3D Print a Snap-Fit Lid: Gap, Catch and Groove Sizes

A printed box lid that clicks shut needs a lip about 7 mm deep, a 0.25 mm gap per side between the lip and the box, small catches that stand 0.5 mm off the lip, and a 45 degree groove inside the rim for them to drop into. Put the catches on the middle of each side, not the corners, and both parts print without supports.

Those are the numbers in our box with a snap-on lid, which you can size and order in one click. Below is why each one is what it is, what changes if you draw your own, and what a box like this costs to print.

The numbers

For PLA or PETG with a 0.4 mm nozzle and 0.2 mm layers:

Why the gap is 0.25 mm

A printed wall is never perfectly flat. Each layer bulges a little, and the outside of a part usually comes out 0.1-0.2 mm larger than drawn. Two printed surfaces that are meant to slide past each other need about 0.2-0.3 mm per side, which is the sliding-fit row in our tolerances guide.

So the lip is drawn 0.5 mm smaller than the inside of the box in both width and depth. At 0.1 mm per side it binds on the layer lines; at 0.5 mm it rattles and the catches barely reach the groove.

Why the catch is so small

The snap is the catch height minus the gap: 0.5 mm minus 0.25 mm, so 0.25 mm. That sounds like nothing, but it is enough to hear and feel.

The limit is the plastic. Our snap-fit guide keeps the bend at or under 1/8 of the beam's length. The catch sits 5.5 mm down the lip, so 1/8 is about 0.69 mm, and 0.25 mm leaves plenty of margin. Work it out as strain and the lip would see about 2% if it took all of the bend (1.5 x 1.6 mm thick x 0.25 mm, divided by 5.5 mm squared). It doesn't take all of it: the box wall is 40 mm tall and only held at the corners, so the middle of each side bows out a little too.

Double the snap to 0.5 mm and the lip would see about 4%, more than PLA takes on a lid that opens again and again. A lid that is hard to close is not a better lid. If yours is too loose, drop the gap to 0.2 mm before you make the catch bigger.

Why catches only on the middle of each side

A rounded corner is stiff in every direction. A catch that wraps around it has to stretch the corner to get past the groove, and the corner won't. A full ridge all the way around either jams or splits at a corner.

The middle of a side flexes easily, like a ruler held at both ends. So each catch covers the middle 40% of its side, four in total, and the corners only locate the lid. The groove can still run all the way around, so the lid fits either way round.

Why everything is at 45 degrees

Both parts print without supports because nothing hangs over air at more than 45 degrees:

• The box prints open side up. The groove is a V cut into the inside of the wall, so its upper face slopes at 45 degrees instead of forming a flat ceiling.
• The lid prints top face down, so the top comes out flat and smooth from the bed. The lip grows straight up from it, and each catch has a 45 degree underside.

The angles also help in use. The lower face of the catch lets the lid slide in, and the upper face lets it pop back out with a firm pull. A square catch would lock in place, and you would break the lip getting it open.

Where the lip ends

The lip reaches 7 mm into the box, so dividers or posts inside have to stop at least 8 mm below the rim, as ours do.

PLA or PETG

PLA is stiff, so the click is crisp, but it fatigues. Fine for a box of SD cards opened a few times a week. PETG bends further before it cracks and takes more heat before it softens, so pick it for a pill box, a tackle box, a box that rides in the car or anything opened every day. PETG is made to order in black or white, costs 20% more on the print and adds 3 to 5 business days.

What it costs

Live quotes from our quote API on November 8, 2026, for our box with a snap-on lid, box and lid together, one set in PLA, shipped by USPS Ground Advantage anywhere in the US, before tax. Sizes are inside dimensions.

The same 80 x 60 x 40 mm box is $13.89 in PETG. Two of them are $16.15 and four are $23.67, about $5.92 each, because the $3.50 order fee and most of the postage are paid once.

Drawing your own

If you design the box yourself, start from these numbers and change one thing at a time:

1. Draw the lip 0.5 mm smaller than the inside of the box in width and depth, and keep its corner radius 0.25 mm smaller too.
2. Put the catch about three quarters of the way down the lip and the groove at the same depth below the rim.
3. Give the groove a V or trapezoid profile at 45 degrees, at least 0.2 mm deeper than the catch stands proud.
4. Keep walls at 2 mm or more where the groove cuts in, so at least 1.2 mm is left.
5. Print one corner first if the box is large: a 30 mm strip of rim and lid costs little and tells you whether the fit is right on that printer.

Get it printed

The quickest way is the box with a snap-on lid: set the inside width, depth and height and up to three dividers, and the price updates as you type. Every size up to 120 x 100 x 80 mm inside prints without supports, and the quote shows the ship date before you pay.

Have your own design? Drop the STL, 3MF or STEP file in the box below for an instant price in PLA or PETG. Add the file check for $5 and we look at the fit before printing and email you if the lip or catches look off. No file at all? Describe the box to the AI designer, with the inside size and what goes in it.