Design for Printing¶

Rules for modelling parts that print cleanly and fit first time on an FDM printer: clearances, holes, overhangs, orientation, small features, fasteners, and mechanisms. Settings live in the slicer; this page is about the geometry you hand it.

Tolerances & clearances¶

Extruded plastic squeezes out slightly past the toolpath, so a 10.0 mm pin will not go into a 10.0 mm hole. You have to design in a gap. Sources quote it two ways — per side (radial) or total (diametral, = 2 × per side) — so always check which one a number means.

Sources span this whole range: Sovol suggests 0.05–0.15 mm per side for a press fit and 0.2–0.3 mm per side for sliding; a university fab-lab guide quotes 0–0.05 mm (press) to 0.15–0.2 mm (sliding) for a Bambu machine and a much looser 0.2–0.3 mm per side for an Ultimaker; Prusa says at least 0.3 mm for moving parts; Hubs uses 0.5 mm as a nominal FDM clearance for snap-fits.

• Put the clearance on the hole, not the pin. Keep the load-bearing male part at full size and enlarge the socket. When the other part is bought hardware (a screw, bearing, magnet), all the clearance goes into the print.
• Apply clearance once per mating pair — not to both parts.
• Chamfer lead-ins on pins and sockets so parts self-align (Sovol).

Holes¶

Vertical holes print undersize¶

Holes along Z come out smaller than modelled — Hubs, Stratasys and Prusa all note it. Causes include CAD exporting circles as polygons whose flats sit inside the true circle, and the nozzle cutting corners as it traces a tight arc. The amount depends on printer, material and hole size (small holes suffer most), so measure rather than guess.

Horizontal holes droop¶

A hole running sideways through a wall has an unsupported ceiling that sags into the bore.

• Teardrop profile — extend the top of the circle into a 45° point so the whole hole is self-supporting (the shape on the RepRap logo).
• Truncated teardrop / diamond — flatten the point into a short bridge if the tip would poke above another feature.
• Rotate the part so the hole is vertical (Hubs' first suggestion).
• Drill or ream afterwards if the hole must be truly round.

Overhangs & bridges¶

Angles here are measured from vertical (a wall is 0°, a ceiling is 90°).

Fix order: reorient the part, then chamfer the overhang to 45°, then teardrop holes, then split the part, and only then add supports.

Orientation vs strength¶

FDM parts are anisotropic — strong along the extruded lines, weakest across the layer bonds (the Z direction).

CNC Kitchen puts layer adhesion at typically ~50 % of in-plane strength, higher for some materials and much lower for, e.g., ABS printed with heavy cooling. Note the elongation collapse: across layers PLA breaks after very little stretch, which is why clips and hooks printed standing up snap.

• Orient so tension and bending run along the layers, not across them. A hook, bracket or clip printed lying flat is far stronger than one printed standing.
• Screw bosses pulled along Z try to peel layers apart — add gussets or load them in shear.
• Angled printing is a middle ground: CNC Kitchen measured 44 MPa at 30°, 40 MPa at 45° and 36 MPa at 60° (vs 31 MPa vertical). It helps less than it sounds — small tilts gain little and angled parts bring support and warp problems.
• More walls/perimeters add strength more efficiently than infill — see Walls, top/bottom & infill.

Minimum walls & features¶

Design to whole line widths. With a 0.4 mm nozzle PrusaSlicer extrudes about 0.45 mm lines, so walls come out roughly 0.45 / 0.9 / 1.35 / 1.8 mm for 1–4 perimeters. A wall between those values leaves a sliver the slicer fills badly or skips.

Chamfers vs fillets on the bed edge¶

The first layer is squashed onto the bed and spreads out — elephant's foot (see Troubleshooting). It ruins flat mating faces and binds sliding and print-in-place parts.

Pair the chamfer with the slicer's elephant foot compensation — Prusa suggests about 0.2 mm for a 0.4 mm nozzle (Sovol: 0.1–0.2 mm).

Threads & heat-set inserts¶

In order of preference for a thread that has to hold:

For printed threads, give them the same clearance as a sliding fit above, print the axis vertical so each turn is a smooth helix, and expect them to wear. Small metric threads (M3–M5) are better served by inserts.

Heat-set insert holes¶

These are the straight-knurled heat-set inserts sold for 3D printing by CNC Kitchen and ruthex (both brands publish the same hole diameters). Tapered industrial inserts (e.g. PEM, Spirol, McMaster tapered heat-set) need a tapered hole — use that manufacturer's datasheet, not this table.

• Depth: for a blind hole, make it about 1 mm deeper than the insert so displaced plastic has somewhere to go (CNC Kitchen). A through-hole only needs to be as long as the insert.
• Straight, not tapered, for these inserts; a lead-in chamfer is normally not needed.
• Printed holes shrink, so print a test plate with a few diameters (CNC Kitchen publishes one) and adjust in 0.1 mm steps.
• Stratasys Direct's rule of thumb for heat-staked inserts is a wall of ≥ 2 mm beyond the insert's outer diameter — more conservative than the table, and a good default for anything structural.

Installation (iron temperature, technique, jigs) is covered in Post-processing → Heat-set inserts.

