Picking an appropriate manufacturing method for a new product is often easier said than done. CNC machining and plastic injection molding are both commonly used and cost-effective ways to produce parts. While both methods are similar in those regards, they are completely different fabrication methods. CNC machining is a subtractive manufacturing process that uses metal cutters to remove material from a workpiece to create the final part shape. Plastic injection molding forces liquid plastic to conform to the shape of a mold to make the finished part. Three major differences exist between the two methods. These are materials that can be used, the volume of production that can be attained, and part dimensional tolerances and precision. 

These differences lead to significantly different costs for the two processes. In this article, we discuss everything there is to know about CNC machining vs. plastic injection molding, including the advantages and disadvantages of each, and process alternatives to both.

CNC Machining Definition and Comparison to Plastic Injection Molding

CNC machining is a subtractive manufacturing process that uses machines like mills, lathes, drill presses, and saws, to make highly precise parts. Computer-Aided Design (CAD) data is developed during the product design phase. This CAD data is then used as the basis for programming and optimizing the machine tool sequences and paths. The material is then processed using cutters, such as end mills and drill bits to create the parts. Auxiliary machinery, including honing, hobbing, or grinding machines, may also be needed to complete parts to customer specifications.

While manual machining of parts has existed since 1300 BC, CNC machining wasn’t developed until the mid-1940s. The first control mechanisms that eventually were used on CNC machines were used to punch code tapes as a way of communication. In 1949, the first experimental 3-axis milling machine was created to make helicopter blades and stiffer skin for various aircraft. In the 1970s, CAD and CAM software began to be integrated into CNC systems. Now, CNC machining is one of the most widely used manufacturing methods in the world. It is used to make products for several industries, from automotive to agriculture. CNC machining is often preferred to injection molding due to the ability to easily accommodate product design changes, its ability to produce tight tolerance parts, and the possibility of using several different materials.

What Are the Advantages of CNC Machining Compared to Plastic Injection Molding?

Listed below are some advantages of CNC machining over injection molding:

1. Beautiful surface finishes and tight tolerances are easily obtainable with CNC machining since small-diameter tools can be used and each process parameter can be optimized.
2. A wide range of materials can be shaped using CNC machining since both plastic and metal materials can be cut.
3. It is easy to modify part designs since programs containing the CNC machine instructions and fixtures can quickly be changed to match successive design iterations.
4. CNC machining has faster tooling lead times than injection molding since complex molds don’t have to be made.

What Are the Disadvantages of CNC Machining Compared to Plastic Injection Molding?

Listed below are the disadvantages of CNC machining vs. injection molding:

1. Large production volumes and price-per-part are more expensive for CNC-machined products due to the longer per-part cycle time needed to make parts.
2. Labor-intensive since workpieces, completed parts, tools, and tool holders, must be fixed to and removed from the machine depending on the job.