Cantilever Snap Fit Design: Hook Sizes for PLA and PETG

A cantilever snap fit works when the hook is no taller than the arm can bend without cracking: hook height = 0.67 x strain x length² / thickness for a straight arm, or 1.09 x strain x length² / thickness for an arm that tapers to half its thickness at the tip. Use a strain of about 2% for PLA and 2.5% for PETG on a joint you close once or rarely, and half that for a clip opened every day.

So a PLA arm 12 mm long and 1.6 mm thick at the root can clear a 1.2 mm hook if it is straight, or a 2 mm hook if it is tapered. The tables below do the arithmetic for common sizes. Our snap-fit guide covers the parts of a joint and how to print it; this page is the math.

The two formulas

These are the standard cantilever equations from the plastics industry's snap-fit design guides (Bayer, now Covestro, and BASF). Measure the length from the root of the arm to the point where the hook touches the mating part, not to the very tip.

L is the length, h is the thickness at the root and strain is a fraction (2% is 0.02). The tapered arm clears a 63% taller hook at the same strain, because the bend spreads along the arm instead of piling up at the root. That is why tapering is the first fix for a clip that cracks.

Where 2% and 2.5% come from

Strength divided by stiffness gives, roughly, the strain at which a material reaches its full bending strength. From the Bambu Lab datasheets for the PLA and PETG we print, measured in the direction a flat-printed arm bends:

The design strains are about 70% of the strain at full strength, the margin the Covestro guide gives non-crystalline plastics for a single snap, rounded down. For joints opened again and again the guide drops to 60%; we use half, because printed parts have layer lines and small voids that molded ones don't. Treat these as starting points and print a test.

Hook height tables

The largest hook height in mm, straight arm / tapered arm. Width doesn't change these numbers, only the force. If the mating part leaves a gap, the arm only bends by the hook height minus the gap, which adds margin.

PLA, closed once or rarely (2%)

PETG, closed once or rarely (2.5%)

For a clip opened every day, halve the number. Hooks over about 2 mm are rarely worth it even when the arm allows them: the joint gets hard to close and the hook takes up room inside the part.

When the 1/8 rule is wrong

A common rule says an arm can bend 1/8 of its length. Run it through the strain formula and the answer depends on thickness: a straight arm bent by 1/8 of its length sees a strain of 0.19 x h / L. An arm 15 mm long and 1.5 mm thick (10 to 1) comes out at 1.9%, fine for PLA. An arm 10 mm long and 2 mm thick (5 to 1) comes out at 3.8%, more than PLA's 2.8% at full strength, and it cracks. The rule holds only for arms at least ten times longer than they are thick.

How hard it is to push together

The force to bend the arm, times a factor for the lead-in angle and friction, is the force to close the joint. For an arm 12 mm long, 1.6 mm thick and 6 mm wide at its design strain, assuming a friction of 0.3:

12.5 N is about the weight of 1.3 kg: a firm push with a thumb. The same formula with the return angle gives the force to pull it apart. A 90 degree return face never lets go without breaking something; a 45 degree return comes apart with a firm pull, which is what you want on a lid or a battery door.

Print it lying flat

Bending strength drops when the arm bends across layer lines: PLA goes from 76 MPa to 59 MPa, PETG from 75 to 56. Draw the arm as a side profile and print it lying on its side, so every layer contains the whole arm. Round the inside corner at the root with a radius of at least half the thickness.

What a test pair costs

Before printing a whole enclosure, print the joint alone. We drew a test pair: a plug with two arms 1.6 mm thick at the root tapering to 0.8 mm, 6 mm wide, 0.8 mm hooks with a 30 degree lead-in and a 45 degree return, and a socket with a 0.25 mm gap per side, so each arm bends 0.55 mm. Live quotes from our quote API on November 10, 2026, plug and socket together, one set, shipped by USPS Ground Advantage anywhere in the US, before tax:

All three stay under PLA's 1% everyday strain, so they survive being clicked over and over. Most of the price is the $3.50 order fee and postage, so put the test joint on the same order as the real part if you can. PETG is made to order in black or white and adds 3 to 5 business days.

Get it printed

Drop your STL, 3MF or STEP file in the box below for an instant price in PLA or PETG, and add the file check for $5: we look at the clips before printing and email you if a hook looks too tall for its arm. No file? Describe the part to the AI designer with the arm length and hook height from the tables. For a box with a lid that clicks shut, the box with a snap-on lid is ready to size and order, and the quote shows the ship date before you pay.