Under Formlabs’ “SLS 3D Printing Explained,” a viewer said the required blasting step “sometimes results in parts that are too loose or tight,” and asked how to get more consistent results. In a follow-up, they added that the process is sensitive to rapid cooling too: parts shrink, and the fit ends up too tight to use. SLS assembly clearance has to absorb all of that, so it starts with the design.
What SLS assembly clearance should you design in?
Formlabs’ Fuse Series design guide gives minimums that scale with feature size. For parts printed separately and assembled afterward, it lists 0.2 mm for features under 20 mm² and 0.4 mm for features over 20 mm². Integrated assemblies printed in one piece need more: 0.3 mm and 0.6 mm. Parts meant to stay separate need at least 1.0 mm, and 5.0 mm is recommended.
The guide also notes that some powders need more. For Nylon 11 and TPU 90A, it asks for at least 1 mm between the pieces of an integrated assembly.
Why does the same design fit differently?
Orientation is one reason. Formlabs says mating parts should be printed with their interfacing surfaces parallel, because small errors in X, Y and Z accuracy can spoil the fit if the parts are oriented differently. Cooling is another. Formlabs found that parts unpacked hot warp more, and the viewer had already spotted cooling as a suspect.
Scaling is the third. If one machine prints undersized, every fit on it shifts the same way. Formlabs’ X/Y scaling tool measures its test part after media blasting, so the correction already covers the blasted surface.
How do you get consistent fits?
Test before the full build. Formlabs suggests printing several test pieces at once, starting tight and stepping up 0.1 mm at a time, in the same orientation as the real part. Then keep the unpack temperature and blast cycle the same for that coupon and every production run, so the fit you approved is the fit you ship.
Related on the Solvi blog: SLS Scaling Calibration: Fixing Undersized Parts; SLS Cooldown: How Hot Can You Unpack a Build?; MJF Tolerances: Clearance for Holes and Nut Pockets.
Sources: YouTube: SLS 3D Printing Explained (Formlabs), viewer question and follow-up; Formlabs: Fuse Series SLS design guide; Formlabs: Model orientation best practices for SLS printing; Formlabs: X/Y scaling on Fuse 1 generation printers.