Design for 3D printing: minimum wall thickness, 45° overhangs, supports, hole tolerances and FDM vs SLA vs SLS vs MJF

Design rules compared: FDM, SLA, SLS and MJF

Design for additive manufacturing (DfAM) starts with the process, because the smallest feature you can rely on varies a lot: FDM extrudes 0.4 mm lines, so detail is limited by line width; SLA / DLP cures resin with light, giving the finest detail but nearly always needing supports; SLS and MJF fuse nylon powder, need no supports and handle complex internals, but small holes must be cleared of powder.

These are values that print reliably, not machine limits. For structural parts go thicker: FDM 1.2–1.6 mm, SLA 1–2 mm, SLS / MJF 1–3 mm. The tolerance column applies to small dimensions; on large parts expect roughly ±0.3–0.5% of the size, whichever is larger.

TIPIf you are unsure, validate shape and assembly on FDM first, then send critical fits and volume runs to SLS / MJF or CNC. Orinovate checks wall thickness, holes and orientation as part of every quote.

Minimum wall thickness: line width, shrinkage and warping

FDM walls are made of extruded lines about 0.4–0.45 mm wide with a 0.4 mm nozzle, so 0.8 mm is two lines and 1.2 mm three. A wall that is not a whole number of lines leaves a sliver the slicer cannot fill, or gets treated as infill, and ends up weaker (the newer Arachne wall generator helps, but whole multiples are still the safest).

FDM wall ≈ n × line width　(0.4 mm nozzle: 0.8 / 1.2 / 1.6 / 2.0 mm = 2 / 3 / 4 / 5 perimeters)

• Enclosures and boxes: 1.2–2 mm; thicken locally into bosses of at least 2.5 d where screws go.
• Storage tray dividers: 0.8–1.5 mm is plenty — thicker only costs material and time.
• SLA: unsupported thin walls and fins ≥ 1 mm or they warp and break during peeling and washing; hollow parts need drain holes (≥ 3.5 mm).
• SLS / MJF: keep walls uniform — abrupt changes cool unevenly and warp or sink; hollow out solid blocks and leave two or more ⌀5 mm powder escape holes.

TIPThe box generator offers 1.2, 1.6, 2 and 3 mm walls with a 0.2 mm per-side lid clearance, and the parts tray offers 0.8–2 mm dividers — all sized for FDM line widths.

The 45° overhang rule, supports and bridging

Each FDM layer must sit on the one below. The steeper the overhang, the further each layer sticks out; once that exceeds roughly half a line width the filament has nothing to rest on and droops. That is where the 45° rule comes from (measured from vertical).

Offset per layer = layer height × tan θ　(θ: angle of the face from vertical)

• Use 45° chamfers instead of fillets on downward-facing edges: the bottom of a fillet is almost horizontal, a chamfer stays at 45° all the way.
• Bridges (horizontal spans supported at both ends) are reliable up to about 10 mm on FDM and 20–30 mm with tuned cooling; beyond that add supports or reorient.
• The top of a horizontal hole is an overhang: from ⌀3 mm up use a teardrop shape (45° point on top); smaller holes print fine as circles.
• SLA overhangs are handled by supports, not angles: tilt the part 15–45° to reduce peel area per layer and keep support points off cosmetic faces.
• SLS / MJF parts are supported by unfused powder, so there is no overhang limit — but powder trapped in a closed cavity can't get out, so always add escape holes.

TIPIf reorienting removes an overhang, prefer that to supports: supports waste material and leave marks that spoil mating surfaces.

Holes, pins and fit clearances

FDM holes almost always come out small: circles become polygons, material bulges inward at corners, cooling shrinks inner contours and horizontal holes sag at the top. SLA over-cures slightly, and small SLS / MJF holes clog with stuck powder. Add process compensation in the design.

CAD hole = shaft + fit clearance + process compensation　e.g. ⌀5 mm steel pin, FDM vertical hole, slide fit → 5 + 0.25 + 0.2 = 5.45 mm

• Printed pins (outer circles) come out close to nominal or slightly large. Keep FDM pins ≥ 3 mm and print them lying flat so layers don't shear off.
• For precise holes, print 0.1–0.2 mm undersize and ream or drill to size — faster than iterating compensation.
• If you need an ISO 286 grade such as H7, machine the part on CNC or press a metal bushing into the print.

TIPThe hole compensation calculator gives the CAD diameter, polygon segment count and teardrop apex for any process, orientation and fit. Print a test plate (2.8–3.4 mm in 0.1 mm steps) once to calibrate your own printer.

Embossed text and small features

• Embossed text on a top face is the most reliable: on FDM use strokes ≥ 0.6 mm and a height ≥ 0.6 mm (three 0.2 mm layers); ironing makes the surrounding surface flatter.
• Text on side walls: character height ≥ 5 mm, engraving depth 0.4–0.6 mm; thinner layers give smoother curves.
• Engraved text on the bottom (bed) face has the sharpest edges, but elephant's foot fills gaps under 0.5 mm, so use strokes ≥ 0.8 mm.
• Pick a bold sans-serif font. CJK characters are dense, so keep them ≥ 8 mm tall on FDM; Latin letters and digits work from 5 mm.
• SLA resolves 0.3 mm details, but engraved text traps resin — go ≥ 0.4 mm deep and wash thoroughly. On SLS / MJF keep embossing and engraving ≥ 0.5 mm so powder clears.

The same logic applies to thin pins, snap hooks and ribs: free-standing FDM features under 2 mm wobble during printing and deform because each layer has too little cooling time. Make them bigger, print several parts at once to stretch layer time, or slow those layers down.

TIPFor labels and markings, engraving wears better than embossing and won't snag on other parts during assembly.

Orientation and strength: a final check for functional parts

FDM parts are anisotropic: strongest along the lines (XY) and weakest between layers (Z), where the bond is often substantially weaker than the material itself. Orientation therefore decides not only appearance and support use but also where the part will break.

1. Identify the main load and let tension and bending run in the XY plane; snap-fits, cantilevers and hooks should be printed lying down.
2. Print gears and pulleys flat (axis vertical) so the teeth are formed by perimeters; the spur gear generator recommends module ≥ 1 and 0.1–0.15 mm backlash for FDM.
3. Fillet vertical edges and inside corners to spread stress; use 45° chamfers on downward-facing edges.
4. Large flat plates (especially ABS and nylon) warp: add ribs and fillets, or split the part.
5. Dimension and tolerance only what matters and let the process handle the rest; include fit requirements when you send the file to Orinovate for a quote.

TIPSLS / MJF parts are nearly isotropic and a whole build volume can be packed at once — for tens to hundreds of functional parts the unit price is often lower than FDM, with more consistent quality.