December 18, 2023

While both methods can provide high-quality parts, it’s important to understand how each process converts raw materials into a component that is ready for secondary operations or shipping.

CNC stands for computer numerical control. CNC machines used for plastic processing are typically advanced multi-axis systems that are computer-controlled for precision and accuracy. Machining is considered a subtractive manufacturing method because the machine, whether mill, lathe, or router, removes material from a stock shape or injection molded blank. The number of axes depends on the machine and its function. But generally speaking, the more axes a machine has, the more complex part it can produce with reduced manual handling.

The shape of the part determines which machine, or machines, is used. A CNC lathe, which rotates the workpiece against the cutting tools to make a shape, is ideal for symmetrical objects like spheres, cylinders, or cones and can also create features such as drill holes, bores, and threads. With a mill, the cutting tool rotates around the workpiece, allowing it to make complex shapes. Routers operate on a three-axis spindle at a higher speed than a mill.

Comparatively, injection molding is a formative technology. The process involves melting a polymer and injecting the molten plastic into a mold, where it cools and solidifies to form the desired part. In addition to traditional molding, overmolding and insert molding are options for more complex products. Overmolding is when one plastic material is molded over another. Insert molding is when plastic is molded over non-plastic parts, such as a soft-touch handle on a metal scalpel.

Other differences that are important to consider are design, cost and volume, and materials:

• Design Considerations  – Working with your plastic fabricator from the design phase is beneficial because the part design features can differ depending on the method used to create your part. For example, removing a part from an injection mold requires appropriate draft angles. Walls must be of similar thicknesses to avoid uneven shrinkage and warpage. Neither is required for CNC machining. For CNC parts, the depth of the cavity should be four times its width, a limitation not found with injection molding. Similarly, the overall size of the part is limited by the size of the machine.
• Cost and volume – Upfront costs are higher with injection molding. However, for large volumes, the ROI is seen in speed, repeatability, and long tooling lifespan when properly maintained. CNC machining does not have upfront costs for tooling, but there is a point as volumes increase that it is no longer cost-effective.
• Materials – CNC machining works best with hard materials. Soft plastics may melt from the friction of machining. Some engineering and high-performance plastics require annealing or curing, which requires special equipment that not all injection molders will have.

Like any manufacturing process, there are pros and cons to CNC machining plastics. One should consider the impact of each on the product.

• Precision – CNC machining can produce parts with tight tolerances.
• Customization – CNC machining allows for the creation of products with complex design features that may be challenging or more costly for injection molding, including:
  • Undercut removal
  • Holes that penetrate completely through a part
  • Complex internal threads
  • Variable wall thicknesses
  • Non-uniformed cross-sections, such as tapered or conical sections
• Material flexibility – A wide range of materials can be machined, including PVC and PTFE.
• Less set up – Since no molds must be built, CNC machining can be faster for lower volumes.
• Modifications – Design iterations aren’t a problem since programs can easily be updated.Cost – Since no molds are required, CNC machining is most cost-effective for small to medium volumes.

• Plastic knowledge required – You must work with a machine shop that specializes in plastics. Plastics have a higher coefficient of thermal expansion than metal, and since machining creates heat, the material will expand and, in some cases, melt. Understanding the polymer is critical for ensuring proper heat regulation.
• Material waste – Since CNC machining is a subtractive process, material waste is inherent.
• Limited capabilities – Being a subtractive process, it is limiting or requires more steps for products that benefit from overmolding or insert molding.

Likewise, plastic injection molding has its own advantages and disadvantages.

• High volume efficiency – Plastic injection molding provides faster production options for high volumes since multiple cavity molds can create several parts at once.
• Complex geometries – Like CNC machining, some features are better suited to injection molding than CNC machining, including:
  • Thin walls
  • Intricate internal channels
  • Small features
  • Complex contours
  • Fine surface textures
  • Over molding
  • Insert molding
  • Fine details and lettering
• Repeatability – With a precision machined mold or inserts, injection molding produces exact parts consistently.
• Cost – Injection molding is most cost-effective for large runs since properly maintained molds have long lives.

• Design Changes – Once the mold is built, design changes require a new mold, which can be expensive.
• Upfront investment – Building molds requires a sizeable upfront investment.
• Limited capabilities – Some design features, such as internal threads and undercuts, can be challenging and require special considerations for mold design, which can add cost, or require secondary operations.

Having to choose between plastic injection molding and machining isn’t always necessary. Sometimes, molding the part and then doing post-mold machining, often referred to as near-net injection molding, makes sense. A near-net shape is molded to the approximate shape of the part and machined to the customer’s dimensions and tolerances to create the final part. It dramatically reduces the waste, and a less precise mold can be used. Post machining can also be used to remove flash, prepare the part for further processing, or achieve tighter tolerances.

• Material waste reduction – Much less material is wasted with near-net molding and machining than with machining alone.
• Tight tolerance –Tighter tolerances can be achieved than injection molding alone.
• Complex geometries  –  A part can be injection molded, and challenging features can be machined.
• Cost savings – Less material waste, especially with expensive high-performance plastics, and shorter machining times reduce costs over machining alone. Molds are less precise and, therefore, less costly to produce.

• Project dependent – It may not be cost-effective for projects using low-cost polymers or those with loose tolerances or simple designs.
• Volumes – Volumes must be enough to offset the cost of the mold but not so high that machining becomes cost-prohibitive.