Introduction
Every manufacturing process covered in engineering so far starts with more material than the finished part needs. Machining cuts metal away as chips. Forming pushes it into shape. Casting pours it into a mould that already defines the outer surface.
Additive manufacturing does something none of them can. It builds the part from nothing, one thin layer at a time, adding material only where the design says material should be.
That inversion has a consequence worth stating clearly: complexity becomes free. A machined part costs more as it gets more complicated, because every extra feature is another operation. A 3D printed part with internal lattices, curved cooling channels and hollow sections costs almost exactly the same as a solid block of the same size, because the machine is depositing layers either way.
This guide covers what additive manufacturing actually is, the seven official process categories, how a part travels from CAD file to finished component, and why 3D printed parts are weaker in one direction than another. Written in plain language for mechanical and production engineering students.
What Is Additive Manufacturing?
ASTM defines additive manufacturing as the process of joining materials to make parts from 3D model data, usually layer upon layer.
Three ideas sit inside that definition:
- Additive — material is added where needed, not removed from a larger piece
- Layer upon layer — the part is built as a stack of thin 2D cross-sections
- From 3D model data — the geometry comes directly from a digital file, with no tooling, mould or fixture defining the shape
That last point is the economically important one. Casting needs a pattern and a mould. Forging needs dies. Injection moulding needs a tool costing thousands. Additive manufacturing needs no tooling at all, which is why it is unbeatable for one-off parts and prototypes, and why it struggles to compete on cost once volumes get large.
Additive Manufacturing vs 3D Printing
The two terms are used interchangeably in everyday speech, but there is a real distinction worth knowing.
3D printing originally described one specific process depositing binder onto a powder bed with inkjet heads. In popular use it now covers everything, including desktop hobby machines.
Additive manufacturing is the formal, standards-based term, and it carries an industrial connotation production parts, qualified processes, engineering materials.
Strictly, 3D printing is a subset of additive manufacturing. In practice, if the context is a factory or an aerospace component, “additive manufacturing” is the correct word. If it is a desktop machine printing a phone stand, “3D printing” fits better.
Additive vs Subtractive Manufacturing
| Factor | Additive manufacturing | Subtractive manufacturing |
|---|---|---|
| Basic action | Adds material layer by layer | Removes material from solid stock |
| Starting point | Nothing; builds from a platform | A block, bar or billet |
| Material waste | Very low; only supports are wasted | High; material becomes chips |
| Tooling required | None | Cutters, fixtures, jigs |
| Cost vs complexity | Roughly flat complexity is free | Rises steeply with complexity |
| Internal features | Easy, including enclosed channels | Impossible if the tool cannot reach |
| Surface finish | Rough; layer lines visible | Excellent straight off the machine |
| Dimensional accuracy | Moderate, ±0.1 to ±0.3 mm typical | High, ±0.01 mm achievable |
| Material properties | Anisotropic; direction dependent | Isotropic; same in all directions |
| Material choice | Limited range of printable grades | Almost any machinable material |
| Speed for one part | Fast, no setup | Slow, needs programming and setup |
| Speed for 10,000 parts | Slow, each takes the same time | Fast once running |
| Best suited to | Prototypes, complex shapes, low volume | Precision parts, high volume |
The trade-off is clean and worth memorising: additive wins on complexity and low volume, subtractive wins on precision and high volume.
In practice the two are increasingly used together. A metal part may be printed to near-net shape, then machined only on the surfaces that need tight tolerance combining the geometric freedom of one with the accuracy of the other.
Additive Manufacturing Process Steps
Every AM process, from a desktop filament printer to an aerospace metal machine, follows the same eight-stage workflow.
1. CAD modelling. The part is designed in 3D CAD, or captured by 3D scanning of an existing object.
2. Conversion to STL. The model is exported as an STL file, which approximates all surfaces as a mesh of triangles. More triangles means a smoother approximation and a larger file. STL is the long-standing standard, though 3MF and AMF formats are replacing it because they also carry colour, material and metadata.
