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Login with my Xometry account Snap-Fit Design Checker for Injection Molding
Cantilever snap-fit strain calculator for injection-molded thermoplastics. Check strain usage, assembly force, and moldability against the Bayer and BASF design guide methodology across six materials.
Default R = 0.4 × h, the ratio where the stress concentration curve starts to flatten. Current Kt: 1.85
This version models uniform cross-section arms only. Tapered arm support is planned.
Side view of the cantilever snap-fit with a uniform cross-section of thickness h, as the calculation assumes. Entry and retention angles are measured from the dashed datum line. The mating edge detail shows the recess the hook catches in; its height equals the undercut depth Y. The colored root zone tracks strain usage live based on your inputs.
of allowable strain for this material and lifecycle
The hook is an undercut. Add a through-slot in the wall directly under the hook so a straight-pull core can form it. Without the slot, the tool needs a lifter or side action, which raises tooling cost and lead time on your quote.
Apply at least 0.5 degrees of draft per side on the arm faces. Place ejector pins near the root, not on the flexing arm, to avoid witness marks at the highest-stress region.
Induced strain at the root
The classical cantilever beam relation from the Bayer and BASF snap-fit design guides. Strain falls with the square of arm length, which is why lengthening the arm is the most effective fix.
ε = 3 × Y × h / (2 × L²)The verdict: strain usage
Induced strain divided by the allowable strain for your material and lifecycle. Under 75 percent is a comfortable pass. Between 75 and 100 percent is marginal, because material scatter and molding variation can consume the remaining margin. Over 100 percent exceeds the material limit.
usage = ε / εallow × 100%Stress at the weak point
Nominal stress is the strain-dependent secant modulus times strain. The peak value applies a stress concentration factor Kt driven by the root fillet ratio R/h. The Kt curve is a smoothed engineering approximation, see the reference notes below.
σmax = Kt × Es × ε
Push-on and pull-off forces
Both come from the same friction wedge relation. The shallow entry angle gives a low assembly force, the steep retention angle gives a high holding force. That asymmetry is the entire point of a snap-fit.
F = FD × (tan θ + μ) / (1 − μ × tan θ)Reference notes: The stress concentration factor Kt uses a smoothed engineering approximation pending full digitization of a published Kt chart. Treat the peak stress value as a screening number, not a certified one. Tapered arms are not yet supported. All results assume a uniform cross-section along the arm length.
Data based on the Bayer and BASF snap-fit design guide cantilever beam methodology, with a stress concentration factor derived from a smoothed engineering approximation pending full digitization of a published Kt chart, and material properties from typical published ranges. Values are for reference and preliminary screening only. Xometry assumes no liability for design failures. Final design validation must consider material strength, joint geometry, load conditions, cycle count, and safety factors, and should be confirmed by FEA for critical or high-cycle applications.
Frequently asked questions
What formula does this calculator use for snap-fit strain?
ε = 3Yh / 2L², the classical cantilever beam relation from the Bayer and BASF snap-fit design guides, where Y is deflection, h is root thickness, and L is arm length.
Why does the verdict use strain instead of stress?
Allowable limits for plastics are published as strain, not stress, because plastics behave non-linearly well before failure. Strain is also the direct output of the beam equation, so it avoids compounding errors from an assumed modulus.
What counts as single use versus multiple use?
Single-use assumes the joint engages once and is never disengaged, so it can be strained closer to the material’s yield point. Multi-use assumes repeated engagement and reduces the allowable strain to roughly half, to guard against creep and fatigue.
Why does glass-filled nylon fail where unfilled nylon passes?
Glass fill raises stiffness dramatically but cuts allowable strain, since the fibers reduce the material’s ability to flex without cracking. The same geometry that’s safe in unfilled nylon can exceed glass-filled nylon’s strain limit.
How do I mold the hook without a side action?
Add a through-slot in the wall directly beneath the hook. A straight-pull core then passes through the slot to form the undercut face, and the tool opens without a lifter or side action. This is the most common cost-saving detail in snap-fit design and it also gives the mold a natural shutoff surface.
Can I use these results for final snap-fit design validation?
No. This tool is a preliminary screening aid. Material properties use typical published ranges, the Kt curve is an approximation, and real joints see temperature, moisture, and creep effects that the beam model does not capture. Confirm critical or high-cycle designs against supplier datasheet values and FEA before release.
What other snap-fit types exist besides the cantilever?
Annular snap-fits work well for round parts like caps and tube fittings, torsional snap-fits suit rocker-style latches, and U-shaped arms pack a long effective length into a short space. The cantilever covered by this tool is the most common type and the easiest to mold. For selection guidance across all snap-fit types, see the Xometry Pro guide to snap-fit joints for plastics.
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