engineering-skills/additive-manufacturing/references/dfam-guidelines.md

DfAM Guidelines — Design for Additive Manufacturing

Universal Rules

Part Orientation

Orientation is the most impactful decision in AM. Consider:

1. Mechanical strength: The XY plane is always stronger than the Z direction (layer-by-layer build)
   • FDM: Z/XY difference can be 40–60% for UTS
   • LPBF: anisotropy ~10–20%
   • SLS/MJF: nearly isotropic (<10% difference)
2. Surface finish: Surfaces parallel to the build plane (top surface) are smoother
3. Supports: Minimize by orienting critical surfaces upward or reducing overhangs
4. Distortion: Prevent warping by orienting long axes in XY, not in Z

Overhangs and Support Angles

Process	Limit angle without support	Notes
FDM	45° from vertical	Depends on material and cooling
SLA/DLP	45° from vertical	Supports also required for floating islands
SLS/MJF	No structural limit	Powder acts as support
LPBF	45° — below requires support	Metal supports are difficult to remove
EBM	~35°	Better metallostatic behaviour than LPBF

Minimum Wall Thicknesses

Process	Min. wall (mm)	Min. feature (mm)
FDM (0.4mm nozzle)	0.8–1.2	0.4 (XY planes only)
FDM (0.6mm nozzle)	1.2–1.6	0.6
SLA/DLP	0.2–0.5	0.2
SLS	0.7–1.0	0.6
MJF	0.5–0.8	0.5
LPBF	0.3–0.4	0.2
Binder Jetting	1.0–1.5	0.8

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Guidelines for Specific Features

Holes

• Vertical holes (Z-axis, FDM): Accurate, tolerance ±0.1–0.2mm
• Horizontal holes (FDM):
  • ≤ ø5mm: printable without support (deform into ellipse, ~5–10%)

  • ø5mm: require support or teardrop profile

• Teardrop profile: Modifies the upper cross-section to a point — eliminates supports for horizontal holes
• Post-drilling: For tight tolerances (H7/H8), always plan for reaming/drilling post-AM
• Threads: M3+ printable in SLS/LPBF; FDM → metal heat-set inserts are far more reliable

Radii and Fillets

• Internal fillets: ≥ 0.5mm (SLA), ≥ 1.0mm (FDM), ≥ 0.5mm (SLS/LPBF)
• Sharp edges: Avoid for AM metal parts — stress concentrators + process difficulty
• Rule of thumb: R_internal ≥ wall thickness

Internal Cavities and Channels

• Lattice geometries: SLS, MJF, LPBF only — FDM requires planning for internal supports
• Conformal cooling channels (tooling): Ideal application for LPBF
  • Minimum channel diameter: ≥ ø1.5mm (LPBF), ≥ ø3mm (Binder Jetting post-sinter)
  • Avoid horizontal channels > ø8mm without teardrop profile (LPBF)
• Powder entrapment (SLS/MJF): Provide powder escape holes ≥ ø5mm for closed cavities

Text and Embossing

• Engraved/embossed text: Min. height 1.5mm (FDM/SLS), 0.5mm (SLA)
• Orientation: Always on XY plane for optimal readability

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Anisotropy and Directional Orientation

Anisotropy is the most overlooked property in AM: the part is not isotropic. Strength, fatigue and ductility depend on the direction relative to the XY build plane.

Anisotropy factors by process (UTS_Z / UTS_XY)

Process / Material	Z/XY factor (as-built)	Post-HIP	Note
FDM PLA/ABS/PETG	0.40–0.60	n/a	Layer bonding is the weak point — avoid loading in Z if possible
FDM PA12/PC	0.50–0.65	n/a	Heated chamber improves but does not eliminate anisotropy
FDM PEEK	0.60–0.75	n/a	Heated chamber ≥ 90°C improves significantly
SLS PA12	0.90–1.00	n/a	Nearly isotropic — main advantage over FDM
MJF PA12	0.88–0.98	n/a	Slightly worse than SLS on top surfaces
LPBF Ti-6Al-4V	0.80–0.92	0.95–1.00	HIP practically eliminates anisotropy
LPBF AlSi10Mg	0.70–0.85	0.90–0.98	More anisotropic than Ti; HIP important
LPBF 316L	0.80–0.95	0.95–1.00	Ductile → anisotropy less critical
LPBF 17-4PH	0.75–0.90	n.d.	Depends on post-HIP aging
LPBF IN718	0.75–0.88	n.d.
EBM Ti-6Al-4V	0.90–0.98	—	Preheated chamber → greatly reduced anisotropy

