration at base)
Kt at sharp corner: Kt ≈ 3; with r = h/4: Kt ≈ 1.5 → doubles allowable cycles
Arm width:
b ≥ 0.5 × h (minimum); b = h to 2h typical
Wider → higher force (linear); more flexible in twisting
Undercut geometry:
y_undercut = 0.5 to 0.8 × y_max_allow (use safety factor; material variability)
Total engagement: y_undercut = perpendicular displacement available for locking
Lead angle α:
Easy assembly: α = 15–30°; standard: α = 30°; tamper-evident: α = 45°
No lead (α = 0°): requires tool for assembly (permanent snap for serviceability restraint)
Mold Design Considerations
Undercut Release
Draft angle: minimum 0.5–1° per side for arm sides to eject without binding
Side action / side core: required for undercut perpendicular to mold opening direction
Alternative: self-releasing (flexible arm naturally releases as mold opens — only if α allows)
Self-releasing criteria:
β_release + α_lead ≤ 90° AND arm can flex enough during ejection
Arm must remain deformed long enough to clear mold → verify with ejection simulation
Gate location: away from snap arm root; flow direction should not create weld line at high-stress root
Multiple-Cycle Fatigue
S-N approach:
For N cycles, use S_N = S_endurance_limit × CF
CF = fatigue correction factor (surface, notch, environment)
Compute σ_max_snap = E × ε_root = E × (3/2) × h × y / L²
Compare with S_N; verify σ_max_snap × Kt ≤ S_N (modified endurance)
POM fatigue limit:
σ_endurance ≈ 28 MPa (at 10⁷ cycles, R = -1); adjust for mean stress (Goodman)
For snaps with only one-direction loading (R = 0): σ_endurance ≈ 0.5 × σ_y (conservative)
Standards and References
| Standard | Scope |
|---|
| DuPont "Design Handbook for Living Hinges and Snap-Fits" | Primary design reference |
| Bayer AG "Snap-Fit Joints for Plastics" | European reference guide |
| Tres "Designing Plastic Parts for Assembly" | Textbook |
| ASTM D638 | Tensile properties of plastics (source for E, σ_y, ε_y) |
| ISO 527 | Tensile testing of plastics |
Output
Provide: application (assembly frequency: single/multi-cycle; N_cycles; operating T [°C]; environmental: humidity/chemical exposure), material selection (polymer grade; E [MPa] at operating T; single-cycle ε_s [%]; multi-cycle ε_s [%]; moisture correction if PA), snap type (cantilever/annular/torsional; basis), arm geometry (L [mm]; h [mm]; b [mm]; taper ratio h_t/h; r_root [mm]), deflection check (y_max from formula [mm]; undercut y_design [mm]; margin = y_max - y_design > 0), strain verification (ε_root [%] = 1.5×h×y/L² ≤ ε_s_allow: pass/fail), assembly force P [N] (with lead angle α [°]; friction µ; formula used), retention force P_ret [N] (with retention angle β [°]), fatigue check (if N > 1: σ_max_snap [MPa] vs. S_N [MPa]; Kt at root; fatigue life estimate [cycles]), mold release (self-releasing: yes/no; side action required: yes/no; draft angle [°]), and reference (DuPont Design Handbook; Bayer Snap-Fit Guide).
