The decision between CNC machining and injection molding is the most expensive one you’ll make for your plastic parts. Choose molding too early and you waste money on tooling for a design that’s still evolving. Choose CNC for too long and you overpay per part on volumes that should have transitioned to a mold months ago. This guide provides a concrete decision framework.
Cost Break-Even Analysis
| Volume (parts/year) | CNC Unit Cost | IM Unit Cost (incl. tool amortization) | Winner |
|---|---|---|---|
| 50 | $15-40 | $50-100 | CNC |
| 500 | $10-25 | $5-15 | Depends on tooling cost |
| 5,000 | $8-20 | $0.80-3.00 | Injection Molding |
Decision Framework
Choose CNC machining when: your annual volume is under 500-1,000 parts, the design may change (prototyping), you need fast turnaround (days vs weeks for mold making), or the part geometry is un-moldable (deep undercuts, large flat plates, threaded features from multiple directions).
Choose injection molding when: your annual volume exceeds 1,000 parts, the design is frozen and validated, per-part cost is the primary driver, or the material requirements demand glass-filled nylon or other molded-only grades.
Frequently Asked Questions
What’s the cheapest way to get 100 nylon parts?
CNC machining from rod stock. 100 units is well below the break-even threshold for even a simple single-cavity mold ($3,000-5,000 tooling cost). CNC gives you production-grade material properties and surface finish without any tooling investment.
Can I use CNC-machined parts to validate my injection mold design?
This is a smart approach. Machine 10-50 parts in the intended production material (e.g., acetal or nylon rod stock), test them in the application, and refine the design before committing $5,000-25,000 to a production mold. The CNC prototypes cost more per unit but save $10,000+ in mold revisions.
How long does it take to go from CNC prototype to injection molded production?
After finalizing the CNC-validated design, mold manufacturing takes 3-6 weeks, followed by 1-2 weeks of mold trials (T0, T1 samples), and 1 week for process optimization. Total transition: roughly 6-10 weeks from design freeze to production parts.
What if my volume is between 500-2,000 parts/year?
This is the “gray zone.” Consider: (1) aluminum prototype tooling ($2,000-5,000, good for 5,000-10,000 shots), (2) family molds that produce multiple different parts in one cycle to amortize tooling cost, or (3) continuing CNC machining if your part price can absorb the higher unit cost.
Not sure whether to CNC machine or injection mold?
Send us your 3D file and target volume — we’ll run the numbers for both processes and give you the honest cost comparison.
