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Should I machine or injection mold my plastic part?
Machine plastic parts for prototypes and runs up to low thousands a year: there is no tooling, and acetal, nylon or PEEK parts hold ±0.005 in (±0.13 mm) or better. Switch to injection molding above roughly 5,000 parts a year, where a $3,000–$40,000 mold and 15–60 second cycles cut piece price to a fraction of machining.
Which is cheaper at your quantity?
The same housing, each process in a suitable material (PC/ABS and PC).
| Quantity | Injection molding | CNC machining |
|---|---|---|
| 10 | $1,047 | $169 |
| 50 | $209 | $125 |
| 100 | $105 | $115 |
| 500 | $21.50 | $87.60 |
| 1,000 | $11.10 | $78.20 |
| 5,000 | $4.24 | $72.40 |
| 10,000 | $2.50 | $72.40 |
| 50,000 | $1.19 | $72.30 |
| 100,000 | $1.02 | $72.30 |
Where each pays off
Plastic injection molding
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- Tooling
- 3–14 weeks
- First article
- 1–2 weeks after tool completion
- Production
- 2–4 weeks per release
±0.005 in (±0.13 mm); ±0.002 in (±0.05 mm) on small critical features
Production machining, turning, and sawing
1101001k10k100k1M+
- First article
- 1–3 weeks
- Production
- 2–6 weeks per release
±0.005 in (±0.13 mm), or ISO 2768-m
Which should you pick?
Pick Injection molding when
- Volume is above about 5,000 parts a year
- The part uses thin walls, snap fits or living hinges
- Color and texture should be molded in
- The resin is glass-filled or flame-rated
Pick CNC machining when
- You need prototypes through low thousands of parts
- Sections are thicker than 0.25 in (6 mm) and would sink in a mold
- The design is still changing
- Low-volume PEEK or acetal parts need tight tolerances
Key differences
| Injection molding | CNC machining | |
|---|---|---|
| Tooling | $3,000–$15,000 aluminum; $10,000–$40,000 P20 steel | None |
| Lead time | 3–14 weeks for tooling, then 1–2 weeks of sampling | 1–3 weeks to first article |
| Walls | 0.040–0.150 in (1.0–3.8 mm) nominal; core out thick sections | 0.060 in (1.5 mm) minimum in plastic; thick sections are fine |
| Draft | 1–2° typical | None |
| Materials | Pellet grades, including glass-filled, flame-rated and TPE overmolds | Stock rod, plate and sheet such as acetal, nylon, PEEK, HDPE, ABS and PC |
| Piece price with quantity | Drops sharply once the mold is paid | Falls from 1 to about 100 parts, then flattens |
Questions
When should I switch from machining to injection molding?
Usually around 5,000 parts a year for a stable design. Aluminum molds make 1,000–10,000 part runs practical, so the switch can come sooner for small parts with long machining cycles.
Can I machine a part designed for injection molding?
Usually. Draft and uniform walls machine fine, but thin, tall ribs and deep narrow slots are slow or impossible to cut. For prototypes, thicken ribs slightly or print those parts instead.
Are machined plastic prototypes representative of molded parts?
Close, not identical. Machined parts come from extruded or cast stock, while molded parts have flow orientation, knit lines and molded-in stress. Validate critical tests on molded samples.
Try both on your part.
The builder prices the same geometry in every suitable process and material, with DFM for each.