Two materials better suited to CNC Machines than 3-D Printing are polypropylene and polyoxymethylene (commonly known as acetal). Let’s discuss them one at a time:

Polypropylene (PP):

As of right now, none of the major rapid prototyping companies offer PP as a 3D printable material. The only way to prototype it is to use the CNC machine. There are some SLA and Polyjet materials that claim to be “polypropylene-like”, but there are limitations to the material property similarities. For example, making a prototype living hinge in one of these “PP-like” materials will not last more than a few bending cycles; it will simply break. Or, to keep the prototype hinge from breaking, you may need to adjust its design. Oftentimes this defeats the original purpose of prototyping a production-ready design and so, in a practical sense, it is not sufficient. Using the CNC machine to cut a PP living hinge is really the best option available with the present technology on the market. Creative Mechanisms specializes in this process and you can view a full suite of relevant polypropylene living hinge data on our dedicated webpage. For an in-depth discussion of polypropylene plastic read here.

Polyoxymethylene (POM; a.k.a. Acetal):

Similar to the case with polypropylene, none of the major rapid prototyping companies offer acetal as a 3D printable material. Even if you could print with acetal, you would lose some of the key properties that acetal offers. For example, acetal is often used because its surface is inherently slippery (it has a low coefficient of friction). This feature makes it a great plastic for complex mechanisms involving gears or contact surfaces. Generally speaking - and this is the case for all 3D printed surfaces not just acetal - 3D printed surfaces will not be as smooth as a molded or machined part. This means that,all else being equal, the coefficient of friction will go up. 

In the case of acetal this is particularly relevant because one of the desired features you are looking for is low friction. Adding quite a bit of additional friction to a part that is intended to be slippery is going to make it difficult to test the functionality of the part in preparation for larger manufacturing processes. For this reason, CNC machining is typically the best way to prototype novel designs with acetal that require some sort of functionality testing. For an in-depth discussion of the properties of acetal plastic read here.

In general, the utility of 3D printing and CNC machining depends on the desired results (and of course, the available machine and material technology). It may be the case that 3D printing with “specific polymer-like” materials is sufficient for design iterations and rapid prototype development in the initial stages of the design and engineering process. That said, if the prototype requires any type of functionality testing prior to manufacturing then it is useful to test the prototype in the actual material it will be mass produced in. The least expensive and most effective way to do this with a small number of prototypes is with the CNC machine. If more rigorous testing is required on a larger number of prototypes, small scale production in an injection molding machine is the logical next step. Read more about this here. In all of these cases, Creative Mechanisms can help.