Short answer
Make the arm long and thin rather than short and thick, keep engagement shallow, put a generous radius at the root, and print so the layers run along the arm rather than across it. Most broken clips fail on that last point.
Why printed clips get a bad reputation
A snap fit works by bending elastically, holding, and bending back. That demands a material that tolerates repeated flexing without cracking, and a shape that spreads the bending along its length rather than concentrating it at one point.
Printed clips fail for two reasons far more often than any other: they were printed with the layers running across the arm, so the flex tries to peel the layers apart; or the arm meets the body at a sharp corner, so all the strain lands in one place. Both are fixable for free.
Get the proportions right
The useful rule is that a cantilever arm should be at least five times longer than it is thick. A 2 mm thick arm wants to be 10 mm or more long. Longer and thinner arms bend more gently along their whole length; short stubby ones concentrate all the strain at the root and crack.
If space forces a short arm, reduce the engagement depth to match — less deflection means less strain. It is better to have a clip that holds lightly and survives than one that grips hard once.
Engagement depth
Engagement is how far the hook projects, and it sets how far the arm must bend to clear. For PETG on a well-proportioned arm, 0.8 to 1.5 mm is a sensible range. Deeper feels satisfying on the first assembly and is where clips break on the third.
The angle of the lead-in face matters as much as the depth. A shallow ramp of 25 to 30 degrees lets the parts push together easily. The retaining face behind it should be steeper — 45 degrees for something you want removable, near vertical for a permanent assembly.
Radius the root
Where the arm meets the body, put a fillet. A sharp internal corner is a stress concentrator, and it is exactly where a printed clip splits along a layer line. A radius roughly half the arm thickness — 1 mm on a 2 mm arm — spreads that load and dramatically extends the life of the clip.
This is the single highest-value change most people can make to a clip design, and it costs nothing.
Orientation is not a detail
An arm printed lying down, with layers running along its length, flexes as intended: the bending stretches the plastic within layers, which is where it is strong. The same arm printed standing up has the layers stacked across the direction of bend, and flexing pulls directly on the layer bonds. Those parts break almost immediately.
Because you cannot control that from the file alone, mention it. "The two arms on this part are clips" is enough.
Design in a way out
If the part will ever be disassembled, give the clip somewhere for a thumbnail or a small screwdriver, and say in your notes whether it needs to release. Clips that must be levered blindly get broken by whoever services the thing later, which is often you.
What this means when you order
Tell us that the part is a clip when you order. It changes how we lay it on the plate, and that single decision is usually the difference between a clip that lasts and one that breaks first use.
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