The Overheating Enclosure
A small Sydney firm designs a portable battery pack for outdoor events. The enclosure is 180 mm × 120 mm × 60 mm, moulded in ABS, wall thickness 2.5 mm, with four 6 mm ventilation slots on one face. During summer field trials the internal temperature reached 71 °C against a target ceiling of 55 °C, and two units showed slight warping near the slot edges. The client wants to keep the same external dimensions and the same mould tool, which cost $38,000, because the launch date is fixed in ten weeks. Options raised in a meeting were: switch to polycarbonate, add internal aluminium heat-spreader plates, enlarge the slots, or add a small fan. A junior designer argues that polycarbonate alone will fix it because it has a higher heat-deflection temperature.
1. Evaluate the four options and recommend a course of action, justifying your reasoning with reference to material properties, tooling, and the fixed launch date.
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Framing: the problem is thermal, not just material. The enclosure traps heat faster than the slots can remove it, so any fix must either reduce heat generation, improve heat path to the outside, or raise the material's tolerance.
- Polycarbonate: higher heat-deflection temperature than ABS, so warping near the slots would reduce. But it does not lower the internal air temperature of 71 °C, which is the real fault. It also needs higher moulding temperatures and may need tool adjustments (gates, shrinkage allowance), risking the fixed launch. The junior is only half right.
- Aluminium heat-spreader plates: conduct heat from the cells to the enclosure walls, increasing the effective radiating area. Adds mass, cost, and assembly steps, but needs no tool change because it sits inside the existing cavity. Strong candidate.
- Enlarging slots: improves convective flow but weakens the wall around the slots where warping already appears, and changes the tool, costing time and money.
- Fan: actively moves air and would likely meet the target, but adds a power draw, a failure point, noise, and a new tool for the fan mount.
Recommendation: combine internal aluminium spreader plates with a modest enlargement of the slots only if trials show it is needed, and keep ABS for this launch. This addresses the heat path without a new tool, protects the ten-week date, and preserves the $38,000 tool. Schedule a thermal re-test at 40 °C ambient within two weeks; if the target is still missed, the fan becomes the fallback.
2. The client insists on keeping the existing mould tool. Explain how this constraint shapes which design changes are permissible, and identify one change that would force a new tool.
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Principle: an existing mould tool fixes the external form, the wall thickness in the cavity, the position of the gate, and the position and size of any features cut into the steel. Changes that only add or remove material inside the cavity, or that use separate parts, are cheap; changes that alter the steel are not.
Permissible under the constraint:
- Adding internal aluminium spreader plates as loose or clipped-in parts.
- Changing the polymer grade within a family with similar shrinkage, provided trials confirm dimensions.
- Adding a separate external heat sink or vent cover that clips on.
Not permissible without a new tool:
- Enlarging the ventilation slots, because the slot geometry is cut into the mould steel; recutting or welding and remachining the tool is effectively a tool rebuild.
- Changing wall thickness, which alters the cavity and core dimensions.
- Moving the gate, which changes weld-line positions and flow.
Implication: the constraint pushes the design toward added parts and internal changes rather than geometry changes. This is a classic trade-off between tooling cost and design freedom, and it should be recorded in the design folio as a constraint that shaped the final solution.