Cost-Benefit Analysis: CNC Machining vs. Injection Molding for Low-Volume Plastics
The Low-Volume Dilemma
When production volumes fall between 50 and 5,000 units, the choice between CNC machining and injection molding is not obvious. Injection molding offers lower per-part cost at volume but requires $5,000-$100,000+ in tooling. CNC machining eliminates tooling amortization but incurs higher per-part cost — and for complex geometries, may not be feasible at all. The break-even point varies dramatically by part geometry, material, tolerance requirements, and surface finish specifications.
Cost Model
Injection Molding Total Cost = Tooling Amortization + (Material Cost × Part Weight × Scrap Factor) + Machine Hourly Rate × Cycle Time + Post-Processing (degating, trimming). CNC Machining Total Cost = (Material Cost × Stock Volume) + Machine Hourly Rate × Cycle Time + Fixturing Cost + Post-Processing (deburring, surface finishing).
Break-Even Analysis by Material
| Material | Tooling Cost (Mold) | IM Part Cost @ 500 pcs | CNC Part Cost @ 500 pcs | Break-Even Volume |
|---|---|---|---|---|
| ABS | $8,000-15,000 | $2.50 | $35 | ~850 units |
| Acetal (POM) | $10,000-18,000 | $3.80 | $42 | ~700 units |
| PEEK | $18,000-35,000 | $22 | $85 | ~450 units |
Key insight: For PEEK and other high-performance polymers, the break-even point shifts dramatically toward injection molding at lower volumes because the raw material cost dominates CNC economics — PEEK stock shapes (rod, plate) cost 3-5× more per kg than PEEK injection molding pellets. This is a crucial factor missed by generic cost models that assume equivalent material pricing.
When CNC Machining Wins — Regardless of Volume
- Very tight tolerances: CNC can hold ±0.025 mm; injection molding typically ±0.1 mm for PEEK due to anisotropic shrinkage.
- Thick sections (>6 mm): Injection molding thick PEEK sections generates sink marks and internal voids from crystallization shrinkage; CNC from solid stock eliminates this.
- Prototype validation before tooling investment: CNC machine 5-10 prototypes for fit/function testing; invest in injection mold tooling only after design freeze.
References & Industry Standards
- ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
- ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
- National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
- UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
- MatWeb — Material Property Data. matweb.com