Posted December 10, 2020

Comparing CNC Machining vs. Injection Molding: Which Method Is Best?

A plastic injection mold for mass production

Creating items from a mold is a common practice. Most kitchens have a collection of cookie cutters, cake tins, or Jello molds. Mold-made objects fill our homes, automobiles, and hospitals. They’re also essential to aerospace and defense engineering. So whether it be a plastic dinosaur or parts of a spacecraft, molds are common. 

Manufacturers use injection molding to fabricate parts for many consumer and industrial applications. The method involves melting raw material and injecting it into a mold at high pressure. At high quantities, injection molding offers low costs and relatively competitive speeds. For parts requiring greater precision, such as those commonly used in medical devices or for defense and aerospace applications, other manufacturing methods may offer better results. 

In these instances, consider computer numerical control, or CNC machining. CNC machine shops create plastic parts with software-controlled milling, turning, and drilling equipment that removes material in layers from a solid block. This method offers distinct advantages when it comes to time, cost, precision, and performance for the product you’re designing.  

Here’s a closer look at the advantages and disadvantages of CNC machining vs. injection molding to help you make the best choice for your next prototype or part’s production.

The Pros and Cons of Injection Molding

Comparing and contrasting injection molding and CNC machining starts with understanding the advantages and disadvantages of injection molding.

Advantages of Injection Molding

When it comes to creating high volumes of parts that are virtually identical, most manufacturers choose injection molding. This process supports everything from toys to automobile engine components—anything that needs to be mass-produced out of moldable plastic. 

Injection molding offers a wide variety of materials, and more are being regularly created to fulfill the need for high-performance plastic parts. While CNC machining cannot accommodate softer parts requiring TPE or rubber material, injection molding can.

Disadvantages of Injection Molding

In higher quantities, injection molding offers a low piece-part-price. The costs associated with fabricating the actual molds for injection molding drive up start-up costs. Certain materials, such as fiberglass enhanced plastics, require molds built out of hardened tool steel to handle the increased pressures. 

Another drawback to injection molding relates to costs associated with part changes, which usually require the production of a new tool or mold. Additionally, molds consist of two halves, which must be brought together for injection, leading to potential surface imperfections. The injection system can introduce air bubbles in the materials, creating further imperfections.

There are many good reasons to use injection molding, especially when it comes to the mass-production of parts. Based on your product’s use, material specifications, and desired quantity, CNC machining may offer a better solution for sourcing high-quality plastic parts.

The Benefits of CNC Machining vs. Injection Molding for Precision Parts

Here are four areas to consider as you evaluate injection molding and CNC machining:

  • Quantity: Generally, CNC machining offers faster delivery and lower cost for lower volumes of parts. The exact quantity threshold where injection molding offers lower costs depends on part size, part complexity, and material selection.
  • Speed and expense: CNC machining offers greater speed for low quantities. or rapid prototypes or limited production runs of parts you need quickly without breaking your development budget, machining offers faster turn times at a lower cost. For production quantities in the tens or hundreds of thousands, injection molding typically makes more sense.
  • Accuracy: A machined part gives you more control and fewer variables when working with precision tolerances. Machining focuses on the precise specifications of the part itself, as opposed to injection molding, which considers tolerances of the mold instead of the part. When absolute precision is required for the final products, especially for aerospace, medical and defense applications, CNC machining generally delivers more accurate results.
  • Performance: For high-performance hard plastics that can’t be molded, manufacturers choose CNC machining. Certain applications require hard plastics that must be machined. The melting and re-hardening of plastics in injection molding may introduce unwanted variances to the material properties of the final part.

Both injection molding and CNC machining offer high-quality methods designed to produce the best products possible, each with separate strengths and weaknesses. Many final products include a combination of components made from each method. To make the best choice for your production needs, enlist the help of manufacturing experts who understand these methods best.

Choosing the Right Method for Your Next Project

Machine shops see a wide variety of part requests and understand the capabilities of the different manufacturing processes. They have first-hand practice with a wide variety of materials and make informed choices for your product. With this knowledge and experience, experts will advise you on the best method for your components. 

In many cases, product development teams start with CNC machining to build prototypes and then transition over to injection molding for the full production run. The material requirements for your parts may dictate which production method best supports your overall project. 

In many cases, product development teams start with CNC machining to build prototypes and then transition over to injection molding for the full production run.

Whatever your needs, consulting experts who specialize in precision parts manufacturing allows you to obtain the best advice on how to proceed.

At Plethora, we specialize in manufacturing precision parts using the industry's most advanced CNC machining processes. Not only can we produce the parts you require, but we’re happy to quote the fabrication of the molds and tools for your injection molding projects. We are available to answer your questions when it comes to comparing CNC machining vs. injection molding, and we look forward to partnering together with you on your next project. To get started, upload your design files to Quote My Part or call us at 415-726-2256.

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The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.