3. Orientation and support generation. The part is positioned on the virtual build plate, and support structures are added under overhangs. This decision quietly determines strength, surface quality, print time and how much finishing work follows.
4. Slicing. Software cuts the model into hundreds or thousands of horizontal layers, typically 0.02 to 0.4 mm thick, and generates the tool path for each. The output is machine code usually G-code.
5. Machine setup. Material is loaded, the build platform is levelled and cleaned, and process parameters such as temperature, laser power or layer height are set.
6. Building. The machine executes the layers one at a time. Builds run from a few hours to several days.
7. Part removal. The part is taken off the build plate. Powder-based processes require excess powder to be removed and recovered; resin parts need washing.
8. Post-processing. Supports are removed, then any combination of curing, heat treatment, machining, surface finishing or infiltration is carried out.
Step 8 is the one students underestimate. For metal AM especially, post-processing frequently takes longer and costs more than the printing itself stress relief, wire EDM to cut the part off the plate, support removal, hot isostatic pressing, and machining of critical faces.
The 7 Categories of Additive Manufacturing
In 2010, ASTM Committee F42 classified all additive processes into seven families, now standardised as ISO/ASTM 52900. Every AM technology, whatever its trade name, belongs to one of these seven.
| Category | How it works | Materials | Common trade names |
|---|---|---|---|
| Vat photopolymerization | Liquid resin cured selectively by light | Photopolymer resins | SLA, DLP, CLIP, LCD |
| Material extrusion | Molten filament pushed through a heated nozzle | Thermoplastics, composites | FDM, FFF |
| Powder bed fusion | Powder fused by laser or electron beam | Polymers, metals | SLS, SLM, DMLS, EBM, MJF |
| Material jetting | Droplets of material jetted and cured | Photopolymers, waxes | PolyJet, MJP, DOD |
| Binder jetting | Liquid binder printed onto powder | Metals, sand, ceramics, polymers | Binder jetting, 3DP |
| Sheet lamination | Sheets stacked and bonded, then cut | Paper, polymer, metal foil | LOM, UAM |
| Directed energy deposition | Material melted as it is deposited | Metal powder or wire | LENS, WAAM, LMD |
Vat Photopolymerization
A vat of liquid photopolymer resin is cured selectively by ultraviolet light. SLA traces each layer with a laser point by point. DLP projects an entire layer image at once, so it is faster. LCD (mSLA) uses an LCD screen as the mask, which made the process cheap enough for desktop machines.
Strengths: the finest resolution and smoothest surfaces of any AM process. Weaknesses: resins are brittle, degrade in UV light, and need washing and post-curing. Used for: dental models, jewellery patterns, hearing aid shells, investment casting patterns.
Material Extrusion
Thermoplastic filament is fed through a heated nozzle, melted, and deposited along the layer path where it fuses to the layer beneath. FDM is a trademarked term; FFF is the generic equivalent.
Strengths: cheapest and most accessible process, wide range of engineering thermoplastics. Weaknesses: visible layer lines, weakest inter-layer bonding of all AM processes, needs supports. Used for: prototypes, jigs and fixtures, tooling aids, education.
Powder Bed Fusion
A thin layer of powder is spread across the build platform, and a laser or electron beam fuses the cross-section. The platform lowers, fresh powder is spread, and the cycle repeats. Unfused powder supports the part as it builds.
This is the most important category for engineering, because it produces fully dense metal parts with properties approaching wrought material.
Strengths: excellent mechanical properties, complex internal geometry, no support needed for polymer versions. Weaknesses: expensive machines, powder handling hazards, significant post-processing. Used for: aerospace brackets, turbine components, medical implants, tooling with conformal cooling.
Material Jetting
Droplets of photopolymer are jetted from print heads exactly like an inkjet printer and cured immediately by UV light.
Strengths: very high resolution, and the only process that easily produces multi-material and full-colour parts in one build. Weaknesses: materials are expensive and relatively weak; support material must be dissolved away. Used for: realistic prototypes, medical models, colour-coded assemblies.
