What is a snap-fit in injection moulding?

A snap-fit is a moulded-in fastening feature — a deflecting beam or ring with a hook — that clips two parts together without screws or tools. It eliminates hardware and assembly labour, and it is the most common fastening method in plastic housings.

How do you calculate snap-fit strain?

For a constant-cross-section cantilever, maximum strain is approximately ε = 1.5 × h × δ / L², where h is beam thickness at the root, δ is deflection and L is beam length. Keep calculated strain below the material's permissible limit — roughly 2-4% for ABS/PC, more for flexible resins.

Which plastics are best for snap-fits?

Flexible, tough resins: PP and POM tolerate 6-8% strain, conditioned PA6/PA66 about 4-6%, PC and PC/ABS around 3-4%, ABS 2-3%. Glass-filled and brittle materials (PMMA, SAN, GF grades at 1-2%) make poor snap-fit beams — use them only with very gentle geometry.

Why do snap-fits break at the root?

Strain concentrates at the beam root, and a sharp internal corner multiplies it. Add a generous root radius (at least 0.5× beam thickness), taper the beam so bending spreads along its length, and avoid placing weld lines across the root — each measure materially raises fatigue life.

Can snap-fits be moulded without side actions?

Often yes. A hook whose face is drafted and shallow enough can release by 'bumping off' — the part flexes over the core during ejection — especially in flexible resins. Deeper or stiffer hooks need lifters or side actions, which add tooling cost. This is decided at DFM stage; Nordmould flags it in every review.

How many times can a snap-fit be assembled?

A snap-fit strained well below its permissible limit can survive hundreds to thousands of cycles; one assembled at 80-100% of its limit may survive only a few. Design service snap-fits (battery doors) for repeated assembly at 50% or less of the permissible strain, and verify with prototypes.

Snap-Fit Design Guide: Cantilever Geometry, Strain Limits & Materials

Snap-fits replace screws, clips and labour with geometry: a beam flexes during assembly, a hook locks, and the joint holds. Done well they are the cheapest fastening in manufacturing; done badly they are the first thing to snap in the field. The engineering is a strain calculation, a material choice, and a moulding check for undercuts — all three covered here.

What are the main snap-fit types?

Type	Geometry	Typical use	Notes
Cantilever hook	Straight beam with hook at tip	Housings, covers, battery doors	Most common; easiest to analyse and mould
Tapered cantilever	Beam thickness reduces toward tip	Same as above, higher cycles	Spreads strain; better fatigue life
Annular (ring)	Circumferential hook on round part	Caps, lids, lens mounts	Strong; assembly force depends on wall stiffness
Torsion bar	Arm rotates around a pivot	Clips, latches	Good where linear deflection space is limited
Ball-and-socket / catch	Local dimple engagement	Light latching, detents	Low force, wears over cycles

How do you size a cantilever snap-fit?

The working formula for a constant-section cantilever estimates maximum root strain:

ε ≈ 1.5 × h × δ / L² — where h is beam thickness at the root, δ is the deflection needed to clear the hook, and L is beam length.

The design levers follow directly: longer beams strain less (strain falls with L²), thinner roots strain less, and shallower hooks need less deflection. A beam 2 mm thick at the root, 20 mm long, deflecting 1.5 mm sees ε ≈ 1.5 × 2 × 1.5 / 400 ≈ 1.1% — comfortable for ABS. The same hook on a 10 mm beam sees about 4.5% — past the limit for ABS and into failure after a few cycles.

Tapering the beam (thicker at root, thinner at tip) spreads bending along the length and cuts peak strain for the same stiffness. Keep beam thickness at or above 50% of the adjoining wall so it fills and packs properly, and draft the beam faces 0.5-1° like any rib.

What strain can each material tolerate?

Indicative permissible strain for a single assembly at room temperature; reduce for repeated assembly, elevated temperature, or moulded-in stress. Confirm against the grade datasheet.

Material	Permissible strain (single assembly)	Snap-fit character
PP / PE	6-8 %	Excellent — the classic clip resin
POM (acetal)	5-7 %	Excellent, springy, low friction
PA6 / PA66 (conditioned)	4-6 %	Very good; note dry-as-moulded is stiffer
TPE / TPU	10 %+	Extremely flexible, low retention force
PC	3-4 %	Good; watch chemical exposure
PC/ABS	2.5-3.5 %	Good general housing choice
ABS	2-3 %	Adequate for gentle clips
PBT / PA + 30% GF	1-2 %	Poor — only shallow deflections
PMMA / SAN / PS	1-2 %	Brittle — avoid snap-fits

Why do snap-fits fail in practice?

Three causes account for most field failures. Root stress concentration: a sharp internal corner at the beam root multiplies local strain — specify a root radius of at least 0.5× beam thickness. Weld lines across the beam: a knit line through the root can halve practical strength, so gate location must keep weld lines off snap features — a gate-location decision, not luck. Creep: a permanently deflected snap relaxes over weeks; if the hook must stay engaged under load, check the resin's creep data or add a positive lock.

What does a snap-fit do to the mould?

A snap hook is an undercut. The three tooling outcomes, cheapest first: bump-off release, where a shallow drafted hook flexes over the core at ejection (works in PP/POM/PA, risky in brittle resins); lifters, angled pins that form the hook and release it during ejection; and side actions for external hooks. Lifters and sliders typically add from €1,000-5,000+ per direction to the tool.

Orientation is free money: rotating the hook so it releases in the opening direction, or windowing the wall behind the hook so a shut-off forms it, can eliminate the mechanism entirely. Nordmould's DFM review checks every snap-fit for bump-off feasibility and weld-line exposure before quoting the tool.