Snap-fits¶

A cantilever snap-fit is a flexible beam with a hook that deflects as it slides past a lip, then springs back.

Sizing the beam. The classic Bayer design guide gives the permissible deflection y of a rectangular beam of length l and root thickness h for a chosen strain ε (as a fraction, e.g. 2 % = 0.02):

A longer, thinner, tapered beam lowers the strain for the same undercut.

Strain limits. Bayer's guidance: amorphous plastics can be strained to about 70 % of their yield strain for a single snap; partly crystalline ones almost to yield; for frequent assembly use about 60 % of that. Printed parts have no published snap-fit tables, so derive one from the TDS:

These derived figures are estimates — test-print before relying on them.

• Orientation: the beam must flex within the layer plane — print it lying flat so bending runs along the extruded lines. A beam that bends across layers will snap (see the 1.8 % figure above).
• Root fillet: at least 0.5 × beam thickness (Hubs) — the root is where they break.
• Width: Hubs suggests at least 5 mm.
• Material: prefer tough, ductile materials — PETG, ABS/ASA, nylon, TPU. A PLA test print checks fit only; PLA is too brittle to judge the final part. See Material selection.
• Creep: a clip held permanently deflected slowly loses force. Use snaps to click, and geometry (hooks behind lips) or screws to hold.

Living hinges¶

A living hinge is a thin flexible strip joining two rigid parts.

• Thickness: 0.4–0.6 mm, at least two layers (Hubs). Injection-moulded hinges are 0.25–0.38 mm, which is too thin for FDM (MatterHackers).
• Orientation: lay the hinge so the extrusion runs across the hinge line in one continuous strand into both rigid sections — the hinge should bend along its lines, never between layers.
• Expect a limited life: Stratasys rates FDM living hinges for "a small number of cycles"; Hubs' FDM test hinge lasted 25 cycles. For anything opened daily, use a pin hinge.

Print-in-place joints¶

Hinges, chains and toys printed already assembled.

• Clearance: at the loose end of the fit table — 0.3 mm total minimum (Prusa); Sovol suggests 0.25 mm per side (0.5 mm total) as a first test value in PLA, wider in PETG.
• Orientation: run the moving axis vertical so the clearance gap lies in XY, where the printer is most accurate. Horizontal axles need a teardrop socket so the top bridges cleanly (Sovol).
• Vertical gaps between stacked moving parts must be whole layers (e.g. 2 × 0.2 mm) — a gap that isn't a layer multiple rounds unpredictably.
• Protect the first layer: elephant's foot is "a common cause of failed print-in-place assemblies" (Sovol). Chamfer the bottom edges of pins and sockets, enable elephant foot compensation, and keep any brim away from moving gaps.
• Work the joint free gently after printing before deciding it has fused — first movement is often stiff.

Splitting large parts¶

Split parts that don't fit the bed, need impossible supports, or would print in a weak orientation. Prusa notes seams are less visible on large models.

PrusaSlicer's dowel connectors aren't toleranced automatically — size them with the slip-fit clearance from your tolerance test.

Glue by material (from the Polymaker wiki unless noted):

Floating islands¶

An island is a region that starts mid-air — the first layer of a downward-pointing arm, a ledge whose underside has nothing below it, the bottom of a sideways peg. Each face can pass the 45° rule while the very first strand still has nothing to land on.

• Scrub the slicer preview layer by layer with the layer slider. Watch for any layer where new material appears with nothing under it.
• PrusaSlicer notes that with "Support on build plate only", overhangs that start on the model get no support — check the preview to confirm every part is supported or at a printable angle.
• Fixes, best first: reorient so the feature grows upward; give it a 45° underside back to the body; print it separately and attach it; or use supports (or a support enforcer) on just that region.

Sources & further reading¶

• Prusa Knowledge Base — Modeling with 3D printing in mind
• Prusa Knowledge Base — Elephant foot compensation
• Prusa Knowledge Base — Support material
• Prusa Knowledge Base — Cut tool
• Hubs — How to design parts for FDM 3D printing
• Hubs — Key design considerations for 3D printing
• Hubs — How to design snap-fit joints for 3D printing
• Hubs — How to design living hinges for 3D printing
• Stratasys Direct — FDM design guide (PDF)
• Protolabs — Threading and inserts for 3D printing
• CNC Kitchen — Stop printing flat: the 45° secret for stronger parts
• CNC Kitchen — Tips & tricks for heat-set inserts
• CNC Kitchen threaded inserts — dimension table (3DJake listing)
• ruthex threaded inserts — dimension table (3DJake listing)
• Bayer MaterialScience — Snap-Fit Joints for Plastics: A Design Guide (PDF)
• Polymaker — PolyLite PLA technical data sheet
• Prusament — PETG technical data sheet (PDF)
• OrcaSlicer Wiki — Precision settings (hole/contour compensation, polyholes)
• RepRap Wiki — Logo (the teardrop profile)
• HydraRaptor (nophead) — Polyholes
• Sovol — FDM tolerances & clearances
• Sovol — Print-in-place hinges, joints and moving parts
• UAL Creative Computing Institute — Design the clearance and tolerance
• MatterHackers — Living hinge design guidelines and material selection
• Polymaker Wiki — Glueing