In the world of plastic injection molding, a vast majority of the manufacturing process is handled with computers and machines. Computer Numerical Control, or CNC machining, is the process in production when manufacturing utilizes machines in order to produce and handle very specific tasks. For someone looking at producing a large number of items in the manufacturing world, it is necessary to understand what the difference is in regards to injection molding and CNC machining in plastic production. Once this is determined, it is possible to move on from there and determine which method is the most cost effective and beneficial for a particular company.
The initial design of the product takes on a similar process. With both, the manufacturer takes a design to the plastic production firm, who then goes over the design to make sure it is properly crafted and does not need any corrections in order to be properly produced. From there, the plastic production company is going to create an 3D design using a computer aided design (CAD) software. This 3D design allows the manufacturer and the plastic production company to see what the part is going to look like when produced. Both plastic injection and CNC machining is going to work in a similar way to this.
The production is a bit of a different produce and is where the two start to differ. With plastic injection, a mold is first produced. With this produced mold, the liquid plastic is injected into it, quickly cooled and ejected from the mold so the production can continue on. With CNC machining, the design of the product is saved into the machine, which then receives essentially a block of plastic. The machine cuts the block of plastic down until it finished the final shape. Both CNC machining and plastic injection molding can produce the same looking product (generally speaking, although it does depend on the shape and intricacy).
In general, CNC machining produces more waste than plastic injection. It might take a bit longer to setup the initial mold of the injection, once complete it has almost zero percent waste as it is liquid when first used while CNC cuts away a substantial amount, which much then either be melted back down and formed into another block (which costs more in production time and energy) or completely scrapped, which is expensive and not cost effective.