B3.1 — Material selection

Key concepts

Choosing the right material is one of the most consequential decisions a designer makes. The choice affects performance, cost, manufacturability, aesthetics, and environmental impact. A brilliant concept can fail if the wrong material is selected.

Selection criteria

Material selection balances many criteria:

• Mechanical performance — strength, stiffness, toughness, hardness, fatigue resistance
• Thermal and electrical properties — conductivity, expansion, insulation
• Aesthetic qualities — colour, texture, finish, feel
• Environmental impact — embodied carbon, recyclability, biodegradability
• Cost — raw material and processing
• Availability — supply chain reliability, local sourcing
• Processability — can it be manufactured using available methods?
• Durability and maintenance — lifetime, repairability
• Weight — critical for transport, handheld items, aerospace
• Safety — toxicity, food contact, flammability

No material wins on every criterion. Selection is always a trade-off.

Decision matrices

A decision matrix (or weighted selection matrix) converts material choice into a numerical comparison. The process:

1. List candidate materials as columns.
2. List criteria as rows.
3. Assign a weight (1–5) to each criterion reflecting its importance.
4. Score each material against each criterion (1–5).
5. Multiply score × weight for each cell.
6. Sum the weighted scores. The highest total wins.

Decision matrices force designers to make weighting explicit. Two designers can disagree on the result, but both can point to the criterion where their judgements diverge.

Property-based justification

Good material selection cites properties, not brand names. Saying "I chose aluminium" is weak. Saying "I chose 6061 aluminium for its high strength-to-weight ratio (270 MPa tensile strength at 2.7 g/cm³), compatibility with anodising for aesthetic finish, and recyclability" is strong.

Examiners reward specific numeric justification backed by standard references.

Cost, availability, sustainability

• Cost is rarely just raw material price. Include processing cost, wastage, tooling, and failure risk.
• Availability includes supply chain reliability and local sourcing (important for sustainability and resilience).
• Sustainability considers extraction impact, processing energy, usable lifetime, and end-of-life pathway.

A plastic may be cheaper than timber per kg, but timber may be cheaper per product if it requires less processing. An imported material may fail availability tests even if it is cheaper.

Aesthetics and sensory properties

Users experience materials through touch, sight, smell, sound, sometimes taste. Consider:

• Colour and finish — matt vs gloss, translucent vs opaque
• Texture — smooth, rough, soft, hard
• Thermal feel — wood and polymers feel warm, metal and stone feel cold (thermal conductivity)
• Acoustic quality — tap a ceramic mug vs a plastic one
• Smell — some polymers outgas noticeably when new

These properties often drive perceived quality more than measured performance.

Material–process compatibility

Not all materials work with all processes:

• Thermosets cannot be injection moulded (they cure irreversibly)
• Ceramics cannot be conventionally machined (too brittle)
• Wood cannot be cast (no liquid phase)
• CFRP cannot be cheaply one-off 3D printed

Material selection and manufacturing process must be chosen together. Choosing a material then discovering no accessible process exists to shape it is a common design failure.

Case studies

iPhone materials — early iPhones used polycarbonate/ABS. Apple moved to aluminium unibody for perceived premium feel, then to stainless steel for certain iPhone Pro models for further weight and aesthetic cues. Each material choice was justified by specific sensory and mechanical properties, with acknowledged trade-offs (steel is heavier, aluminium scratches more easily).

Dyson supersonic hairdryer — housing uses a specific polycarbonate blend selected for heat resistance (hairdryer internal temperatures reach 100°C+), impact resistance, and mouldability for Dyson's distinctive forms. The material choice was dictated by function, not style.

Bamboo vs plastic toothbrush — bamboo handles address sustainability criteria (rapidly renewable, biodegradable) but trade off durability (bamboo degrades when wet) and manufacturing cost (cannot be injection moulded at plastic speeds). Illustrates the genuine complexity of "sustainable" material choices.

3D-printed titanium hip implants — medical implants demand biocompatibility, strength, fatigue resistance, and machinability. Titanium Grade 23 (Ti-6Al-4V ELI) is selected because it meets all four. Very expensive, but justified by life-critical application.

