Design and Technology: Product Design (9252) | Selection of materials or components

Hello Future Product Designers! Mastering Material Selection

Welcome to one of the most exciting and crucial chapters in Product Design: the Selection of materials or components. Think of yourself as a chef selecting ingredients. A five-star meal requires the right mix of taste, texture, and quality—just as a successful product requires the perfect materials!

In this chapter, we will learn the deep technical reasons why designers choose specific materials (like plastic over metal, or wood over composite). Getting this choice right ensures your product is functional, safe, economical, and loved by the user.

Section 1: Fitness for Purpose – The Properties Checklist

The first rule of selection is fitness for purpose. Does the material do the job it needs to do? To answer this, we must look closely at its technical properties.

1.1 Aesthetic and Sensory Factors

The material choice heavily influences how the user perceives the product. These factors appeal to our five senses:

• Aesthetics (Sight): This is about visual appeal—colour, texture (surface finish), sheen (glossy or matte), and transparency. Example: A designer might choose polished chrome for a luxury item, but matte black plastic for a tool handle.
• Tactile (Touch): How does the material feel? Is it warm, cold, smooth, rough, or soft? Example: Choosing rubber grips for a bicycle handle because it feels soft and offers good grip, even though it's less durable than metal.
• Aural (Sound): Some products need to be quiet (like a silent keyboard), while others need to make a specific sound (like a warning bell). Material density affects sound transmission.

Key Takeaway: Aesthetics and sensory properties are not just "looks"; they are part of the product's functionality and user experience.

1.2 Mechanical Properties – Dealing with Stress

Mechanical properties describe how a material reacts to external forces, like being pushed, pulled, bent, or hit. These are critical for safety and durability.

Key Mechanical Terms Explained:

• Strength: The ability to withstand force without breaking or permanently bending. We often look at tensile strength (resisting being pulled apart, like a rope) and compressive strength (resisting being squashed, like a concrete pillar).
• Hardness: The ability to resist scratching, abrasion, and indentation. Example: Diamond is very hard; it resists scratching. A wooden desk is softer.
• Toughness: The ability to absorb energy (shocks or impacts) without fracturing. Tough materials deform before they break. Example: The plastic used in a helmet or a phone case is tough because it absorbs the shock of a drop.
• Elasticity: The ability to return to the original shape after the force causing the change is removed. Example: A rubber band or a spring.
• Ductility: The ability to be stretched and drawn into a wire shape without fracturing. This is a property of many metals (like copper wire).
• Malleability: The ability to be hammered, pressed, or rolled into thin sheets without cracking. Example: Aluminium foil.

Quick Review Trick: Think of a car crash.
- The frame needs strength (not to snap).
- The paint needs hardness (not to scratch easily).
- The safety cage needs toughness (to absorb the massive impact).

1.3 Physical and Chemical Properties

These properties relate to how the material interacts with energy, heat, and the environment.

• Density: How heavy a material is relative to its size (mass per unit volume).

  Struggling Student Tip: Density is not the same as weight! A large piece of foam weighs less than a tiny pebble, but the pebble is much denser. Designers choose low-density materials (like aluminium) for products that need to be light (aeroplanes, racing bikes).

• Thermal Conductivity: How well a material allows heat to pass through it.
  • Conductors (metals) are good for cookware and radiators.
  • Insulators (plastics, wood) are good for handles and housing (like a coffee cup sleeve).
• Electrical Conductivity: How well a material allows electricity to pass through it.
  • Conductors (copper, gold) are essential for wiring.
  • Insulators (rubber, ceramic) are essential for safety casings.
• Corrosion/Chemical Resistance: The ability to resist breaking down due to moisture, oxygen, or chemicals (often called rusting or decay). Example: Outdoor furniture needs excellent corrosion resistance (e.g., stainless steel or chemically treated wood).

Section 2: Manufacturing and Production Factors

It doesn't matter how perfect a material is if you can't actually shape it or buy it easily. Designers must consider the manufacturing process when selecting materials.

2.1 Availability and Stock Forms

Manufacturers rarely start with raw materials. They purchase materials in standardised stock forms, which affects cost and efficiency.

