Drain hole: A drain hole is an opening designed into a hollow 3D-printed part so that liquid resin or loose powder inside can be removed after printing.
When a part is hollowed to save material and weight, the inside has to be emptied. Drain holes are the way out: openings in the shell through which resin can be drained and washed out, or powder poured and blown out. Without them the hollow becomes a trapped volume.
A few practical rules apply in most shops:
- use at least two holes, so air can enter as material leaves, and place them far apart;
- put them at the lowest points of the part as printed, and where they will not spoil a visible face;
- make them big enough for the material: powder needs larger openings than resin, often several millimetres;
- for resin, a vent near the build plate also helps avoid the suction effect that can pull a hollow print off the plate.
Example: a hollow resin vase base 80 mm across is given two drain holes of 3 mm diameter on its underside. After printing, the shop tips the part, drains the resin, flushes the interior with solvent and cures it. The holes can later be plugged with a resin-filled cap if the buyer wants a sealed surface.
Drain holes are often confused with vents in moulding, which let air escape from a mould cavity as plastic flows in. Both release something trapped, but they belong to different processes. Read design for 3D printing for the full set of hollowing rules.
Go deeper: Design for 3D printing: walls, overhangs, holes, clearances and orientation