Mailbox hinge in a coastal environment
A communal mailbox bank of 120 boxes stands 300 m from the sea in Paphos. Each box has an injection-moulded hinge 30 mm × 25 mm × 4 mm, with a snap-fit pin and a 95° opening. The hinge carries a 12 N flap load and is operated about 20 times a day. The current ABS hinge cracks at the screw bosses after 14 months; UV exposure is high, salt fog is present, and surface temperatures range from 5 °C to 45 °C. The manufacturer wants to keep the existing mould and change only the polymer, so the replacement must flow in the same tool, snap-fit without lubricant, and cost no more than €0.18 per hinge at 100,000 units. Candidates suggested are ASA, PC/ASA, PA6-GF30, PP-T20 and POM. A local supplier can provide UV-stabilised grades but not all in the same colour.
1. Select the most suitable polymer from the list for the existing mould and justify your choice using at least four properties. Explain why one of the other candidates is unsuitable.
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Choose PC/ASA, or ASA if impact tests show it is adequate. The requirements are UV stability, impact strength at 5 °C, creep resistance under a sustained 12 N load, snap-fit elasticity, flow in the existing tool, and cost below €0.18. PC/ASA combines the UV resistance of the ASA cap with the toughness of polycarbonate, and it can be UV-stabilised. Its shrinkage is close enough to ABS to attempt a trial shot in the same tool, although melt viscosity is higher and injection pressure may need increasing.
PA6-GF30 is unsuitable despite high strength: glass fibres make snap-fits brittle, the material is abrasive, it warps, and its shrinkage differs from ABS, so the existing tool would not produce accurate parts. PP-T20 is cheap and chemically resistant but has low stiffness and creeps under sustained load, so the flap would sag. POM has good snap-fit and friction behaviour but needs special UV stabilisation and is more expensive. Verify the choice with a mould trial, 1,000 open-close cycles and a colour match check.
2. The snap-fit must survive 20 cycles per day for 10 years. Describe a test programme and state two acceptance criteria.
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Twenty cycles per day for ten years is 20 × 365 × 10 = 73,000 cycles. Build a rig that opens to 95° and closes at 45 °C and 50% relative humidity; inspect every 10,000 cycles for cracks at the screw bosses and snap-fit root. Add UV exposure, for example 2,000 h of xenon-arc weathering, then repeat 1,000 cycles and a cold impact test at 5 °C. Add 480 h of salt-fog exposure and check snap retention.
Acceptance criteria: no visible crack at 2× magnification after 73,000 cycles; snap retention force at least 8 N, which is two-thirds of the initial value. These criteria link directly to the 10-year life and the 12 N flap load. A cheaper test is to run 10 hinges to failure and use the weakest result to estimate lifetime, but the full cycle rig is needed to detect wear-out.
3. Explain why simply adding carbon black to ABS might solve the UV problem but not the screw-boss cracking.
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Carbon black absorbs UV radiation and slows surface degradation, so it can extend outdoor life. But it does not change the notch sensitivity of ABS or the stress state at the screw boss. Boss cracking is usually caused by moulded-in stress, a sharp corner, a boss wall that is too thin or too thick, or hoop stress from a thread-forming screw. Adding pigment may even change shrinkage and increase residual stress. The correct response is to combine a tougher polymer such as PC/ASA with a redesigned boss: a larger radius, a suitable pilot hole, and a screw that does not over-expand the boss. This shows why material selection must follow the failure mode, not just the symptom.