Reusable bottle for a school hiking club: when the brief, not the material, is the problem
A school hiking club needs a 600 mL reusable bottle: 800 units, unit cost ≤ HK$45 including cap and logo printing. The club's stated requirements are: survive a 1.5 m drop onto concrete when full; be dishwasher-safe at 70 °C; accept freshly boiled water (about 95 °C) so members can make tea on the trail; weigh ≤ 180 g; show the water level; and not retain odours from sports drinks. Three candidates are on the table: PP copolymer (1.2 mm wall, about HK$9 moulded, translucent, heat deflection temperature around 100 °C but softens and creeps above about 90 °C, known to hold odours); Tritan copolyester (clear, dishwasher-safe, supplier states maximum continuous service temperature 90 °C, about HK$18 moulded); and single-wall 304 stainless steel (inert, handles boiling water, but 260 g and about HK$52). Prototype results: the PP bottle passes the 1.5 m drop at 5 °C, distorts after ten fills at 95 °C, and its neck thread strips after about 25 open/close cycles.
1. Recommend a material and bottle specification. Show how you weigh the requirements, including any requirement you would renegotiate.
Read the worked answer
Build a requirement–property matrix before choosing:
- 95 °C hot fill: Tritan is eliminated as supplied (90 °C maximum continuous service); PP copolymer survives brief contact but creeps and distorts above about 90 °C; stainless steel passes.
- Weight ≤ 180 g at 600 mL: PP at 1.2 mm is roughly 45–55 g, Tritan roughly 70–80 g, single-wall 304 stainless roughly 260 g. Stainless is eliminated.
- Cost ≤ HK$45: all three pass at the quoted prices, stainless only just.
The requirement set is internally contradictory: the hot-fill requirement eliminates the two light materials, and the weight requirement eliminates the only material that meets hot fill. That is the judgement point - the brief, not the material, is wrong. Choosing a material here without naming the conflict would score poorly.
Resolution: separate the functions. Specify a 600 mL ambient-water bottle in Tritan (clear so the level is visible, odour-resistant, dishwasher-safe, about 75 g, within budget) and add a 350 mL vacuum flask as an optional club accessory for hot drinks. Alternatively, if one vessel for boiling water is non-negotiable, state explicitly that the weight limit must rise to about 300 g and the budget to about HK$55 for single-wall stainless steel. The examinable reasoning is: rank requirements as must/want, test them for compatibility, and make the trade-off visible rather than silently dropping a requirement.
2. The PP prototype's neck thread strips after about 25 open/close cycles. Diagnose the failure and propose design changes, giving the reasoning behind each.
Read the worked answer
Stripping after 25 cycles is progressive creep–fatigue, not a single overload. Each closure applies hoop stress at the neck; PP has a low modulus and creeps under sustained stress, so the thread flank yields a little each time until the thread jumps under normal closing torque.
Likely contributors, with the diagnostic evidence:
- Short engagement. If the thread is 3 mm pitch with two turns engaged, engagement is only about 6 mm, on a wall of 1.2 mm. The neck ovalises and the flank contact area is small.
- Sharp roots. A moulded thread with a small root radius concentrates stress; PP is notch-sensitive under repeated loading.
- No positive stop. Without a shoulder, users tighten until the thread yields, so the applied torque is uncontrolled.
- Material. PP copolymer at 1.2 mm is thin for a threaded neck.
Design changes, cheapest first:
- Increase engagement to at least four full turns (about 12 mm) and use a coarse rounded thread with a 0.5 mm root radius.
- Add a hard shoulder stop so the sealing torque is taken by the shoulder, not the thread.
- Thicken the neck to 2.0 mm or add an external reinforcing bead.
- Replace the screw cap with a quarter-turn bayonet or a flip-top: a bayonet removes repeated screwing entirely and tolerates moulded dimensional variation.
Verification: 500 open/close cycles at 40 °C with 1.5 N·m torque, plus a 0.3 bar leak test for one minute.
3. The PP bottle passes the 1.5 m drop at 5 °C but distorts after ten fills at 95 °C. Explain what this tells you about the test plan, and state a valid hot-fill test.
Read the worked answer
The two results are not contradictory; they are measuring different things, and the test plan has confused them.
At 5 °C, PP is still well above its glass transition (about −20 °C), so it remains tough and absorbs the drop by shear yielding rather than brittle cracking. A pass at 5 °C says almost nothing about behaviour at 90–95 °C. Near its heat deflection temperature, PP's modulus falls by roughly an order of magnitude, so the base and neck creep under the hydrostatic and handling loads. Ten fills are enough for cumulative permanent deformation because each fill imposes a thermal strain mismatch between the hot inner wall and the cooler outer wall, and there is no annealing step to relieve the moulded-in stress.
So the cold drop test gave false confidence; it was the wrong test for the stated duty.
A valid hot-fill test:
- Fill with water at 95 °C, cap, and hold for 30 minutes.
- Drop from 1.5 m onto concrete at 60 °C ambient (the realistic post-fill condition).
- Repeat 50 times.
- Measure base diameter and height change; accept ≤ 0.5% permanent deformation and no leakage.
Design changes that would raise hot performance, if hot fill were retained: increase the base corner radius from 1 mm to at least 3 mm, add a petaloid base, and thicken the lower third of the wall from 1.2 mm to 1.8–2.0 mm. The examinable point is that test conditions must reproduce the service conditions, including temperature.