Orientation rules for known loads

Known primary load → orient so that σ_max lies IN THE XY PLANE

  Axial tension on cylindrical rod:
    Rod axis → horizontal (in XY plane)
    Cross-section → parallel to XY plane

  Bending on beam:
    Neutral axis → in XY plane
    Maximum tension and compression fibres → in XY plane

  Torsion on shaft:
    Axis of rotation → in XY plane
    Maximum shear stresses act on XY cross-sections

  Biaxial load (plate):
    Plate → parallel to XY plane
    Both load directions in XY → optimal

When optimal orientation is not possible

1. Complex geometry → Z-direction inevitably loaded
2. Action: for metals, HIP mandatory → reduces anisotropy to < 5%
3. For polymers (FDM): change process → SLS PA12 (anisotropy < 10%)
4. For fatigue in Z direction: see references/fatigue-design.md section 2C for knockdown factors

Fatigue anisotropy — quick summary

Scenario	Z/XY fatigue factor as-built	With HIP
LPBF Ti-6Al-4V	0.60–0.75	0.92–0.98
LPBF AlSi10Mg	0.55–0.70	0.85–0.95
FDM PLA (R=0.1)	0.25–0.40	n/a
SLS PA12	0.88–1.00	n/a

Absolute rule: do not use FDM for fatigue-critical components loaded in the Z direction.

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Tolerance Stack-Up in AM

Tolerance stack-up in AM is managed differently from conventional manufacturing: each process introduces systematic errors (shrinkage, warping) that add to process tolerances.

Typical dimensional errors by process

Process	Systematic error	Random error (±)	Main cause
FDM	−0.1 to −0.5% (shrinkage)	±0.3 mm	Warping, layer shift, thermal
SLA/DLP	−0.1 to −0.3%	±0.15 mm	Curing shrinkage, peel force
SLS PA12	−3.0 to −4.0% XY, −3.5 to −4.5% Z	±0.3 mm	Sintering shrinkage
MJF PA12	−2.5 to −3.5%	±0.25 mm	Similar to SLS
LPBF	−0.2 to −0.5%	±0.1 mm	Solidification shrinkage
EBM	−0.2 to −0.4%	±0.2 mm
Binder Jetting	−18 to −22% linear	±0.4 mm	Sintering shrinkage

Stack-up on multi-part AM assemblies

When multiple AM parts are assembled, errors accumulate in worst-case or RSS:

Minimum clearance on assembly = Nominal clearance − Σ(component tolerances)

Example: LPBF flange (±0.1mm) + gasket + LPBF cover (±0.1mm)
  Total clearance required for guaranteed fit: 0.2mm + safety margin

Rule of thumb: for functional fit on AM-AM assembly → clearance ≥ 0.5mm (LPBF)
               for AM + machined assembly → clearance ≥ 0.2mm (LPBF vs CNC)

Critical surfaces: stock allowance strategy

For surfaces requiring H7/h6 tolerances, precision fits, bearing seats:

1. Design in CAD with stock allowance: +0.3mm on all surfaces to be machined
2. Print with stock allowance
3. Stress relief + HIP (if planned) — HIP modifies dimensions by ±0.05–0.2mm
4. Post-machining to nominal CAD dimensions (CNC, reaming, grinding)
5. Final CMM inspection

DO NOT attempt to achieve H7 tolerances as-built in AM — it is not reliable.

