Metal laser powder bed fusion (LPBF, sold under names like DMLS) is unforgiving of geometry that ignores how the part is actually made: fused one layer at a time into a bed of loose powder, welded to a build plate, and fighting its own thermal stress the entire build. The rules below are the ones that keep a metal part from curling off the plate, trapping powder, or failing inspection – and they are different in kind from the polymer rules most designers learn first. If you are new to choosing between processes, start with our AM process selection guide.
Overhangs, downskin, and the 45° rule
Every down-facing surface is built onto powder, not onto solid material, so it sags and picks up dross. The practical threshold for a self-supporting surface is roughly 45° measured from the horizontal build plate: steeper (closer to vertical) prints cleanly, shallower needs support. Down-facing surfaces – the “downskin” – always come out rougher than up-facing or vertical walls, often by a factor of two or more in Ra, so never put a critical finish on a downskin face.
Supports in metal are not the throwaway scaffolds you know from FDM. They are structural and thermal at once: they conduct heat out of the melt zone and anchor the part against curling. They are also near-solid and must be cut, ground, or wire-EDM’d off by hand, which is real labor. So the goal is to minimize supported area by design, not to eliminate support and hope.
Design every overhang to sit at or above 45° from horizontal wherever you can. For horizontal holes and bores, use a teardrop or diamond cross-section so the top of the hole stays self-supporting instead of collapsing.
Residual stress and warping
The laser creates steep thermal gradients: each new layer heats and shrinks against the cooler, already-solid material beneath it. That mismatch locks in residual stress, and it accumulates over the whole build. The failure modes are curling of large flat down-facing areas, cracking at sharp section changes, and in bad cases the part tearing free of its supports mid-build.
Design mitigations: avoid large uninterrupted solid masses and abrupt jumps in cross-section, add generous fillets at every internal corner, break big flat faces up with ribs, and keep the part well anchored to the plate. Just as important is the post-build sequence – stress-relieve the part while it is still bolted to the plate, then remove it with wire EDM or a bandsaw. Cutting a highly stressed part off the plate first is how flat parts spring into potato chips.
Walls, channels, and minimum features
These are general LPBF figures across common alloys (AlSi10Mg, Ti6Al4V, 316L, IN718). Treat them as a starting envelope and qualify your own machine and parameter set with a test artifact – thin features in particular are highly parameter-dependent.
| Feature | Typical minimum | Notes |
|---|---|---|
| Solid wall thickness | 0.4 mm (design to 0.8–1.0 mm) | Thin walls warp and can lack fusion; thicker is more robust |
| Vertical pin / boss | ~1.0 mm diameter | Below this, heat has nowhere to go and features distort |
| Self-supporting hole (vertical) | up to ~8–10 mm before a bridge sags | Larger holes need internal support or a teardrop top |
| Internal channel diameter | ≥ 2–3 mm for reliable de-powdering | Sub-1 mm channels can print but trap powder |
| Gap / clearance between walls | 0.3–0.5 mm | Below this the walls can fuse together |
| Embossed / engraved detail | ~0.5 mm wide and deep | Finer text and lines wash out |
Powder removal and escape holes
Any enclosed cavity fills with un-fused powder during the build, and that powder is dead weight, a contamination risk, and – for reactive alloys like titanium – a genuine safety hazard. Every internal volume needs a way out. Give each cavity at least one escape hole, ideally two (an inlet and an outlet so you can flush it), sized at ≥ 2–4 mm and placed at the low points powder settles into. Avoid blind dead-end pockets entirely; if you cannot open them up, expect trapped powder and plan a plugging step after cleaning.
For internal channels and manifolds, combine a self-supporting teardrop cross-section with escape holes at the lowest point of each run. That single move usually solves both the overhang problem and the de-powdering problem at once.
Orientation, machining stock, and heat treatment
Orientation is the highest-leverage decision on the whole part. It trades off surface finish, support volume, dimensional accuracy, build time (which is really build height), and the direction of the part’s slight anisotropy. There is rarely a free orientation; pick the one that protects your most important surfaces and load path.
As-printed metal accuracy is only about ±0.1–0.2 mm, so any bore, sealing face, datum, or mating surface that needs to be tight has to be finish-machined – see the numbers in our tolerances guide. Add machining stock of roughly 0.3–0.5 mm on those surfaces, and up to ~1 mm on large faces prone to distortion. Choose a datum scheme that will still exist and be fixturable after the part is cut off the plate.
Finally, plan the thermal post-processing up front: stress relief before cut-off is near-mandatory; then a solution/age or anneal appropriate to the alloy; and hot isostatic pressing (HIP) for any fatigue-critical or pressure-containing part, since HIP closes internal porosity under high pressure and temperature and is what lets a printed part approach wrought fatigue life. Machine after heat treatment, because heat treatment moves the part.
Metal DfAM is a system, not a checklist – orientation, stress, supports, and post-processing all trade against each other. Our courses walk through that decision-making on real parts.
Take Foundations free See Pro pricingMetal AM design checklist
- Keep every overhang at or above 45° from horizontal; use teardrop cross-sections for horizontal holes and channels.
- Kill residual stress by design: fillet internal corners, avoid abrupt section changes and large solid masses, break up big flat faces.
- Respect the minimum feature envelope and validate thin walls and small channels on a test artifact.
- Add escape holes (≥ 2–4 mm) to every internal cavity and delete blind pockets.
- Add 0.3–0.5 mm (up to ~1 mm on large faces) of machining stock to datums, bores, and sealing surfaces.
- Choose an orientation that protects your critical surfaces and load path, not just build time.
- Sequence post-processing correctly: stress-relieve on the plate, cut off, heat-treat / HIP, then finish-machine.