If every big part in a build has the same ridge at the same height, stop blaming your files. SLS layer bulging like that tracks the chamber, not the part. One Fuse 1+ owner kept seeing it and finally pulled a chamber apart to find out why.
What causes SLS layer bulging at one height?
On the Fuse, the culprit is usually the thermal break. Formlabs’ layer shifting guide says most shifts happen near that thin black band, about 2.5 cm below the top of the chamber’s inner wall. It says small shifts there are expected and can’t be fully eliminated.
The owner went further. With the chamber stripped, they found gaps of over 1/16″ around the band. Powder was leaking out through those gaps, which explains the sinkholes they watched form mid-print and the underdosing aborts that followed.
Is it always the chamber?
No, and the thread is honest about that. A second shop traced the same bulges to a faulty batch of print beds. They swapped the beds and haven’t had a problem since, at better than 95% success. In the original owner’s case, a loose coupler on the doser motor was causing the dosing failures, while the bulge at the band stuck around.
Formlabs’ guide lists both leaks and a worn lead screw as causes. If you see shifting on several chambers, or Error 94/4, it points at a slipping Z-drive coupling.
What do operators do about it?
They stop putting parts there. Formlabs told the owner to either leave a gap at the bottom of the chamber or use the newer print settings, which include layer-shift adjustments. Legacy settings showed it worse.
One member shared a pedestal STL that marks the no-go zone so parts get packed above it. Another shop does it for free: drop a placeholder block into PreForm, lock it, pack around it, then delete it before upload. Nothing extra gets sintered.
How do you check your own chambers?
Inspect the band area for gaps and look for powder where it shouldn’t be. If yours has piles of powder between the walls after a print, that chamber is leaking, and it goes in your placeholder rotation until it’s fixed.
Related on the Solvi blog: MJF Packing Density: How Full Should You Pack a Build?; MJF Tolerances: Clearance for Holes and Nut Pockets; SLS Cooldown: How Hot Can You Unpack a Build?.
Sources: Formlabs forum layer bulging and powder leak thread; Formlabs: Layer shifting (SLS).