Building a wall out of bricks and creating a plastic part out of plastic layers are both additive processes. As such, they share several advantages. Both are relatively simple, inexpensive, and repetitive—brick is piled on brick; plastic layer is laid upon plastic layer to form the finished product. But additive processes have limitations as well. One of the most significant is weakness at the joints.
Drive a truck into a brick wall. While the bricks may survive, the wall will give way at the mortar joints. Similarly, if you stress a layered plastic prototype it will tend to fracture along its layers. Brick walls are rarely struck by trucks, but plastic prototypes represent production parts that will almost certainly be subjected to a variety of stresses. If the prototype cannot withstand the same stresses as the production part (which will almost certainly come out of an injection mold as a solid resin) the prototype cannot be used for meaningful functional testing.
An alternative is to replace the additive process with one of two subtractive processes. The first of these is a primary subtractive process: machining a prototype out of a solid block of resin, which will have similar characteristics to solid molded resin. The other is a secondary subtractive process: milling an aluminum mold and injecting resin to produce a prototype with characteristics very similar to those of a production part coming from a steel mold. If these subtractive processes can be made as fast and economical as additive processes like Stereolithography and Fused Deposition Modeling, they will be competitive with the additive processes.
Besides eliminating the layering of additive processes, subtractive processes have several other advantages. They support a range of resins, which allows prototype models to be matched for strength, flexibility, chemical resistance, dielectric properties, and other critical characteristics. Most additive processes focus on a couple of resins.
Subtractive processes also offer a variety of surface finishes, doing away with the “stepped” surfaces often found in many additive processes. This can be functionally important if parts must slide and cosmetically important if the prototypes are to be used in market testing.
Automation is the key to making subtractive prototyping competitive with the additive methods. But while additive automation was a simple matter of “slicing” a CAD model into layers that could be laid one-on-another, subtractive processes faced the more complex chore of turning a CAD model into tool paths for milling machinery. At Proto Labs, a large-scale compute cluster and proprietary software allow CNC machining equipment and mold milling machinery to do just that. As a result they can compete with additive processes for both speed and economy.