Why Do Perfect CAD Snap-Fits Break on the Assembly Line?

You’ve designed a plastic enclosure. The CAD model is flawless. But the moment the assembly team snaps the two halves together, the cantilever hooks shear off at the base.

The culprit is usually a failure to calculate maximum strain during deflection.

Many designers focus entirely on the holding force (the undercut) and forget that the plastic must temporarily survive the bending phase during assembly. If the strain at the base of the hook exceeds the material's allowable short-term yield limit, it will break before it ever locks.

Here is the fundamental equation for calculating maximum strain (ε) at the base of a uniform rectangular cantilever snap-fit:

ε = (3 · Y · h) / (2 · L²)

Where:

• ε = Strain at the base
• Y = Deflection amount (the depth of your undercut)
• h = Thickness of the hook at the base
• L = Length of the cantilever arm

💡 The Example

Let’s say you are molding a part in a standard PC/ABS blend, which typically has an allowable short-term strain limit of about 4.0% (0.04).

You design a hook with an undercut (Y) of 2.5 mm, a thickness (h) of 2.5 mm, and a length (L) of 15.0 mm.

Let's do the math:

ε = (3 · 2.5 · 2.5) / (2 · (15)²) ε = 18.75 / 450 ε = 0.0416 (or 4.16%)

The Result: 4.16% exceeds the 4.0% limit. The moment the operator pushes the parts together, the outer fibers at the base of the hook yield. Even if it doesn't snap off completely, it suffers permanent deformation, resulting in a loose, rattling assembly.

🛠️ The Solution

If you realize your strain is too high, you have three mechanical options:

1. Increase the Length (L): Because length is squared in the denominator, this is your most powerful lever. Increasing L from 15 mm to just 18 mm drops the strain to 2.8%—well within the safe zone, without sacrificing holding force.
2. Decrease the Thickness (h): Thinning the hook will reduce strain, but be careful—it will also reduce your insertion and retention forces linearly.
3. Taper the Beam: Transitioning from a uniform beam to a tapered beam distributes the bending stress more evenly along the length of the arm rather than concentrating it entirely at the base.

Stop guessing if a snap-fit will hold. Run the strain equation first.