SLS internal channels look great in CAD, but every channel prints full of unfused powder. On 3D Printing Stack Exchange, a designer planned 1.5 to 2 mm wire raceways inside a polypropylene SLS part and asked whether they were worth the trouble, and what tools could clear powder around the bends.
Can you clear powder from SLS internal channels?
Yes, but it takes physical work, and small, curved channels are much harder to clear. One answer described a PA print with 5 mm channels, mostly straight, with one opened up near a 90° bend. They cleared the long straight run with a 5 mm drill, then used welding wire in several sizes, bent into different shapes, to poke out the rest. Compressed air finished the job, but only after the powder had been loosened, because it sticks to the walls. They said it took about 30 minutes to clean three prints with about 1 m of channel in total, and that they wouldn’t expect the same method to work as easily in a 1.5 to 2 mm curved channel.
What other risks did they mention?
Small exits can clog later. The same answer warned that with 0.5 mm openings, powder that comes loose during use can block them.
What do design guides say?
- Rapidobject’s SLS/MJF design guidelines say holes and channels can be printed from an internal size of 1.0 mm. They tend to come out undersized, and winding or branching channels need the largest possible bend radius so they can be depowdered.
- For enclosed hollows, the same guide calls for at least two opposing openings of at least 8 mm. It warns that fully powder-free results often can’t be guaranteed in complex parts.
- HP’s MJF handbook shows pneumatic parts split into more than one piece so powder can be removed from the air chambers, with channels sized so all excess powder can come out.
What should a bureau tell the customer?
Ask what the channel is for. If it carries air or fluid, trapped powder is a functional failure, not just a cosmetic one. Suggest larger diameters, straighter runs, cleanout openings, or splitting the part along the channel. Quote cleaning time based on channel length and bends, and agree up front on how you’ll confirm the channel is clear.
Related on the Solvi blog: Depowdering SLS and MJF Parts: When Is Automation Worth It?; SLS Powder Cleanup: How to Keep Dust Under Control; MJF Tolerances: Clearance for Holes and Nut Pockets.
Sources: 3D Printing Stack Exchange internal channels question; Rapidobject SLS/MJF design guidelines (German); HP MJF Design Handbook.