The Seven Categories of 3D Printing

Every 3D printer builds a part in layers from a digital model, but the ways they do it are very different. One squeezes melted plastic through a nozzle, another cures liquid resin with light, another melts metal powder with a laser. The international standard for additive manufacturing, ISO/ASTM 52900, sorts them into seven families. Knowing which family a printer belongs to tells you most of what matters: which materials it can use, how strong and accurate the parts are, and roughly what they cost.

What every 3D printer has in common

The model is sliced into thin horizontal layers, from about 0.02 mm to 0.3 mm thick depending on the process, and the printer builds them one on top of another. A few things follow from that, whatever the machine:

• Layers show. Curved and sloped surfaces have small steps. Thinner layers give smoother parts and longer print times.
• Direction matters. Parts are usually weakest between layers, so the way a part is oriented in the printer affects its strength, its surface and its cost.
• Overhangs need support in most processes. Supports are printed with the part and removed afterward, and they leave marks.
• Post-processing is part of the job: removing supports or powder, curing, heat treating, sanding or machining.

For getting a file ready, see how to prepare a file for 3D printing.

1. Material extrusion (FDM)

A heated nozzle melts plastic filament and lays it down line by line, in layers usually 0.1 to 0.3 mm thick. The common name is FDM (fused deposition modeling), also called FFF (fused filament fabrication). It's the most common and the cheapest way to print a part, and it scales well to big parts.

Materials range from PLA and PETG to ABS, ASA, nylon, flexible TPU and polycarbonate, plus filaments filled with chopped carbon or glass fiber. Industrial machines also print high-temperature plastics such as PEEK and PEI (Ultem). Some printers lay a continuous strand of carbon or glass fiber inside the part for much higher stiffness, and bound-metal filaments can be printed, then debound and sintered into solid metal parts.

The trade-offs are visible layer lines, accuracy around ±0.5 mm, and parts that are noticeably weaker between layers than along them. FDM is the right call for fit checks, jigs and fixtures, and large or low-cost parts.

2. Vat photopolymerization (SLA, DLP and LCD)

A light source cures liquid photopolymer resin one layer at a time. In SLA (stereolithography) a laser traces each layer; DLP printers flash a whole layer at once with a projector; LCD printers, also called MSLA, shine UV light through a screen that masks each layer. Layers run from about 0.025 to 0.1 mm, so surfaces come out smooth and fine detail holds up.

Resins come in standard, tough, flexible, high-temperature, castable and dental grades. After printing, parts are washed in solvent, post-cured under UV light and have their supports removed. Standard resins are brittle, creep slowly under load, and yellow and weaken in sunlight, so working parts should be printed in a tough or high-temperature resin, or in nylon instead. Resin printing is the usual choice for appearance models, small detailed parts and master patterns for casting.

3. Powder bed fusion

A thin layer of powder is spread across the build area, and a heat source fuses the cross-section of the part. The bed drops, a new layer is spread, and the process repeats. This family covers both plastic and metal printing.

Plastic: SLS and MJF

In SLS (selective laser sintering) a laser fuses nylon powder. In MJF (HP's Multi Jet Fusion) a print head jets a fusing agent onto the powder and infrared lamps fuse the treated areas. Either way, the loose powder holds the parts up as they build, so there are no supports, and dozens of parts can be packed into one build. Common materials are PA12 and PA11 nylon, glass-filled nylon and TPU.

Parts are strong, close to equally strong in every direction, and accurate to about ±0.3 mm. Surfaces are matte and slightly grainy, and MJF parts are usually dyed black. This is the usual choice for working plastic parts and for batches from about 10 to a few hundred.

Metal: laser powder bed fusion (DMLS and SLM)

A laser melts fine metal powder in layers around 0.02 to 0.06 mm thick. The process goes by several names, including DMLS, SLM and LPBF. Common alloys are 316L and 17-4 PH stainless steel, AlSi10Mg aluminum, Ti-6Al-4V titanium, nickel alloys such as Inconel 718, maraging tool steel and copper.

