Designing vented enclosures for electronics: a practical guide

Vented enclosures are one of the most common 3D-printing projects and one of the most commonly botched. The shape itself is easy: a box with slits in the walls. The decisions around the shape — where the slits go, how big they are, whether the box should sit flat or stand vertically, whether the corners should be rounded — are where people lose hours of print time to enclosures that warp, fail bed adhesion, or do absolutely nothing for cooling.

Here's a practical guide based on what works for the most common targets: a Raspberry Pi or mini PC, an ESP32 / ESP8266 project, a 18650 cell holder, a small LED driver, or a mesh router or hub.

Where the slits go

Two rules:

1. Slits go on opposite walls. Air doesn't move through a single opening — it moves between two. If you only vent one wall, you've drilled holes in a sealed box, which is very nearly the same thing as a sealed box. Vent at least the front and back, or the left and right.
2. Slits go where the heat is. For most projects that means the wall closest to whatever is producing heat — a CPU, a voltage regulator, a power supply, a battery cell under load. Position the enclosure so the hot surface has a vent in front of it and a vent behind it.

LittleBoxes.ai's vented generator gives you per-wall control on purpose. Most of our users vent the front+back or all four; very few projects benefit from venting only one wall.

How big the slits should be

For passive cooling (no fan), wider isn't always better. The bottleneck for an FDM-printed enclosure is the open area, not the individual slit size. A wall with 7 narrow (1.5 mm) slits has more open area than a wall with 3 medium (2.5 mm) slits, but it also prints faster, looks more refined, and gives the wall more structural strength between slits.

Our defaults — 5 slits at 1.5 mm — are sized for the most common case: a small enclosure (under 150 mm on a side) with passive convection. If you're putting a fan in the enclosure, increase to 7 or 9 slits and use the 2.5 mm width so airflow isn't choked at the wall.

Where things go wrong: the base

The most common 3D-printing failure on a vented enclosure isn't the vents — it's the bottom. Two things people get wrong:

• Rounding the bottom corners. A box with rounded bottom corners has a tiny first-layer overhang at every corner. That overhang is small enough that the slicer rarely flags it, but it's enough to weaken bed adhesion and produce a slightly rough first layer. Always keep the bottom corners sharp.
• Making the bottom too thin. A vented enclosure usually has openings — the bottom is the only place the slicer can do a clean solid infill. Don't go below 1.5 mm, and 2.4 mm or more is better. A thin bottom plus a wobbly weight on top is how enclosures end up bowed.

LittleBoxes.ai's "rounded outer corners" toggle was specifically designed around this — the upper walls round, the base stays sharp and flat. The rounded walls sit directly on top of the solid base slab so there's never an overhang.

Print orientation

Always print vented boxes open side up. The slits are vertical, so they print bridgeless. The base lays flat on the bed. The walls grow up cleanly. Don't print them on their side "to skip supports" — you'll get bridging on the slits, weaker interlayer adhesion across the wall plane, and a worse-looking print.

Material choice

For enclosures that live indoors at room temperature, PLA is fine and prints the best surface finish. If the enclosure will see sustained heat above ~45 °C — a power supply box, a battery enclosure under heavy load, an enclosure near a window in summer — use PETG or PLA+. ABS only when you need it: it warps, smells, and rarely improves on PETG for this use case.

The 30-second checklist

Before exporting, ask yourself:

• Are slits on opposite walls so air can flow through?
• Is the base sharp and at least 1.6 mm thick?
• Will the box print open-side-up?
• Did I leave enough internal clearance for cables, connectors and any heatsinks?
• If I'm using a fan, did I size the open area to match the fan's rated CFM?

Tick all five and the print will work the first time. Skip one and you'll know which one when you read the failure mode.