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Which plastic should I use for a snap fit, and how much can it bend?

Choose a resin that tolerates the strain the snap arm sees as it deflects. Published guide values for a one-off snap are roughly 6 percent or more for polypropylene and acetal, about 4 percent for polycarbonate, 2.5 to 3 percent for ABS and PC/ABS, and often only 1 to 2 percent for glass-filled grades. For snaps that are opened and closed repeatedly, design to about 60 percent of those values.

A snap fit is a hook or bead that bends as it is pushed past a catch, then springs back to hold the parts together. Because plastics creep and their stiffness changes with temperature, designers usually check snap fits against an allowable strain rather than a stress.

Published allowable strains (one-off assembly)

The two sources disagree on ABS, which shows why these are guide values: they come from different grades and test methods. BASF footnotes several of its figures as 70 percent of tensile yield strain. Always ask the resin supplier for the value for your exact grade.

The cantilever check

For a straight hook arm whose thickness does not change along its length, root strain works out to roughly:

strain = 1.5 × t × y ÷ L²

where t is the arm thickness, y the deflection needed to clear the catch, and L the arm length. This form is given in Polycon's guide; BASF's manual adds correction factors because real roots are not perfectly rigid.

Worked example (our numbers): an arm 1.2 mm thick and 12 mm long that must deflect 1.0 mm sees 1.5 × 1.2 × 1.0 ÷ 144 = 1.25 percent strain. That passes for polycarbonate even at the repeated-use limit of 0.6 × 4 = 2.4 percent. Lengthening the arm to 18 mm cuts the strain to about 0.56 percent, because length is squared.

Design points that prevent cracking

• Root radius. BASF recommends a fillet at the root of at least half the arm thickness; going much beyond that adds little strength and can cause sink or voids.
• Make it longer, not thicker. Strain rises with thickness and falls with the square of length.
• Taper the arm. Polycon notes a taper from root to tip spreads strain and can cut peak strain by a third or more.
• Lead-in and retention angles. Polycon suggests a 25 to 35 degree entry ramp; a retention face near 90 degrees is effectively permanent, while about 35 to 45 degrees allows release.
• Keep weld lines away from the root. A weld line at the root is a weak point.
• Plan the mould. Every hook is an undercut, so agree a side action, lifter or pass-through opening with the toolmaker.

Strain values are published guide figures from design manuals. The resin supplier's data for your grade and tests on moulded parts decide the final design.

Questions buyers ask

Why does nylon have two values?

Nylon absorbs moisture, which makes it softer and more ductile. BASF's manual notes that properties change between the dry as-moulded state and normal humidity, so both conditions should be checked.

Why did my 3D printed snap break when the moulded one works?

Printed parts are weaker between layers. Fictiv recommends halving the allowable strain for arms built in the vertical print direction.

Is polypropylene always the safest choice?

It tolerates high strain, but it is less stiff, so the snap holds with less force. Stiffer resins give a firmer click but less margin.

How do I check the design before tooling?

Calculate the root strain, then test printed or cast parts for feel. Confirm on moulded samples, at the coldest service temperature, for the number of cycles the product needs.

Sources

• BASF, Snap-fit design manual (PDF)
• Polycon International, Snap fit design: getting the strain right
• Fictiv, How to design snap fit components