Milling is a plastic forming process in which a cylindrical cutter moves on an axis to carve the plastic material (that is locked into place) into a desired shape. CNC milling can chisel intricate designs, flatten surfaces, machine precise edges, or create recessed cavities such as pockets, slots, and drilled holes. Usually block or disc shaped plastic materials are used in milling.

There are two basic milling operations – up milling (also called conventional milling) and down milling (climb milling). Up milling is when the cutter and the plastic move in opposite directions; down milling is when they move in the same direction. Down milling is preferred for thinner and more precise parts as it results in a better surface finish, but up milling is more commonly used.