Design for 3D printing: walls, overhangs, holes and tolerances that work first time

A model can look perfect on screen and still fail on a printer: an overhang that droops, a hole that comes out too small, a lid that does not fit its box. Good design for 3D printing is mostly a handful of rules about how a printer lays down plastic. This article collects the ones we see matter most on real parts, with numbers you can use straight away. They apply to FDM printing, the kind of printing we do on our Voron 2.4 with a 0.4 mm nozzle.

Wall thickness: design in multiples of the line width

A 0.4 mm nozzle lays lines roughly 0.4 to 0.45 mm wide. Walls are built from whole lines, so a wall of 1 mm prints as two lines with a gap the slicer has to fill, while a wall of 1.2 mm prints as three clean lines.

• Minimum wall: 0.8 mm (two lines). Thinner walls may print poorly or disappear.
• Practical wall: 1.2 to 2 mm for most parts.
• Load-bearing wall: 2 to 3 mm or more, together with a strong print setting.
• Thin vertical features such as pins and ribs: at least 1 mm thick and not much taller than ten times their thickness, or they wobble while printing.

Overhangs: the 45-degree rule

Each layer has to rest on the one below. A slope up to about 45 degrees from vertical prints fine, because each line still sits half on the line beneath. Steeper overhangs droop and need supports. Our Voron handles a little more with strong cooling, but 45 degrees is the safe design rule.

• Replace a flat overhang with a 45-degree chamfer where you can. A chamfer on the underside of a part looks deliberate and prints cleanly; a rounded fillet on the underside is the hardest shape to print.
• Split a part that cannot avoid big overhangs into two pieces that print flat and are glued or screwed together.
• Remember we print your model the way it lies in your file. Choose that orientation to keep overhangs below 45 degrees.

Bridges: flat spans between two supports

A printer can stretch a line across a gap, like a rope between two posts. Short bridges of 10 to 20 mm print well; longer ones sag. A bridge is fine for the top of a window or a slot; for anything longer, add a rib or let the part rest on supports.

Holes and tolerances: plan for the fit

Printed holes come out slightly smaller than designed, and parts that slide into each other need room. These clearances work well as a starting point:

• Horizontal holes (with their axis lying flat) come out slightly egg-shaped because the top of the circle is an overhang. A teardrop or diamond top fixes that.
• Vertical holes are round but run small: add about 0.2 to 0.3 mm to the diameter, or drill them out afterwards.
• Screws: for a self-tapping screw, design the hole slightly smaller than the screw and use a strong print setting with extra walls. For parts you open often, use heat-set brass inserts, as in our Blobifier kits.

Text, detail and the smallest feature

Embossed or engraved text should be at least 0.6 mm deep or high, and the strokes at least 0.5 mm wide. Text on the top or a vertical side prints best; text on the underside rests on the plate or on supports. For fine detail, choose a fine layer height (0.12 mm) and keep features above about 0.5 mm in every direction.

Corners, edges and the first layer

• Sharp inside corners concentrate stress. A small fillet of 1 to 2 mm makes a bracket much stronger.
• The bottom edge spreads out slightly on the first layer (“elephant’s foot”). A 0.4 to 0.5 mm chamfer on the bottom edge hides it and keeps parts that fit together from binding.
• A flat base on the plate is the best start: the part sticks well and the first layer is accurate.

Strength comes from direction

One rule of design for 3D printing is easy to forget: printed parts are strongest along their layers and weakest across them. If a part has to bend, like a clip or a hook, orient it so the bend runs along the layers, not across them. More about this, and about walls versus infill, in our guide to layer height, walls and infill.

Design for 3D printing: checklist before you export

1. Every wall at least 0.8 mm, preferably 1.2 mm or more.
2. Overhangs at 45 degrees or less, or a plan for supports.
3. Bridges shorter than about 20 mm.
4. Clearances added where parts fit together.
5. Text at least 0.6 mm deep and 0.5 mm wide.
6. The model in millimetres, one closed solid, with the flattest side down.

For the wider theory behind these rules, see design for additive manufacturing. For a quick check of your own part, upload it to our instant 3D print quote: it slices the model the way our printer will print it and shows the size and price within seconds. A person checks every model before printing as well, so if something in your design will not print well, we tell you before we start.