How to Choose a Material
A decision framework: process first, then environment, strength, cost and supply
Material selection is a funnel, not a lookup: (1) fix the manufacturing process, (2) list service conditions — temperature, chemicals, loads, regulatory — (3) screen candidates on strength and stiffness, (4) compare cost and supply, (5) validate with a datasheet and a prototype. Skipping the process-first step is the most common error: a material that cannot be machined, printed or molded in your geometry is not a candidate at all.
1. Start from the process
Machining supports nearly any metal and most plastics in solid form; 3D printing adds nylon (PA12), resins, TPU and DMLS metals; sheet metal is restricted to formable alloys; injection molding applies to virtually all thermoplastics and LSR. Your chosen process defines the material universe before any property comparison.
Quantity is the other gate: prototype volumes (1–100 pcs) favor machining and 3D printing; production volumes favor molding and stamping, which may justify a material change (e.g. machined 6061 bracket → molded nylon or die-cast A380 at scale).
2. Service conditions first
Build a requirement list: max continuous temperature, chemical exposure, UV, moisture, wear, fatigue, electrical, regulatory (food, medical, RoHS, flame). Rank them — the binding constraint picks the family: >150 °C → PEEK/PPS/PI; chlorides → 316/titanium; wear → POM/PA66/bronze; transparency → PC/PMMA; biocompatibility → Ti-6Al-4V/LSR/PEEK.
Do not over-specify: many failures are cost failures, not property failures. If the application peaks at 80 °C with mild chemicals, ABS/PC or 6061 aluminum will be dramatically cheaper than PEEK or titanium.
3. Strength, stiffness and density
Screen on specific strength (UTS/density) and specific stiffness (modulus/density) rather than absolute numbers: Ti-6Al-4V, 7075 and CFRP dominate specific strength; magnesium and aluminum compete on stiffness per weight; steel wins on absolute stiffness and cost.
For precision and stability, compare modulus, thermal expansion and creep: aluminum expands ~23 µm/m·K, steel ~12, plastics 70–200. A machined aluminum part will move dimensions with temperature where a steel or Invar part will not.
4. Cost, supply and validation
Rank finalists by price index (aluminum ≈1, steel ≈1.2, stainless ≈3–4, titanium ≈8–12, PEEK ≈30–60) and by availability in your required form (bar, plate, sheet, powder). Small quantities of exotic grades are dominated by material minimums — check stock sizes before finalizing.
Validate with the specific grade datasheet (exact temper/alloy/fill) and a prototype under real loads; datasheet properties vary with condition (printed vs molded, annealed vs hardened). Document the chosen grade on the drawing so the quote and inspection match the intent.
Quick reference
| Step | Question | Typical outcome |
|---|---|---|
| 1. Process | How will it be made, at what quantity? | CNC / 3DP / sheet / molding universe |
| 2. Environment | Temperature, chemicals, UV, moisture, loads | Family shortlist (PEEK vs PA66 vs 316) |
| 3. Strength/stiffness | UTS, modulus, density, creep, expansion | Finalists + grades |
| 4. Cost & supply | Price index, stock forms, minimums | Selected grade |
| 5. Validate | Datasheet + prototype under load | Confirmed choice on drawing |
Sources & references
- ASTM D638 / ASTM E8 — Tensile test methods
- ASM International — Materials handbooks
- ISO 527 / ISO 6892 — Tensile standards
- Xometry Learning Center — Material Selection Guide (public engineering reference)