22-Mec-B5 Product Design and Development · May 2015

Question 3 of 7: Life cycle of a toaster

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams, May 2015 — 07-Mec-B5 Product Design and Development. Three hours. Open book; no calculator is permitted. Question 1 must be completed and is worth 40 marks; four of the six remaining questions are chosen, each worth 15 marks, for 100 marks in total. Only the first five questions as they appear in the answer book are marked. The paper states that most questions require an answer in essay format or the use of tables, figures and charts, and that clarity and organisation of the answer are important.

The paper prints 40 + 6 × 15 = 130 marks and a candidate attempts 40 + 4 × 15 = 100 of them. All seven questions are answered below, because this set is a study resource rather than an examination script. The marking scheme printed on the last source page splits Question 1 as 6 / 9 / 9 / 6 / 4 / 6 and gives the part weights for each 15-mark question, and the answers here are proportioned to that split. Because no calculator is allowed, every calculation is arranged so that it can be carried out on paper in one or two lines.

Reference texts for this subject

K. T. Ulrich and S. D. Eppinger, Product Design and Development — the framework text for this exam code: identifying customer needs, target specifications, concept generation, concept selection by Pugh screening and weighted scoring, design for manufacturing, and product development economics.
G. E. Dieter and L. C. Schmidt, Engineering Design — problem definition, decision matrices, design for the environment and life-cycle assessment, standards and codes, and the four components of a complete design.
G. Pahl and W. Beitz, Engineering Design: A Systematic Approach — the systematic conceptual / embodiment / detail sequence and function structures.
G. Boothroyd, P. Dewhurst and W. Knight, Product Design for Manufacture and Assembly — part-count reduction, handling and insertion time, and the design-for-assembly index used in Question 6.
M. F. Ashby, Materials Selection in Mechanical Design — translate, screen, rank and seek supporting information; the property charts and material indices used in Question 7.
S. Kalpakjian and S. R. Schmid, Manufacturing Engineering and Technology — process selection, process capability, tooling cost and economic batch size.
I. Gibson, D. Rosen and B. Stucker, Additive Manufacturing Technologies — process families and design for additive manufacturing, for Question 2.
W. Preiser and K. Smith (eds.), Universal Design Handbook, and the seven Principles of Universal Design (Centre for Universal Design, North Carolina State University, 1997) — the framework Question 1 asks for.
Canadian context: CSA B125.3, Plumbing fittings; CSA B651, Accessible design for the built environment; the National Plumbing Code of Canada; the Accessible Canada Act framework; CSA Group, Standards Council of Canada and Health Canada under the Canada Consumer Product Safety Act.

Check: the exam gives no data of its own — every question asks the candidate to bring a product, a set of numbers and a method. All quantities used below (operating torques, embodied energies, machine rates, process sigmas, material properties) are stated explicitly as design assumptions drawn from the reference texts and from Canadian standards, and each answer is written so that the method stands whatever numbers a marker would prefer.

Question 3: Life cycle of a toaster (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A two-slice pop-up toaster of 1.40 kg, drawing 900 W and used for four minutes a day over a seven-year life, with the bill of materials and embodied-energy figures below (embodied energies are typical values from Ashby’s tables).

Given data — toaster bill of materials and duty
Item	Mass (kg)	Embodied energy (MJ/kg)	Burden (MJ)
Mild steel body and rack	0.60	30	18.0
ABS end caps and lever	0.40	95	38.0
Aluminium reflector	0.10	200	20.0
Copper cordset and nichrome element	0.15	70	10.5
Mica, glass and fixings	0.15	20	3.0
Total	1.40		89.5
Rated power / daily use / service life	900 W / 240 s per day / 7 years

Find. The five life-cycle stages, the design challenge at each, one environmental intervention at each, and the screening numbers that say which of those interventions is worth the most.

Approach. Run a screening life-cycle assessment first — embodied burden from the bill of materials against use-phase energy — so that the qualitative discussion of the five stages is ordered by where the impact actually is rather than by where it is easiest to talk about.

The screening assessment that orders everything else

The use-phase energy follows from the rated power and the daily duty:

$$E_{\text{use}} = P\,t\,d\,y = (900)(240)(365)(7) = 552\ \text{MJ}$$

taking it in stages so it can be done without a calculator: 900 W for 240 s is 0.216 MJ a day, 78.8 MJ a year, and 551.9 MJ over seven years. Against the 89.5 MJ of embodied burden in the table, the total is

$$E_{\text{total}} = 89.5 + 551.9 = 641.4\ \text{MJ}, \qquad \frac{E_{\text{use}}}{E_{\text{total}}} = \boxed{86\ \%}$$

and on the functional unit that matters — one day’s toasting, of which there are 2 555 in the life — the product costs 0.251 MJ per use. The sensitivity that follows is the whole design lesson. Doubling the service life from 7 to 14 years halves the embodied burden per use but leaves the use phase untouched, improving the per-use figure only from 0.251 to 0.234 MJ, about 7 %. A 15 % faster toasting cycle — a better reflector, a higher watt-density element, a properly closed cavity — cuts the total by 13 % on its own. For this product, in other words, efficiency in use beats durability and beats lightweighting, and that conclusion should drive the answers to parts B and C rather than a general preference for recyclable materials.

Figure 3.1 — The five life-cycle stages of the toaster, with the share of the 641 MJ life-cycle energy attributable to each. The use phase carries 86 % of it; materials, manufacture, distribution and end of life share the remainder.