Binder Jetting
A print head deposits liquid binder onto a powder bed, gluing particles together layer by layer. The result is a fragile green part, which is then debound and sintered in a furnace to fuse the powder into solid material.
The sintering step causes roughly 20% shrinkage, which must be compensated for in the model a source of dimensional difficulty unique to this process.
Strengths: fast, no heat during printing so no residual stress, works with metals, sand and ceramics. Weaknesses: shrinkage during sintering, generally lower density than powder bed fusion. Used for: sand casting moulds and cores, metal production parts, full-colour sandstone models.
Sheet Lamination
Sheets of material are stacked and bonded, with each layer’s profile cut by laser or blade. LOM uses paper or plastic with adhesive. UAM (Ultrasonic Additive Manufacturing) bonds metal foils by ultrasonic welding, then machines the profile a solid-state process that runs cool enough to embed sensors and electronics inside a solid metal part.
Strengths: fast, inexpensive materials, and UAM enables embedded components. Weaknesses: significant waste, weak bonding in the Z direction, limited geometry. Used for: concept models, embedded-sensor metal components.
Directed Energy Deposition
Material metal powder or wire is fed into a focused energy source and melted as it is deposited. Unlike powder bed fusion, there is no powder bed; the nozzle moves in space, often on a robot arm or multi-axis machine.
WAAM (Wire Arc Additive Manufacturing) is worth knowing: it uses a standard welding arc and wire feed to build large metal parts. It is literally arc welding used additively, with deposition rates measured in kilograms per hour rather than grams.
Strengths: very high deposition rates, very large parts, and the ability to add material to existing components for repair. Weaknesses: poor surface finish and accuracy; almost always machined afterwards. Used for: repairing turbine blades and dies, large aerospace structures, marine propellers.
Metal Additive Manufacturing: DMLS vs SLM vs EBM
These three acronyms cause more confusion than anything else in AM, because all three are powder bed fusion of metal.
| Factor | DMLS | SLM | EBM |
|---|---|---|---|
| Energy source | Laser | Laser | Electron beam |
| Atmosphere | Inert gas | Inert gas | Vacuum |
| Powder bed temperature | Moderate preheat | Moderate preheat | High, around 700 °C |
| Fusion mechanism | Melting (historically sintering) | Full melting | Full melting |
| Residual stress | Higher | Higher | Much lower |
| Surface finish | Better | Better | Rougher |
| Resolution | Finer | Finer | Coarser |
| Build speed | Slower | Slower | Faster |
| Materials | Wide range including alloys | Pure metals and alloys | Conductive metals only |
| Support needs | Extensive | Extensive | Minimal |
DMLS and SLM are essentially the same process under different manufacturers’ trademarks. DMLS was originally named for sintering, but modern DMLS machines fully melt the powder, so the practical difference from SLM is branding rather than physics.
EBM is genuinely different. Because it uses an electron beam, it requires a vacuum, and it can only process electrically conductive materials. Its high bed temperature relieves stress as the part builds, so parts come out with far less residual stress and need fewer supports at the cost of a rougher surface. It dominates titanium orthopaedic implant production for exactly this reason.
Materials Used in Additive Manufacturing
| Material family | Examples | Processes |
|---|---|---|
| Thermoplastics | PLA, ABS, PETG, PC, nylon, PEEK | Material extrusion, powder bed fusion |
| Photopolymers | Standard, tough, castable, dental resins | Vat photopolymerization, material jetting |
| Metals | Titanium, aluminium, stainless, Inconel, cobalt-chrome | Powder bed fusion, DED, binder jetting |
| Ceramics | Alumina, zirconia, silicon carbide | Binder jetting, vat photopolymerization |
| Composites | Carbon and glass fibre filled polymers | Material extrusion |
| Sand | Foundry sand with binder | Binder jetting |
| Biomaterials | Hydrogels, living cells | Bioprinting (specialised extrusion or jetting) |
Build Orientation, Anisotropy and Support Structures
This section covers the engineering reality that separates a working knowledge of AM from a superficial one.
Why 3D Printed Parts Are Anisotropic
A machined part is isotropic cut from solid stock, its properties are the same in every direction.
