Additive Manufacturing

1.2.1 Stereolithography (SLA)

SLA is the second most widely used 3D printing technique. It produces highly accurate, isotropic and watertight parts with smooth surfaces and fine details. Feature sizes down to 0.1 mm are readily achievable; certain specialised machines can reach 0.02 mm.

An SLA printer selectively exposes liquid resin to a laser beam. The resin cures only where the laser is focused; the remaining liquid is washed away after printing. Resolution in the XY plane depends on the laser spot size (typically 30–140 µm, non-adjustable). Vertical resolution (Z) ranges from 25 to 200 µm – a trade-off between print speed and surface quality. For comparison, a desktop FDM printer typically works at 150–400 µm per layer.

After printing, the part must be washed with ethanol or a similar solvent to remove uncured resin, then UV post-cured to fully harden the material and improve mechanical stability. Two-directional laser exposure is also possible on certain systems, further increasing resolution.

1.2.2 Digital Light Processing (DLP)

DLP also falls under vat photopolymerisation. The key difference is the light source: DLP uses a digital projector (Digital Micromirror Device) that flashes an image of the entire layer at once, making it generally faster than SLA for single-part builds.

One drawback of DLP is that output quality decreases when printing multiple parts in a single tray, because the fixed pixel grid is spread over a larger area. SLA, which traces geometry with a focused laser, does not share this limitation.

1.3.1 Selective Laser Sintering (SLS) – polymers

SLS machines use a laser to sinter polymer powder layer by layer. The build chamber is held a few degrees below the material melting point, reducing the energy the laser needs to fuse each layer. The chamber is typically filled with nitrogen to limit oxidation and improve part quality. Parts require a cool-down period after the build to achieve high dimensional accuracy.

1.3.2 Direct Metal Laser Sintering (DMLS) – metals

DMLS follows the same process as SLS but uses metal powders instead of polymers. A wide range of engineering alloys is available. The powder is sintered layer by layer, producing functional metal parts with good mechanical properties.

1.3.3 Selective Laser Melting (SLM) – metals

SLM is similar to DMLS but fully melts the powder rather than sintering it, typically yielding higher density parts. It generally builds faster than SLS but requires an inert gas atmosphere, has higher energy consumption and an energy efficiency in the range of 10–20 %. A roller or blade spreads each new powder layer; blade-based systems often vibrate the blade to improve powder distribution.

1.3.4 Electron Beam Melting (EBM) – metals

EBM fuses metal powders using an electron beam controlled by electromagnetic coils, operating in a vacuum. The even temperature distribution during fusion results in very good strength properties, making EBM well suited for high-performance applications in aerospace and medical implants. Compared to conventional CNC machining of titanium, EBM at a 0.1 mm layer thickness can achieve comparable results in shorter lead times and with cost savings of up to 35 %.

1.3.5 Selective Heat Sintering (SHS)

SHS uses a heated thermal print head rather than a laser to fuse thermoplastic powder. This significantly reduces the heat and power levels required. SHS is primarily used for concept prototypes. A typical desktop SHS machine provides a build volume of approximately 200 × 160 × 140 mm with a layer thickness of 0.1 mm.

1.4.1 Material Jetting (PolyJet)

Material Jetting creates parts in a method analogous to a two-dimensional inkjet printer. Liquid photopolymer or wax is jetted onto the build platform via a continuous or Drop-on-Demand (DOD) approach, cured layer by layer under UV light. Support structures are printed simultaneously from a dissolvable material.

Material Jetting step by step:

1. The print head is positioned above the build platform.
2. Droplets of material are deposited onto the required areas using thermal or piezoelectric actuation.
3. Droplets solidify and form the first layer.
4. Subsequent layers are built up on top.
5. Layers are allowed to harden or are cured by UV light; support material is removed in post-processing.

Material Jetting produces near-homogeneous parts with excellent surface finish and is the main process enabling multi-material printing in a single build. Its key limitation is that parts are typically brittle and have poor mechanical properties compared to FDM or PBF, making it best suited to prototyping and visual models.

1.4.2 Binder Jetting

Binder Jetting uses a powder-based material (polymers, metals, ceramics, or glass) and a liquid binder that acts as an adhesive between powder layers. A print head moves in the horizontal plane, depositing alternating layers of powder and binder. After each layer the build platform lowers by one layer thickness. No support structures are required because the surrounding powder supports the part throughout the build.

After printing, parts are in a fragile green state and require a post-processing step before they can be used:

• Infiltration: the part is placed in a furnace to burn out the binder, leaving roughly 60 % porosity. Bronze is then infiltrated by capillary action, resulting in approximately 10 % porosity and good mechanical strength.
• Sintering: the binder is similarly burnt out and the remaining metal particles are bonded together at high temperature, yielding less than 3 % porosity.

Both post-processing routes cause significant shrinkage: parts up to 75 mm typically shrink 0.8–2 %; larger parts average around 3 %. Non-uniform shrinkage must be accounted for in collaboration with the supplier. Even after post-processing, internal porosity remains – approximately 90 % density after infiltration and 97 % after sintering – so mechanical properties are generally inferior to fully dense DMLS/SLM parts.

