Designing parts that print well — nine rules that matter
Practical design-for-manufacture rules for FDM 3D printing, covering overhangs, tolerances, wall thickness, holes and the orientation decisions that decide whether a part works.
A printer builds a part one layer at a time, from the bottom up, with nothing underneath except what it already printed. Almost every design rule for FDM follows from that single fact.
1. 45° is the overhang limit
Any surface leaning more than 45° from vertical needs support underneath. Support costs material and time, and leaves marks you then have to sand.
Chamfer instead of overhanging. A 45° chamfer under a boss prints unsupported; a 90° shelf does not.
2. Design holes as teardrops, or drill them
Horizontal holes print badly. The top of the circle is a 90° overhang, so it sags and the hole ends up out of round.
Two fixes. Either shape the top of the hole as a teardrop so its steepest angle stays under 45°, or design the hole undersized and drill it to final dimension afterwards. For anything that needs to be accurately round, drilling wins.
3. Leave real clearance on fits
Printed dimensions run slightly large — extruded plastic spreads. Design in the clearance rather than hoping.
| Fit | Clearance per side |
|---|---|
| Loose / sliding | 0.4 mm |
| Normal assembly | 0.3 mm |
| Snug press fit | 0.15 mm |
| Threaded insert | Per the insert spec, usually 0.1 mm under |
For parts printed on different machines, add another 0.1 mm.
4. Walls in multiples of the nozzle
Our nozzle is 0.4 mm. Walls that are a clean multiple of that print cleanly: 0.8 mm, 1.2 mm, 1.6 mm. A 1.0 mm wall forces the slicer to either overlap or leave a gap, and you get a weaker, uglier result than 1.2 mm would give.
Minimum for anything structural: 1.2 mm. Below 0.8 mm, do not bother.
5. Layer adhesion is the weak axis
A printed part is markedly weaker along Z — between layers — than within a layer. Often by half.
This is the single most important orientation decision. A hook printed lying down is strong; the same hook printed standing up will snap along a layer line under load. Tell us how the part is loaded and we will orient it accordingly.
6. Fillet the internal corners
Sharp internal corners concentrate stress and are where printed parts crack. A 2 mm fillet on an internal corner can double the load a bracket takes, at no cost in material.
External corners can stay sharp — they print fine and look better.
7. Text needs room
Embossed text should be at least 4 mm tall with 0.6 mm of relief. Engraved text should be at least 0.6 mm deep and 0.8 mm wide.
Smaller than that and the nozzle simply cannot resolve it. Sans-serif fonts survive far better than serif ones.
8. Give the plate something to hold
Tall narrow parts warp and detach. If the footprint is small relative to the height, add a brim in the design, or chamfer the base outward slightly to widen the first layer.
Large flat parts have the opposite problem — corners lift. Rounding the corners of the base in the design helps more than any slicer setting.
9. Design for the plate you have
Ours is 180 × 180 × 180 mm in-house. Parts larger than that get split and bonded, which is fine but adds a seam and a bonding step.
If you are designing something big, design the split yourself. You will place the seam somewhere sensible — along an edge, hidden behind a face — where an automatic split would put it through the middle of the visible surface. Add alignment pins while you are at it.
The quickest sanity check
Before you export, ask: if I printed this standing on the plate as oriented, what is holding up every surface? If the answer is "nothing" for a large area, you have found the problem.
Send us the file and we will do a design-for-manufacture review before printing — for free. We would rather spend ten minutes flagging a wall that is too thin than print a part that breaks in your hand.
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