In 1986, inventor Charles W. Hull developed a “solid imaging process” known as stereolithography, which initiated the rapid prototyping revolution. Stereolithography (SLA) builds three-dimensional models out of liquid photosensitive polymers that solidify when exposed to ultraviolet light.

The process begins with the CAD data of a part. These data are translated into cross section “slices” that are fed into the computer-controlled SLA equipment. For each layer of the model or models, a low-power UV laser beam traces a cross section pattern on the surface of a platform that rests in a vat of liquid epoxy or acrylate resin. As the laser traces out each layer (a process sometimes labeled as printing), its energy solidifies the cross section while leaving excess areas liquid.

The platform lowers into the liquid polymer by a distance equal to the part’s layer thickness (typically 0.003-0.002 in.), and the process repeats until all layers are traced and cured. Afterwards, the solid part is removed from the vat and rinsed clean of excess liquid. Supports are broken off or rinsed away. In some cases, the model is placed in an ultraviolet oven for a final cure. Part surfaces can then be polished, sanded or otherwise finished.

Early SLA prototypes were brittle and prone to curing induced warpage and distortion. Today’s systems have corrected those problems.

Initially, SLA systems were intended for quick, concept model prototyping. Newer systems, though, can produce parts strong enough to be machined or used as master patterns for injection molding, thermoforming, blow molding, and in various metal casting processes. One of the benefits of SLA is that there are virtually no limitations to the shapes of the parts that can be created.

However, SLA is not an inexpensive process. The machines can cost from $100,000 to more than $500,000. The various photo-curable resins are costly as well with some going for up to $800 per gallon. Plus, these systems must be located in a suitably vented space because of the fumes created by the polymers and solvents.

Lastly, part detail may not be as sharp as that available with other processes. Click here to view all the available SLA machines.