Wall Thickness and Minimum Feature Size

What limits a thin feature

Every printer has a smallest thing it can draw. On FDM it is the width of one extruded line, which is set by the nozzle. On resin it is the light's pixel or spot size, plus the forces a thin layer has to survive as it peels from the vat. Below that limit a feature does not print thin: it prints badly or not at all. Formlabs adds that the right minimum also depends on what the part has to do; a wall that survives printing may still be too weak in use.

FDM: think in extrusion lines

Prusa's modeling guide puts it simply: walls thinner than one nozzle perimeter are not printable. For a 0.4 mm nozzle, PrusaSlicer uses an extrusion width of 0.45 mm, which gives these approximate wall thicknesses Rule of thumb (Prusa notes they vary with slicer settings):

Perimeters (0.4 mm nozzle)	Approximate wall thickness
1	0.45 mm
2	0.9 mm
3	1.35 mm
4	1.8 mm

A wall between one and two line widths is the awkward case: there is room for more than one line but not two. PrusaSlicer's Detect thin walls option (Print Settings, Layers and perimeters) merges two paths into a single pass for such walls, but Prusa is clear that it cannot rescue a wall thinner than one perimeter. Formlabs gives a similar rule of thumb from the design side: make the wall thickness a multiple of the nozzle size Rule of thumb. In practice, use multiples of your slicer's actual line width, which is often a little wider than the nozzle: about 0.9 or 1.35 mm for PrusaSlicer's 0.45 mm lines. How the slicer fills walls is covered in Walls and Perimeters.

General figures for FDM, useful only as a first guess before you know your printer Rule of thumb:

• Formlabs' wall-thickness guide lists about 1 mm as a minimum wall for FDM.
• Tinkercad's best-practices page says to keep walls at least 1.5 mm (about 1/16 inch) thick.

The two figures differ because they are generic advice for printers their authors do not make. Your printer maker's guidance and your own test print outrank both.

Small details on FDM

• Raised and engraved text. Each stroke of a letter is a tiny wall. Strokes narrower than one line width vanish in the preview. Make letter strokes at least one line wide, and preferably two.
• Pins and posts. A thin vertical pin is printed as a few short loops per layer and is fragile across the layer lines. Make it thicker than you would expect, or lay it horizontally if the design allows.
• Material. Prusa notes that tougher filaments such as PC, ASA and PETG may not print small details as well as PLA, so a part meant for outdoor or mechanical use may need simpler details.

Resin: much finer, but printer-specific

Resin printers resolve far finer features, and their makers publish tested limits. As one example, Formlabs' design specifications for the Form 4 (tested with Grey Resin V5 at 50 micron layers) recommend Maker spec:

Feature (Form 4)	Recommended minimum	What goes wrong below it
Supported or unsupported wall	0.2 mm	May warp from peel forces, or detach
Vertical wire	0.3 mm up to 7 mm tall; 0.6 mm up to 30 mm tall	Deforms
Embossed detail	0.1 mm	May not be visible
Engraved detail	0.15 mm	May fuse shut
Hole diameter	0.5 mm	May close during printing
Drain hole in a hollow part	0.75 mm	Traps resin or air

These are Formlabs numbers for Formlabs printers. Other resin printers, resins and layer heights behave differently; Formlabs itself notes slight variations with other resins, printers and layer thicknesses. Use your own printer and resin maker's guide, and if there is none, print a test piece with a range of wall and detail sizes.

Thin resin walls have one more risk: Formlabs warns that they can absorb solvents such as IPA during washing and deform, so keep washing time to what the resin maker specifies. Handle uncured resin and wash solvent with the PPE and ventilation in their SDS. Hollow parts and drain holes are covered in Hollowing Resin Prints.

Check a model before you slice

1. In the modeling program. Blender's 3D Print Toolbox has a Thickness check that selects walls thinner than a value you set. Set it to your FDM wall minimum, or to your resin maker's figure, and look at what it selects.
2. In the slicer preview. Thin features that cannot print simply do not appear. Zoom in on text, edges and pins and compare the preview with the model.
3. On a test print. For a feature that matters, print just that region first.

Thicker is not always better

Formlabs points out that walls that are too thick cost more material and time and can even crack. On FDM, a thick wall is mostly perimeters plus infill, so the slicer settings decide how solid it really is. Design for the strength the part needs, then set walls and infill to match; see Resolution and Accuracy for what your printer can hold. Moving parts add a second limit, the gap between pieces, covered in Print-in-Place Hinges and Joints.