Unit 2 Option C: Product Design — Joining Materials, Smart Materials and Composites
Welcome to your study notes for Unit 2 Option C: Product Design! Whether you are designing a product from scratch or analyzing an existing one, choosing the right method to join parts together and selecting the most effective modern and smart materials are vital skills. Don't worry if some of the terminology looks daunting at first — we will break down every concept into clear, simple steps.
---Part 1: Joining Materials
When creating any product, designers must decide whether parts need to be joined permanently (cannot be taken apart without damaging the product) or temporarily (can be disassembled for maintenance, repair, or flat-pack transport).
1. Joining Wood
Wood can be joined using traditional woodworking joints, chemical adhesives, or mechanical fixings.
Common Wood Joints:
• Butt Joint: The simplest and weakest joint; two flat edges are glued together.
• Lap Joint: A step cut into each piece so they overlap, providing more gluing area than a butt joint.
• Housing Joint: A groove cut across one piece of wood to hold the end of another (commonly used for shelving).
• Dowel Joint: Wooden pegs (dowels) are inserted into drilled holes to add strength and alignment.
• Mortise and Tenon Joint: A classic, very strong joint where a peg (tenon) on one piece fits snugly into a rectangular hole (mortise) in the other (used in table and chair frames).
• Dovetail Joint: Interlocking trapezoidal "pins" and "tails" that resist being pulled apart, offering great strength and a decorative look (often used in quality drawer construction).
Wood Adhesives:
• PVA (Polyvinyl Acetate): Standard water-based glue used for general wood-to-wood bonding indoors.
• Cascamite: A waterproof powder resin adhesive mixed with water; ideal for outdoor and marine applications.
• Contact Adhesive: Applied to both surfaces and allowed to become tacky before pressing together; useful for bonding large sheets or laminates to wood.
Mechanical Fixings for Wood:
• Screws: Countersunk screws sit flush with or below the surface; round head screws sit proudly on top of the surface.
• Nails: Oval nails (which reduce the risk of wood splitting along the grain) and panel pins (small, thin nails for delicate work or pinning joints while glue dries).
2. Joining Metals
Metals require high-strength joining methods depending on their function and whether the joint must withstand heat or mechanical forces.
Permanent Metal Joints:
• Welding (MIG/TIG): Melts the base metal edges together along with a filler rod to create an extremely strong joint.
• Brazing: Uses a gas torch on a brazing hearth to heat the metal to a bright red heat, using a brass filler rod called spelter and flux to join the parts.
• Soldering: Uses soft solder (a low melting point alloy) to join thin metal components, commonly found in electrical/electronic circuits and plumbing.
• Riveting: Metal pins passed through aligned holes and deformed at the ends. Includes snap head rivets (formed with a hammer and snap) and pop rivets (installed from one side using a rivet gun, great for hollow sections).
Temporary Metal Fixings:
• Nuts, Bolts, and Machine Screws: Threaded fasteners that allow components to be bolted securely together and unscrewed easily for maintenance or replacement.
3. Joining Polymers (Plastics)
Polymers can be joined chemically or mechanically:
• Solvent Cement (Tensol): Used primarily for acrylic. It does not act like a sticky glue; instead, it chemically dissolves (melts) the plastic surfaces so they fuse together as the solvent evaporates.
• Self-Tapping Screws: Screws with sharp, hardened threads that cut their own mating thread into plastic holes as they are driven in.
Top Exam Tip: Matching Joints and Adhesives
Always justify your choice of joining method! Never suggest PVA for bonding plastic or metal. Also consider the scale of production: hand-crafted joints (like dovetails) suit bespoke furniture, whereas mechanical fixings or simple knock-down fittings are preferred for mass-produced flat-pack furniture.
Key Takeaway: Choose permanent joints (welding, brazing, gluing) when parts must never move, and temporary fixings (screws, nuts and bolts) when products need servicing, assembly at home, or recycling.
---Part 2: Smart Materials
A smart material is defined as a material that has one or more properties that can be significantly changed in a controlled fashion by an external stimulus (such as temperature, light, pressure, or electricity).
Important Pitfall: Do not confuse "smart" with "modern" or "high-performance". A material is only smart if it actively responds to an environmental change!
