SLA uses a photo sensitive liquid polymer that is hardened in thin, precise layers by a laser beam. After the laser traces out a layer of the design on the surface of the liquid polymer, a platform beneath the object sinks slightly into the photosensitive goo in preparation for the next layer. Over time (and many, many layers), a 3D object emerges within the liquid.
The other technique, SLS, is also a laser-based additive manufacturing process, but uses a photo-reactive powder instead of a liquid polymer. Again, a laser is used to harden the top layer of the powder (this time by sintering or fusing particles together) as a platform drops beneath the object.
Why use two different processes? Simple: the different materials render objects with different qualities and properties. For instance, sometimes a designer is creating something that requires a bit of flex, like a plastic hinge, yet other times may value a material with more rigidity. SLA and SLS processes allow GM engineers to fabricate parts with different attributes in order to meet different prototyping needs.
The most impressive thing about modern rapid prototyping is that it's basically a WYSIWYG process, or what you see is what you get. Parts come out of the machines fully assembled, just as they appeared "in math" on a computer screen. You can build extremely complex objects with gaps, spaces, and moving parts, so long as you work within the limits the prototyping materials. The object comes out fully assembled--all that's needed is a bit of cleaning, dusting, and baking at high temperatures to harden the finished product.
A third type of 3D prototyping that wasn't explored in TRANSLOGIC 66 is CNC (computer numerical control) milling. Unlike SLA and SLS, CNC is a subtractive process that uses drills to mill an object from a block of solid material. An example of this can be seen in TRANSLOGIC 2 (02:45) during our conversation with Reberto Velozzi, who used the technology to render a prototype of his turbine-powered Velozzi supercar.
Click the image below to watch TRANSLOGIC 66: Rapid Prototyping: