For reliable snap-fits use PA11: 31% elongation withstands repeated flex cycles far better than PA12. Design the snap lip at least 2 mm thick at the base, with a fillet R >=1 mm, and keep 0.2-0.3 mm clearance in the fit. Test one iteration before you run a series; snap behaviour varies with geometry.
Why PA11 for snap-fits, not PA12
A snap-fit flexes elastically during clicking, returns to rest once closed. With PA12 (18% elongation) fine for one-time or a few cycles. On repeated use the snap lip risks fatigue crack.
PA11 with 31% elongation handles hundreds to thousands of click cycles without cracking. For snap-fits on product parts that end users open often (battery covers, service hatches, product housings) PA11 is therefore the standard choice.
Design rules for SLS snap-fits
Base thickness – At least 2 mm where the snap lip attaches to the part. Thinner risks cracking at the attach point.
Fillets – All internal corners R >=1 mm. Stress concentrations are the fastest route to cracking.
Snap-lip angle – 30-45° on the contact surfaces. Steeper = higher click force, shallower = less holding force.
Fit clearance – 0.2-0.3 mm between the snap protrusion and the mating opening. Too little: it will not click. Too much: it will not hold.
Cantilever length – 5-8x the base thickness for enough flex.
When a prototype test is necessary
For one-time (permanent) snap-fits often calculation suffices.
For repeatedly opened parts, or if you want to know exactly how firm the click feels: always a prototype test with 3-5 parts. Click open and closed, test at max load, measure click force with a spring gauge.
Based on results adjust geometry (thicker/thinner, more/less flex) before you commit to a series. Our engineers gladly join the DfAM check to spot potential crack points up front.