Key Smart Materials You Must Know:
• Shape Memory Alloys (SMA) (e.g., Nitinol):
Stimulus: Heat.
How it works: Can be deformed when cool, but returns to its pre-programmed "remembered" shape when heated above a certain temperature.
Applications: Dental braces (which gently pull teeth into position as mouth heat acts on them) and surgical stents (which expand inside blocked blood vessels).
• Thermochromic Pigments:
Stimulus: Temperature.
How it works: Changes color as the temperature rises or falls.
Applications: Baby feeding spoons (warning if food is too hot), mood rings, and battery power indicators.
• Photochromic Pigments:
Stimulus: Light intensity (specifically Ultraviolet / UV light).
How it works: Darkens or changes color when exposed to bright UV light, and turns clear again indoors.
Applications: Reactive lenses in sunglasses and transition eyeglasses.
• Quantum Tunnelling Composite (QTC):
Stimulus: Pressure / Compression.
How it works: A flexible polymer containing tiny metal particles. In its resting state, it is an electrical insulator. When squeezed or compressed, it becomes an electrical conductor.
Applications: Touch-sensitive membrane switches, wearable controls in smart clothing, and mobile phone buttons.
• Piezoelectric Materials:
Stimulus: Mechanical stress (squeezing) OR electric current.
How it works: Generates an electric voltage/spark when mechanically stressed or squeezed; conversely, it changes shape or vibrates when an electric current is passed through it.
Applications: Spark igniters in gas lighters and BBQ starters, and audio sounders in musical greeting cards.
Quick Memory Aid:
• Thermo- = Heat / Temperature
• Photo- = Light
• Piezo- = Pressure to Electricity
• QTC = Squeeze to Conduct
Key Takeaway: Smart materials react dynamically to their surroundings, allowing products to sense changes and respond automatically without complex electronic sensors.
---Part 3: Composite Materials
A composite material is formed when two or more distinct materials are combined to produce a new material with improved physical and mechanical properties. The separate materials do not dissolve into each other; they work together (usually as a reinforcing fiber embedded in a matrix/resin binder).
Key Composites You Must Know:
• Glass Reinforced Plastic (GRP):
Composition: Fine glass fibers embedded in a polyester resin matrix.
Key Properties: High strength-to-weight ratio, waterproof, corrosion-resistant, and relatively easy to mould into complex curves.
Applications: Boat hulls, kayak shells, and lightweight car body panels.
• Carbon Fiber Reinforced Plastic (CFRP):
Composition: Strands of carbon fibers held together by a thermosetting resin matrix.
Key Properties: Extremely rigid, exceptionally high tensile strength, and ultra-lightweight, but expensive to manufacture.
Applications: Aerospace components, Formula 1 racing cars, and high-end competition bicycles.
• Medium Density Fibreboard (MDF):
Composition: Wood fibers bonded together under heat and pressure using synthetic resin glue.
Key Properties: Uniform strength in all directions (no grain direction), smooth surface finish, and easy to machine without splitting.
Applications: Flat-pack furniture, interior paneling, and architectural moldings.
Top Exam Tip: Avoid the "Strong" Trap!
Examiners frequently penalize students for simply writing that a composite is "strong". Always be specific! Use precise technical terms such as:
• High strength-to-weight ratio (strong, but very light)
• Rigid / High stiffness (does not bend under load)
• Corrosion-resistant (does not rust or rot when wet)
Key Takeaway: Composites combine the best features of fibers (high tensile strength) and resins (shape, protection, and compression strength) to make materials that outperform traditional single materials.
---Quick Review Summary
• Wood Joining: Dowels, mortise & tenon, dovetails; PVA for general woodwork, Cascamite for outdoors.
• Metal Joining: Welding, brazing, soldering (permanent); nuts & bolts (temporary).
• Plastic Joining: Solvent cement (Tensol) fuses acrylic chemically.
• Smart Materials: Must change properties with an external stimulus (SMA = Nitinol, QTC = pressure-sensitive conductor, Thermochromic = heat, Photochromic = light, Piezoelectric = squeeze to spark).
• Composites: Combined materials with superior properties (GRP, CFRP, and MDF).