Full-colour Binder Jetting is possible by simultaneously jetting ink and binder onto plaster or PMMA powder. The parts are subsequently coated with cyanoacrylate infiltrant and optionally an epoxy layer to improve strength and colour vibrancy. These full-colour parts are very brittle and not recommended for functional use.

1.4.3 Multi Jet Fusion (MJF)

MJF shares similarities with Binder Jetting – both deposit powder layer by layer and jet agents onto it – but works fundamentally differently. Binder Jetting uses a single adhesive binder to bind the powder layers together. MJF uses two agents: a fusing agent defining where material will melt and a detailing agent precisely defining part edges. Together, they cause the polymer chains within the material to entangle and fuse, rather than just bind, producing isotropic, functional parts with properties more comparable to injection-moulded components.

1.5.1 Laser Metal Deposition (LMD)

LMD uses a high-power laser to melt metal powder or wire feedstock. It offers high precision, excellent surface finish, and the ability to deposit a wide range of alloys. LMD is well suited for adding local features (ribs, bosses, wear pads), rebuilding worn surfaces, and applying corrosion- or wear-resistant cladding layers.

1.5.2 Electron Beam Additive Manufacturing (EBAM)

EBAM uses an electron beam as the heat source and typically processes wire feedstock in a vacuum chamber. It is particularly suited to reactive and high-temperature alloys – titanium, nickel-based superalloys – where oxidation must be avoided. Deposition rates are high, often several kilograms per hour, making EBAM attractive for large structural components. All fixtures and components must be compatible with the vacuum environment.

2.1.1 Geometry and Feature Size

Table 1 – Geometry and feature size for FDM

Aspect	Guideline	Why it matters
Minimum wall thickness	≥ 2 × nozzle diameter (0.4 mm nozzle → ≥ 0.8 mm)	Thin walls may not bond properly, leading to gaps or delamination.
Overhangs	≤ 45° without support; steeper angles require support structures	Unsupported overhangs sag because the extruded filament has nothing to rest on.
Bridges	Keep spans ≤ 10 mm (material-dependent); use a low print speed	Long bridges droop as the filament cools before it can solidify.
Small details	≥ 0.5 × nozzle diameter for holes/slots; larger for raised features	Tiny features can be clogged or incompletely formed due to extrusion limits.
Fillet radius	≥ 0.5 mm (or approximately 1 × nozzle diameter)	Sharp internal corners trap material and cause poor flow or weak points.
Orientation-dependent features	Align tall, slender elements vertically to reduce support volume	Reduces support material and improves surface quality on critical faces.
Maximum build envelope	Up to 900 × 600 × 900 mm on industrial FDM; desktop units typically ≤ 200 × 200 × 200 mm	Parts exceeding the envelope must be split and joined.

2.1.2 Part Orientation

Part orientation significantly affects strength, surface finish, and the amount of support material required. Key considerations:

• Strength direction: FDM parts are strongest along the raster (extrusion) direction and weakest across layer interfaces. Align the primary load path with the raster direction where possible.
• Support minimisation: Rotate the model to keep overhangs under 45°, limit contact points, and place the largest flat surfaces on the build plate.
• Surface finish: Faces that require a good visual finish should face upward or be placed against the build plate. Support-contact surfaces always have a rougher finish.
• Print speed: Orientation can influence print time but is usually a secondary concern.

2.1.3 Material Selection

Table 2 – Common FDM materials and their functional properties

This list is not exhaustive. Other commonly available filaments include Polycarbonate (PC), PolyEtherImide (PEI), ASA, and various composite grades. Consult supplier datasheets for specific mechanical and thermal properties.

2.1.4 Tolerances and Clearance

• Dimensional tolerance: ±0.2 mm for well-calibrated printers; ±0.5 mm for less precise machines.
• Moving parts clearance: Add at least 0.4–0.6 mm (≈ 1–1.5 × nozzle diameter) between mating surfaces.
• Threaded features: Print threads as a separate insert (metal or nylon) or design them approximately 10 % oversized and tap after printing.

2.1.5 Supports

• Support type: Use "tree" supports for delicate overhangs to reduce contact area and ease removal.
• Support density: 15–30 % infill for most plastics; increase to 40 % for ABS or Nylon to counteract warping.
• Interface layers: Add a few dense interface layers (≈ 0.2 mm) between the part and support to improve the surface finish where supports contact critical areas.

2.1.6 Print Settings That Influence Design

Table – Print settings and their effect on part design

2.1.7 Post-Processing Considerations

• Sanding and smoothing: Design with extra material where sanding is planned (e.g., thicker walls).
• Acetone vapour smoothing (ABS only): Avoid tight tolerances in areas to be smoothed; the process removes approximately 0.1 mm from the surface.
• Annealing: Allow for dimensional change of up to 2 % after annealing; maintain sufficient clearance in mating features.

2.1.8 Common Pitfalls and Fixes

Table – FDM common pitfalls and corrective actions

2.3.1 Minimum Wall Thickness

Table – Minimum wall thickness by PBF process

2.3.2 Embossed / Engraved Details

Table – Minimum embossed/engraved detail size

Smaller features may be possible; discuss requirements with the supplier.

