Plastic CNC Machining vs Injection Molding: When to Choose Each
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Introduction
You need plastic parts. Two manufacturing methods come to mind: CNC machining and injection molding. Both can produce high-quality plastic components — but they serve fundamentally different purposes.
Choosing incorrectly can mean paying 10× more per part or waiting weeks longer than necessary. This guide provides a clear, data-driven comparison so you can decide with confidence.
1. At a Glance: The Core Difference
| CNC Machining | Injection Molding | |
|---|---|---|
| **How it works** | Cuts plastic from a solid block | Injects molten plastic into a mold |
| **Setup cost** | Low ( tooling) | High (,000-,000+ for mold) |
| **Per-part cost** | Medium-high | Very low (at volume) |
| **Lead time** | 3-10 days | 4-8 weeks (mold + sampling) |
| **Materials** | 20+ engineering plastics | 100+ thermoplastics |
| **Tolerances** | ±0.025mm typical | ±0.05-0.1mm typical |
| **Surface finish** | Visible tool marks | Mold finish determines surface |
| **Design changes** | Easy — just update CAD | Expensive — modify or replace mold |
Rule of thumb:
- **Under 100 parts**: CNC machining is almost always cheaper
- **100 – 5,000 parts**: Depends on complexity — compare both
- **Over 5,000 parts**: Injection molding is usually more economical
2. Cost Comparison: Real Numbers
Let's compare a typical plastic housing (100mm × 80mm × 40mm):
CNC Machining
| Quantity | Per-Part Cost | Total |
|---|---|---|
| 1 | ||
| 10 | ||
| 50 | ,400 | |
| 100 | ,200 | |
| 500 | ,000 | |
| 5,000 | ,000 |
Injection Molding
| Quantity | Mold Cost | Per-Part Cost | Total |
|---|---|---|---|
| 1,000 | ,000 | .20 | ,200 |
| 5,000 | ,000 | .80 | ,000 |
| 10,000 | ,000 | .55 | ,500 |
| 100,000 | ,000 | .30 | ,000 |
Break-even point: For this example part, injection molding becomes cheaper at approximately 500-800 units. But this varies dramatically by part complexity.
3. When to Choose CNC Machining
Ideal Scenarios:
Prototyping & Design Iteration
- Design is still evolving — easy CAD updates
- You need functional prototypes in days, not weeks
- Testing multiple design variations simultaneously
Low-Volume Production
- 1-500 parts typically
- Bridge tooling while injection mold is being made
- End-use parts for niche/low-volume products
Large or Thick Parts
- Parts too thick for molding (>6mm walls warp)
- Parts larger than typical molding machines accommodate
Tight Tolerances
- When ±0.05mm or better is required
- Critical mating surfaces and fits
Material Requirements
- Materials that don’t injection mold well (PEEK, PTFE)
- Glass-filled materials that wear molds rapidly
Advantages of CNC Machining for Plastic:
- No sink marks, weld lines, or flow marks
- Isotropic strength (vs anisotropic in molded parts)
- Better dimensional stability in thick sections
- No draft angles required
4. When to Choose Injection Molding
Ideal Scenarios:
High-Volume Production
- 5,000+ units per year
- Consumer products with long product lifecycles
- The mold cost amortizes over volume
Complex Internal Features
- Snap fits, living hinges, complex ribs
- Features that would require 5-axis CNC (expensive)
Cosmetic Parts
- High-gloss, textured, or colored surfaces
- Consistent appearance across millions of parts
- In-mold decoration and labeling
Multi-Material / Overmolding
- Soft-touch grips over rigid substrates
- Multi-color parts in a single cycle
Advantages of Injection Molding:
- Lowest per-part cost at volume
- Excellent repeatability
- Wide range of commodity plastics (PP, PE, PS)
- Automated production — minimal labor
5. The Hybrid Approach: CNC for Prototypes, Molding for Production
The most common strategy among product developers:
`
Phase 1: CNC Machining
→ 10-50 prototypes for testing and validation
→ Iterate design based on feedback
→ Duration: 1-4 weeks
Phase 2: Bridge Tooling (optional)
→ Aluminum or soft steel mold for 500-5,000 parts
→ Lower mold cost (,000-,000)
→ Validate molding process before committing to production tool
Phase 3: Production Injection Molding
→ Hardened steel mold for 100,000+ parts
→ Optimized cycle time and part cost
→ Full-scale manufacturing
`
6. Material Considerations
| Material | CNC Machining | Injection Molding |
|---|---|---|
| **ABS** | Good | Excellent |
| **POM (Delrin)** | Excellent | Good |
| **PEEK** | Excellent | Difficult (high melt temp) |
| **Nylon** | Good (absorbs moisture) | Excellent |
| **PTFE (Teflon)** | Good | Very Difficult |
| **Polycarbonate** | Good | Excellent |
| **Acrylic (PMMA)** | Good (prone to cracking) | Excellent |
| **PP / PE** | Poor (too soft) | Excellent |
| **Ultem (PEI)** | Good | Good (high mold temp needed) |
7. Decision Checklist
Ask yourself:
- **What quantity do I need?**
– Under 100 → CNC
– Over 5,000 → Molding
– 100-5,000 → Compare both
- **Is my design final?**
– Still iterating → CNC
– Design locked → Molding
- **What tolerances do I need?**
– ±0.05mm or better → CNC
– ±0.1mm is acceptable → Molding
- **What’s my timeline?**
– Need parts in <2 weeks → CNC
– Can wait 6-8 weeks → Molding
- **What material am I using?**
– Engineering plastic (PEEK, PTFE) → CNC often better
– Commodity plastic (PP, ABS) → Molding better
Get Expert Advice on Your Plastic Parts
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Published: July 2026 | Leland CNC — Precision Manufacturing Since 2009