Choosing the Right Stuff: Selecting Materials and Components
Welcome! When you’re designing a product, one of the most important decisions you’ll make is what to make it out of. Imagine trying to make a frying pan out of cardboard or a pillow out of concrete—it just wouldn't work! In this chapter, we are going to look at the characteristic properties of materials and the other important factors that help designers choose the perfect match for their ideas.
Don't worry if some of these terms seem technical at first. We’ll break them down using things you see every day!
1. Understanding Material Properties
Every material has its own "personality"—scientists and designers call these properties. These properties tell us how a material will behave when we use it.
Key Characteristic Properties
Here are the main properties you need to know for your exam:
Density: How heavy a material is for its size. Example: A small lead fishing weight is much denser than a giant piece of polystyrene foam.
Strength: The ability to withstand a force without breaking or shattering. There are different types, like tensile strength (pulling) and compressive strength (squashing).
Hardness: How well a material resists scratches, dents, or being worn away. Example: A diamond is extremely hard, which is why it can cut through glass.
Durability: How well a material lasts over time, especially when it's outside in the rain or sun. Example: Teak wood is very durable, making it great for garden benches.
Strength-to-Weight Ratio: This is a "superpower" for materials like carbon fiber or titanium. It means they are very strong but also very light.
Stiffness: The ability to resist bending. A stiff material keeps its shape under a load.
Elasticity: The ability to be stretched or squashed and then return to its original shape. Example: A rubber band has high elasticity.
Impact Resistance: How well a material can absorb a sudden shock without breaking. Example: A cricket helmet needs high impact resistance to protect the player.
Plasticity: The ability to be permanently changed in shape without breaking. Example: Modeling clay has high plasticity.
Corrosive Resistance: How well a material resists rusting or being eaten away by chemicals or weather. Example: Stainless steel is used for cutlery because it doesn't rust when it gets wet.
Flammability: How easily a material catches fire. Example: Cotton is very flammable, so it often needs special treatments for use in children's pajamas.
Absorbency: How much liquid or moisture a material can soak up. Example: A paper towel is designed to be highly absorbent.
Thermal and Electrical Conductivity: How easily heat or electricity flows through the material. Copper is a great electrical conductor (used in wires), while Wood is a good thermal insulator (used for pan handles so you don't burn your hand).
Quick Review: The Difference Between Hardness and Strength
Common Mistake: Many students think "hard" and "strong" mean the same thing. They don't!
Example: Glass is very hard (it's hard to scratch), but it is not strong in terms of impact resistance (it shatters easily if dropped).
Key Takeaway: Designers choose materials based on these properties to ensure the product is fit for its purpose. A bike frame needs a high strength-to-weight ratio, while a kitchen sponge needs high absorbency.
2. Physical and Working Properties
Once we know what the material is like, we have to think about how it behaves in the workshop.
How easy is it to work with?
Some materials are "friendly" to makers, while others are difficult. Designers consider:
• Machinability: Can it be easily cut, drilled, or shaped?
• Joinability: Can it be glued, welded, or screwed together easily?
• Forming: Can it be bent or molded into complex shapes?
Fulfilling the Function
The material must be able to do the job it was designed for in its specific context. If you are designing a reusable water bottle, the material must be non-toxic, impact-resistant (if dropped), and lightweight for the user to carry.
Did you know? Some materials are chosen specifically because they are "easy to work with" for mass production, which helps keep the price of the product down!
Key Takeaway: A great design isn't just about the final look; it's about making sure the material can be manufactured efficiently and performs its job perfectly.
3. Other Factors Influencing Selection
Properties aren't the only thing on a designer's mind. There are several "big picture" factors that can make or break a design.
Functionality and Aesthetics
• Required Functionality: Does it actually work? This is the most important question. If a chair collapses when you sit on it, it doesn't matter how pretty it is!
• Aesthetic Attributes: How does it look, feel, smell, or even sound? Example: A high-end smartphone often uses glass and metal to feel "premium" and "cool" to the touch.
Environmental and Ethical Factors
• Environmental Considerations: Is the material sustainable? Can it be recycled? Does it come from a renewable source (like timber) or a finite source (like oil for plastics)?
• Social, Cultural, and Ethical: Was the material sourced fairly? For example, designers might look for Fair Trade textiles or ensure that mining for metals hasn't harmed local communities.
Availability and Cost
• Cost of Stock Forms: Materials come in standard sizes (like sheets, rods, or tubes). Some are much cheaper than others. If the material is too expensive, people won't buy the finished product.
• Availability: Can you get the material easily? If a designer chooses a rare wood that takes six months to ship, the production will be too slow.
Memory Aid: The "C.A.F.E. S.O.D.A." Mnemonic
To remember these wider factors, think of CAFE SODA:
Cost
Aesthetics
Functionality
Environmental impact
Social/Ethical
Origin (where it comes from)
Durability
Availability
Key Takeaway: Choosing a material is a balancing act. You have to find the sweet spot between cost, performance, looks, and impact on the planet.
Summary Checklist for Students
When you are asked to justify why a material was chosen in your exam, ask yourself these three questions:
1. Physical Properties: Is it strong enough? Is it the right weight? (e.g., "The steel was chosen for its high tensile strength.")
2. Working Properties: Can we make it easily? (e.g., "The polymer is suitable for injection molding into complex shapes.")
3. Wider Factors: Is it affordable and eco-friendly? (e.g., "The cardboard is cheap and easy to recycle.")