Surface Finish
RP: Ra 100 to 600 in. (2.5 to 15 microns)
Without any secondary operations, some, if not all, surfaces will be rough. While RP has progressed to layer thicknesses of 0.0005 to 0.001 inch (0.0125 to 0.025 mm) for some technologies, the layer striations and stairstepping still affect surface finish.

If desired, secondary operations can improve the smoothness to any level, but this can alter the dimensional accuracy of the part. Also, these operations can add time and cost to the project.

 CNC: Ra 20 to 200 in. (0.5 to 5 microns)
Unlike RP, machining can produce surface finishes suitable for prototypes, patterns and tooling. As with RP, secondary operations (sanding, polishing) can further improve the surface finish, but they may adversely affect accuracy, time and cost.

 Reliability
RP: Moderate
For most technologies, reliability increases as the product matures. The age of RP technologies ranges from one to fifteen years, which means that one can expect varying degrees of reliability. With less time and fewer resources, some of the RP manufacturers have not had sufficient time to refine the components of the systems to improve reliability.

 CNC: Moderate to high
With more than thirty years of research and development, CNC is a mature technology that is dependable and reliable. Over the years, continuous improvement has eliminated system elements that diminish reliability.

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