Injection Molding Basics
How molds are built, why tooling costs, and what drives molded part price
Injection molding melts plastic pellets and injects them into a closed steel/aluminum cavity under pressure; the part cools, shrinks and is ejected. The mold is the investment — aluminum/prototype tools from ~$2k–8k, production steel tools $10k–100k+ — and unit cost collapses with volume. Design rules (draft, wall uniformity, gating) are locked at tool build, so DFM review before tooling is the highest-leverage engineering step.
1. The molding cycle
Each cycle: clamp the mold, inject melt (~0.1–2 s per mm of wall), pack under pressure, cool (the longest step), eject. Cycle time scales with wall thickness — a 2 mm wall part cycles in ~20–40 s; thinner walls cycle faster but need higher pressure and better flow.
Production volumes reach 100k–1M+ parts per tool with hardened steel; aluminum tools are good for 1k–100k and cheaper. T1 samples (first shots) are as fast as 5 working days on prototype tools — the platform quotes tooling separately, amortized in per-part comparisons.
2. Tooling classes and cost drivers
Mold cost scales with: number of cavities (1-cavity prototype vs 8+ production), complexity (side actions, lifters, inserts, textures), steel grade and size. A simple 1-cavity aluminum tool ≈ $2k–8k; a 4-cavity steel tool with slides ≈ $20k–80k.
The quoting engine prices tooling from part volume and feature complexity: toolingBase + toolingPerVol × part volume, plus per-feature charges for undercuts, threads and texture. Mold ownership transfers to the customer.
3. Part cost structure
Molded unit cost = resin cost (weight × price/kg) + machine cycle cost + finishing/assembly + margin, with quantity tiers discounting setup. Resin is usually a small fraction; wall thickness and cycle time dominate — every 1 mm of extra wall adds seconds per cycle.
At 10k+ units, per-part cost can drop below $1 for small parts; at 100k+, molding is typically 10–50× cheaper per part than machining the same geometry.
4. DFM checklist before tooling
Uniform walls (1–4 mm), draft 1–2° on all vertical faces, ribs 0.5–0.6× wall, no zero-draft undercuts unless side actions are budgeted, gates/ejector marks on non-cosmetic faces, generous radii at transitions, and shrinkage allowance per resin (ABS 0.4–0.7%, PP 1–2.5%).
Get a DFM review before cutting steel: the platform flags wall ratio, draft, and feature issues on upload, and recommends tooling class by quantity.
Quick reference
| Tool class | Material | Shots | Lead time | Typical cost |
|---|---|---|---|---|
| Prototype (Class 105) | Aluminum / P20 | 1k–10k | 5–15 days | $2k–8k |
| Bridge (Class 103) | P20 / H13 | 10k–100k | 15–25 days | $8k–30k |
| Production (Class 101) | Hardened steel | 100k–1M+ | 25–40 days | $20k–100k+ |
| High-cavity (8+) | Hardened steel | 500k–1M+ | 35–60 days | $50k–200k+ |
Sources & references
- DIN 16742 — Plastic part tolerances
- ISO 294-4 — Shrinkage
- SPI mold classification
- Xometry Learning Center — Injection Molding Basics (public engineering reference)