For plastic parts, the CNC vs injection molding decision is driven by volume, material and how settled the design is. Injection molding injects melted plastic into a steel or aluminum mold and produces parts in seconds, but the mold is a large up-front cost. CNC machining cuts parts from plastic rod and plate with no tooling, which makes it the practical choice for prototypes, low volumes and high-performance materials that are hard to mold.
CNC vs Injection Molding: Comparison Table
Figures are typical for a small functional plastic part. Cost is a relative per-part index with mold cost amortized, where one machined part at quantity 1 equals 100.
| Factor | CNC machined plastic | Injection molding |
|---|---|---|
| Relative cost, qty 1 | 100 | 3,000 to 8,000 |
| Relative cost, qty 10 | 40 | 300 to 800 |
| Relative cost, qty 100 | 20 | 35 to 85 |
| Relative cost, qty 1,000 | 14 | 5 to 10 |
| Tooling | None beyond fixtures | Aluminum or steel mold, typically thousands to tens of thousands of dollars |
| Typical tolerance | ±0.002 to ±0.005 in, material dependent | ±0.003 to ±0.010 in, shrink dependent |
| Surface finish | Machined, Ra 32 to 125 µin | Replicates mold finish, cosmetic textures possible |
| Materials | Acetal, nylon, PEEK, Ultem, PTFE, UHMW, polycarbonate and more from stock shapes | Wide range of moldable resins, filled grades, colors |
| Lead time | Days to a few weeks | 4 to 12 weeks for tooling, then fast |
| Strength | Stock shape properties, no weld lines | Good, but weld lines and gate stress possible |
| Typical use case | Prototypes, low volume, thick or high-performance parts | High volume consumer and industrial parts |
When CNC Machining Wins
- Low volumes. Below a few hundred to a few thousand parts, mold cost usually outweighs the per-part savings.
- Design still evolving. Changing a machined part is a program edit. Changing a mold means cutting steel or starting over.
- Thick sections. Molded parts need uniform thin walls to avoid sink and voids. Machined plastic can be any thickness.
- High-performance materials. PEEK, Ultem, PTFE and UHMW are commonly machined from stock shapes because molding them is costly or impractical for small runs.
- Bridge production. Machined parts can ship while the mold is being built and qualified.
When Injection Molding Wins
- High volumes. At thousands to millions of parts, molding cost per part drops well below machining.
- Cosmetic parts. Textures, colors and consistent appearance come straight from the mold.
- Snap fits, living hinges and thin walls. These features are natural to molding and awkward to machine.
Common Mistakes
- Prototyping a molded design by machining, then being surprised when the molded part warps or sinks. Machined prototypes do not reveal shrink, warp or weld lines.
- Designing for machining and then sending it to a molder. Molded parts need draft, uniform walls and no undercuts without side actions.
- Applying metal tolerances to plastic. Most plastics move with temperature and moisture, so ±0.001 in may be unrealistic outside a controlled environment.
- Ignoring material certification. For regulated applications, confirm the stock shape grade matches the intended molded resin.
Planning a Path From Machined to Molded
A sensible sequence is to machine the first functional prototypes, then run a small machined batch for field testing, then commit to a mold once the design is frozen. If early production must ship before the mold is ready, a machined bridge run keeps orders moving. Keep a drawing revision for each process so tolerances and features match the method being used.
How This Applies at Wexmar
Wexmar does not do injection molding. We machine engineered plastics including acetal, nylon, PEEK and Ultem on our lathes, horizontal machining centers and Swiss-type machines, which suits prototypes, low-volume production and bridge runs. Small turned plastic parts to 32 mm diameter run well on our Maier Swiss machines. See materials we machine, Swiss machining, and the guide on Delrin vs nylon.
Getting a Quote
Send your plastic part model and drawing with the quantity you need. You can upload a drawing for an instant estimate, with every quote confirmed by a human estimator, or call (315) 400-2654.