Additive Manufacturing: AM Processes, Materials, and Defects

Additive manufacturing (AM)

Building a part by adding and joining material from digital 3D geometry, usually layer by layer.

Subtractive manufacturing

Making a part by removing material from a larger block using processes such as milling, turning, drilling, or grinding.

Seven AM categories

Vat photopolymerization, material extrusion, powder bed fusion, material jetting, binder jetting, sheet lamination, and directed energy deposition.

Vat photopolymerization

Using light to selectively cure liquid photopolymer resin into a solid part.

Material extrusion

Depositing softened polymer, paste, gel, or biological material through a nozzle.

Powder bed fusion

Using thermal energy to selectively sinter or melt regions of a powder bed.

Material jetting

Depositing droplets of the actual part material that are cured, dried, or solidified.

Binder jetting

Depositing liquid binder onto powder; the powder provides most of the part volume.

Laser SLA

Uses a focused laser that scans and cures resin point by point.

DLP-SLA

Uses a projector and digital micromirror device to expose an entire resin layer at once.

MSLA

Uses a UV backlight and LCD mask to expose an entire resin layer at once.

Laser SLA vs. DLP/MSLA

Laser SLA scans point by point, while DLP and MSLA expose a complete layer simultaneously.

Volumetric 3D printing

Creates a part inside a volume of resin using overlapping light exposures instead of stacking many individual layers.

CAD

Computer software used to create and edit the digital geometry of a part.

STL

A file format that approximates a CAD surface with triangular facets for slicing.

AMF

A file format that can store geometry, units, materials, colors, and multiple objects.

Tessellation

Converting smooth CAD surfaces into small triangles for manufacturing.

Watertight model

A closed digital model with no holes, gaps, duplicate faces, or incorrect surface normals.

Part orientation

The position of a part in the build volume; it affects supports, strength, surface quality, and build time.

Nesting

Arranging multiple parts in the build volume while maintaining proper spacing and thermal conditions.

Supports

Temporary structures that hold overhangs, transfer heat, or prevent distortion during printing.

Post-processing

Operations after printing, such as washing, depowdering, support removal, heat treatment, machining, sanding, or curing.

Photoinitiator

A resin ingredient that absorbs light and creates reactive species to start polymerization.

Free-radical polymerization

Acrylate-based curing mechanism involving initiation, chain propagation, and termination.

Cationic polymerization

A curing mechanism commonly used for epoxy resins in which cations open epoxy rings and grow the polymer network.

Critical exposure, Ec

The minimum light exposure required to begin curing resin.

Penetration depth, Dp

A measure of how deeply light travels into resin before its intensity decreases significantly.

Cure depth, Cd

The thickness of resin cured by a specific exposure.

Working curve equation

Cd = Dp ln(E/Ec), relating exposure energy to cured depth.

SLS

Selective laser sintering; a laser heats polymer powder particles so they bond together without fully melting the entire powder bed.

Laser powder bed fusion

A powder bed process in which a laser selectively melts or fuses material to build a part.

Sintering

Bonding particles through diffusion and surface-energy reduction, usually below the material's full melting temperature.

Melting

Heating material above its melting point so it becomes liquid and solidifies into a dense part.

Particle size effect

Smaller particles usually sinter faster because they have more surface area per volume and a stronger surface-energy driving force.

Fine powder disadvantage

Fine powder can flow poorly, agglomerate, become airborne, and create inhalation or fire hazards.

Powder flowability

The ability of powder to spread uniformly across the build platform.

Powder packing

The way particles arrange and fill space; better packing generally improves density and consistency.

Preheating

Heating the powder bed to reduce thermal gradients, residual stress, and distortion.

PBF energy density equation

Ea = P/(vs × s), where P is laser power, vs is scan speed, and s is scan spacing.

Effect of increasing laser power

Increases energy density and can improve bonding, but excessive power can cause overheating, keyholing, or degradation.

Effect of increasing scan speed

Decreases energy density when power and spacing stay constant, which can cause weak bonding or lack of fusion.

Effect of increasing scan spacing

Decreases energy density and track overlap, which can leave unprocessed gaps.

Lack of fusion

A defect caused by insufficient energy, leaving incomplete bonding between tracks or layers.

Balling

A defect where surface tension breaks a molten track into separate spheres.

Keyholing

A defect caused by excessive energy creating a deep unstable vapor cavity that traps pores.

FDM

Fused deposition modeling; a material-extrusion process that deposits heated thermoplastic roads.

Road

A single deposited strand of material from an extrusion nozzle.

Layer bonding

The joining of adjacent roads or layers through polymer-chain diffusion while the material is hot.

FDM anisotropy

Direction-dependent strength caused by weaker bonding between roads and layers than along continuous roads.

Warping

Distortion caused by uneven cooling and thermal contraction.

Stringing

Thin unwanted strands formed when material leaks from the nozzle during travel movements.

Delamination

Separation between printed layers caused by weak interlayer bonding or thermal stress.

Material jetting vs. binder jetting

Material jetting deposits the actual part material; binder jetting deposits binder onto powder.

Drop-on-demand jetting

Ejects droplets only when commanded by the printhead.

Continuous jetting

Produces a continuous droplet stream and directs selected droplets using charging or deflection.

Thermal inkjet

Uses a small heated bubble to force a droplet out of the nozzle.

Piezoelectric jetting

Uses a piezoelectric element that changes chamber pressure when voltage is applied.

Viscosity

A fluid's resistance to flow; excessive viscosity can prevent reliable droplet ejection.

Surface tension

The tendency of a liquid surface to contract; it affects droplet shape, spreading, and satellite formation.

Wetting

How well a droplet spreads and bonds to the receiving surface.

Satellite droplets

Small unwanted droplets that form beside the main droplet and reduce placement accuracy.

Material jetting advantages

High resolution, smooth surfaces, full color, and multimaterial printing.

Material jetting limitations

Limited jettable materials, nozzle clogging, support removal, and sometimes limited heat or mechanical performance.