As-built additive surfaces are rough for reasons that finishing cannot always reach
Partially melted powder adheres to down-facing surfaces and internal channels. Where a tool cannot reach, the roughness stays.
A powder bed fusion part comes off the plate with a surface roughness typically an order of magnitude worse than machining. Where that matters, finishing follows. The complication is that the roughness is not uniform, and the worst surfaces are frequently the least accessible.
Where the roughness comes from
Adhered partially melted powder. Particles at the edge of the melt pool receive enough energy to stick but not to melt fully. They remain attached, and they dominate roughness on down-facing and steep surfaces.
The staircase effect. Layer-wise construction approximates a sloped surface as a series of steps. The effect is worst at shallow angles to the build plane and disappears on vertical walls.
Melt pool geometry. The scan track profile leaves a periodic waviness on up-facing surfaces.
Spatter. Ejected material landing on the surface and fusing.
Why orientation dominates
A vertical wall is comparatively smooth. An up-facing surface is smoother still. A down-facing surface at a shallow angle, supported by powder rather than solid material, is the worst case — heat is not conducted away, more powder adheres, and the result can be several times rougher than a vertical wall on the same part.
That means surface finish is set at orientation planning, and a part oriented for build height or support minimisation may put the roughest surface where the drawing requires the best finish.
The accessibility problem
External surfaces can be machined, polished, blasted or tumbled. Internal channels — which are frequently the reason a part is made additively — cannot be reached by tooling.
Options for internal surfaces are limited to abrasive flow machining, chemical or electrochemical polishing, and their effectiveness depends on the channel geometry allowing flow. A tortuous internal passage may be unfinishable in practice.
The design consequence
Where an internal surface has a finish requirement — flow resistance in a heat exchanger, cleanability in a medical device, fatigue performance in a loaded region — the achievable finish must be established before the geometry is committed.
Designers who specified internal finish requirements on the assumption that finishing would follow have produced parts that cannot be made to specification, which is a more expensive discovery than orienting differently would have been.
This article was produced by the LasersNews AI desk and reviewed by our editors.
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