Parts come out over 99 percent dense, and after heat treatment many alloys reach strengths close to wrought metal. The process makes shapes nothing else can, like internal cooling channels and lattices. It's also the most expensive option: parts are built on supports fused to a steel plate, then stress relieved, cut off the plate, and machined on any surface that seals, mates or carries a bearing. See conformal cooling in injection molds for one place it earns its cost.

Electron beam melting (EBM)

EBM uses an electron beam in a vacuum instead of a laser, and keeps the powder bed hot during the build, which lowers residual stress and reduces the supports needed. Surfaces come out rougher. It's used mostly for titanium, including orthopedic implants with porous surfaces that bone can grow into.

4. Material jetting (PolyJet)

Print heads like an inkjet printer's jet tiny droplets of photopolymer, which are cured by UV light as they land, in layers as thin as about 0.016 mm. Because different heads can jet different materials, one part can combine rigid and rubber-like areas, clear sections and full color. Stratasys PolyJet is the best-known version.

Parts are very smooth and look close to finished products, which makes material jetting popular for appearance models, overmold mock-ups and anatomical models. The materials are expensive and fairly weak, and like other photopolymers they don't hold up to heat or sunlight.

5. Binder jetting

A print head deposits a liquid binder onto a bed of powder, gluing the particles together layer by layer without melting them. For metal parts, the printed "green" part is cured, then sintered in a furnace, where it shrinks by roughly 15 to 20 percent in each direction as the particles fuse. That shrinkage has to be designed in, and it limits accuracy on large parts.

Binder jetting is faster and cheaper per part than laser powder bed fusion, so it suits small stainless and tool steel parts in the hundreds or thousands. It's also used to print sand molds and cores for metal casting, with no pattern needed, and full-color gypsum models.

6. Directed energy deposition (DED)

Metal powder or wire is fed into a melt pool made by a laser, an electron beam or a welding arc, and a robot or CNC head moves the pool along a path to build up material. Wire arc additive manufacturing (WAAM) uses standard welding equipment to build large metal parts quickly.

DED deposits material fast but roughly, so parts are machined afterward. It's used to build large near-net-shape parts, to add features to existing parts, and to repair worn components such as turbine blades and mold surfaces. Hybrid machines combine a deposition head and a milling spindle in one setup.

7. Sheet lamination

Sheets of paper, plastic or metal foil are bonded on top of each other and cut to the outline of each layer. Ultrasonic additive manufacturing welds thin metal foils together at low temperature, so sensors or fibers can be embedded inside a metal part. Sheet lamination is a niche today, used mostly for specialized parts such as heat exchangers and parts with embedded sensors.

Side by side

Which one to use

• Checking size and fit cheaply: FDM.
• Appearance models and fine detail: resin (SLA or DLP), or material jetting for color and several materials in one part.
• Working plastic parts, alone or in batches: SLS or MJF.
• Complex metal parts: laser powder bed fusion. Small metal parts in the thousands: binder jetting.
• Large metal parts: DED or WAAM, followed by machining.
• Simple metal parts: usually CNC machining, which is faster, cheaper and more accurate. See 3D printing vs CNC machining vs injection molding.

For the four processes most printed parts use, there's a closer comparison in FDM vs SLA vs SLS vs MJF, and costs are broken down in what sets the price of a 3D printed part.

We print in FDM, resin, nylon powder and metal. Send a STEP or STL file and a note on what the part has to do, and we'll recommend a process when we quote. See our 3D printing service.

Related reading

• FDM vs SLA vs SLS vs MJF: which process to use
• 3D printing cost factors
• How to prepare a file for 3D printing
• 3D printed prototypes by development stage
• Rapid prototyping methods compared

Terms are defined in the glossary, and every guide is listed in the knowledge base.