Glossary

• Material selection — the process of choosing a material that best meets a design's requirements.
• Decision matrix — a tool for comparing candidate materials numerically against weighted criteria.
• Weighting — the relative importance assigned to each selection criterion.
• Property-based justification — defending a material choice by citing specific measurable properties.
• Material-process compatibility — the requirement that a chosen material can be shaped by an available process.
• Embodied carbon — the total CO₂ emissions associated with producing a material.
• Sensory properties — the qualities of a material perceived through touch, sight, smell, or sound.

Check your understanding

1. Explain how a decision matrix is used to select a material.

Candidate materials are listed as columns, selection criteria as rows. Each criterion is assigned an importance weighting (1–5). Each material scores against each criterion. Scores are multiplied by weights and summed for each material. The highest total score indicates the best-fitting material. The matrix makes judgements visible and auditable — critics can challenge specific weightings or scores rather than the final choice.

2. Give two examples of material–process incompatibility and explain why they matter.

Thermosetting polymers cannot be injection moulded: once cured, they cannot be re-melted. They must be shaped by compression moulding or casting while still liquid. Ceramics cannot be conventionally machined: their brittleness causes chipping under cutter loads, so they are shaped in a green state and fired. Designers who select a material without checking processability risk discovering mid-project that their design cannot be made.

3. Why is "I chose aluminium because it is strong and light" a weak justification?

It cites no specific grade, no numerical property, no comparison to alternatives, and no connection to the product's requirements. "Aluminium" covers hundreds of alloys with dramatically different properties. A stronger justification names the alloy (6061-T6), cites properties (270 MPa yield, 2.7 g/cm³, weldable), explains the performance requirement the material meets, and shows that competing materials (steel, magnesium, CFRP) were considered and rejected with reasons.

4. A student claims bamboo is always more sustainable than plastic. Critique this claim.

"Sustainable" depends on the full life cycle, not just the feedstock. Bamboo is rapidly renewable and biodegradable at end of life, but its durability can be lower (shorter product life = more replacements), its processing may involve imported labour and transport (carbon footprint), and some bamboo products require adhesives that negate the biodegradability advantage. Plastic products with long service life and effective recycling pathways may have lower lifetime environmental impact. Proper comparison requires a life-cycle assessment (see C2.3), not surface assumptions.

Teacher's notes — additional examples and activities

Properties types — quick distinction

• Physical properties (objective) — colour, density, melting point, conductivity. Don't change the substance.
• Mechanical properties (objective) — response to forces: strength, hardness, elasticity.
• Chemical properties (objective) — interactions with other substances: flammability, corrosion.
• Aesthetic characteristics (subjective) — textures, shapes, colours perceived as pleasing.

Objective properties can be measured and compared; aesthetic characteristics require user judgement.

Material selection as trade-off

Material selection is almost always a compromise. Example: a kitchen chopping board.

• Natural hardwood — tough, sustainable, aesthetically pleasing — but expensive, heavier, needs oiling.
• Plywood / MDF — cheaper, easier to source, easier to work — but less sustainable, water-damages.
• Plastic HDPE — dishwasher-safe, lightweight, cheap — but degrades aesthetically, not recyclable in all regions.

The "best" material depends entirely on user, context, and task.

ACCESS FM applied to material choice

Material influences every ACCESS FM criterion:

• Aesthetics — colour, texture, finish
• Cost — raw and processing
• Customer — user acceptance, safety
• Environment — extraction, processing, disposal
• Size — density affects product weight
• Safety — toxicity, thermal, food contact
• Function — does it perform the required task?
• Materials — availability and supply chain

Aesthetic moodboard exercise

Before specifying materials, students build a moodboard exploring: - Textures - 3D forms - 2D shapes - Colours

Anchors material choice in a coherent visual language.

Product disassembly task

A hands-on investigative task:

1. Disassemble a product (old toaster, remote, torch)
2. Photograph each component as it comes apart, numbered and labelled
3. For each component identify:
4. Material used (specific name if possible — e.g. polypropylene, ABS, 6061 aluminium)
5. Manufacturing process (moulded, cast, CNC-machined, thermoformed)
6. Additive/subtractive/joining technique
7. Properties that justify the choice (mechanical, chemical, physical)
8. Annotate each component (≤10 words) with the reasoning

Builds material and process literacy through direct observation.

Why multiple materials in one product?

Every product combines materials because no single material satisfies all requirements. A saucepan, a kettle, a phone, a chair — each uses several materials chosen for complementary properties. This is the essence of informed material selection.