22-Mec-B8 Engineering Materials · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2013 — 07-Mec-B8 Engineering Materials. Three hours, open book; any non-communicating calculator permitted. Eight questions, all of equal value; any FIVE constitute a complete paper, so each question is worth 20 marks. Candidates are urged to state any assumptions made. All eight questions are solved below, because the set as a whole is the study resource.
Reference texts (22-Mec-B8 Engineering Materials).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Given. An enclosure mounted on a bracket bolted directly to an automobile engine, with two candidates: ABS (acrylonitrile–butadiene–styrene), an amorphous thermoplastic, and a phenolic (phenol–formaldehyde) moulding compound, a highly cross-linked thermoset normally supplied filled with wood flour, mineral or chopped glass.
Find. (a) a comparison across the six stated attributes, and (b) a selection with justification tied to the service environment.
The decisive feature of this application is not stated as a number in the question, so it must be supplied by engineering knowledge: a box bolted to an engine lives in the under-bonnet environment. Ambient air temperature there reaches 100–120 ℃ in traffic on a hot day, and a component conducting heat through its bracket from the block or sitting near the exhaust manifold will run hotter still, 150 ℃ or more. It is also splashed with engine oil, coolant, fuel, brake fluid and road salt, and shaken continuously at engine order frequencies. Everything below follows from that environment.
| Attribute | ABS (thermoplastic) | Phenolic (thermoset) |
|---|---|---|
| Strength | Tensile 35–50 MPa, modulus 2–2.5 GPa; ductile, yields before it breaks. Creeps measurably under sustained load, and creeps much faster as temperature rises | Tensile 35–60 MPa (higher with glass filler) but compressive strength very high, 150–250 MPa; modulus 6–9 GPa. The cross-linked network essentially does not creep |
| Impact resistance | Much better. The dispersed polybutadiene rubber phase crazes and blunts cracks: notched Izod typically 150–400 J m−1 | Poor. Brittle, notch-sensitive, typically 15–30 J m−1 for a wood-flour grade; chopped glass improves it but never to ABS levels |
| Manufacturing methods | Injection moulding, extrusion, blow moulding, thermoforming. Cycle times of seconds, no chemical reaction to wait for, sprues and rejects regrind and re-run | Compression, transfer and (with modern compounds) injection moulding, but the tool must be held hot while the resin cures. Longer cycles, and flash and rejects are scrap — a thermoset cannot be remelted |
| Chemical resistance | Moderate at best. Dissolved or crazed by ketones, esters, chlorinated solvents and aromatic fuels; stress-cracks in the presence of some oils and brake fluid | Excellent. The cross-linked network is insoluble in essentially all engine-bay fluids — oil, fuel, coolant, dilute acids and salt. Attacked only by strong alkalis and strong oxidisers |
| Heat resistance | Weak. Glass transition around 105 ℃, heat-deflection temperature 85–100 ℃, continuous-use limit about 70–80 ℃. Softens and creeps above that; the butadiene phase also oxidises and embrittles with prolonged heat | Excellent. No melting point — the network chars rather than flows. Heat-deflection temperature 150–200 ℃, continuous use to 150–180 ℃, and inherently flame retardant with low smoke |
| Cost | Resin moderate; total part cost low because cycles are fast and scrap is recovered | Resin cheap (phenolics are among the least expensive thermosets), but slower cycles, deflashing and unrecoverable scrap push the finished part to roughly comparable or slightly higher cost |
(b) Selection. I would select the phenolic moulding compound, preferably a mineral- or glass-filled grade. The selection turns on a single disqualifying constraint rather than on an overall score. ABS wins clearly on impact resistance and on ease of manufacture, and those are genuine advantages — but a material whose heat-deflection temperature is 85–100 ℃ and whose continuous-use limit is around 75 ℃ cannot be bolted to an engine. It would soften, sag under its own fasteners, creep at the bolt bosses until the joint lost preload, and progressively embrittle as the butadiene phase oxidised. The chemical exposure compounds the problem: an oil or fuel film on a part that is simultaneously above its glass transition and carrying bolt-preload stress is a textbook environmental-stress-cracking situation.
The phenolic accepts the thermal and chemical environment without qualification, holds bolt preload because it does not creep, is dimensionally stable, and brings useful flame retardancy in a compartment that contains both fuel and ignition sources. Its brittleness is the price, and it is a manageable one for an enclosure: generous radii, no sharp moulded-in notches, metal compression limiters in the bolt holes so the fastener cannot crush the boss, and rubber-isolated mounts to keep engine vibration and stone impact out of the part. If subsequent testing showed that impact really did govern — say the box is in a position exposed to road debris — the right answer would not be to revert to ABS but to move up to a glass-filled engineering thermoplastic such as PA66-GF33 or PPS, which keeps most of the phenolic’s temperature and fluid resistance while restoring toughness.
