When to Use 3D Printing, CNC Machining, or Injection Molding

6 min read

Written by Guntis Endziņš — Mechanical Engineer & Founder, Guenzo Engineering

Choosing the right process before detailed design is the biggest cost decision in product engineering. Here is how the three main processes compare.

3D Printing

Best for: prototypes, low volumes (1–100), complex geometry, internal channels, fast iteration.

• Lead time: 1–3 days
• Per-part cost: dominated by build time and material
• Tolerances: ±0.2 mm typical (better on SLA/SLS, worse on FDM)
• Materials: photopolymers, nylon, ABS-like, some metals (DMLS)

CNC Machining

Best for: low to medium volumes (1–10,000), real engineering materials, tight tolerances, structural parts.

• Lead time: 1–3 weeks
• Per-part cost: drops with quantity but never approaches molding at scale
• Tolerances: ±0.025 mm achievable, ±0.1 mm easy
• Materials: any machinable metal or plastic — aluminum, steel, stainless, brass, POM, PEEK

Injection Molding

Best for: high volumes (1,000+), thin-walled plastic parts, consistent appearance, low unit cost.

• Lead time: 4–12 weeks (tooling)
• Per-part cost: very low after tooling is paid off
• Tolerances: ±0.1 mm typical; depends on shrink and warp
• Materials: ABS, PC, PP, PA, TPU, glass-filled variants

How to choose

Example

An enclosure ordered at 5 units: 3D printing wins (€40 each, 2 days). At 500 units: CNC or short-run molding (€18 each). At 50,000 units: injection molding (€1.20 each plus €8,000 tooling).

Common mistakes

• Designing for one process and switching late — geometry rules differ.
• Molding small volumes because "plastic should be molded".
• Machining shapes that would be 10x cheaper to print.