Parts A, B and C — the five stages, their challenges, and one intervention each

Stage 1 — Raw material extraction and processing. Ore is mined and reduced, bauxite is smelted, crude oil is cracked and polymerised, and the result is 1.40 kg of stock carrying 89.5 MJ before anything has been made. Challenge: the highest-burden materials are not the heaviest ones — the 0.10 kg aluminium reflector carries 20 MJ, more than the 0.60 kg steel body’s 18 MJ, because aluminium costs 200 MJ/kg to win from ore. Designers routinely optimise mass and miss this. Good design: select on burden per unit of function rather than on mass, and prefer materials whose recycled fraction is high and whose recycled route is genuinely cheaper in energy. Environmental intervention: specify secondary (recycled) aluminium for the reflector, which is produced at roughly a twentieth of the primary energy, cutting that 20 MJ to about 1–2 MJ — the largest single materials-side saving available in this product.

Stage 2 — Manufacture and assembly. Sheet is blanked and formed, ABS is injection moulded, the element is wound, and the parts are assembled and electrically tested. Challenge: process scrap and rework, and the fact that a design fixed for appearance often forces expensive, high-scrap processes; assembly of many small fasteners drives both cost and defect rate. Good design: design for manufacture and assembly — fewer parts, self-locating features, no fasteners where a snap or a formed tab will do, and tolerances matched to process capability, as Question 6 develops. Environmental intervention: mono-material sub-assemblies — keep the moulded parts to a single polymer family so that runners, sprues and rejects can be reground into the same product rather than downcycled.

Stage 3 — Distribution and retail. Packaging, palletisation, ocean and road freight, warehousing and the retail display. Challenge: a toaster is mostly air, so distribution burden is set by packed volume rather than by mass, and it is decided by the packaging designer months after the product is frozen. Good design: design the carton and the product together, so that the packed cube is minimised and the pack is stable enough to survive transit without expanded-polystyrene inserts. Environmental intervention: replace moulded polystyrene with die-cut moulded-pulp or corrugated inserts, which are recyclable in every Canadian municipal stream, and size the carton so that pallet fill exceeds 90 %.

Stage 4 — Use. Seven years of daily toasting, 552 MJ, 86 % of the total. Challenge: almost all of the product’s impact sits in a phase the manufacturer does not control and the buyer cannot see, because a toaster carries no efficiency label. Heat leaves through open slots, through an uninsulated body and into a cold cavity at the start of every cycle. Good design: attack the cycle energy directly — a polished or coated reflective cavity, higher watt-density elements that reach temperature faster, insulated outer panels, and controls that measure browning by moisture release rather than by a fixed timer, so that the second slice does not run the full first-slice time. Environmental intervention: a reliable auto-shutoff and a genuinely zero-draw off state, together with the faster cycle above — a 15 % cycle reduction saves 83 MJ over the life, which is more than the entire embodied burden of the product.

Stage 5 — End of life. Retirement, collection, and either recovery or landfill. Challenge: small appliances are the classic case of a product that is technically recyclable and practically discarded: 0.85 kg of the 1.40 kg — 61 % — is steel, aluminium and copper with real scrap value, but it is riveted and bonded to ABS and mica, and consumers have no convenient route. Good design: design for disassembly — a small number of accessible fasteners of a single drive type, no adhesive between dissimilar materials, moulded resin-identification marks, and a metals fraction that separates in one operation. Environmental intervention: make the product accepted by a provincial extended-producer-responsibility programme for electrical and electronic equipment, and design the joint scheme so that the copper cordset and the steel body separate in under two minutes, which is what determines whether a processor recovers them or shreds the whole unit.

Question 3 — results
Quantity	Value
Embodied burden of the bill of materials	89.5 MJ
Use-phase energy over 7 years	551.9 MJ
Life-cycle total	641.4 MJ
Use-phase share	86 %
Energy per use (2 555 uses)	0.251 MJ
Effect of doubling service life to 14 years	0.234 MJ per use, a 7 % improvement
Effect of a 15 % faster toasting cycle	558.6 MJ total, a 13 % improvement
Recyclable metal fraction	0.85 kg of 1.40 kg, 61 %

Topic. Life-cycle thinking and screening life-cycle assessment as a design-priority tool.

Key relations. Embodied burden E_m = Σ m_i H_i; use-phase energy E_use = P t d y; impact per functional unit E_total/(number of uses); the sensitivity test of doubling life against improving efficiency.

Why this works. A screening assessment does not need a database or software; it needs a bill of materials, a table of embodied energies, and an honest duty cycle. Its value is not the absolute number, which is uncertain by tens of percent, but the ratio between phases, which is robust: when one phase carries 86 % of the burden, no plausible error in the material figures changes which phase deserves the design effort. Products divide cleanly into two families on this test. Energy-using products — toasters, kettles, motors, vehicles — are use-phase dominated, and their design lever is efficiency. Passive products — packaging, furniture, structures — are materials-dominated, and their lever is material choice, mass and life extension. Applying the second family’s remedies to the first is the most common error in design-for-environment, and the arithmetic above is what exposes it.

Common pitfall. Reaching for recyclability and lightweighting because they are visible, when the assessment says efficiency. Recycling this toaster perfectly recovers a fraction of 89.5 MJ; making it 15 % faster saves 83 MJ. Both are worth doing, but only one is worth doing first. A second pitfall is quoting life-cycle results without a functional unit — “641 MJ” means nothing until it is 641 MJ per seven years of daily toast.

Source. Dieter & Schmidt, Engineering Design, ch. 9 (Design for the Environment / life-cycle assessment); Ashby, Materials Selection in Mechanical Design, ch. 20 (Materials and the environment) and the embodied-energy data tables; ISO 14040/14044 for the LCA framework.

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