An additively manufactured part is anisotropic. Within each layer, the material is continuous and strong. Between layers, strength depends on how well one layer bonded to the last, and that bond is always weaker than the bulk material.
The practical result: strength in the Z direction (perpendicular to the layers) is significantly lower than in the XY plane. In extrusion processes the difference can be dramatic; in metal powder bed fusion, where each layer is fully melted into the one below, it is much smaller but still measurable.
The design rule follows directly: orient the part so the principal load runs within the layer plane, not across it. A printed hook loaded in a direction that peels the layers apart will fail at a fraction of its expected strength.
Build Orientation Affects Everything
One decision how the part sits on the build plate controls:
- Strength, through the anisotropy above
- Surface finish, since curved and angled surfaces show stair-stepping
- Support volume, and therefore material cost and finishing labour
- Build time, because height usually matters more than footprint
- Accuracy, as supported surfaces come out rougher than free ones
Support Structures
Any surface overhanging beyond roughly 45° from vertical needs support, because there is nothing beneath it to build onto.
Supports cost material, add print time, and leave witness marks that must be finished off. In metal AM they also serve a second purpose: conducting heat away and anchoring the part against thermal distortion, which means they cannot simply be minimised.
Good design reduces the need for them chamfering overhangs to 45°, using teardrop-shaped holes rather than circular ones for horizontal bores, and orienting critical faces away from support contact.
Design for Additive Manufacturing (DfAM)
DfAM means designing to exploit what AM does well rather than copying a design meant for machining.
Exploit complexity, since it is free. Internal lattices, conformal cooling channels that follow a mould’s contours, and organic topology-optimised shapes cost nothing extra to print.
Consolidate parts. An assembly of twelve machined components with fasteners can often be printed as one piece eliminating joints, fasteners, assembly labour and leak paths. This is where AM delivers its largest real-world savings.
Remove unnecessary material. Since you pay for material and time, hollow sections and lattice infill reduce both while retaining stiffness.
Design for support minimisation. Self-supporting angles, teardrop holes, chamfers instead of overhangs.
Respect minimum feature size. Walls and details below the process resolution simply will not form.
Plan post-processing from the start. Add machining allowance on tight-tolerance surfaces, and ensure supports can physically be reached and removed supports inside a sealed internal channel are unremovable.
Common Defects in Additive Manufacturing
| Defect | What happens | Main cause |
|---|---|---|
| Porosity | Voids inside the part | Insufficient energy (lack of fusion) or excessive energy (keyhole collapse) |
| Warping | Part curls off the build plate | Thermal contraction and residual stress |
| Delamination | Layers separate | Poor inter-layer bonding, temperature too low |
| Stair-stepping | Visible steps on curved surfaces | Layer thickness too great for the geometry |
| Balling | Rough beaded surface in metal AM | Unstable melt pool, wrong scan parameters |
| Residual stress | Distortion after removal from the plate | Rapid heating and cooling cycles |
| Support scarring | Rough marks where supports touched | Unavoidable; managed by orientation |
| Dimensional error | Part outside tolerance | Shrinkage, thermal distortion, poor calibration |
| Powder contamination | Inclusions and weak spots | Recycled powder degradation, mixed materials |
Porosity is the defining problem of metal AM, because it is internal and invisible. Two opposite causes produce it: too little energy leaves unmelted gaps between tracks, while too much energy vaporises metal and creates keyhole voids. This is why hot isostatic pressing (HIP) high temperature and pressure that closes internal voids is standard for aerospace and medical AM parts.
Applications of Additive Manufacturing
- Aerospace – fuel nozzles consolidated from dozens of parts into one, lightweight brackets, turbine blade repair by DED
- Medical – patient-specific titanium implants, surgical guides, dental crowns and aligners, prosthetics
- Automotive – prototypes, jigs and fixtures, motorsport components, spare parts on demand
- Tooling – injection moulds with conformal cooling channels that no machining could produce
- Foundry – binder-jetted sand moulds and cores, eliminating pattern making
- Energy – turbine components, heat exchangers with complex internal geometry
- Consumer – customised eyewear, footwear midsoles, hearing aid shells
- Construction – 3D printed concrete structures and formwork
The hearing aid industry is the classic case study: it converted almost entirely to additive manufacturing within a couple of years, because every shell must be unique to one ear canal exactly the situation where tooling-free production wins outright.
