Additive manufacturing processes

Why it matters

Additive manufacturing (AM) builds a part layer by layer straight from a CAD model, with no dies, moulds or fixtures. It is now used for prototypes, jigs and fixtures, patient-specific implants, lightweight aerospace brackets and conformal-cooled moulds. A mechatronics engineer must choose the right process, orient and slice the part sensibly, and estimate build time, surface quality and cost.

Key ideas

Process chain

1. CAD solid model.
2. Export to STL (a triangle mesh; finer triangles reduce chordal error but enlarge the file) or AMF/3MF, which also carry colour, material and units.
3. Choose build orientation (affects support volume, staircase error, strength in z, build height and time), add supports, and place parts in the build volume.
4. Slice into layers and generate tool paths (contours, infill, hatch).
5. Build, layer by layer.
6. Post-process: remove supports or depowder, cure, heat-treat or HIP, machine critical surfaces, and finish.

The seven ISO/ASTM 52900 process categories

• Vat photopolymerisation (SLA, DLP): a UV laser or projector cures liquid photopolymer resin layer by layer. Fine detail and smooth surfaces; brittle resins, needs supports and post-curing.
• Material extrusion (FDM/FFF): thermoplastic filament (PLA, ABS, PETG, nylon, PEEK) is melted and extruded through a heated nozzle. Cheap and simple; visible layers and weaker in the build direction.
• Powder bed fusion: a laser or electron beam fuses regions of a thin powder layer. Polymer SLS needs no supports because the unfused powder supports the part. Metal laser PBF (SLM/DMLS) fully melts steels, titanium, aluminium and nickel alloys; it needs supports to anchor the part and conduct heat, and stress relief afterwards. EBM works in vacuum with a preheated bed, giving lower residual stress but a rougher surface.
• Material jetting (PolyJet): droplets of photopolymer are jetted and UV-cured; multi-material and colour, high accuracy.
• Binder jetting: a liquid binder glues powder (sand, metal, ceramic); the green part is then sintered (metal) or used directly as a sand mould. Fast, no supports, but shrinkage on sintering.
• Directed energy deposition (LENS, laser cladding, wire-arc AM): powder or wire is melted by a laser, electron beam or arc as it is deposited; used for large parts and repair; coarse resolution.
• Sheet lamination (LOM, ultrasonic AM): sheets are bonded and cut to the layer outline.

Quality issues

• Staircase effect: an inclined or curved surface is approximated by steps of layer thickness t. Thinner layers reduce it but increase layer count and build time roughly in proportion.
• Anisotropy: strength in the build (z) direction is lower, because layer interfaces are weaker.
• Residual stress, warping and porosity in metal PBF from steep thermal gradients; controlled by scan strategy, preheating, supports, and HIP.
• Shrinkage in SLS, binder jetting and photopolymers, compensated by scale factors.

Advantages: geometric freedom (lattices, internal channels, topology-optimised shapes), part consolidation, no tooling, economical at very low volume and for customisation, little material waste. Limitations: slow and costly per part at volume, limited build size, anisotropy, surface finish and accuracy that often need machining, and qualification of metal parts for critical use.

Rapid tooling: AM is also used indirectly, for sand moulds, investment-casting patterns and injection-mould inserts with conformal cooling channels.

Formulas

Number of layers n = H / t

• H = build height (mm), t = layer thickness (mm).

Build time ≈ n · (t_scan + t_recoat) (powder bed and vat processes)

• t_scan = time to scan or expose one layer (s), t_recoat = time to spread the next layer (s).

Extrusion volumetric rate Q = w · t · v, so build time ≈ V / Q

• w = bead width (mm), t = layer thickness (mm), v = print speed (mm/s), Q in mm³/s, V = deposited volume (mm³).

Cusp height c = t · cos θ

• θ = angle between the surface normal and the build direction (equal to the angle of the surface from the horizontal). For vertical walls θ = 90° and c = 0. Valid for inclined surfaces; a flat horizontal face that coincides with a layer boundary has no cusp.

Volumetric energy density E = P / (v · h · t) (laser powder bed fusion)

• P = laser power (W), v = scan speed (mm/s), h = hatch spacing (mm), t = layer thickness (mm); E in J/mm³. Too low gives lack-of-fusion porosity; too high gives keyholing and spatter. The right window depends on the alloy and machine (take from process data).

Worked examples

Example 1 (standard): SLS build time. A part 45 mm tall is built with layer thickness 0.15 mm. Each layer needs 12 s of laser scanning and 8 s of recoating.

1. n = H / t = 45 / 0.15 = 300 layers.
2. Time per layer = 12 + 8 = 20 s.
3. Build time = 300 × 20 = 6000 s = 100 min (excluding warm-up and cool-down, which on SLS can take longer than the build itself).

Example 2 (GATE level): staircase error, layer choice and energy density. A face of a part is inclined at 30° to the horizontal; the part is 30 mm tall. The drawing allows a cusp height of at most 0.10 mm. The machine is a laser PBF unit with P = 200 W, v = 1000 mm/s, h = 0.10 mm.

1. With t = 0.20 mm: c = t · cos θ = 0.20 × cos 30° = 0.20 × 0.866 = 0.173 mm, more than the 0.10 mm allowed.
2. Required t ≤ c_max / cos θ = 0.10 / 0.866 = 0.1155 mm. Choose the next standard layer, t = 0.10 mm, giving c = 0.087 mm.
3. Layers: n = 30 / 0.10 = 300, twice the 150 needed at 0.20 mm, so build time roughly doubles.
4. At the PBF's usual metal layer of t = 0.03 mm: E = P / (v · h · t) = 200 / (1000 × 0.10 × 0.03) = 200 / 3 = 66.7 J/mm³.

Common mistakes

• Using the angle from the vertical in the cusp-height formula; check that a vertical wall gives zero cusp.
• Forgetting post-processing, support removal and cool-down when estimating total lead time.
• Saying SLS needs supports (polymer SLS does not; metal PBF does).
• Assuming AM parts are isotropic; FDM and PBF parts are weaker in the build direction.
• Mixing units in build-time problems (mm versus cm, s versus min).
• Choosing AM for high-volume simple parts, where casting or machining is cheaper.

For GATE ME

Questions are mostly conceptual: match the process to its energy source and material (SLA resin and UV, SLS powder and laser, FDM filament and heated nozzle), identify which processes need supports, and pick the right process for a use. Numericals involve number of layers and build time, staircase or cusp height, and laser energy density. Practise the process-material-energy matching table.

Quick check

1. Which AM category does FDM belong to?
2. Why does polymer SLS not need support structures?
3. A part 24 mm tall is sliced at 0.08 mm. How many layers?
4. What is the cusp height on a vertical wall?
5. Which file format is the traditional triangle-mesh input to AM machines?

Answers: 1. Material extrusion. 2. The unfused powder around the part supports it. 3. 300. 4. Zero. 5. STL.