Internal threads: compensation standards

Process	Recommended approach
FDM, SLS, MJF	Print undersized (−0.3mm) + tap post-print for M6+ threads
LPBF	Print hole +0.3–0.5mm undersized + tap or ream
M3 and smaller threads (FDM)	Heat-set inserts (e.g. Ruthex, CNC Kitchen) — more reliable
M3 and smaller threads (SLS/LPBF)	Printable if parameters are optimised, verify with first part

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Topology Optimization and Consolidation

When to apply topology optimization

• Structural parts in metal AM where weight is critical
• Brackets, supports, aero/automotive components
• Tools: nTopology, Altair Inspire, Ansys Discovery, SolidWorks Simulation, Fusion 360 Generative Design

Guidelines for optimised topology

• Enforce minimum thickness = 2× minimum thickness of the process
• Set overhang angle as a constraint in the solver
• Final smoothing mandatory (optimised meshes create stress concentrations)
• Verify manufacturability with AM-aware tool in the solver

Component Consolidation (Assembly Consolidation)

Typical opportunities:

• Bolted joints → single-piece geometries in SLS or metal AM
• Hydraulic/pneumatic ducting with fittings → monolithic body with internal channels
• Hollow structures (sandwich) → integrated lattice

Decision rule: If assembly cost + tolerances + gaskets > AM cost of the consolidated part → consolidate

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Tolerances and Compensations

Shrinkage compensations (to be applied in CAD or in print software)

Process	Typical shrinkage
FDM	0.1–0.5% (material-dependent)
SLS PA12	3.0–4.0% XY, 3.5–4.5% Z
SLS PA11	2.5–3.5%
LPBF AlSi10Mg	0.3–0.5%
LPBF Ti-6Al-4V	0.2–0.4%
LPBF 316L	0.2–0.3%
Binder Jetting (metals)	18–22% (sintering)
SLA/DLP resins	0.1–0.3%

Fits and Mating

• Clearance fit (moving): Add 0.3–0.5mm per side (FDM), 0.15–0.25mm (SLS), 0.05–0.1mm (LPBF after machining)
• Press fit / interference: Prefer post-machining for H7/p6 or tighter tolerances
• Snap fit: Design with SLS PA12 or TPU; avoid PLA (brittle under cyclic fatigue)

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Post-Processing Decision Tree

Printed part
├── Metals (LPBF/EBM)
│   ├── Stress relief → ALWAYS (before support removal)
│   ├── HIP → if critical application or biomedical
│   ├── Heat treatment → if required (e.g. 17-4PH H900, IN718 aging)
│   ├── Support removal → manual + machining
│   ├── Surface finishing → shot peening, vibratory, EDM, polish
│   └── Inspection → CT scan for critical parts, UT, HV mapping
│
├── Metals (Binder Jetting)
│   ├── Debinding (solvent/catalytic/thermal)
│   ├── Sintering (specialised furnace)
│   └── HIP optional → if critical
│
├── SLS/MJF Polymer
│   ├── Blasting (glass/sand) → standard for uniform finish
│   ├── Dyeing → uniform colouring (black or colours)
│   ├── Vibratory finishing → improved Ra
│   ├── SLS coating (e.g. Ceracoat) → waterproofing
│   └── Epoxy impregnation → alternative waterproofing
│
├── SLA/DLP Resin
│   ├── IPA wash (10–15 min) → MANDATORY
│   ├── UV post-curing → MANDATORY (900–1200 mJ/cm²)
│   └── Support removal → before or after curing (resin-dependent)
│
└── FDM
    ├── Support removal (mechanical or soluble if dual extrusion)
    ├── Sanding + primer → for painting
    ├── Acetone smoothing → ABS only (changes dimensions ~0.1–0.3mm)
    └── Heat-set inserts → for reliable threads (M3+)

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Pre-print Checklist

• Minimum thicknesses met for the chosen process?
• Overhang angles within limits (or supports planned)?
• Horizontal holes: teardrop or support?
• Closed cavities with powder escape holes (SLS/MJF)?
• Shrinkage compensated in CAD?
• Internal fillets ≥ process minimum?
• Orientation optimised (strength + finish)?
• Post-processing defined and included in cost?
• Critical tolerances reserved for post-machining?