The Overweight Electronics Enclosure
A design team is developing a handheld field instrument. The enclosure is currently injection-moulded ABS, 2.5 mm nominal wall, mass 180 g. A customer requires the unit to survive a 1.2 m drop onto concrete and to weigh no more than 150 g. The team proposes switching to polycarbonate, which is tougher but denser, and thinning the wall to 2.0 mm to save mass. Prototypes at 2.0 mm PC pass the drop test at room temperature but crack at −10 °C, and the moulding supplier reports short shots and visible weld lines near the battery hatch. The project has one prototype cycle left before tooling is committed.
1. Evaluate the proposed change. Does thinning the wall achieve the mass target without unacceptable risk?
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The proposal trades one requirement for another and does not clearly win.
Mass. PC is denser than ABS (roughly 1.20 vs 1.04 g/cm³, about 15% more). Reducing wall from 2.5 to 2.0 mm cuts volume by about 20%, so the net mass change is roughly 0.80 × 1.15 ≈ 0.92 of the original - about 166 g. That is still above the 150 g target, so the change does not actually meet the requirement. This is the key judgement: the team has assumed thinning solves mass, but the density penalty eats most of the saving.
Drop performance. PC is tougher than ABS, which is why room-temperature drops pass. But the −10 °C cracking shows the thinner wall has reduced stiffness and left less material to absorb impact; PC's toughness is notch-sensitive, and weld lines are weak points.
Moulding. Short shots and weld lines at 2.0 mm indicate the wall is near the process limit for the flow length. Thinner walls raise injection pressure and make weld-line strength worse - exactly where the cracks appear.
Conclusion: the change is not acceptable as proposed.
2. Recommend an alternative design direction and justify it against the requirements.
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Separate the requirements and solve them with geometry rather than wall thickness.
Option A - ribbed 2.5 mm PC shell. Keep the 2.5 mm wall for mouldability and impact, and add internal ribs to restore stiffness lost by removing material elsewhere (e.g. pockets in low-stress areas). Ribs add stiffness far more efficiently than wall thickness. Move the battery hatch away from the weld-line location or add a boss/gusset so the weak line is not load-bearing.
Option B - material hybrid. Use PC/ABS blend or a toughened grade with better low-temperature impact, or add a separate elastomer bumper at the corners where drop stresses concentrate. Corner protection is cheap and targets the failure mode directly.
Option C - reduce mass elsewhere. The 150 g target includes internals; a smaller battery or lighter fasteners may recover 15–20 g without touching the structural wall.
Justification. Each option keeps wall thickness in the mouldable range, keeps weld lines away from impact zones, and addresses low-temperature toughness. Option A is preferred because it fixes stiffness and mass together with one tooling change. Use the final prototype cycle to test a ribbed PC shell at −10 °C with a drop from 1.2 m.