23-Ind-B2 Manufacturing Processes · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2015 — 98-Ind-B2 Manufacturing Processes. Closed book; candidates may use one of two calculators, the Casio or Sharp approved models. 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, and automation/numerical control; Montgomery, Introduction to Statistical Quality Control, 8th ed. — where quality-control concepts are referenced.
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.
The standard textbook answer (Kalpakjian & Schmid) names two trends: (1) the mechanization and automation of the casting process, and (2) the growing demand for high-quality castings with close dimensional tolerances.
Trend 1 — Mechanization and automation. Casting has traditionally been a labour-intensive, craft-dependent process. It is being reshaped by automated molding lines, automated or robotic pouring, shakeout, cleaning and finishing, and computer control of melting and pouring. These cut the dependence on scarce skilled labour, remove workers from hot and hazardous tasks, and raise productivity and consistency. Computer-aided design is part of the same trend: solidification-simulation software predicts shrinkage porosity and optimizes gating and riser placement before any mold is made, and rapid tooling (3D-printed sand molds and patterns) shortens pattern lead time from weeks to days.
Trend 2 — Demand for high-quality castings with close dimensional tolerances. Customers, especially automotive and aerospace, now expect castings that are near-net-shape, have few defects, and need little machining. This has driven the development of precision processes (investment, ceramic-mold, plaster-mold, lost-foam, squeeze and semisolid casting), better control of melt quality (degassing, filtration, inclusion control), and more non-destructive inspection. It has also moved production away from ordinary green-sand casting wherever tight tolerances or thin sections are needed.
Also relevant — environmental and regulatory pressure. Foundries are energy-intensive and historically significant sources of air emissions (metal fume, sand-binder off-gassing), solid waste (spent sand, slag), and worker-exposure hazards (silica dust, molten-metal handling). Increasingly strict environmental regulation and workplace-safety requirements are driving foundries toward cleaner binder chemistries (e.g. moving away from traditional resin/oil-sand binders toward inorganic or water-based binder systems), sand-reclamation and recycling to cut landfill waste, better fume/dust capture, and energy-efficient melting (induction rather than cupola furnaces in many shops) — all of which raise capital cost but are now a competitive necessity, not an option.
Plaster-mold casting is an expendable-mold process in which the mold is made from a plaster of Paris (gypsum, CaSO4·½H2O) mixture rather than sand. The process: a slurry of plaster, water, and talc/sand additives (to control setting time and reduce cracking on drying) is poured over a pattern (usually made of metal or plastic, since plaster molds require a smoother, more dimensionally stable pattern than green sand); the slurry sets around the pattern in roughly 15 minutes; the pattern is withdrawn; the mold halves are then baked in an oven (several hours, at a few hundred °C) to drive off retained moisture, since trapped water would otherwise flash to steam and cause gas defects when molten metal is poured. The dried mold halves are assembled, and the casting is poured (limited to relatively low-melting-point non-ferrous metals — aluminum, magnesium, zinc, and copper-based alloys — because plaster cannot withstand the temperatures of ferrous pours and because plaster has very low permeability and thermal conductivity, which would cause problems with iron/steel's higher pouring temperatures). Once solidified, the plaster mold is broken away (it is a one-use, expendable mold) to release the casting.
The very fine grain size and low permeability of plaster give plaster-mold castings an excellent surface finish and fine dimensional detail and tolerance — substantially better than sand casting — which makes the process well suited to intricate, thin-walled non-ferrous parts such as pump/valve components, gears, and ornamental hardware in low-to-medium production quantities.
"Expendable-mold" casting (the printed "expandable" is a typo) covers every process whose mold is destroyed to remove the casting. That means sand casting mainly, plus shell, plaster, ceramic, investment and lost-foam casting. The pattern may be reusable (sand, shell, plaster) or consumed (investment wax, lost foam). The most significant design considerations are:
