22-Mec-A4 Design and Manufacture of Machine Elements · December 2016
Question 2 of 6
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Examinations, December 2016 — 07-Mec-A4, Design and Manufacture of Machine Elements. Three hours, open book, any non-communicating calculator. Six questions in two parts: Part A (Q1–Q3) is qualitative manufacturing-process theory, Part B (Q4–Q6) is quantitative machine-element design. The candidate answers two from Part A and two from Part B; four questions of equal value (25 % each) constitute a complete paper. All six are solved here, because the set is a study resource rather than an exam script.
Reference texts.
S. Kalpakjian and S. R. Schmid, Manufacturing Engineering and Technology, 8th ed. — Part A (sheet-metal forming, casting, machine-tool selection).
M. P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes and Systems, 7th ed. — fine blanking, die casting, turning-machine classes.
R. G. Budynas and J. K. Nisbett, Shigley's Mechanical Engineering Design, 11th ed. — Part B (welded joints Ch. 9, shafts Ch. 7, bolted and riveted joints Ch. 8).
R. L. Norton, Machine Design: An Integrated Approach, 6th ed. — corroborating weld and riveted-joint treatments.
Check — weld allowable stress basis (Q4). The paper gives an electrode ultimate strength (E60, Su = 60 ksi) and a safety factor of 3.0, but not the strength theory. This solution uses the distortion-energy shear strength of the deposited metal, Ssu = 0.577 Su, divided by the given factor — the treatment used throughout Shigley Ch. 9. The alternative code route (AISC allowable 0.30 Su, which already embeds its own reserve) is worked out at the end of Q4 and gives a smaller leg. State whichever basis you adopt; the marker is looking for the weld-as-a-line method, not the code table.
i) Process selection for an intricate, good-finish casting
(a) Solid casting. The requirement combines fine detail with a good as-cast surface, which points at the expendable-mould, expendable-pattern family, where no draft, no parting line and no core withdrawal constrain the shape.
Investment casting (lost wax) — the first choice. A wax pattern is built (injection-moulded for production quantities), assembled onto a runner tree, invested by repeated dipping in fine ceramic slurry and stuccoing, dewaxed and fired, then poured. Tolerances of roughly ±0.05–0.1 mm and 1.3–3 µm Ra are routine, undercuts and thin sections reproduce faithfully, and any alloy including superalloys can be poured.
Ceramic-mould (Shaw) casting — the same fine ceramic face but with a reusable pattern, so it suits larger or fewer parts, for example dies and impellers.
Plaster-mould casting — excellent finish and detail at lower cost, restricted to non-ferrous alloys (aluminium, zinc, copper) by the plaster's temperature limit and long drying cycle.
Die casting — if the alloy is a low-melting non-ferrous one and the quantity is large, since the steel die reproduces the finish and detail directly; see part (ii).
Lost-foam (evaporative-pattern) casting — free-form shape without a parting line, but the finish is coarser than investment casting.
(b) Hollow, with a cavity of complex shape. The difficulty shifts entirely to the core: an intricate internal cavity means the core cannot be drawn out mechanically, so it must either be destroyed or dissolved after solidification.
Investment casting with a preformed ceramic core — the industry answer for internally cooled turbine blades. The ceramic core is placed in the wax-injection die, the wax is moulded around it, the ceramic investment is built up outside, and after casting the core is leached out chemically (caustic autoclave). Serpentine internal passages a millimetre across are produced this way.
Investment casting with a soluble (water-soluble wax or urea) core — the core is dissolved before the ceramic investment is even fired, leaving a hollow wax pattern. Cheaper than ceramic cores where the passage geometry allows it.
Sand casting with resin-bonded (hot-box or cold-box) cores — collapsible cores assembled into a core package; the standard route for engine blocks and manifolds. Detail and finish are inferior, so use it only if the "good finish" requirement is confined to machined faces.
Lost-foam casting — the pattern is the cavity former; complex internal passages are glued up in polystyrene and simply vaporize. No cores at all, and no core removal problem.
The one family to rule out is ordinary die casting with permanent metal cores: a mechanical core must be able to retract along a straight line, so a genuinely complex internal cavity cannot be produced in a conventional die-casting die.
ii) The four principal die-casting processes — verbal description
All four force molten metal into a permanent steel die under pressure; they differ in how the melt is delivered and how much pressure is applied, which is the point the question asks to be made clear.
Hot-chamber die casting. The injection cylinder is submerged in the molten-metal bath. On the return stroke a port uncovers and metal fills the "gooseneck" cylinder by gravity; on the forward stroke a plunger closes the port and drives that metered charge through the gooseneck and nozzle into the die at pressures of roughly 7–35 MPa. Cycles are very fast because there is no ladling step, but the immersed steel hardware limits the process to low-melting alloys that do not attack it — zinc, magnesium, lead and tin.
Cold-chamber die casting. The melting furnace is separate from the machine. A measured charge is ladled into a horizontal (occasionally vertical) shot sleeve and a plunger then rams it into the die at much higher pressure, typically 20–150 MPa. The extra ladling step lengthens the cycle and the free surface in the sleeve allows some air entrainment, but nothing is immersed in the melt, so aluminium, brass and magnesium can be cast.
Low-pressure permanent-mould (low-pressure die) casting. The die sits above a sealed holding furnace and is connected to it by a refractory riser (stalk) dipping into the melt. Dry air or inert gas pressurizes the furnace to about 0.03–0.1 MPa, pushing metal quietly up the stalk and filling the cavity from the bottom. The gentle bottom-up fill is the whole point: it avoids the turbulence and entrained air of high-pressure injection, so the castings are sound enough to heat-treat and weld. Solidification proceeds from the top down while the riser feeds shrinkage, and the un-solidified metal in the stalk drains back when the pressure is released.
Vacuum die casting (vacuum-assisted / vacuum permanent-mould). The die cavity is evacuated before and during the shot. In the vacuum-permanent-mould variant, evacuating a die whose fill tube dips into the melt is by itself enough to draw metal up and fill the cavity at a differential of about one atmosphere; in the more common vacuum-assisted high-pressure variant, the cavity is evacuated through vent blocks while a cold- or hot-chamber plunger injects. Either way, removing the air removes the gas porosity, so the castings can be heat-treated and welded and their mechanical properties approach those of permanent-mould castings.
If a fifth process is wanted for contrast, squeeze casting pours a metered charge into the lower die half and closes the upper half onto it, applying a high pressure through the die faces themselves during solidification rather than through a gate — a hybrid of casting and forging that eliminates both porosity and the sprue.
