Plastic Injection Molding vs. CNC Machining for Low-Volume Production

Choosing the right manufacturing process before you start production can save both time and money, especially when you’re making a plastic part in low volume. Two of the most common methods are CNC machining and plastic injection molding, and while both can produce high-quality parts, they serve very different purposes depending on your goals.

In this post, we’ll break down the key differences in CNC vs injection molding for low-volume production. You'll learn how to evaluate unit cost, lead time, part complexity, and long-term goals to decide which method fits your project best. Whether you're prototyping a new design or fulfilling a small batch order, understanding when to use CNC machining or injection molding is essential for smart manufacturing decisions.

CNC Machining for Plastic Parts

CNC machining is a subtractive manufacturing process where plastic blocks or rods are milled, drilled, or cut into the desired shape using precision-controlled tools. It's widely used for prototyping and low-volume production because it doesn’t require molds or tooling, making it ideal when speed and flexibility are priorities.

One of the main advantages of CNC machining for plastic parts is the ability to produce highly accurate components with tight tolerances. One value of CNC machining plastic parts is when a part requires critical dimensions or complex geometries that need to be validated before investing in hard tooling. Additionally, CNC supports a wide range of plastics, such as ABS, polycarbonate, POM, and nylon, allowing designers to test performance with production-grade materials.

Because there's no upfront tooling investment, CNC machining is often the most cost-effective option for low-volume production. It also allows for design changes between batches, which is valuable during early-stage development when designs are still evolving.

However, CNC machining comes with some limitations. The process generates more material waste since it involves cutting away from a solid block, and the per-unit cost remains high as volume increases. Cycle times will also be longer than injection molding.

For low-volume production or prototyping, CNC machining offers speed and precision, making it a go-to option when fast iteration and functional testing matter more than the unit price.

Plastic Injection Molding for Low-Volume Runs

Plastic injection molding is a process where plastic resin is injected into a custom mold cavity under high pressure. Once solidified, the part is ejected, and the cycle repeats. While it's often associated with mass production, injection molding can also be a viable option for low-volume runs, especially when the upfront tooling cost is justified by the lower unit price. 

One of the biggest advantages of injection molding is the low per-unit cost once the mold is ready. After the tooling is complete, production cycles are fast and consistent, allowing manufacturers to deliver thousands of identical parts quickly. 

For low-volume production, manufacturers often use aluminum tooling instead of hardened steel to reduce cost and lead time. While aluminum tools may not last as long, they’re sufficient for runs in the range of 1,000 to 10,000 units. This makes injection molding a strong choice for pilot runs, crowdfunding campaigns, or initial product launches where a higher volume of consistent parts is needed.

However, the main drawback is the upfront investment. Even with aluminum molds, the tooling cost can range from a few thousand to tens of thousands of dollars, depending on part complexity. Tooling can also take 3–6 weeks or longer to complete, which means injection molding is less ideal when speed or design flexibility is your top priority.

For projects where volumes are expected to grow or where unit cost matters more than agility, plastic injection molding becomes a better long-term solution, especially once the design is locked in and stable.

Key Factors to Consider

Choosing between CNC vs Injection Molding for low-volume plastic parts comes down to aligning the process with your project's priorities. Factors like unit cost, speed, part complexity, and long-term plans all play a role. Below, we break down the most important considerations to help you make the right decision at the right stage.

Volume Thresholds and Break-Even Point

The most critical factor when choosing between CNC vs Injection Molding is volume. CNC machining doesn’t require tooling, so it’s ideal for low-volume production runs. However, as volume increases, the per-part cost of CNC remains relatively high, while injection molding becomes more economical thanks to repeatability and rapid cycle times.

The tipping point often occurs around 1,000 units, depending on the part's complexity, material, and mold cost. At that point, even with an initial tooling investment, the total cost of injection molding can start to undercut CNC.

Key takeaways:

• CNC is more economical for fewer parts of just a few hundred.

• Injection molding becomes cost-effective at around 1,000+ parts.

• Always calculate the total cost of ownership, including tooling and per-unit cost.

