SLS and Metal Additive Basics#

Overview#

Powder-bed fusion (PBF) processes build parts inside a bed of powder. A recoater spreads a thin layer of powder, an energy source (laser or heat plus fusing agent) selectively joins it, the bed lowers, and the cycle repeats. The two families you will meet are:

• Polymer powder bed fusion: selective laser sintering (SLS), multi jet fusion (MJF) and related processes. Typically nylon (PA12, PA11), TPU and filled variants. The unsintered powder surrounds and supports the part, so most geometry needs no support structures.
• Metal powder bed fusion: selective laser melting (SLM), direct metal laser sintering (DMLS), electron beam melting (EBM) and related processes, in stainless steel, aluminum alloys, titanium, nickel superalloys and others. Metal parts need support structures because of heat and residual stress, not just gravity, and almost always need post-processing such as stress relief, support removal, machining and surface finishing.

Other metal additive processes exist (binder jetting, directed energy deposition, bound-metal filament printing), each with different rules. This page concentrates on the two PBF families because their design guidance carries most directly into Fusion modeling. Autodesk's Manufacture workspace supports additive setups for SLS, MJF and MPBF machines. The metal PBF (MPBF), eBeam and DED workflows, and additive process simulation, are marked as Fusion Manufacturing Extension features. See the additive settings page. The process descriptions on this page are general industry knowledge, not Autodesk data.

Where to find it#

Step by step#

Polymer PBF (SLS / MJF) design#

1. Pick the material and vendor first, then read the vendor's design guide. Minimums vary: for PA12, a typical wall minimum is 0.7-1.0 mm, and minimum detail around 0.3-0.5 mm.
2. Hollow parts need escape holes. Loose powder trapped in a cavity is wasteful and heavy. Provide at least two holes, usually 4-5 mm or larger (service-specific; many suggest 5 mm minimum), placed so powder can pour out.
3. Avoid very long unsupported thin flat parts; they warp as they cool. Add ribs and make walls uniform.
4. Design clearances for assemblies. Moving assemblies can be printed assembled with clearances of roughly 0.3-0.6 mm, so powder can be removed from the gap. Tighter gaps fuse shut.
5. Allow for the surface. The surface is slightly grainy. Add 0.2-0.3 mm of stock if the surface will be machined or smoothed.
6. Plan the packing. Parts are nested in 3D. Smaller parts and a good density improve cost, and spacing between parts prevents fusing. See the build plate page.

Metal PBF design#

1. Orient first. Metal orientation controls support volume, residual stress, surface quality and distortion. A part standing on its smallest base needs the most thermal management; a part tilted at about 45 degrees to the plate often reduces recoater collisions.
2. Keep overhangs above a critical angle. For many metals the self-supporting overhang limit is around 45 degrees from horizontal (measured from the plate, surfaces shallower than about 45 degrees need supports). Anything below that angle needs supports. Machine and alloy specific; verify with your vendor.
3. Design for supports. Supports in metal conduct heat and anchor the part against warping. They must be removable. Leave access for cutting tools and wire EDM. Avoid internal supports in channels you cannot reach, by making channels teardrop, diamond or self-supporting.
4. Add machining stock on critical surfaces: 0.3-1.0 mm depending on the machine and process. Datum features, threaded holes and sealing faces are typically finish-machined.
5. Thicken thin features to the vendor minimum, often 0.4-1.0 mm for walls and larger for pins and posts.
6. Round internal corners and avoid sharp transitions between thick and thin sections to limit stress concentrations.
7. Include powder removal paths for internal cavities, same as polymer PBF.
8. Consider stress relief. Parts are usually stress relieved while still attached to the plate, before cutting off. Design with that in mind, keeping a flat bottom for a clean cut.
9. Check the build volume. Typical metal PBF volumes are small, such as 250 x 250 x 300 mm or less, but vary by machine.
10. Simulate. For MPBF machines, Fusion's process simulation is a thermo-mechanical analysis that predicts displacement and print failures such as interference with the recoater blade, and can create compensated designs. It requires the Fusion Manufacturing Extension.

Using Fusion for metal additive preparation#

With the Manufacturing Extension, Autodesk's MPBF workflow is:

1. Model the part in Design.
2. Switch to the Manufacture workspace and create an additive setup with an MPBF machine and a print setting (the print setting is specific to the raw material).
3. Position the part, for example with an Automatic Orientation study ranked by support volume, support area, height and center-of-gravity height, then adjust manually.
4. Add supports (volume, bar or polyline types are listed for MPBF), set by overhang angle among other parameters.
5. Generate and simulate the additive toolpath, optionally run a process simulation and compensate for predicted displacement.
6. Generate a 3MF file or a machine-specific print file.

