With CNC machining a solid block of plastic is clamped into a CNC mill and cut into a finished part. This method produces superior strength and surface finish to any additive process. It also has the complete, homogenous properties of the plastic because it is made from solid blocks of extruded or compression molded thermoplastic resin, as opposed to the additive processes which use “plastic like” materials and are built in layers. The wide range of material choices allows parts to be made with the desired material properties, such as: tensile strength, impact resistance, heat deflection temperatures, chemical resistance and biocompatibility. Good tolerances yield parts suitable for fit and functional testing. Prototypes can be delivered in days like additive processes.
Because the process is removing material instead of adding it, milling undercuts can sometimes be difficult. Machining also tends to be somewhat more expensive than the additive processes.
Pros: Machined parts have a good surface finish and they are very strong because they use real plastic resins.
Cons: There are some geometry limitations associated with CNC machining, and it is much more expensive to do this in-house than the additive processes due to the cost of the programmers and machinists needed to create CNC toolpaths and fixturing for the parts.