Skip to content 
**Injection Molding (IM)** requires a significant upfront investment in tooling but offers a low per-unit cost, making it the most cost-effective solution for high-volume production (typically >500–1,000 units). **Computer Numerical Control (CNC) Machining** has near-zero setup costs but a high per-unit cost due to machine time and material waste, making it ideal for prototyping and low-volume runs. The choice hinges on finding the “breakeven point” where tooling amortization is surpassed by the savings in unit price.
Definitions
- Injection Molding (IM): A formative manufacturing process where molten material (usually thermoplastic) is injected under high pressure into a custom-machined metal mold. It is a standard for mass production.
- Computer Numerical Control (CNC) Machining: A subtractive manufacturing process where computer-controlled cutting tools remove material from a solid block (stock) to shape the final part.
- Breakeven Point: The production quantity at which the total cost of Injection Molding (Tooling + Unit Cost × Quantity) equals the total cost of CNC Machining (Unit Cost × Quantity).
Injection molding becomes more cost-effective than CNC machining as production volumes increase past the breakeven point.True
While molding has high upfront tooling costs, the low cycle time and unit cost mean total project costs drop significantly per part at high volumes.
CNC machining is always more expensive than injection molding for producing plastic parts.False
For low volumes (1–100 parts) or prototypes, CNC is cheaper because it avoids the expensive mold fabrication costs required for injection molding.
Key Cost Drivers and Parameters
The following table compares the primary economic and technical variables influencing the cost of both processes.
| Parameter | Injection Molding (IM) | CNC Machining |
|---|---|---|
| Upfront Cost (NRE) | High ($3,000 – $100,000+) for Mold Design & Fabrication | Low ($0 – $500) for CAM Programming & Fixturing |
| Unit Cost | Low ($0.10 – $10.00) | High ($5.00 – $200.00+) |
| Cycle Time | Seconds (15s – 60s typical) | Minutes to Hours |
| Material Waste | Low (Runners can often be reground/recycled) | High (Subtractive process creates chips/swarf) |
| Tolerance Standard | DIN 16742 / ISO 20457 (Looser) | ISO 2768 (Tighter) |
| Scaling Cost | Linear (Cost drops rapidly with volume) | Constant (Cost remains high per unit) |
| Surface Finish | Smooth/Textured (Defined by mold polish) | Visible tool marks (Requires post-processing) |
Advantages and Disadvantages
Injection Molding
| Advantages | Disadvantages |
|---|---|
| Economies of Scale: Extremely low unit cost for large runs. | High Barrier to Entry: Molds are expensive and take weeks to build. |
| Repeatability: Millions of identical parts with minimal deviation. | Design Restrictions: Requires draft angles, uniform wall thickness. |
| Material Options: Vast array of polymers and additives available. | Difficulty of Modification: Changing a steel mold is costly and slow. |
| Complex Geometry: Can create complex shapes with sliders/lifters. | Initial Lead Time: T1 samples can take 2–8 weeks. |
CNC Machining
| Advantages | Disadvantages |
|---|---|
| Speed: Parts can be made in 24–48 hours. | High Variable Cost: Machine time is expensive and linear. |
| Precision: Superior tolerances (±0.01mm is standard). | Geometry Limits: Cannot easily machine undercuts or deep internal pockets. |
| Design Freedom: No need for draft angles; uniform walls not required. | Material Waste: You pay for the block, not just the final part volume. |
| Strength: Parts are isotropic (uniform strength in all directions). | Tool Wear: Cutters degrade, affecting surface finish over long runs. |
CNC machined parts typically possess isotropic structural properties.True
Because CNC parts are cut from a solid extruded or cast block, the material properties are generally uniform in all directions, unlike molded parts which have flow lines.
You can use the exact same 3D CAD file for both CNC machining and injection molding without modification.False
Injection molding requires specific design for manufacturing (DFM) features like draft angles and consistent wall thickness which are not required for CNC.
Application Scenarios: When to Use Which?
Choose CNC Machining When:
- Volume is Low: Production runs under 100–300 units.