Options reference#

These are indicative values drawn from common vendor guidance and not from Fusion; verify against your actual machine, powder and vendor.

Tips and pitfalls#

Tip: Treat vendor design guides as authoritative. Rules such as "2 mm minimum wall in this alloy" reflect their calibrated process and are better than any general rule of thumb.

Warning: Closed internal voids in a metal or polymer PBF part trap loose powder. Without escape holes, the powder cannot be removed, the weight is wrong, and the part may be rejected.

Tip: Lattices and topology-optimized shapes are a good fit for PBF because internal geometry costs no extra. Leave pathways for powder removal, and check that struts are above minimum size after meshing.

Warning: Metal PBF is expensive and unforgiving. Small oversights, such as a downward-facing flat roof of a bore, can cause a complete build failure. Run simulation or consult the vendor for first-of-kind geometry.

Tip: Keep the STL/3MF resolution high for metal. Facets on curved sealing faces or on thin lattices can alter geometry, so use a fine surface deviation (for instance 0.005-0.01 mm for small parts) or deliver STEP if the vendor accepts it.

Warning: Safety. Metal powders can be combustible or hazardous. Handling is a trained-operator task and is outside the scope of this guide.

Tip: Design with the process chain in mind. A part that requires wire-EDM support removal needs wire access; a part that will be bead-blasted shouldn't have blind small holes that collect media.

Practice exercise#

Design a hollow aluminum-style bracket (as if for metal PBF). Create a bracket body about 80 x 40 x 30 mm, then use Shell to hollow it with a 2 mm wall. Add two 5 mm powder escape holes on opposite faces. Chamfer all downward-facing horizontal edges to remove overhangs steeper than 45 degrees. Add a 0.5 mm machining allowance to the mounting face by offsetting it outward with Press Pull (Offset Face), and note it as a parameter. Expected result: a part that avoids flat roofs, has two escape routes, and includes stock on the face that will be milled. Then, if you have the additive setup, orient it at 45 degrees and generate supports; compare support volume with the flat orientation.

Related#

• Orientation and Support Generation
• Additive Settings and Machine and Material Libraries
• Preparing Models for Printing
• Design for manufacture (08-sheet-metal-plastic)

Why powder-bed rules differ from FDM rules#

In FDM the main constraint is gravity: molten plastic cannot be deposited onto air. In polymer powder-bed fusion, the surrounding powder does that job, so gravity stops mattering and the constraints become thermal and logistical. In metal PBF, heat is the dominant problem.

Heat in metal. Each laser pass melts a thin track of metal that cools extremely quickly. The part shrinks as it solidifies, and the layers beneath resist that shrinkage, building residual stress. Large flat cross-sections accumulate the most stress, and thin overhanging regions curl upward toward the recoater blade. The recoater can then crash into the curled edge and stop the build. Supports in metal act as heat sinks and anchors. That is why orientation advice for metal differs: an inclined part presents smaller cross-sections per layer, reduces stress, and lets the blade meet the part gradually.

Powder in polymer PBF. The powder near the part gets hot and partially fuses ("growth"), making holes smaller and gaps close. This is why the minimum gap in assemblies is larger than in resin, and why small holes close. The thermal bleed also means thin walls print slightly thicker than modeled. Vendors publish compensation numbers.

Depowdering. After the build, the part is dug out of the powder cake and cleaned by brushing, air blasting or media blasting. Long internal channels need wide access; follow vendor advice on diameter versus length.

Lattices, topology optimization and Fusion#

PBF suits lightweight structures. Fusion includes generative design tools; see the generative design section for requirements. When you take generated geometry to print:

1. Convert the result to a solid body so that you can apply holes, flats and machining stock.
2. Add interface features such as bolt bosses and datum faces that the optimization did not include.
3. Check the minimum feature size of struts against the process minimum. Optimization happily produces 0.2 mm filaments that nobody can build.
4. Check for powder-escape paths and for closed voids.
5. Re-run Section Analysis across several planes to find thin necks.
6. If the final mesh is enormous, reduce it in the Mesh workspace (see the mesh-repair page).

Deciding when metal additive makes sense#

Metal PBF is justified when geometry is impossible or very expensive by subtractive methods: internal conformal cooling channels, consolidated assemblies, lightweight lattices, patient-specific implants, and low-volume complex components. It is rarely justified for simple prismatic parts that a CNC mill can make in an hour. In Fusion, you can compare the two by building a milling setup and an additive setup from the same model and checking time and cost, which is one of the strongest arguments for staying in a single CAD/CAM environment.