Advantages and Limitations of Additive Manufacturing
Advantages
- No tooling, so no setup cost and immediate production
- Geometric complexity is essentially free
- Internal features impossible by any other process
- Part consolidation removes assembly, fasteners and joints
- Very low material waste
- Mass customisation is economically viable
- Rapid design iteration, from file to part in hours
- Distributed and on-demand manufacturing, including spares
Limitations
- Slow for high volumes; cost per part barely falls with quantity
- Limited material range compared with conventional processes
- Anisotropic properties requiring orientation-aware design
- Surface finish generally needs post-processing
- Lower dimensional accuracy than machining
- Build size limited by machine envelope
- Expensive machines and materials
- Powder handling and resin exposure are real safety hazards
- Certification and quality assurance remain difficult for critical parts
Frequently Asked Questions (FAQ)
1. What is additive manufacturing in simple words?
Additive manufacturing builds a part by adding material layer upon layer directly from a 3D model, rather than cutting it from a solid block. Because no mould or tooling is needed, the shape is defined entirely by the digital file.
2. What are the 7 categories of additive manufacturing?
Vat photopolymerization, material extrusion, powder bed fusion, material jetting, binder jetting, sheet lamination, and directed energy deposition. These were defined by ASTM Committee F42 and are standardised in ISO/ASTM 52900.
3. What is the difference between additive manufacturing and 3D printing?
They describe the same family of processes, but additive manufacturing is the formal standards term with an industrial connotation, while 3D printing is the popular term that also covers desktop hobby machines. Strictly, 3D printing is a subset of additive manufacturing.
4. What is the difference between additive and subtractive manufacturing?
Additive builds a part by adding material layer by layer with almost no waste and no tooling. Subtractive cuts material away from solid stock, producing higher accuracy and better finish but more waste. Additive suits complex low-volume parts; subtractive suits precision and high volume.
5. What are the steps in the additive manufacturing process?
CAD modelling, conversion to STL, orientation and support generation, slicing into layers, machine setup, building, part removal, and post-processing.
6. Why are 3D printed parts weaker in one direction?
Because the part is built as a stack of bonded layers. Within a layer the material is continuous, but between layers the bond is weaker than the bulk material. Strength perpendicular to the layers is therefore lower, which is why build orientation must match the load direction.
7. What is the difference between SLS, SLM and DMLS?
All three are powder bed fusion. SLS generally refers to polymer powders sintered by laser. SLM and DMLS both fully melt metal powder with a laser and are essentially the same process under different manufacturers’ trademarks.
8. Why do additive manufactured metal parts need post-processing?
Because parts come off the build plate with supports attached, residual stress from rapid heating and cooling, rough surfaces, and possible internal porosity. Stress relief, support removal, machining of critical faces and often hot isostatic pressing are required before the part is usable.
Conclusion
Additive manufacturing reverses the assumption every other process is built on. Instead of starting with too much material and taking away what is not needed, it starts with nothing and adds only what is.
That reversal produces one enormous advantage and one stubborn limitation, and both come from the same source. Because there is no tool to reach into the part and no mould to open, geometry stops being a constraint internal channels, lattices and consolidated assemblies become possible, and complexity costs nothing extra. But because the part is a stack of bonded layers rather than continuous material, it is anisotropic, its surfaces are rough, and its accuracy trails machining by an order of magnitude.
So the question is never whether additive is better than subtractive. It is which constraint matters more for the part in front of you. Need one complex titanium bracket next week, with internal passages and no tooling budget? Print it. Need fifty thousand identical steel pins to ±0.01 mm? Turn them.
The seven categories, the materials and the defect list are all detail hanging off that single trade-off. Get the trade-off right and the rest follows.