Materials, Manufacturing, and Cost
Properties Drive Selection
Material selection starts from function: what loads, temperatures, environments, and duty cycles must it survive? Key property families: mechanical (strength, stiffness/modulus, hardness, toughness, fatigue limit), thermal (conductivity, expansion coefficient, service temperature), electrical, and chemical (corrosion resistance).
Know the broad classes and their trade-offs: metals (strong, stiff, conductive, dense), polymers (light, cheap, formable, low stiffness, temperature-limited), ceramics (hard, heat-resistant, brittle), composites (high specific strength, anisotropic, costly).
Manufacturing choices follow from material and volume: machining (flexible, low volume), injection molding (high tooling cost, low per-unit cost at volume), casting, stamping, additive manufacturing (no tooling, slow per unit).
Cost has several components: fixed/tooling, variable/material, labor, and lifecycle costs (maintenance, energy, disposal). Design decisions lock in a large share of lifetime cost early.
Traps
- Comparing materials by strength alone - specific strength (strength per density) often decides aerospace and portable products.
- Ignoring anisotropy: composites are strong along fibers, weak across them.
- Break-even confusion: high tooling is justified only above a volume threshold.
- Stiffness vs. strength: a part can be strong enough yet deflect too much; stiffness is a geometry-and-modulus question.
- Hidden costs: assembly labor, fasteners, finishing, scrap rate.
Rule of Thumb
Break-even volume = fixed cost difference ÷ per-unit cost savings. If molding tooling costs $50,000 and saves $5 per part, you need 10,000 parts to break even.
Worked example: Machining costs $20/unit with no tooling; molding costs $50,000 tooling plus $5/unit. At 1,000 units: machining $20,000; molding $55,000 - machine it. At 10,000 units: machining $200,000; molding $100,000 - mold it. The crossover is exactly 10,000/3.33… ≈ 3,333 units once you solve 20N = 50,000 + 5N.