There are many different kinds of 3D printing with varying levels of sophistication. The unifying principle between common 3D printing methods is “additive manufacturing,” or manufacturing by building something layer by layer rather than creating a mold. This technology is especially useful when an individual or organization wants to print a limited quantity of items (without paying for tooling) or when you need a complex part that can’t be achieved with traditional manufacturing.

Some 3D printing technology is just for fun, or elementary prototyping at best. However, the most advanced 3D printing machines can make highly complex parts that can be used in virtually limitless real world applications, like working engine parts and prosthetic limbs.

The three most common 3D printing methods are SLS, SLA, and FDM.

Here’s how they work:

SLS: Selective Laser Sintering

SLS is a great choice for functional products as well as prototypes. The final product is a strong, flexible, chemically resistant part with a high tolerance to heat. These are large and expensive machines, not your consumer-grade printer.

To use SLS technology, you start with a CAD model. A highly sophisticated 3D printer reads the model one layer at a time, in turn then sintering powder-based materials layer by layer with extreme precision using a powerful laser. The typical materials are nylon-based and melt into a durable plastic. However, composite powders are also common depending on the look and function of the desired product. Each layer is incredibly thin–about 0.004 inches. Each time the laser sinters a new layer, the build platform lowers by a layer’s thickness and a leveling roller pushes a new layer of powder across the top to be sintered to the previous layer. Put more simply, it’s like building a loaf of bread one layer at a time.

SLA: Sterolithography

Sterolithography is also very useful for functional products as well as prototypes. 

Like SLS, SLA is a professional-level 3D printing technology with the ability to make complex parts based on a CAD model. The material for SLA is a vat of liquid UV photopolymer “resin.” To create each layer, an ultraviolet laser beam traces a cross-section of the design on the surface of the liquid resin, then the build platform descends by one layer and a resin-filled blade lays down fresh material for the next layer. The UV laser cures and solidifies the pattern, and support structures hold the cross sections in place. These structures are removed following the printing process. Once the complete model is printed, the products go into a chemical bath to remove excess resin. Finally, they are cured in a UV oven.

FDM: Fused Deposition Modeling

Fused deposition modeling is very different from the aforementioned methods, typically using thermoplastics (which soften at a certain temperature and solidify when cool) and requiring support structures to act as scaffolding during the build process. 

An FDM machine reads a 3D model and slices it up, then it lays down material in thin layers, which are generally thicker than the layers created by more sophisticated 3D printers. The consumer printer Makerbot uses FDM, and of course more sophisticated FDM machines exist for professional purposes. The building material—either plastic filament or metal wire—passes through a heated extrusion nozzle that melts the material as it lays it down. At times, a support structure is necessary to hold the model in place during construction. The machine will print this structure along with the model, to be removed after completion.

Fused deposition modeling can be used for many of the same purposes as SLS and SLA, but its comparative drawbacks include poorer resolution, limited material options, greater waste, chemical use, and slower production time.