CNC Machining vs. Plastic Injection Molding

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.

To learn more, see our guide on What is CNC Machining.

The Benefits of CNC Machining vs. 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.

Some Cons of CNC Machining vs. 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.

Plastic Injection Molding Definition and Comparison to CNC Machining

Plastic injection molding is a manufacturing method that uses an injection molding machine (IMM). These machines contain components such as injection screws, injection barrels, and fabricated molds. Plastic pellets are fed into the barrel and are melted through the rotation of the screw and heater bands. The molten plastic is injected into the mold and formed into the geometry of the part. The IMM holds pressure on the mold until products have been properly formed and cooled. The completed parts are ejected and the cycle restarts.

Brothers Isaiah and John Hyatt developed injection molding in 1872 and used it to mold hair combs, buttons, and other small items. In the 150 years since that first machine, injection molding has blossomed into a multi-billion dollar global industry. Injection molding is considered a more cost-effective and efficient way to make plastic parts than CNC machining.

For more information, see our guide on what is plastic injection molding.

Benefits of Plastic Injection Molding vs. CNC Machining

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

1. It can easily produce large volumes of parts in a rapid and repeatable fashion since multiple-cavity molds can create many parts in one injection cycle.
2. Injection molding has the lowest price per part of any manufacturing process, since the molds have long lives, and multiple-cavity molds can increase the capacity of each machine to support high-volume orders.
3. Minimal post-processing is required because parts are often ready to be shipped immediately after ejection depending on the type of gate (entry point from the runner into the cavity) that is used.

Cons of Plastic Injection Molding vs. CNC Machining

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

1. It is difficult to accommodate product design changes since rework on complex molds with several plates, cams, and complex geometries can be troublesome.
2. Imperfections on the interior of the mold can result in finished parts with surface flaws.
3. High upfront investment is required for mold fabrication since multi-cavity molds can cost anywhere from a few thousand to hundreds of thousands of dollars.