23-Ind-B2 Manufacturing Processes · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examinations, December 2018 — 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, since all questions carry equal (20-mark) value.
Reference texts. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 6th ed. (primary text for this subject — material selection, casting, polymer processing, machining, and composites).
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.
A casting rarely leaves the mold in finished, usable form, so it typically passes through several downstream steps. Trimming/fettling removes the gating system, risers, and any flash at the parting line (by cutting, sawing, or breaking off, depending on the metal). Cleaning follows — shot blasting, tumbling, or core/sand removal from internal passages, especially for sand castings where residual sand and burned-on scale must be stripped from the surface. Heat treatment (annealing, normalizing, or quench-and-temper depending on the alloy) relieves the residual stresses locked in by non-uniform solidification and sets the final mechanical properties/microstructure. Machining brings critical dimensions, mating surfaces, and holes to final tolerance and surface finish, since as-cast surfaces are rarely precise enough on their own. Inspection (dye penetrant, X-ray/radiography, or ultrasonic testing) checks for internal defects such as porosity, shrinkage cavities, or cracks that are invisible from the outside. Finally, surface finishing (painting, plating, anodizing) may be applied for corrosion protection or appearance.
(1) Feed reservoir against shrinkage. Metal shrinks as it cools and solidifies; the riser is a reservoir of molten metal, sized and placed to solidify AFTER the casting itself, so it can continue feeding liquid metal into the casting to compensate for that shrinkage — without it, the shrinkage would instead show up as a cavity or porosity inside the part.
(2) Venting / fill indicator. An open riser also provides an escape path for air and mold gases displaced by the incoming metal, and it gives the founder a visual sign that the mold cavity has been completely filled once molten metal appears at the riser's top — a blind (closed) riser can additionally be used to control the solidification sequence and applied pressure on the melt.
An expendable mold is destroyed to remove the finished casting, so a new mold must be made for every part. Besides sand casting (already covered elsewhere in this paper), two further expendable-mold processes are investment (lost-wax) casting — a disposable wax pattern is coated in a ceramic slurry to build a one-use ceramic mold, which is broken away after the metal solidifies — and plaster-mold casting, which uses a disposable gypsum-plaster mold cast around a reusable pattern, ideal for non-ferrous alloys and fine detail/surface finish. Both molds are single-use, in contrast to permanent-mold processes such as die casting.
