Hollow MJF Parts: Escape Holes and Real Savings

Shelling a chunky print looks like free money. Then the hollowed file goes up for a quote and the number barely moves, or goes up. With hollow MJF parts, the savings depend less on how thin you make the shell and more on whether the loose powder inside can get out.

Does Hollowing a Part Make an MJF Print Cheaper?

It can, when the cavity is open to the outside through escape holes. Seal it and the powder inside stays put, so the part keeps that weight and the cavity may not save anything.

The question is an old one. A thread on r/3Dprinting titled "Hollowing out .stl = more expensive?" tells the story: a solid model quoted at $250 at one service, and the hollowed version came back at $377. One reply guessed the extra inside surface area was driving the price.

Why a shell isn't automatically cheaper

In a powder bed, unfused powder fills every void, and a closed cavity just holds a slug of it. Shelling also adds a whole inside surface.

Our 3D printing page says the price comes from the mesh: "bounding box, surface area and solid volume", combined with the material's per-unit cost, packing factor and finish multiplier. Hollowing cuts solid volume, adds surface area, and leaves the bounding box alone. Whether the total drops depends on the part, so if it matters, ask us when you quote.

Hollow MJF parts: walls and escape holes

Materialise's PA 12 MJF design guidelines suggest hollowing a solid model when the wall thickness exceeds 20 mm, with a 2 to 3 mm shell and at least two holes with a minimum diameter of 2 mm for powder removal. It also warns that longer internal channels can be hard to clear, especially if the powder is partly fused, and that complex holes need larger diameters.

Use two holes, not one, so air can get in while powder comes out, and put them at opposite ends of the cavity. Treat 2 mm as the floor, not the target. If looks matter, put the holes on a face nobody sees.

Thin shells bring their own tolerance behavior, covered in why MJF wall thickness tolerance reads tighter on thin features. If you're hollowing to chase cost at volume, see where MJF crosses over with injection molding. The same powder logic applies on SLS, and the process differences are in MJF vs SLS for a functional nylon part.

What our quote checks

The 3D printing page lists "Watertightness and triangle count" and "Volume and surface area for accurate material use" among its automatic checks. A shelled model with escape holes is still one watertight mesh. We don't publish a minimum escape hole or shell thickness for MJF Nylon, so ask us when you quote.