Materials and their applications

A cosmetics company needs (a) an eye-catching printed retail box that will sit on a shelf and (b) a protective outer carton to ship 24 boxes. Select an appropriate board for each and justify the choice.

1. 01Identify the requirement

   The retail box must print crisply, look premium and be food/skin-safe adjacent; the outer carton must be cheap, stiff and cushion the contents in transit.

2. 02Match a board to (a)

   Folding boxboard or solid white board: smooth, takes high-quality print, folds cleanly and is rigid enough to stand on a shelf - the printability and finish justify the higher cost for a retail-facing box.

3. 03Match a board to (b)

   Double-wall corrugated card: its fluted core gives a high stiffness-to-weight ratio and impact cushioning at low cost and it recycles easily - print quality is irrelevant for an outer carton, so cost and protection dominate.

Result: Folding boxboard for the printed retail box (printability and finish) and corrugated card for the outer carton (cushioning stiffness at low cost) - each board chosen by matching its properties to that item's requirement.

A company mass-produces a flat-pack wardrobe sold at a budget price with a light oak-effect finish. Evaluate whether MDF or chipboard is the better carcass material.

1. 01Set the requirements

   Large stable panels, low cost, a smooth surface to take a wood-effect melamine foil or veneer, machinable for knock-down fittings, and adequate strength for shelving - moisture exposure is low (indoor bedroom).

2. 02Compare the two boards

   Chipboard is cheapest and stiff enough in compression but crumbles at screw holes and swells if it gets damp; MDF is a little dearer but has a dense, smooth, uniform surface that machines cleanly, holds knock-down fittings better and takes a foil or paint finish superbly.

3. 03Evaluate and decide

   For a budget wardrobe MDF gives the better finish and fixing reliability at an acceptable cost; melamine-faced chipboard is the even cheaper alternative if margins are tighter. Whichever is chosen, the designer must manage the disadvantage - MDF's weight and hazardous dust, or chipboard's poor moisture resistance and weak screw holding.

Result: MDF is the better carcass for a budget wood-effect wardrobe because its smooth, uniform surface takes the finish and holds fittings well at low cost; the designer must control its dust and weight (or accept melamine-faced chipboard as the cheaper option).

A designer must choose a metal for the frame of a lightweight folding camping chair that will be carried outdoors and must resist corrosion. Recommend a metal and justify it against the requirements.

1. 01Identify the decisive properties

   Light (it is carried), corrosion-resistant (used outdoors, gets wet), strong and stiff enough to take a person's weight, and cheaply formed into tube - cost matters for a consumer product.

2. 02Consider the options

   Mild steel is strong and cheap but heavy and rusts unless coated; aluminium alloy is light, naturally corrosion-resistant and easily extruded into tube; stainless steel resists corrosion but is heavy and dear.

3. 03Recommend and justify

   Aluminium alloy tube best meets the brief: its low density keeps the chair light to carry, its natural oxide layer resists corrosion outdoors without a finish, and it extrudes readily into the tube sections needed - at an acceptable cost for a mid-market product.

Result: Aluminium alloy tube is recommended: light for carrying, corrosion-resistant outdoors and easily extruded, it matches the folding chair's key requirements better than heavier steel or costlier stainless steel.

A manufacturer must specify polymers for (a) a transparent, weatherproof shop sign and (b) the moulded body of a mains electrical plug. Recommend a suitable polymer for each and justify the choice using the thermoplastic/thermoset distinction.

1. 01Analyse (a) the sign

   Needs transparency, weather resistance and rigidity, and could be made in volume and recycled - a thermoplastic. Acrylic (PMMA) is rigid, clear and weather-resistant and can be laser-cut, line-bent and eventually recycled.

2. 02Analyse (b) the plug body

   Must resist heat, never soften in a fault, insulate electrically and be dimensionally stable - a thermoset. Urea-formaldehyde is hard, heat-resistant and a good electrical insulator that will not soften if the plug warms.

3. 03Justify with the distinction

   The reheatability of the thermoplastic acrylic suits a mouldable, recyclable, transparent sign; the permanent cross-linked network of the thermoset urea-formaldehyde suits a safety-critical electrical part that must not soften with heat.

Result: Acrylic (a thermoplastic) for the sign - clear, weatherproof and recyclable; urea-formaldehyde (a thermoset) for the plug body - hard, heat-resistant and insulating - each justified by its response to heat.

Evaluate the use of carbon-fibre reinforced polymer (CFRP) instead of aluminium alloy for a high-performance racing bicycle frame.

1. 01State the advantage

   CFRP has an outstanding stiffness- and strength-to-weight ratio, so the frame can be lighter and stiffer than aluminium for the same strength; fibre direction can be laid up to match the loads, and complex aerodynamic shapes can be moulded seamlessly.

2. 02State the disadvantages

   CFRP costs far more in material and skilled labour (often hand lay-up and autoclave curing), can fail suddenly rather than bending, is hard to repair, and the thermoset matrix makes end-of-life recycling difficult.

3. 03Reach a justified recommendation

   For a professional racing team where performance is decisive and cost is secondary, CFRP is justified by its weight and stiffness advantage; for a budget consumer bike, aluminium alloy is the better trade-off because it is far cheaper, tougher in a crash and more recyclable.

Result: CFRP is recommended for the professional racing frame - its stiffness-to-weight advantage outweighs cost when performance dominates - but aluminium alloy is better for a consumer bike, showing the choice depends on the priority given to performance versus cost and recyclability.

A weight-critical bracket could be made from a mild steel of tensile strength 400 MPa and density 7.85 g/cm^3, or an aluminium alloy of tensile strength 300 MPa and density 2.70 g/cm^3. Determine which has the better strength-to-weight ratio and comment on the choice.

1. 01Recall the measure

   Strength-to-weight is the specific strength = tensile strength / density. A higher value means more strength per unit of density.

2. 02Calculate for steel

   Specific strength = 400 / 7.85 = 50.96, so about 51 (MPa per g/cm^3).

3. 03Calculate for aluminium

   Specific strength = 300 / 2.70 = 111.1, so about 111 (MPa per g/cm^3).

4. 04Interpret

   Although the steel is stronger in absolute terms (400 versus 300 MPa), the aluminium's specific strength (111) is more than double the steel's (51), so for the same strength the aluminium bracket can be much lighter - which is why weight-critical products favour it. A tensile test to failure on samples of each would confirm the strengths used.

Result: Aluminium alloy has the far better strength-to-weight ratio (about 111 versus 51 MPa per g/cm^3), so despite the steel's higher absolute strength the aluminium is preferable for a weight-critical bracket - confirmed by a tensile test.