CNC Machining vs Metal Injection Molding

Computerized numerical control (CNC) machining and metal injection molding (MIM) are two common manufacturing processes in metal parts production. While both are widely used production methods, CNC machining and metal injection molding have distinct differences in process and applications.  

CNC machining subtractive manufacturing

What is CNC Machining?

CNC Machining is a manufacturing process where a set of factory tools and machinery create parts using specific instructions from a pre-programmed computer software. CNC machining follows a subtractive process where layers of a workpiece are removed to form the end part. The machine ‘subtracts’ material from the workpiece, meaning the resulting product has less material compared to when the process started. The common CNC machine tools used in this process include mills, lathes, routers, and grinders.

The computer-aided design (CAD) software gives the designers the capabilities to realize a 2D or 3D model of the product and its necessary properties, such as dimension and geometries. Tool geometry and tool access need to be considered when designing the part. Most CNC tooling is cylindrical, so the produced parts will have curved corner sections. Achieving sharp corners is difficult, expensive, and sometimes not possible. The tool also must access the workpiece from a specific angle, denoted by the tool path encoded in the computer-aided manufacturing (CAM) software. Features of the part that cannot be accessed via this angle will be impossible to create.

CNC machining can produce parts with high accuracy and tight tolerances, with typical tolerances ranging from +/- 0.001 to 0.0001 inches. The material removal process can be optimized for the specific material being machined to achieve the desired finish and surface quality. However, as mentioned earlier, sharp corners and tight internal geometries can be challenging to machine, and may require additional post-processing steps such as drilling, tapping, or broaching.  CNC machined parts will usually have tool marks, or small imperfections left by the tooling, that need to be removed or smoothened by a secondary process.  

Strengths of CNC Machining

  • High precision and accuracy: CNC machining can achieve very tight tolerances, making it ideal for applications that require precise and accurate parts. The machine can produce parts with tolerances as low as +/- 0.0001 inches.
  • Versatility: CNC machines can be used to manufacture a wide range of parts, from simple to complex geometries. They can also work with a variety of materials, including metals, plastics, wood, ceramics, and composites.
  • Automation: Some machines are fully automated and can run for extended periods without human intervention. Lights out manufacturing is common in advanced production facilities. This makes it ideal for mass production runs where consistency is critical.
  • Flexibility: CNC machines can be easily reprogrammed to manufacture different parts, making them a flexible manufacturing option.
  • Speed: Parts can be produced quickly, making them ideal for time-sensitive applications.

Weaknesses of CNC Machining

  • High initial cost: Machines are expensive to purchase, install, and maintain. This makes it difficult for small businesses or startups to invest in machining equipment.
  • Limited internal geometries: CNC machines are limited in their ability to produce parts with tight internal geometries and complex shapes.
  • Tool marks: The cutting tools used in CNC machining can leave tool marks or burrs on the surface of the part. This requires additional post-processing to remove them.
  • Production rate: Most parts are made one at a time which limit the production rate compared to other manufacturing methods like metal injection molding or stamping.
  • Design limitations: CNC machining is limited by the available cutting tools and their access to the part being machined. This can limit the design options for certain parts.
CNC tool marks
CNC machined part showing tool marks
Formative Manufacturing Metal Injection Molding

What is Metal Injection Molding?

Metal injection molding (MIM) is a formative process which combines a powdered metal with a binding material to create an intermediate mixture, called a “feedstock”.  The feedstock is typically composed of 60-70% metal powder and 30-40% binder material, which can be a thermoplastic or thermosetting polymer.  The feedstock is then solidified and shaped using injection molding to create the final shape. The binding material is removed in a process called sintering, yielding the final product. The upfront production costs (tooling, materials, etc.) may be higher for MIM, initially; however, this cost is minimized with medium to higher volume productions. MIM is well-suited for small to medium-sized parts, with typical production runs ranging from 10,000 to 1 million + parts annually. MIM offers unique geometric capabilities that exceed the abilities of CNC machining. MIM can produce parts with complex geometries and thin walls, as well as tight dimensional tolerances of +/- 0.1% or better. Because metal injection molded parts do not endure stresses or pressure induced by machining, they are often stronger than their CNC counterparts.

Strengths of Metal Injection Molding

  • High precision and accuracy: MIM can produce parts with tight dimensional tolerances, making it ideal for applications that require high precision.
  • Versatility: MIM can be used to manufacture a wide range of parts, from small to medium-sized parts with complex geometries and thin walls.
  • Cost-effective: MIM can be a cost-effective manufacturing method for producing high-volume production runs of complex metal parts, as it can produce near-net-shape parts with minimal waste.
  • Consistency: MIM can produce consistent parts with uniform density and mechanical properties, making it ideal for applications that require high-quality and reliable parts.
  • Excellent material properties: MIM can produce parts with excellent material properties, comparable to wrought metal parts. The parts produced by MIM have high strength, good corrosion resistance, and excellent surface finish.

Weaknesses of Metal Injection Molding

  • High initial cost: MIM requires expensive tooling and equipment, which can make it less cost-effective for small production runs.
  • Limited part size: MIM is generally limited to producing small to medium-sized parts, due to the limitations of the injection molding process.
  • Limited material selection: MIM is limited to metals and metal alloys, which can be a disadvantage if other materials are required for the application. The most commonly used metals in MIM include stainless steel, low-alloy steel, tool steel, nickel alloys, cobalt alloys, copper alloys, and tungsten alloys.
  • Long lead times: MIM can have long lead times due to the time required for tooling design, manufacturing, and testing.
  • Difficult to modify: MIM parts are difficult to modify once they are produced, as the sintering process makes it difficult to machine or modify the part in any way.
Metal Injection molded part with complex geometry
Metal injection molded part with a complex geometry with no finish machining

A cost comparison of CNC Machining and Metal Injection Molding

Details
Part Description:
Small, complex-shaped screw used in a high temperature application
Material:
NIckel steel alloy (Inconel)
Annual Volume:
10,000 pieces
Metal Injection Molding
Tooling cost:
$6,500 ÷ 10,000 = $0.65 per part
Unit cost:
$1.25 per part
Total cost for 10,000 parts:
($0.65 + $1.25) x 10,000 = $18,000
CNC Machining
Tooling cost:
$0
Unit cost:
$3.73 per part
Total cost for 10,000 parts:
$3.73 x 10,000 = $37,300

In this example, the MIM process is more cost-effective, with a total cost of $18,000 compared to $37,300 for CNC machining. However, it’s important to note that this cost comparison is based on the specific part and production volume, and other factors such as lead time, design flexibility, and quality requirements may also need to be considered when selecting a manufacturing process.

Choosing Between CNC Machining and Metal Injection Molding

The choice to use CNC machining or metal injection molding for a project depends on various factors including geometry, precision, secondary processes, and production volume. MIM is preferred for mass producing geometrically intricate components. CNC machining is better suited for complex shapes and tighter tolerances for lower production volumes. MIM parts are close to finished parts which minimizes the required post-processing.

Size comparison of Metal Injection Molded vs CNC Machined part

Size of metal injection molded part
Metal injection molded next to a penny
DSC02343
Multi-axis CNC machined part

Amfas offers a wide range of CNC machining capabilities and metal injection molding services, and our team can help you decide which manufacturing process is best for your project.