23-Ind-B2 Manufacturing Processes · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2016 — 98-Ind-B2 Manufacturing Processes. Closed book; Casio or Sharp approved calculators only. Any five of the seven questions constitute a complete paper; all questions are of equal value (20 marks each). Answers are written in point form but fully, with all calculations shown, as instructed. Complete answers to all seven questions follow.
Reference texts: Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 6th ed. — material selection, casting, metal-cutting theory, welding processes, polymer processing, statistical process control; Montgomery, Introduction to Statistical Quality Control, 8th ed. — acceptance sampling, control charts, the Deming/Taguchi quality philosophies.
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.
Non-ferrous alloys in general. As a class, non-ferrous alloys (those not based on iron) share several characteristics that distinguish them from steels and cast irons: generally lower density, giving a favourable strength-to-weight ratio that makes several of them the material of choice wherever weight matters (aerospace, transportation, portable equipment); superior corrosion resistance in most environments, since none of the common non-ferrous metals form the destructive, self-propagating iron-oxide (rust) film that plain carbon steel does — instead many form a thin, adherent, protective oxide (aluminum, magnesium) or are inherently more noble (copper, nickel); generally lower melting points than steel, which makes them easier and cheaper to cast and to machine at high speed; typically non-magnetic, which matters for electronic, instrumentation and shipboard applications; and, as a group, a wider spread of properties and cost than ferrous alloys — from inexpensive, highly formable aluminum sheet to costly, high-temperature nickel superalloys — so the "non-ferrous" label covers a very broad design-property space rather than one characteristic material.
Aluminum alloys. Low density (about 2.70 g/cm³, roughly a third that of steel); a thin, tenacious, self-healing aluminum-oxide film gives excellent atmospheric corrosion resistance without plating; good electrical and thermal conductivity (used for both power transmission and heat-exchanger/engine applications); excellent machinability and castability, and most wrought alloys are readily formed and welded (though the high-strength 2xxx/7xxx aerospace series are comparatively difficult to weld by fusion methods); can be strengthened substantially by cold work and, in the heat-treatable series (2xxx, 6xxx, 7xxx, alloyed principally with copper, magnesium/silicon, or zinc/magnesium), by solution treatment and precipitation (age) hardening; strength and stiffness fall off rapidly above roughly 200°C, limiting use in sustained high-temperature service.
Magnesium alloys. The lightest of the commonly used structural metals, about 1.74 g/cm³ (roughly two-thirds the density of aluminum and a quarter that of steel), giving the best stiffness-to-weight and strength-to-weight ratios of any common structural alloy family; excellent damping capacity, useful in housings and brackets subject to vibration; readily die-cast (low melting point, low heat of fusion) and machined at very high speeds with low cutting forces, but fine machining chips and dust are a genuine fire/explosion hazard and must be controlled (cutting fluids, chip collection, no accumulation of fines); poorer inherent corrosion resistance than aluminum, especially in the presence of chlorides or in contact with more noble metals (galvanic corrosion), so alloying (high-purity, low-iron/nickel/copper "HP" grades) and surface treatment (anodizing, chromate/paint) are usually required; hexagonal close-packed crystal structure gives limited cold formability, so magnesium components are more often cast or hot-formed than cold-worked.
Copper alloys. Outstanding electrical and thermal conductivity, second only to silver among engineering metals, which drives its dominant use in wire, busbar, and heat-exchanger applications; good corrosion resistance in most atmospheres and fresh/salt water; readily cold-worked and cast, and both alloy families are weldable and easily joined by soldering/brazing; brasses (copper-zinc) offer good strength, ductility and machinability at moderate cost and are widely used for fittings, fasteners and decorative hardware; bronzes (copper-tin, plus aluminum bronze and silicon bronze variants) offer higher strength and superior wear and fatigue resistance, making them the standard choice for bearings, bushings, gears and marine hardware; copper alloys are also naturally antimicrobial, a property increasingly exploited on frequently touched surfaces.
Advantages of plastics compared with metals. Much lower density (typically 0.9–2.2 g/cm³ versus 2.7–8.9 g/cm³ for common structural metals), giving substantial weight savings for a given volume; inherent corrosion and chemical resistance — plastics do not rust or corrode electrochemically, so many applications need no protective coating at all; can be processed directly into complex, near-net final shapes (injection molding, blow molding) in a single low-energy step, eliminating the multiple machining/joining operations a metal part often needs; electrical and thermal insulating properties are usually inherent, not added (useful for housings, connectors, and handles); lower processing temperatures (typically 150–350°C versus 600–1600°C for metals) mean lower energy consumption and simpler, lower-cost tooling for a given production volume; colour, transparency and surface texture can be built into the material itself, often removing the need for a separate finishing/painting operation; and good vibration-damping and impact-energy-absorption characteristics in many grades.
General characteristics of plastics. Lower strength and stiffness (modulus) than metals on an absolute basis, although specific (per-unit-weight) strength and stiffness can be competitive, especially once fibre-reinforced; properties are strongly time-, temperature- and rate-dependent (viscoelastic behaviour), so a plastic that performs well in a short-duration test can still creep or relax significantly under a sustained load — a distinction that has no direct metallic analogue at room temperature; maximum continuous-service temperatures are generally far lower than for metals (most commodity plastics soften well below 150°C); most plastics are inherently poor electrical and thermal conductors (an advantage for insulation, a disadvantage for heat dissipation); properties can degrade with prolonged UV, ozone or chemical-solvent exposure unless stabilized; and the property range across the plastics family is extremely wide — from soft, flexible elastomers to rigid, glass-reinforced engineering resins — so "plastic" describes a material family, not a single set of properties, in the same sense that "non-ferrous alloy" does in part (i).
Development of plastics is currently driven along several converging fronts. Sustainability and the circular economy is the most visible: growth of bio-based feedstocks (polylactic acid and other bio-polyesters derived from renewable sources), genuinely biodegradable/compostable grades for single-use applications, and — of greater volume impact — design-for-recyclability (mono-material assemblies, easily separable components) together with expanding mechanical and, increasingly, chemical (depolymerization back to monomer) recycling capacity. Fibre-reinforced and nanocomposite plastics continue to displace metal in weight-critical structures: carbon- and glass-fibre-reinforced thermoplastics and thermosets in automotive and aerospace primary and secondary structure, and nanoclay/graphene/carbon-nanotube nanocomposites that raise stiffness, barrier performance and flame resistance at very low filler loadings without the weight penalty of conventional fillers. High-performance engineering and specialty resins (PEEK, polyimides, liquid-crystal polymers) are steadily displacing metal in under-hood, aerospace and medical-implant applications that demand high continuous-service temperature or biocompatibility together with light weight. Additive manufacturing with engineering thermoplastics (and, increasingly, continuous-fibre-reinforced filament) is moving from prototyping into genuine low-volume production of end-use parts. Smart and functional polymers — shape-memory polymers, inherently conductive polymers for flexible electronics, and self-healing resins that reflow microcracks under mild heat — are moving from the laboratory into niche commercial products. Finally, processing technology itself keeps advancing: multi-material and multi-shot injection molding, in-mold decoration/assembly, and real-time, sensor-based process control are together cutting cycle time, scrap rate and the number of downstream finishing operations a moulded part requires.