- Speed is Critical: You need functional parts in hand within 1–3 days.
- Design is Fluid: The design is still iterating; changing a CAD file is free, changing a mold is not.
- Tight Tolerances: The part requires precision fitments (e.g., H7 fits) impossible to hold with shrinkage-prone plastics.
- High Wall Thickness: The part requires thick solid sections that would cause sink marks in molding.
Choose Injection Molding When:
- Volume is High: Production exceeds 500–1,000 units annually.
- Cost Per Part Matters: The target BOM (Bill of Materials) cost is strict.
- Complex Surface Finishes: Specific textures (e.g., VDI 3400) or high-gloss finishes are required out-of-the-box.
- Insert Molding: You need to encapsulate metal inserts or threaded brass fittings during the process.
- Lightweighting: The design utilizes thin walls and ribs for strength-to-weight efficiency.
Stepwise Process: Calculating the Breakeven Point
To determine the exact point where Injection Molding becomes cheaper than CNC, follow this calculation workflow:
-
Obtain Quotes:
- Get a firm quote for the Mold (Tooling) cost [T(im)] and Molded Unit cost [U(im)].
- Get a quote for the CNC Unit cost [U(cnc)] (Setup is usually amortized into unit cost or minimal).
-
Apply the Formula:
The breakeven quantity (Q) is where the total costs align.
T(im) + [U(im)×Q] = U(cnc)×QSolving for Q:
Q = T(im)/[U(cnc)- U(im)] -
Analyze the Result:
- If your required volume < Q, stick with CNC.
- If your required volume > Q, invest in Injection Molding.
-
Factor in Lead Time:
- If the break-even is 400 units, but you need them next week, the cost saving of molding is irrelevant due to the 4-week tooling lead time.
Injection molding allows for the integration of complex surface textures directly from the tool.True
Mold cavities can be chemically etched or EDM textured to apply specific finishes (like leather grain or matte) to every part automatically.
CNC machining produces less material waste than injection molding.False
CNC is subtractive and turns significant portions of the raw block into scrap chips, whereas injection molding adds material only where needed and often recycles runners.
FAQ: Injection Molding vs. CNC Cost
1. What is the typical breakeven quantity between CNC and Injection Molding?
While it varies by part complexity, the breakeven usually falls between 200 and 500 units. For simple parts, CNC stays cheaper longer. For complex parts where CNC machining takes hours, molding becomes cheaper faster (often around 100 units). Check ZetarMold’s Cost Guide1 for current market averages.
2. Why is CNC machining more expensive per unit?
CNC costs are driven by machine time. If a part takes 30 minutes to machine, you pay for 30 minutes of machine operation plus labor and overhead for every single part. Injection molding might make that same part in 30 seconds.
3. Can I prototype with a "Soft Mold" instead of CNC?
Yes. Aluminum tooling (Rapid Tooling) bridges the gap. It is cheaper and faster (1–2 weeks) than steel production molds but offers true molded parts for testing before mass production.
4. How does tolerance requirements affect cost?
Tight tolerances increase cost in both, but exponentially in molding. CNC naturally holds ±0.05mm. Achieving that in molding requires high-precision molds and scientific process control, increasing the NRE (Non-Recurring Engineering) cost. Refer to ISO 2768 standards2 for machining limits.
5. Is material cost higher for CNC or Molding?
CNC material cost is higher because you pay for the initial block size (the stock), and the removed material is often sold as low-value scrap. In molding, you pay for the net weight of the part plus a small runner percentage.
Summary
The decision between Injection Molding and CNC Machining is strictly an equation of Volume vs. Complexity. CNC is the superior choice for speed, high precision, and low volumes (under 300 units), as it eliminates the risk of tooling investment. Injection Molding is the undisputed king of scalability; once the initial "tuition" of tooling is paid, it delivers identical parts at a fraction of the cost and time. For successful product launches, engineers often start with CNC for validation and transition to Molding as market demand is confirmed.
EN 
ES 
FR 
DE 
PT 
AR 
RU 
JA 
KO 
IT 
NL 
TR 
PL 