Citrus Juicer: Fixing the Lever Without Blowing the Print Budget
A student is developing a handheld citrus juicer for a school enterprise market. Target retail price is NZ$12, batch of 150. The body is 3D-printed PLA, the lever is printed in the same PLA, and the juicing cone is a bought food-grade polypropylene part. Testing with 30 repetitions on limes and lemons showed the lever arm cracked at the pivot after an average of 22 squeezes when a 40 N force was applied at the handle. The pivot is a 5 mm printed pin. The student's proposed solution is to make the whole lever 2 mm thicker and to change infill from 20% to 50%. Print time per unit rises from 4 h to 7 h. The class has two printers running 30 h a week for five weeks. The client wants a product that lasts at least 200 squeezes.
1. Evaluate the student's proposed fix using the numbers in the setup. What does the arithmetic tell you?
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The fix targets stiffness, but the reported failure is at the pivot - a 5 mm printed pin loaded in shear and bending, plus layer adhesion across the lever. More infill raises stiffness somewhat but does not reliably fix a small printed pin; a bought M5 bolt, a larger printed boss, or a change of lever material is a more direct response to the failure mode.
The production arithmetic is decisive:
- Proposed: 150 units × 7 h = 1,050 print hours.
- Available: 2 printers × 30 h/week × 5 weeks = 300 hours.
- Even the original 4 h/unit gives 600 h - already double capacity.
So the proposal cannot be manufactured at the stated batch size. Options: reduce batch to about 42 units (300 ÷ 7), add printer capacity, or redesign for a faster process such as a laser-cut plywood lever. Durability is also far short: 22 squeezes against a 200-squeeze target means the design needs a test-to-200 protocol before any batch decision.
2. Propose a revised development path and explain what evidence would justify it.
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Revised path:
- Replace the printed 5 mm pin with an M5 stainless bolt through a reinforced boss, or change the lever to laser-cut 6 mm plywood with a bolted pivot.
- Run a controlled cycle test: five units to 40 cycles each at 40 N, recording cycles to failure and the failure location. If one survives 200, test three more.
- Re-check print time and cost against the 300-hour capacity.
- Document the decision in a matrix comparing strength, cost, print time, food safety, and assembly complexity.
Evidence to justify it: a dated test log (force, cycles, failure mode), photographs of fracture surfaces, a costed bill of materials, and a revised time budget. The failed prototype should appear in the portfolio alongside the change and the reason - the standard rewards evidence of informed iteration, not an unblemished final product.
3. How should the student communicate the durability-versus-print-time trade-off to the client?
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Use a compact evidence set rather than every test:
- A table: option, print time per unit, batch time, estimated unit cost, cycles achieved.
- A simple graph of cycles to failure against print time (or infill), so the 200-cycle target is a visible line.
- A written recommendation naming the option that reaches 200 cycles within 300 hours, and the residual risk (bolt corrosion, cleaning around the pivot, food-safe surfaces).
A client needs the trade-off, the recommendation, and what would change the recommendation - not the full experimental record. Plain language and quantified claims ('1,050 hours against 300 available') are more persuasive than adjectives.