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.

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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

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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.

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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

`

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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)

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7. Decision Checklist

Ask yourself:

1. **What quantity do I need?**

– Under 100 → CNC

– Over 5,000 → Molding

– 100-5,000 → Compare both

1. **Is my design final?**

– Still iterating → CNC

– Design locked → Molding

1. **What tolerances do I need?**

– ±0.05mm or better → CNC

– ±0.1mm is acceptable → Molding

1. **What’s my timeline?**

– Need parts in <2 weeks → CNC

– Can wait 6-8 weeks → Molding

1. **What material am I using?**

– Engineering plastic (PEEK, PTFE) → CNC often better

– Commodity plastic (PP, ABS) → Molding better

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Get Expert Advice on Your Plastic Parts

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Published: July 2026 | Leland CNC — Precision Manufacturing Since 2009