• Stock Forms: Materials (metals, polymers, timber) are sold in common, standardized shapes like sheets, plates, bars (square, round, hexagonal), tubes, granules (for moulding), or powder.
• Standard Sizes: Stock is usually sold in common dimensions (e.g., 8x4 foot plywood sheets, or specific diameters of metal rod).
• Why this matters: Using a standard stock size minimises waste, reduces time spent cutting, and lowers costs compared to ordering a non-standard or bespoke size. Availability means you can keep production moving.

2.2 Compatibility with Manufacturing Processes

Different materials require different methods of forming and joining.

• Forming: Can the material be easily cast, moulded, pressed, or machined? Example: Thermoplastics are chosen because they can be easily melted and injected into complex moulds (injection moulding).
• Joining: How will the parts be held together?
  • Metals can be welded or brazed.
  • Timber can be glued, jointed, or screwed.
  • Polymers usually require solvent welding or specific adhesives.
  The choice of material dictates the joining method, which impacts production time and cost.
• Finishing: How easily can the material be finished (painted, plated, polished)? Some plastics are notoriously difficult to paint without special surface treatment.

2.3 Standard vs. Bespoke Components

When selecting ready-made parts to include in a product, designers have two choices:

• Standard Components: These are ready-made, mass-produced parts (like screws, nuts, bolts, hinges, electrical switches, wheels) that conform to international standards (e.g., ISO, British Standards).

  Advantages: They are extremely cheap, easy to replace, readily available globally, and reliable because they are tested.

• Bespoke Components: These are custom-designed parts made specifically for a single product line. Example: A unique hinge mechanism for a very specific folding chair.

  Advantages: They offer unique functionality, aesthetic differentiation, and perfect fit. Disadvantages: They are much more expensive to design, tool, and manufacture, and difficult for the consumer to replace if broken.

Key Takeaway: Use standard components wherever possible to save time and money, and only use bespoke components when absolutely necessary for performance or aesthetic reasons.

Section 3: Economic and Environmental Responsibility

A great product must not only perform well but also be made responsibly—both financially and ecologically.

3.1 Cost and Economics

Cost is usually the biggest factor when deciding between two technically suitable materials.

• Raw Material Cost: Price changes based on availability and market demand (e.g., gold is expensive; steel is relatively cheap).
• Processing Cost: How much energy and time is required to shape the material? Materials that require high heat or complex machining will increase the unit cost.
• Tooling Cost: If a material requires expensive, complex moulds (tooling) for manufacturing (common with plastics or die-casting metals), this initial cost must be considered.
• Maintenance and Lifespan Cost: Choosing a cheaper material that rusts quickly might seem good initially, but the long-term cost of maintenance and replacement increases the product's overall cost to the consumer.

3.2 Environmental Considerations (Sustainability)

Designers have a responsibility to minimise the product's negative impact on the planet, often measured through a Life Cycle Assessment (LCA).

Sustainable Selection Principles:

• The 3 R's:
  • Reduce: Can we use less material in the product? (Use thinner walls, clever structural design).
  • Reuse: Can the product or its components be reused for another purpose? (Designing products that can be easily disassembled).
  • Recycle: Is the material easily recyclable? Choosing a material with high recycling rates (like aluminium or certain PET plastics).
• Source: Where does the material come from? Selecting materials that are harvested ethically, rapidly renewable (like bamboo), or come from recycled sources.
• Embodied Energy: This is the total energy required to produce a material, from mining and manufacturing to transporting it. Designers often choose materials with lower embodied energy to reduce the carbon footprint. Example: Aluminium has a very high embodied energy compared to wood.
• Disposal: What happens at the end of the product's life? Selecting materials that are non-toxic and biodegradable, or easily separated for safe recycling.

Did You Know? Choosing only one type of plastic (instead of a mix of three) for a complex product vastly increases its recyclability because sorting becomes easier. This is a crucial sustainability decision made during material selection!

Final Key Takeaway: Material selection is always a compromise. You rarely find a material that is cheap, sustainable, incredibly strong, and beautiful. Successful design is about finding the optimum balance between technical properties, cost, and environmental impact.

Quick Review: The Essential Material Selection Checklist (P.M.A.C.E)

When choosing a material, always ask:

• Properties: Does it meet the functional (Mechanical, Physical, Sensory) requirements?
• Manufacture: Can it be easily and cost-effectively processed using available tools/methods?
• Availability: Is it easily sourced in the required stock forms and quantities?
• Cost: Is the material and its processing economically viable?
• Environment: What is its impact (Embodied Energy, Recyclability, Disposal)?