Additive manufacturing processes

Why it matters

Additive manufacturing (AM) builds a part layer by layer straight from its CAD model, with no tooling. It began as rapid prototyping and now makes production parts: aerospace fuel nozzles, dental crowns, orthopaedic implants, conformal-cooled mould inserts and low-volume spares. A production engineer needs to know which process suits which material and part, and how layer thickness, orientation and build time trade against each other.

Key ideas

The AM process chain.

1. CAD solid model.
2. Export to a tessellated file — STL (triangles only) or the richer AMF/3MF formats.
3. Orientation, support generation and nesting in the build volume.
4. Slicing into layers and generating the toolpath (laser scan, nozzle path or print pattern) for each layer.
5. Build.
6. Post-processing — remove supports and powder, cure, heat-treat or HIP, machine critical faces, finish the surface.

ISO/ASTM 52900 process categories.

• Vat photopolymerisation — stereolithography (SLA), digital light processing (DLP): a UV laser or projector cures liquid resin. Excellent detail and finish; brittle photopolymers; needs supports and post-curing.
• Material extrusion — fused deposition modelling (FDM/FFF): a heated nozzle deposits thermoplastic (PLA, ABS, PETG, nylon, PEEK). Cheap and common; visible layer lines, strongly anisotropic strength.
• Powder bed fusion — selective laser sintering (SLS) of polymers (nylon) needs no supports because the loose powder holds the part; metal processes (selective laser melting / DMLS, electron beam melting) fully melt metal powder (Ti-6Al-4V, Inconel, 316L, AlSi10Mg) and need supports to anchor the part and conduct heat.
• Material jetting — droplets of photopolymer jetted and UV-cured; multi-material and multi-colour.
• Binder jetting — a liquid binder glues powder (sand, metal, ceramic); metal parts are then sintered, with large shrinkage. Fast, used for sand-casting moulds and cores.
• Directed energy deposition (DED) — powder or wire fed into a laser, electron-beam or arc melt pool; large parts and repair of blades and dies.
• Sheet lamination — laminated object manufacturing (LOM), ultrasonic consolidation: sheets bonded and cut.

Design and quality issues.

• Staircase effect: inclined and curved surfaces show steps whose height depends on layer thickness and surface angle. Thinner layers improve finish but raise build time.
• Anisotropy: parts are usually weakest across layers (in the build direction), so orient load paths within layers.
• Supports and overhangs: overhangs beyond about 45° from vertical usually need supports in FDM, SLA and metal PBF.
• Residual stress and distortion in metal PBF from steep thermal gradients; porosity and lack of fusion need control of laser power, speed and hatch spacing.
• Design freedom: lattices, topology-optimised shapes, internal conformal channels and part consolidation are possible because complexity is nearly free.

Economics. AM cost is driven mainly by build time and material, not by part complexity or tooling; it wins for low volumes, custom parts and complex geometry, and loses to moulding or machining at high volumes of simple parts.

Formulas

n = H / t

• n number of layers; H part height in the build direction (mm); t layer thickness (mm).

T_build = n · (t_layer + t_recoat)

• Build time (s); t_layer time to process one layer (s); t_recoat time to spread a new powder or resin layer (s).

t_layer = A / (h · v)

• Laser scanning time per layer (s); A cross-sectional area scanned (mm²); h hatch spacing (mm); v scan speed (mm/s).

Q = w · t · v

• Deposition rate of an extrusion nozzle (mm³/s); w bead width (mm); t layer thickness (mm); v print speed (mm/s).

c = t · cos θ

• Staircase (cusp) height (mm); θ angle between the surface normal and the build direction, for 0° < θ < 90°.

Worked examples

Example 1 (standard). An FDM printer lays a 0.4 mm wide bead with a 0.2 mm layer thickness at 50 mm/s. Estimate the time to deposit 30 cm³ of material, and the number of layers if the part is 30 mm tall. Ignore travel moves.

1. Q = w · t · v = 0.4 × 0.2 × 50 = 4 mm³/s.
2. Time = 30 000 mm³ / 4 mm³/s = 7500 s = 2.08 h.
3. n = H / t = 30 / 0.2 = 150 layers.

Example 2 (GATE level). An SLS part is 60 mm tall, built in 0.1 mm layers. Each layer has a scanned area of 1500 mm², hatch spacing 0.1 mm, scan speed 2000 mm/s, and recoating takes 10 s per layer. Find the build time. Also find the staircase height on a face whose normal is at 60° to the build direction.

1. n = H / t = 60/0.1 = 600 layers.
2. Scan path per layer = A/h = 1500/0.1 = 15 000 mm; t_layer = 15 000/2000 = 7.5 s.
3. T_build = n · (t_layer + t_recoat) = 600 × (7.5 + 10) = 10 500 s = 2.92 h.
4. Note that recoating takes more time than scanning here — for small cross-sections, nesting more parts in the same build is almost free.
5. c = t · cos θ = 0.1 × cos 60° = 0.05 mm.

Common mistakes

• Treating SLS and SLM as the same: SLS partially fuses polymer powder; SLM/DMLS fully melts metal.
• Assuming AM parts are isotropic like wrought material.
• Forgetting recoat time, which can dominate for small parts.
• Ignoring post-processing time and cost when comparing AM with machining.
• Thinking an STL file carries units, tolerances or colours — it stores only triangles.

For GATE PI

Expect MCQs matching processes to their energy source and feedstock (SLA–photopolymer, FDM–filament, SLS–polymer powder, binder jetting–binder plus powder, DED–powder or wire), on STL files, supports and anisotropy. Numericals ask for number of layers, build time from scanning and recoating, deposition rate or staircase height. Practise keeping mm, s and h consistent.

Quick check

1. Which AM process needs no support structures for polymer parts?
2. What does an STL file contain?
3. A part 40 mm tall is built in 0.05 mm layers with 12 s scanning and 8 s recoating per layer. Find the build time.
4. Find the staircase height for t = 0.15 mm and θ = 30°.
5. Which process is commonly used to repair turbine blades?

Answers: 1. SLS (the powder bed supports the part). 2. A triangulated surface mesh — vertices and facet normals only. 3. 800 × 20 s = 16 000 s = 4.44 h. 4. 0.15 × cos 30° = 0.13 mm. 5. Directed energy deposition.