Designing internal channels for L-PBF and binder jetting

Internal channels are the headline AM capability for thermal and fluidic parts. They are also the most common source of unprintable geometry. On L-PBF, the practical minimum diameter is 0.5 mm; the maximum unsupported span for a horizontal channel is roughly 6× the diameter before the downskin droops; channels steeper than ~40° from horizontal self-support. Binder jetting has a softer envelope — minimum diameter is closer to 1.5 mm because of powder cohesion during depowdering.

• L-PBF: D_min = 0.5 mm, horizontal span ≤ 6×D, angle ≥ 40° from horizontal
• Binder jet: D_min = 1.5 mm, horizontal span ≤ 10×D, depowder access required at both ends
• EBM: D_min = 1.5 mm, but powder cake makes depowdering harder than L-PBF

A printable channel is not the same as an evacuable channel. Closed-end channels and tight serpentines trap unmelted powder that cannot be removed by vibration or compressed air. The trapped powder partially sinters during stress-relief, becoming a permanent inclusion that distorts thermal performance and creates inspection signals.

Always design channels with at least two access points, ideally on opposite faces of the part, so depowdering can flow through rather than pull back. Add temporary depowder ports that are plugged or welded after evacuation when the final geometry requires a closed end.

As-built internal channel surfaces are rough: Ra 15–25 μm on L-PBF, Ra 6–12 μm on binder jet. For fluid pressure-drop calculations, treat the as-built channel as roughly 2× the equivalent machined channel in friction factor. Abrasive flow machining (AFM) brings the surface down to Ra 1–3 μm and is the only practical finishing path for channels too small to reach mechanically.