Unlike the desktop printer you use to print documents, Stereolithography (SLA) machines don't extrude ink or some other liquid onto a surface. Instead, a SLA machine starts with an excess of liquid plastic, some of which is cured, or hardened, to form a solid object.
SLAs have four main parts: a tank that can be filled with liquid plastic (photopolymer), a perforated platform that is lowered into the tank, an ultraviolet (UV) laser and a computer controlling the platform and the laser.
In the initial step of the SLA process, a thin layer of photopolymer (usually between 0.05-0.15 mm) is exposed above the perforated platform. The UV laser hits the perforated platform, "painting" the pattern of the object being printed.
The UV-curable liquid hardens instantly when the UV laser touches it, forming the first layer of the 3D-printed object.
Once the initial layer of the object has hardened, the platform is lowered, exposing a new surface layer of liquid polymer. The laser again traces a cross section of the object being printed, which instantly bonds to the hardened section beneath it.
This process is repeated again and again until the entire object has been formed and is fully submerged in the tank.
The platform is then raised to expose the three-dimensional object. After it is rinsed with a liquid solvent to free it of excess resin, the object is baked in an ultraviolet oven to further cure the plastic.
The amount of time it takes to create an object with stereolithography depends on the size of the machine used to print it. Small objects are usually produced with smaller machines and typically take between six to twelve hours to print. Larger objects, which can be several meters in three dimensions, take days.
Stereolithography is an ideal solution for creating prototypes because it creates highly accurate, durable objects fairly quickly and relatively inexpensively. SLA machines can even create oddly shaped objects, which can be difficult to produce using traditional prototyping methods.
Many industries — from medical to manufacturing — use stereolithography to build prototypes and, on occasion, final products.
A car manufacturer, for example, might use stereolithography to create a prototype casting of a car door handle. Such a prototype can be tested for fit and form and, once perfected, can serve as the master pattern for a machined auto part.
Similar to SLA, the Selective laser Sintering (SLS) process starts with a tank full of bulk material, but this time in powder form. As the print continues, the bed lowers itself for each new layer as done in the SLS process. Overhangs are naturally supported by the excess bulk powder material making those features much more simple to print. The bulk material is typically heated to just under its transition temperature to allow for faster particle fusion and print moves.
This method is particularly exciting because of the flexibility of materials that can be used. Both plastics and metals can be fused in this manner, creating much stronger and more durable prototypes. Although the quality of the powders is dependent on the suppliers proprietary processes, the base materials used are typically more abundant than photopolymers, and therefore cheaper. However, there are additional costs in energy used for fabricating with this method which may reverse any savings realized in the material cost.
Speed and resolution of SLS typically match that of SLA, with industry averages at around 0.1mm tolerances. Due to the relatively slow fabrication speed, this method is only suitable for low volume production runs of small, precise parts.
SLS is the preferred rapid-prototyping method of metals and exotic materials