CNC Machining vs. Injection Molding: Which technique is Right for Plastic Parts?

CNC (Computer Numerical Control) machining is a process in which pre-programmed computer software controls the movement of industrial tools and machinery. The method removes material from a solid block and makes it possible to manufacture complex shapes that are difficult or impossible to achieve with manual machining.

This manufacturing method makes plastic parts by injecting molten material into a mold. The injection molding process supplies parts for a broad range of industries, from the tiniest components to entire car body panels.

1. Choice of material:

   CNC machining can be a good choice for plastic parts because it accommodates a broad range of materials, from plastics to metals. With plastics, the hardness of the material is an essential factor. CNC machines cut rigid plastics most effectively, whereas softer plastics can deform during machining and require more care. Hence, the highest precision and tightest tolerances are easiest to achieve with harder materials.

   Injection molding, by contrast, handles soft materials well. This makes it a good choice if you want to work with thermoplastics and other injection-moldable materials, such as polypropylene (PP), acrylonitrile butadiene styrene (ABS), and polyether ether ketone (PEEK). These materials can be molded without compromising their strength. Note that even flexible materials can be molded using this process.

   Norck provides a high-quality CNC machining service for demanding applications across industries such as robotics, aerospace, industrial, automotive, energy, electronics, and defense.

2. Production volumes and cost:

   Both processes are widely used in manufacturing, but they suit different production volumes. Because CNC machining requires no tooling, rapid prototypes can be completed quickly, depending on the material. However, at mass-production volumes, the cost per part remains comparatively high, and production may take a long time to complete.

   Injection molding, on the other hand, is less suited to swift prototyping because it first requires a custom mold. Once the mold is made, however, it is the better choice for mass-production volumes and lowers the cost per part far more efficiently. Most of the time and cost in injection molding goes into designing and making the custom molds. Hence, at high volumes, injection molding proves to be more affordable than CNC machining.

3. Design flexibility:

   Manufacturers consider a CNC machining service the best choice for design flexibility. Engineers value the ability to change plastic parts after production has started, and with CNC machining, any required changes can be incorporated easily.

   With injection molding, design modifications may require a modified or new mold, which adds to the cost. Because of this limitation, CNC machining is preferred over injection molding when it comes to design flexibility. Norck employs rigorous quality control to ensure excellent accuracy, superior part quality, and tight tolerances.

4. Tolerances:

   It is vital to consider tolerances, i.e., how much dimensional variation is acceptable in a functional plastic part. Tolerances ensure that end-use parts fit together and work as intended. Injection-molded parts typically hold tolerances of about 0.005”, whereas CNC machining can hold tolerances as tight as 0.001”. Machining molds to very tight tolerances is costly, which in turn makes plastic injection molding expensive. So, for parts that need very tight tolerances, producing the plastic parts directly by CNC machining can be more efficient.

5. Performance and strength:

   For plastic parts, injection molding produces parts with consistent mechanical properties and strength. This makes the process well suited to plastic parts that must withstand stress and strain consistently across their surface. Since a wide range of plastics is available for injection molding, the process offers options for different strength requirements.

   CNC machining, by contrast, is well known for its ability to work with an extensive range of materials, including metals and plastics. The performance and strength of CNC-machined parts depend largely on the chosen material. Because machined parts can, in some cases, be stronger than molded parts, they may offer better performance for certain plastic parts.

6. Surface finish:

   When it comes to surface finish, the injection molding process uses ejector pins to push the cooled part out of the mold. This can leave minor cosmetic marks on the surface of the product. This may not matter for many applications, but if a surface must be free of such marks, CNC machining may be the better choice for plastic parts. Contacting Norck gives you access to custom manufacturing and engineering solutions tailored to your needs, across industries.

The in-depth comparison above covers aspects such as design flexibility, production runs, costs, tolerances, and more. A CNC machining service offers more control over quality and works with a wide range of machinable materials. As a result, it is preferred over injection molding for specific parts that need excellent precision, tight tolerances, and durability. Injection molding, on the other hand, is better suited to the swift, cost-effective, high-volume production of lightweight plastic parts, from simple to complex. The final decision depends on your requirements and project specifications, such as part precision, volumes, and dimensional tolerances.