Drain holes and powder escape holes
A closed cavity is a trap. Liquid resin or loose powder goes in during the build and cannot get out. The fix is the same everywhere, a way out, but the sizes differ by technology.
| Technology | What gets trapped | Smallest hole | Note |
|---|---|---|---|
| FDM | nothing, cavities stay empty | not applicable | internal supports inside a closed cavity cannot be removed |
| SLA | liquid resin | 3.5 mm | two holes per hollow part |
| SLS | loose powder | 5 mm (working assumption) | powder adds mass and cannot be recycled |
| Metal LPBF | metal powder | 3 mm | trapped metal powder caps the score at 39 |
Where to put the holes
Two holes. One at the lowest point, so resin or powder leaves by gravity. One as far from the first as the part allows, so air can enter. The drain hole study did exactly that: the first hole as low as possible next to the build plate, the second as far as possible from the first. Short holes through thin walls are easier to clean, so put them on faces that do not matter.
A real example
A resin housing with 2 mm walls and a closed interior held a cavity of 125.8 ml. That is 138 g of liquid resin that would stay inside until two holes of Ø3.5 mm were added.
What Dokimion does
- Finds closed cavities and reports their volume in ml.
- For resin, can drill the holes and return an STL with a closed mesh, ready for preparation.
- For metal, reports the trapped powder volume and caps the score at 39 when it is more than traces.
In CAD
- Open the cavity if the part does not need it closed.
- Add holes of at least the size in the table, at the low point and at the far end.
- Replace a solid region with a lattice or ribs, which are open by nature.
- Plug the holes after printing only if the application needs a sealed part.
Find closed cavities in your part
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