CNC Machining vs. Plastic Injection Molding: Differences and Comparison

Selecting the correct manufacturing method for a new product can be challenging. CNC machining and plastic injection molding are both widely used and cost-effective, but they differ completely in how parts are made. CNC machining is a subtractive manufacturing process that uses cutting tools to remove material from a workpiece to create the final part shape. Plastic injection molding injects molten plastic into a mold cavity, where it cools and solidifies into the final part shape. The key differences between the two methods lie in the range of materials they can process, the achievable production volumes, and the attainable tolerances and precision.

These differences also affect the overall manufacturing cost of each process. 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 equipment such as mills, lathes, drill presses, and saws to create 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 cut using tools such as end mills and drill bits to form the desired part geometry. Auxiliary machinery, such as honing, hobbing, or grinding machines, may also be required to produce parts to customer specifications.

Manual machining dates back several centuries, but the concept of numerical control emerged in the late 1940s with early servo-controlled machines. Early numerical-control machines used punched tape or cards to encode coordinate commands for tool motion. In 1949, MIT developed the first experimental three-axis numerically controlled milling machine for the U.S. Air Force, intended for the precise machining of aircraft components such as rotor blades and structural skins. 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 over injection molding because it easily accommodates design changes, achieves tight tolerances, and supports a wide variety of materials.

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

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

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

1. Excellent surface finishes and tight tolerances are achievable with CNC machining because cutting parameters and tool geometries can be precisely controlled.
2. CNC machining can process metals, plastics, and composites, offering access to a wide range of materials.
3. Design modifications are simple, as programs and fixtures can be quickly updated to match new iterations.
4. Tooling lead times are shorter since there is no need to manufacture complex molds.

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

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

1. Producing large volumes by CNC machining results in a higher cost per part because each piece requires a longer cycle time.
2. The process can be labor-intensive, particularly for low-volume or prototype runs where automated loading systems are not used.

Plastic Injection Molding Definition and Comparison to CNC Machining

Plastic injection molding is a manufacturing method that uses an injection molding machine (IMM) consisting of major components such as an injection unit (screw and barrel), a clamping unit, and a mold. 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 maintains holding pressure after injection to compensate for material shrinkage until the parts have solidified sufficiently to be ejected. 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 multibillion-dollar global industry. Injection molding becomes more cost-effective than CNC machining for high-volume production, whereas CNC machining may be more economical for low-volume or prototype runs.

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

What are the Advantages of Plastic Injection Molding Compared to 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 offers one of the lowest per-part costs among manufacturing methods for large production volumes because molds have long service lives, and multi-cavity tools can greatly increase output.
3. Minimal post-processing is needed because many parts can be shipped immediately after ejection, depending on the gate type used.

What are the Disadvantages of Plastic Injection Molding Compared to 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 mold surface or issues like venting, temperature imbalance, or contamination can result in surface defects on molded parts.
3. A high upfront investment is required for mold fabrication, as multi-cavity molds can cost anywhere from a few thousand to hundreds of thousands of dollars.

Comparison Table Between CNC Machining and Injection Molding

Table 1 compares key attributes and properties of CNC machining and plastic injection molding.

With CNC machining, tighter-tolerance parts can be made from a broader range of materials, including metals, plastics, and composites, compared to injection molding, which is mainly limited to plastics. Injection molding can produce parts at a much faster rate, often requiring little or no post-processing, which makes it ideal for high-volume production. Injection molding requires a high upfront investment for mold fabrication; however, the cost per part decreases significantly as production volume increases, making the process cost-effective for large-scale runs.

CNC Machining vs. Injection Molding: Lead Cost Comparison

Tooling costs for CNC machining are significantly less than for injection molding. CNC machining costs are associated with the fabrication of fixtures and jigs and the procurement of raw materials and tools. Molds for injection molding can cost from a few thousand dollars for simple single-cavity molds to several hundred thousand dollars for complex, multi-cavity production molds. An injection mold becomes cost-effective when its cost is distributed over a sufficiently large production volume. Assuming the necessary machinery is available and only tooling, such as molds, fixtures, and jigs, must be arranged, upfront costs for CNC machining are lower.

CNC Machining vs. Injection Molding: Speed Comparison

Both CNC machining and injection molding can produce parts relatively quickly, but injection molding achieves much shorter cycle times once tooling is complete. For small-volume runs, CNC machining is often preferred. Injection molding is preferred for larger runs because multiple-cavity molds can rapidly produce parts and reduce per-piece cost. Considering the mold-fabrication lead time, series production typically starts later with injection molding than with CNC machining. Consider using CNC machining for small production runs and injection molding for large runs.

CNC Machining vs. Injection Molding: Volume Comparison

Injection molding can produce far more parts per machine-hour and per dollar invested than CNC machining once production tooling is available. Molds used in injection molding can have multiple cavities, ranging from one to several hundred. Hence, injection molding can quickly produce large numbers of parts. In CNC machining, parts are typically produced sequentially; however, multi-part fixtures or pallet systems can enable limited batch production. While CNC machining is fast at producing parts, injection molding is even faster.

CNC Machining vs. Injection Molding: Materials Comparison

A broader range of materials can be used in CNC machining processes than in injection molding. CNC machining is capable of producing parts from various plastics, as well as metals such as aluminum and steel. Many elastomers and some thermoplastics are too soft or flexible for conventional CNC machining but can be readily molded into parts through injection molding.