Snap-fit joints: how to design one that lasts

How a snap-fit works

One part has a thin flexible arm with a small hook, the other has a recess or edge for the hook to catch behind. When you push the parts together the arm bends and then springs back. The different types, from single-use to detachable, are described on Wikipedia. You find it on boxes, covers, battery compartments or lids.

In 3D printing there is one weak spot: the part is built in layers, and an arm bends best when the layers run along it. An arm lying flat survives far more bends than one standing upright, which snaps between layers. The orientation of the arm in the model therefore decides the life of the joint more than the material does.

Dimensions and clearance

A few rules that work with a 0.4 mm nozzle and 0.2 mm layers:

• Arm thickness at least 1.2 to 1.6 mm, that is three to four nozzle passes. A thinner arm does not print properly and cracks quickly.
• Arm length preferably longer. A long arm bends more gently for the same deflection, so it is less stressed.
• Clearance between parts about 0.2 to 0.3 mm per side. With zero clearance the parts fuse together, with too much they will rattle. More on this in the entry on tolerances.
• A fillet at the root of the arm. A sharp corner is where it breaks. A fillet spreads the stress.

PLA or PETG

PLA is stiff and brittle. As a snap-fit it survives a few snaps and then the arm cracks, so it suits joints that are snapped once and left. PETG is tougher and bends without breaking, so it is better where the joint is opened repeatedly, for example on a box lid.

We do not print other materials used for flexible joints, such as nylon or polypropylene. If you need a joint for hundreds of cycles, that is the limit of PLA and PETG, and you are better off using a screw.

What to do when ordering

Design the clearance and the latch right in the model, nothing is finished after printing. For an important joint, first order one complete set to try. You see the price per piece and the lead time straight after uploading. If the parts do not fit, change the clearance by a tenth of a millimetre and order again while the setting is still fresh in your head.