Snap fits remove screws and assembly time, but only if they survive assembly and use. How to calculate strain, the allowable limits for common plastics, and the geometry tricks that make snap fits strong and forgiving.
The housing assembled beautifully at the prototype stage. In production, around one in fifty cantilever snap arms cracked at the root during assembly, and a few more broke the first time a technician opened a unit for service. The material was fine and the moulding was fine. The arm was simply too short and too thick for the deflection it needed, which put the strain at its root well beyond what the polymer could tolerate.
Snap fits are one of the best ways to remove fasteners, assembly time and cost from a product, as our snap-fit enclosure case study showed. Designing them comes down to one number: strain.
Why strain, not stress
Plastics do not behave like metals. Their stiffness changes with temperature, time and strain rate, so stress values from a datasheet can mislead. Strain, the amount the material stretches as a fraction of its length, is a far more reliable design limit. Material suppliers publish allowable short-term strain values for snap-fit assembly, which is a single or occasional deflection, and lower values for repeated use.
Typical allowable strains for a one-off assembly are roughly:
- PC: 4%
- ABS: 2.5%
- PC/ABS: 3%
- Nylon 66 (dry): 3%; (conditioned): 6%
- Acetal (POM): 6%
- Polypropylene: 6% or more
- Glass-filled grades: often 1–2%, much lower than the unfilled resin.
For snap fits that are released and re-engaged repeatedly, use around 60% of these values. Always confirm with the material supplier for the specific grade.
Calculating strain on a cantilever
For a straight cantilever arm of constant thickness, the strain at the root is approximately:
ε = 1.5 × t × y / L²
where t is the arm thickness, y is the deflection needed to clear the undercut and L is the arm length. Two things jump out of this formula:
- Strain rises in direct proportion to thickness.
- Strain falls with the square of length. Doubling the length cuts the strain by four.
So the first fix for a cracking snap arm is almost always to make it longer, not thicker. Tapering the arm, so it is thicker at the root and thinner at the tip, spreads strain more evenly along it and can reduce peak strain by a third or more.
Geometry that makes snap fits forgiving
- Root radius. A sharp corner at the root multiplies strain locally. A radius of about 0.5 times the arm thickness reduces stress concentration dramatically.
- Lead-in angle. A shallow entry ramp, typically 25–35°, reduces assembly force.
- Retention angle. A steep retention face, around 90°, makes the joint permanent; a lower angle, around 35–45°, makes it releasable.
- Arm placement. Keep arms away from areas where flow fronts meet, because a weld line at the root is a weak point.
- Undercut and moulding. Every snap feature needs a way to release from the tool: a slide, a lifter or a through-hole in the opposite wall. Agree this with the toolmaker early.
Beyond cantilevers
Annular snaps for round parts, torsional snaps and U-shaped arms that increase effective length in a small space are all options when space is tight. The same strain-based thinking applies.
Validate before steel
Use the strain formula to size arms at concept, then FEA to check the real geometry, including the root radius and the tool-side behaviour of the mating part. Confirm on machined or soft-tool samples in the production material, tested at the lowest operating temperature, where most plastics are least forgiving. Then test the assembly with the actual assembly method, not just by hand in the lab.
