Snap-Fit Design with FEA: How to Design Plastic Snap Fits That Don't Break

• Cantilever: a flexible arm with a hook at the end; by far the most common type.
• Annular: a ring that stretches over a bead, as on pen caps and some bottle closures.
• Torsional: a latch that twists about a small shaft, often used where the snap must be released by hand.

For a cantilever arm of constant rectangular section, beam theory gives a quick first estimate:

• Strain at the root: ε = 1.5 × t × y / L², where t is the arm thickness, y the deflection and L the arm length.
• Deflection force: P = b × t² × E × ε / (6 × L), with b the arm width and E the modulus (a secant modulus at the expected strain for plastics).
• Assembly force: W = P × (μ + tan α) / (1 − μ × tan α), where α is the lead-in angle and μ the friction coefficient.

These formulas are a good starting point, but they assume a sharp, perfectly fixed root and a uniform arm, and real snap fits have neither.

The cover image shows an FEA of an ABS hook with a 1.5 mm thick, 2 mm wide arm, a 0.5 mm root radius and a 30° lead-in, deflected 1.5 mm, the height of the hook, as it is pushed into place:

• The force needed to deflect it is about 1.8 N, close to the beam formula.
• Beam theory predicts about 37 MPa at the root. The FEA shows 44 MPa at the root radius, about 1.9 % strain: the local stress concentration a hand calculation misses.
• The highest stresses sit at the top and bottom of the root, the places where snap fits crack first.

That level of strain is close to the limit for a one-time snap in many unfilled plastics, so this design would be refined, as explained below, before tooling.

• Snap fits are designed on strain rather than stress, because plastics have no single clear yield point.
• The allowable strain comes from the resin supplier's design data. It is typically a few per cent for unfilled amorphous plastics such as ABS and PC, higher for tough semi-crystalline ones such as PP and PE, and much lower for glass-filled grades.
• Snap fits that are opened and closed many times need extra margin; suppliers commonly recommend around 60 % of the one-time value.
• Temperature and moisture matter: nylon, for example, gets softer and tougher as it absorbs water from the air.

• Increase the root radius: a sharp internal corner concentrates strain. A radius of around half the arm thickness is a common starting point.
• Lengthen the arm: strain falls with the square of the length, so a little extra length helps a lot.
• Taper the arm: making it thinner towards the tip spreads the strain along its length, so it can deflect further at the same peak strain.
• Reduce the hook height: just enough to hold, no more.
• Choose the angles: a lead-in of around 25° to 35° keeps the assembly force low, while the retention angle sets the holding force (90° for permanent, shallower for releasable).
• Choose the material: a tougher grade can solve a strain problem without changing the geometry.

• A nonlinear analysis with large deflection, contact and friction between the hook and the mating part.
• Material data for the actual grade, at the right temperature and moisture.
• Checks of peak strain against the allowable, insertion force against an ergonomic target, retention force, and permanent set after release.
• For snaps under constant load, a check of creep and relaxation over time.

Material choice often matters as much as geometry. In our FEA material comparison of a polymer rod guard, PA11 deflected 2.58 mm at 8 N while polycarbonate deflected 9.39 mm, about 3.6 times further, under exactly the same load. Our FEA of a nylon injection-moulded part shows the same attention to fillets and mesh refinement that snap fits need.

Every hook is an undercut. It needs a side action, a lifter or a pass-through hole in the part so a simple core can form it, and each option affects tooling cost. Keep the arm thickness in line with the surrounding walls to avoid sink marks, and add draft where the tool needs it. Our guide to plastic part design for injection molding covers walls, ribs and draft in more detail, and our guide to drop test simulation explains how to check that snaps hold when a product is dropped.

• 3D CAD of both mating parts, or drawings for our 3D CAD team to model.
• The plastic grade, or the candidates you are choosing between.
• Whether the snap is permanent or opened often, and the target assembly and retention forces.
• Operating temperature range and environment.