Lead Time and Speed to Market

CNC machining offers quick turnaround of just a few days making it an excellent choice for prototyping and time-sensitive projects. Injection molding, on the other hand, requires mold design, tool trials, and fabrication, which can take several weeks before the first part is ready.

If your timeline is tight or you’re still finalizing the design, CNC allows for rapid iteration. Once your design is frozen and you're producing in larger volumes, molding becomes worth the wait.

Key takeaways:

• CNC has no tooling lead time which is great for fast development.

• Injection molding takes 3–8+ weeks to create molds.

• Use CNC when speed matters more than cost per unit.

Part Complexity and Tolerance Requirements

Both CNC and injection molding can produce complex geometries, but in different ways. CNC allows for precision features, sharp edges, and tight tolerances, which is important for functional prototypes or components with mechanical interfaces.

Injection molding is excellent for repeatable geometries but may require optimizing your plastics design, like adding draft angles or removing undercuts, to make the part moldable. If your part requires extremely tight tolerances or sharp internal corners, CNC might be the better fit, at least initially.

Key takeaways:

• CNC offers higher precision and better tolerance control.

• Injection molding requires DFM-friendly geometry.

• Use CNC when tolerances are critical or moldability is uncertain.

Material Selection and Surface Finish

CNC machining supports a wide range of engineering-grade plastics, including some that are difficult to mold. It also allows testing of different resins with no tooling changes. This is ideal for early-stage testing when you’re exploring mechanical properties, UV resistance, or chemical compatibility.

Injection molding is more limited by what can be successfully injected and cooled in a mold, but it delivers more consistent surface finishes and texture options, important for cosmetic parts or consumer-facing products.

Key takeaways:

• CNC allows flexible resin selection with no setup changes.

• Injection molding provides surface finish consistency.

• Consider CNC for early-stage material testing and molding for final appearance.

Tooling Investment and Design Flexibility

Tooling is the biggest upfront cost in injection molding, which makes it risky if your design is likely to change. CNC machining lets you iterate freely without sunk costs. Once your part is finalized, investing in a mold becomes a more sensible long-term move.

Keep in mind that modifying injection molds, can still be expensive and time-consuming. That’s why many companies prototype and validate with CNC first, then mold once the design is locked in.

Key takeaways:

• CNC is ideal for iterative design and early-stage development.

• Injection molds are costly and harder to modify once made.

• Use CNC to finalize designs before transitioning to molding.

Transition Strategy: From CNC to Injection Molding

For many companies, the most effective approach isn’t choosing between CNC vs injection molding, it’s using both at different stages. Starting with CNC machining allows teams to iterate quickly and validate designs before committing to the costs and timelines of plastic injection molding. Once the design is locked in and the volume justifies it, transitioning to injection molding becomes a sensible next step.

This staged approach reduces risk and helps ensure that the mold you eventually invest in is based on a proven design. It also allows you to fine-tune your design for manufacturing, which can lower the cost of the mold, prevent future revisions, and speed up time to production.

Before moving into injection molding, there are a few steps to take:

• Conduct tolerance and fit checks using CNC parts to ensure that assemblies function as expected.

• Get feedback from beta testers to make any final design adjustments.

• Work closely with your contract manufacturer to review draft angles, wall thickness, and other DFM considerations.

Conclusion: CNC vs Injection Molding

When it comes to CNC vs injection molding for low-volume production, there’s no universal answer. The answer depends on your current stage, priorities, and goals. CNC machining offers unmatched speed, flexibility, and precision for prototyping and small batches. It’s the ideal way to iterate quickly and validate your design without committing to tooling.

Injection molding, on the other hand, is built for scale. Once your design is finalized and your volume increases, the lower per-unit cost and repeatable quality make it the smarter long-term option, even for relatively modest production runs.

Many successful hardware teams use both processes in tandem: CNC to get started, injection molding to grow. By understanding when to use each one, you’ll reduce waste, manage costs, and set your product up for a smoother path from prototype to production.

If you're evaluating options and want to better understand the costs involved, feel free to contact us. Whether you're considering CNC machining or plastic injection molding, we can help you assess the best path forward based on your part design, volume, and goals.