CNC machining and injection molding are two dominant manufacturing methods used for producing precision parts.
Each method serves different purposes, with key differences in cost, lead time, volume suitability, material selection, and surface finish.
What Is CNC Machining?
CNC machining is a subtractive process that removes material from a solid block using computer-controlled tools.
It supports metals, plastics, and composites for prototyping or production at tight tolerances.
Key features of CNC machining:
- Tolerances down to ±0.005 mm
- Ideal for low-to-mid volume production
- Compatible with metals and engineering plastics
- No need for molds or tooling
- Flexible for custom or complex geometries
What Is Injection Molding?
Injection molding is an additive manufacturing process that injects molten material into a mold cavity.
It is optimized for mass production of identical plastic parts with low per-unit cost at high volumes.
Core characteristics:
- High upfront mold costs
- Tolerances typically ±0.1 mm (but varies)
- Extremely efficient for 1,000+ part runs
- Limited material compatibility (mostly thermoplastics)
- Complex internal features possible with mold design
CNC vs. Injection Molding: How Do They Compare?
| Attribute | CNC Machining | Injection Molding |
|---|---|---|
| Process Type | Subtractive (material removed) | Additive (material injected into mold) |
| Best For | Prototypes, small batches, high precision | Mass production of plastic parts |
| Materials Supported | Metals, plastics, composites | Thermoplastics (ABS, PP, Nylon, etc.) |
| Lead Time | 1–5 days for prototypes | 3–6 weeks (includes mold creation) |
| Upfront Cost | Low setup cost | High mold tooling cost ($5,000–$50,000+) |
| Tolerance | ±0.005 mm or better | ±0.1 mm typical |
| Design Flexibility | Easily modifiable | Limited after mold creation |
| Surface Finish | Machined (can be polished or coated) | Molded (can be textured or glossy) |
| Volume Efficiency | Cost-effective up to 1,000 parts | Ideal for 10,000+ parts |
Which Is Better for Prototyping?
CNC machining is more suitable for prototyping due to its flexibility and lack of tooling requirements.
Design changes are implemented directly in the CAD model without needing new molds.
Use case:
Medical device companies often prototype surgical tools or implant components using medical CNC machining services for fast, accurate iterations.
Which Is More Cost-Efficient for Mass Production?
Injection molding becomes more cost-effective than CNC when part volumes exceed 10,000 units.
Although initial tooling is expensive, the low per-unit cost makes it ideal for high-volume runs.
Example:
Plastic housings, enclosures, and product casings for consumer electronics are often injection-molded in bulk.
Which Process Offers Better Precision?
CNC machining offers higher precision and tighter tolerances than injection molding.
It is preferred in applications where dimensional stability is critical, such as aerospace and orthopedic implants.
Precision comparison:
- CNC: ±0.005 mm or better
- Injection Molding: ±0.1 mm typical, improved with tighter tooling
What About Material Versatility?
CNC machining supports a broader range of materials, including metals like titanium, stainless steel, and aluminum.
Injection molding is largely limited to thermoplastics, though some advanced techniques support metal or ceramic-filled plastics.
Typical CNC materials:
- Aluminum, brass, stainless steel, titanium
- POM, ABS, nylon, PTFE
Injection molding materials:
- Polypropylene, ABS, PC, nylon, TPU
Summary: When to Use CNC Machining vs. Injection Molding
| Situation | Recommended Method |
|---|---|
| Low-volume production (<1,000 parts) | CNC Machining |
| Rapid prototyping | CNC Machining |
| High precision and tight tolerances | CNC Machining |
| High-volume production (>10,000 parts) | Injection Molding |
| Complex molded features (e.g. snap fits) | Injection Molding |
| Fast design modifications | CNC Machining |
