23-Ind-B2 Manufacturing Processes · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examinations, May 2019 — 17-Ind-B2 Manufacturing Processes. 3-hour closed-book exam; candidates may use a Casio or Sharp approved calculator. Any five questions constitute a complete paper (only the first five as they appear are marked officially); all seven are answered below as a full study resource.
Reference texts. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 6th ed. (primary text for this subject — material selection, casting, polymer processing, metal forming, powder metallurgy, and machining).
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.
Why avoid it. Turbulent flow of molten metal while filling the mold is harmful in three compounding ways: (1) it accelerates erosion of the mold and gating surfaces, and the eroded sand/mold material becomes an inclusion defect embedded in the casting; (2) turbulent flow repeatedly exposes fresh metal surface to air, promoting oxidation — the resulting oxide film/dross can fold into the bulk metal instead of floating clear, again becoming an inclusion; and (3) turbulent, splashing flow entrains air bubbles into the melt, which become trapped gas porosity once the metal solidifies around them. All three defect types (sand inclusions, oxide/dross inclusions, gas porosity) weaken the casting and can act as crack-initiation sites in service.
How to avoid it. Turbulence is controlled primarily through gating-system design: use a tapered (rather than straight-walled) sprue whose cross-section narrows to match the accelerating gravity-driven flow, preventing the metal stream from separating from the sprue wall and drawing in air; provide a rounded sprue/runner base (see Question 2(i)) to smooth the vertical-to-horizontal direction change rather than turning it sharply; size runners and gates with sufficient (and, where possible, increasing) cross-sectional area so the metal's fill velocity stays below the critical threshold above which flow becomes turbulent; and use filters/strainers in the runner system to steady the flow and catch inclusions before they reach the cavity. Keeping the pouring rate and pouring temperature within the process's normal range (avoiding overly fast, splashy pouring) is also part of the same control.
A crankshaft is one of the most severely and repeatedly loaded components in an engine: it experiences millions of high-cycle bending and torsional load reversals over its service life. The most important reason a manufacturing engineer would prefer forging (or stamping/cold-forming, for the relevant geometry) over casting is fatigue strength and toughness: hot forging plastically deforms the metal so that its internal grain flow follows the contours of the finished part (main journals, crankpins, webs) rather than being cut through or left randomly oriented, and this continuous, aligned grain-flow structure is substantially more resistant to fatigue-crack initiation and propagation under cyclic loading than the grain structure of an as-cast part. A cast crankshaft can also retain internal porosity, shrinkage cavities, or non-metallic inclusions from solidification, and any of these act as stress-concentration sites where a fatigue crack can start — forging, by working the metal in the solid state under high pressure, closes up such internal discontinuities rather than creating new ones. For this reason, forged (typically alloy steel) crankshafts are the standard choice in higher-performance or higher-duty-cycle engines, while cast crankshafts (commonly ductile/nodular iron) remain acceptable only in lower-stress, cost-sensitive applications where the fatigue margin is less critical.
