Designing for FDM: Tolerances, Overhangs, and Wall Thickness
Tolerances and Fit
FDM parts expand very slightly as they cool, and the degree of expansion varies by material and print profile. The gap you model is rarely the gap you get off the printer.
As a working rule, design 0.2 mm total clearance for a press fit, where two parts should lock together firmly and require force to separate. For a sliding fit, where parts need to move freely against each other, use 0.3 to 0.4 mm total clearance. These figures apply per mating surface, not per side.
For threaded inserts or captured hardware, model to the nominal dimension and test a single piece before committing to a full production run. Shrinkage on features like hex pockets or M3 holes can vary enough to affect function, and it is far cheaper to iterate on a single prototype than to reprint a batch.
Overhangs and Bridging
FDM deposits material layer by layer. Any feature that extends horizontally without support underneath it is either bridging (spanning between two supported points) or overhanging (extending from one supported edge). How well these print depends on the angle and the span.
The 45-degree rule is a reliable starting point. Overhangs up to 45 degrees from vertical will typically print without support material and with acceptable surface quality. Beyond 45 degrees, surface quality degrades, and at steeper angles the print may fail entirely.
Bridging is more forgiving than pure overhangs. The A1 can bridge cleanly across gaps of up to approximately 60 mm in good conditions: the filament is moving in a straight line between two supported points and cooling quickly. Longer bridges or bridges that change direction mid-span are less reliable.
Before adding support structures, consider whether redesigning the geometry is simpler. Chamfering a horizontal feature so it overhangs at 45 degrees rather than 90 degrees, or splitting a part into two pieces that bond together, can eliminate supports entirely and produce a cleaner result. Supports add print time, material, and post-processing work.
Wall Thickness
At a 0.4 mm nozzle diameter, the minimum printable wall is a single extrusion width: approximately 0.4 to 0.45 mm. A single-perimeter wall has no structural integrity and will be fragile. The minimum for any wall that needs to hold its shape is three perimeters, which gives you approximately 1.2 mm.
For parts under any mechanical load, 2 to 3 mm wall thickness is the practical recommendation. Below that, the wall may flex or fracture at layer lines under stress. For structural brackets, clips, or anything load-bearing, err on the side of more material.
Layer Height and Infill
Layer height controls the balance between detail and speed. Our standard profile is 0.2 mm, which suits the majority of parts. For fine surface detail, text, or miniatures, 0.12 mm produces noticeably better results. For functional prototypes where appearance is secondary, 0.28 mm draft mode reduces print time substantially.
Infill density affects stiffness and strength, not just weight. For display models or non-structural enclosures, 15% infill is sufficient. For parts that will carry load or experience mechanical stress, 40% or above is the right starting point. Gyroid and cubic infill patterns distribute stress more evenly than grid.
File Preparation
Export as binary STL with units set to millimetres. Check for non-manifold edges and open surfaces before uploading — most CAD tools have a repair or analysis function that identifies these. A non-manifold mesh will either fail to slice or produce unpredictable results.
If you are not certain your file is ready, upload it anyway. Our auto-quoting system will flag common issues, and our team reviews files before production. Getting a second opinion before you commit to quantity is always worthwhile.