2.3.3 Text (Minimum Font Height)

Table – Minimum font height by PBF process

2.3.4 Tolerances

Table – Typical dimensional tolerances

2.3.5 Feature Size (Pins, Protrusions)

Table – Minimum reliable feature size

2.3.6 Distance Between Parallel Features

Table – Minimum gap between parallel features

For metal processes, features can be placed somewhat closer together, but ensure the gap is large enough for the powder recoater to fill the space completely. If the gap is too tight, voids or incomplete fusion may result.

2.3.7 Holes

Table – Minimum reliable hole size

For metal PBF processes, holes can be smaller before they close off compared to polymer processes, as metal powders sinter more completely. However, unsupported vertical holes can trap unmelted powder; adding a vent hole or planning a "drill-out" post-processing step is recommended.

2.3.8 Escape / Vent Holes (for Hollow Sections)

Table – Minimum vent hole size for hollow sections

For metal builds, powder removal is easier due to higher part density, so vent holes can be slightly smaller. However, thermal stresses in metal builds mean at least one vent per enclosed section is still required.

2.3.9 Maximum Build Envelope

Table – Maximum build envelope by PBF process

If a part exceeds the build envelope, split it into sub-assemblies and join them afterwards (welding, brazing, or mechanical fasteners). The same principle applies when the build volume cannot accommodate the required quantity of parts.

2.3.10 Materials

Table 4 – Materials for PBF printing

2.4.1 Material Jetting (PolyJet)

Material Jetting produces near-homogeneous parts and supports multi-material printing. Its principal limitation is brittle mechanical behaviour.

Table 5 – Geometry and feature size for Material Jetting

Aspect	Guideline	Why it matters
Minimum wall – supported	> 1 mm	Minimum wall thickness for major supported walls.
Minimum wall – unsupported	> 0.5 mm	Minimum for all other walls.
Pin diameter	> Ø 0.5 mm	Minimum recommended pin diameter.
Details	As small as 0.25 mm is achievable	Material Jetting offers very high resolution for fine details.
Embossed / engraved details (including text)	≥ 0.5 mm depth/height	Ensures small details are reliably visible.
Minimum hole diameter	> Ø 0.5 mm; orient holes vertically where possible	Vertical orientation maximises hole circularity.
Clearance – moving parts / hinges	0.15 – 0.2 mm clearance on all sides	Allows support material removal from gaps.
Dimensional tolerance	±0.1 mm to ±0.3 mm (geometry and material dependent)	Sets the baseline for fit design.
Maximum build envelope	~290 × 200 × 140 mm (contact supplier)	Larger parts must be split.

Table – Material Jetting materials overview

2.4.2 Binder Jetting

After Binder Jetting, parts are in a fragile green state and require post-processing (infiltration or sintering) before use.

Table 6 – Geometry and feature size for Binder Jetting

Aspect	Guideline	Why it matters
Minimum wall – supported	> 2 mm	Ensures the part can be handled in the green state without damage.
Minimum wall – unsupported	> 3 mm	Unsupported walls (fins, ribs) are at higher risk during green-state handling.
Unsupported edges	< 20 mm in length	Longer unsupported edges are prone to breaking during green-state handling.
Embossed / engraved details	≥ 0.5 mm	Ensures details are visible after depowdering and post-processing.
Fillets	Radius > 1 mm on all edges where possible	Reduces risk of damage in the green state; aids powder removal from internal cavities.
Minimum hole diameter	> Ø 1.5 mm	Ensures holes print successfully and can be cleared of powder.

2.4.3 Multi Jet Fusion (MJF)

MJF enables production of functional parts at reduced cost and lead time compared to SLS and injection moulding, especially for low to medium volumes.

Table 7 – Geometry and feature size for MJF

2.5.1 Laser Metal Deposition (LMD)

Table 8 – Geometry and feature size for LMD

2.5.2 Electron Beam Additive Manufacturing (EBAM)

Table 9 – Geometry and feature size for EBAM

2.6.1 Laminated Object Manufacturing (LOM)

LOM is best suited to relatively large, simple geometries with moderate surface detail. Avoid features relying on high interlaminar strength. Thin freestanding elements and sharp re-entrant features are particularly difficult to produce reliably.

Typical design considerations for LOM:

• Prefer broad, well-supported geometries; avoid long, fragile cantilevers.
• Keep internal corners and transitions smooth where possible to reduce tearing and improve layer registration.
• Account for the sheet thickness in the Z-direction; stepped surfaces become more visible on shallow slopes and curved surfaces.
• Plan post-processing – sanding, sealing, or coating – when visual appearance is important.

Table 10 – Geometry and feature size for LOM

2.6.2 Ultrasonic Additive Manufacturing (UAM)

UAM (also called ultrasonic consolidation) bonds metal foils or ribbons in the solid state using ultrasonic vibration and pressure, with intermediate CNC machining to define the final geometry. It is particularly attractive for multi-material metallic structures, embedded sensors or wires, and parts with internal channels that would be difficult to produce by conventional machining alone.

Table 11 – Geometry and feature size for UAM