The container I made too well

The question

Our D-shaped box lid snapped on with small ridges and came off fine. The same idea on a round box made a lid nobody could open. Why, and what do we do instead?

The lid nobody could open

The round box used the snap that worked on our D-shaped one: a 45° seat and ridges 0.55 mm proud all the way round. It clicked on and sat perfectly flush. Then it never came off. Not with fingernails, not even pulling on the knob screwed into the top.

The ring test

Before printing a whole box again, we printed only the joint: the top of the box as a ring, and three thin lid rings with softer ridges, 0.25, 0.35 and 0.45 mm, marked with one to three dots. A few hours of printing instead of a day.

Even the softest ring, one dot, was so tight it took two fingers jammed in from the inside to pry it off. The little thumb scoops were too small to get a fingernail under, let alone a big thumb. We never even tried the tighter two.

Why a circle won't let go

A snap fit works by bending a wall out of the way while the ridge passes. A flat wall bends easily. A closed circle cannot: to get past ridges all round, the whole ring has to stretch at once. That is why it barely helped to make the ridges smaller.

Our first box was D-shaped, with long flat sides that could flex. Same ridges, completely different result.

And a lid that sits perfectly flush has nothing to hold. Grip has to be designed in, sized for the biggest hands, not added afterwards.

What we are trying next

Round things close the way jars do: a twist lock. The lid drops in at a mark, turns a little until a small bump clicks, and turns back to lift off. Nothing to pull against. The plug is looser (0.30 mm), there is a groove at the seam for your fingers, and the click comes from a small spring arm cut into the lid wall, so only that arm bends, not the ring.

It is printing now: the same ring test, with a soft, medium and firm click. Part 2 will show which one wins.