FDM Design Guidelines#

Overview#

Fused deposition modeling (FDM), also called fused filament fabrication (FFF), melts a thermoplastic filament and extrudes it through a nozzle, building the part one layer at a time. It is the most common desktop process and the cheapest per part, which makes it the default target for most Fusion users. It is also the process with the most visible "grain": layer lines, weak inter-layer bonds, bridging sag and stringing are all properties of the process, not defects in your model.

Good FDM design works with those properties instead of against them. This page collects the Fusion-side design moves that matter: shaping geometry to be self-supporting, choosing wall and rib dimensions, handling shrinkage and warp, designing fasteners and joints, and controlling surface quality. Numbers are typical for a 0.4 mm nozzle and 0.12-0.28 mm layers; always confirm on your machine.

Where to find it#

The relevant Fusion tools are ordinary solid-modeling commands used with print constraints in mind.

Step by step#

Designing a self-supporting part#

1. Start from a flat bottom face that sits on the bed. If a part has no natural flat face, add a flat "land" with a Cut or Extrude, or plan to print with supports.
2. Review every downward-facing surface. FDM generally prints surfaces up to about 45 degrees from vertical without support. Beyond that, the layers hang in air.
3. Convert 90-degree overhangs to 45-degree chamfers. In Fusion, select the edge and use Chamfer with an equal-distance or distance-angle option. A chamfer on an internal corner under a horizontal shelf is the classic fix.
4. Replace horizontal circular holes with teardrop profiles (point up) or with a hexagon. For holes that must be circular, consider a sacrificial thin bridging layer: model a 0.2-0.3 mm membrane across the hole that the slicer can bridge, then drill it out after printing.
5. Make bridges short. Keep unsupported spans under about 10 mm where possible; for longer ones add an intermediate rib or accept sag.
6. For horizontal slots (nut traps) that enter from the side, model the slot open to the top and plan to pause the print to insert the nut, or use a stepped bridge: two thin layers that narrow the opening step by step.

Strength#

1. Identify the load path. Layers are weakest in tension perpendicular to the layer plane.
2. Orient so that load runs along the layers, not across them. Hooks, clips and cantilevers should be printed flat.
3. Add fillets at the base of any cantilevered feature. A 1-2 mm fillet is a huge strength gain.
4. Use ribs and gussets instead of simply increasing overall thickness; they add stiffness with little material.
5. Increase perimeter count in the slicer for load-bearing walls (3-5 perimeters) rather than only raising infill. Perimeters carry most of the load.
6. For screws that thread into plastic, make the boss walls at least as thick as the screw diameter and consider heat-set threaded inserts for repeated assembly.

Shrinkage and warping#

1. Choose the material first. PLA shrinks very little; PETG a little more; ABS, ASA and nylon shrink noticeably and warp on large flat footprints.
2. Avoid sharp corners on large flat bottoms. Add a 3-5 mm vertical-edge fillet.
3. Avoid abrupt thickness changes, which cause uneven cooling.
4. For high-shrink materials, scale compensation of 0.3-1.5 percent may be applied in the slicer. Apply it only after measuring a calibration part.
5. Add mouse-ears (small brims) in the slicer on sharp corners if they lift.

Fasteners and inserts#

1. For through-bolts, model holes at nominal plus clearance, for example M3 at 3.2-3.4 mm.
2. For nuts, model a hex pocket 0.2-0.3 mm larger across flats than the nut, with a depth of the nut plus a small allowance.
3. For heat-set inserts, follow the insert manufacturer's recommended hole diameter (usually slightly smaller than the insert body) and add depth for displaced plastic.
4. For tapped threads in printed plastic, avoid fine threads; use coarse threads or thread-forming screws. Fusion's Thread tool can model threads, but printed threads below M6 are unreliable at standard resolutions. See the hole-thread page in solid modeling for the tool.

Surface quality#

1. Layer lines are perpendicular to the Z axis. Curved surfaces show stair-stepping where the slope is shallow. Orient shallow-sloped surfaces away from visible faces, or reduce layer height.
2. Put seams (where each loop starts and ends) on concave corners; this is a slicer setting.
3. Model small text and logos as raised rather than engraved, with strokes at least 0.6 mm wide and 0.4 mm tall; engraved text fills in.
4. For smooth finishes, plan to sand or chemically smooth, and add 0.1-0.2 mm of material thickness where you will sand.

Options reference#

The table summarizes typical geometry limits for FDM at 0.4 mm nozzle. Treat as starting points.

Tips and pitfalls#

Tip: When a feature is tricky, print a small test slice of just that feature (for example a 15 mm tall section containing the nut trap) before committing to a 6-hour print.

Tip: Use Fusion's Rib command to add gussets under shelves and brackets; they can also serve as self-supporting transitions.

Warning: Do not dimension a printed part to the exact nominal size of mating hardware. A "6 mm hole for a 6 mm rod" will almost always bind. Use a clearance parameter.

Warning: Thin vertical features with a small footprint (tall narrow spires) wobble under the nozzle and print poorly. Add a wider base or print multiple items together so each layer has time to cool.

Tip: Brittle materials such as PLA fail suddenly in snap-fits. For repeated flexing choose PETG, nylon or TPU, and design a long, gentle cantilever with a fillet at the root.

Warning: Do not rely on small fillets to hide layer lines. They are layer-limited; a 0.2 mm fillet is simply a stair step. Use fillets of at least a few layer heights.

Tip: Thread and gear teeth benefit from higher resolution. Reduce layer height and slow the print for those regions using a slicer modifier.

Material notes#

Material selection interacts with geometry, so decide early.

• PLA - easy, stiff, low warp, brittle, softens near 55-60 C. Good for prototypes and display parts.
• PETG - tougher, slightly flexible, stringing prone, higher temperature tolerance. Good for functional parts.
• ABS / ASA - strong, temperature resistant, needs an enclosure, shrinks and warps. Good for outdoor and automotive uses (ASA).
• Nylon (PA) and composites - tough and wear resistant, absorbs moisture, high shrinkage. Must be kept dry.
• TPU (flexible) - elastic, requires slow printing and direct-drive extruders; design thicker walls and avoid long bridges.
• Carbon-fiber and glass-filled - stiffer but more brittle, abrasive to brass nozzles.

These are general descriptions. Specific temperatures and profiles come from the filament manufacturer and your slicer.

Practice exercise#

Design a wall-mounted hook that holds 2 kg. First, sketch the hook profile and extrude 12 mm. Decide on an orientation that puts the layers parallel to the hook's side profile (hook lying flat on the bed). Add a 3 mm fillet at the junction between the plate and the hook arm, and a 1 mm fillet around the arm's tip. Add two countersunk screw holes in the plate: the horizontal holes are vertical in this orientation, so they print round. Expected result: a hook with layers that run along the load direction, no supports, and a root that does not crack under load. Then reorient the hook upright in your slicer and observe how many supports and weak layer-crossing regions appear; compare.

Related#

• Preparing Models for Printing
• Orientation and Support Generation
• Print-in-Place and Tolerance Design
• Lip, groove and snap-fit (08-sheet-metal-plastic)
• Hole and thread (03-solid-modeling)