23-Ind-B2 Manufacturing Processes · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 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.
Key factors that must be controlled in any casting operation are: (1) pattern and mold design, including shrinkage allowance, draft angles, and parting-line placement so the casting can be removed without damage; (2) gating and risering system design, so molten metal fills the mold cavity completely and quickly before it freezes, and risers feed liquid metal into the casting as it solidifies to offset shrinkage; (3) solidification control, generally by directional solidification (progressing from the section farthest from the riser toward the riser) to avoid trapping shrinkage porosity inside the casting; (4) pouring temperature and rate, hot and fast enough to fill thin sections before freezing but not so hot that gas porosity, oxide inclusions or excessive shrinkage result; and (5) mold material and permeability, so mold gases can escape rather than becoming trapped porosity in the casting.
Casting is chosen over other manufacturing methods when: the part geometry is complex (internal passages, thin walls, irregular external shape) and would be difficult or impossible to produce by machining or forming from solid stock; the material itself is difficult to machine or form in the solid state; very large parts are needed (engine blocks, machine bases) where forming or machining from solid billet is impractical; or the production volume justifies the tooling cost and casting produces a near-net shape that minimizes downstream machining, saving both material and machining time.
Shell molding is an expendable-mold casting process in which a thin (a few millimetres), rigid mold — the "shell" — is formed by dropping or blowing fine sand pre-coated with a thermosetting resin binder onto a heated metal pattern; the resin cures on contact, bonding a hard sand skin to the pattern, which is then stripped off, cured further in an oven, and joined with its matching half to form the complete mold, backed with loose sand or shot for support during pouring.
Advantages: good dimensional accuracy and a smooth cast surface finish (reducing downstream machining), the ability to produce thin sections and fine detail reliably, high permeability that lets mold gases escape, a mold light enough to be handled and stored, and a process well suited to automated, high-volume production.
Disadvantages: the metal patterns and the resin-coated (pre-coated) sand are relatively expensive, which only pays off at moderate-to-high production volumes; casting size is limited compared with sand casting from loose green sand; and the process generates resin fumes during curing, which must be controlled for worker health and environmental compliance.
In permanent-mold casting (including its variants die casting and centrifugal casting) the mold is a reusable metal tool rather than a one-use sand mold, so it can be poured (gravity permanent-mold), pressure-injected (die casting), or spun during pouring (centrifugal casting) to distribute the metal.
Advantages: a reusable, precision mold gives good dimensional accuracy, a fine and consistent surface finish, and finer grain structure/better mechanical properties than sand casting because the metal mold extracts heat quickly, giving faster solidification and a shorter cycle time suited to high-volume production; die casting in particular achieves thin walls and complex detail with essentially no machining allowance.
Limitations: the metal mold itself is expensive to design and machine, so the process is only economical at high production volumes that amortize the tooling cost; the mold cavity is generally limited to relatively simple external shapes because a rigid metal mold cannot collapse the way sand does, restricting the use of complex internal cores; usable casting alloys are restricted to those with melting points well below the mold material's melting point (so die casting is largely limited to non-ferrous alloys — aluminum, zinc, magnesium, copper alloys); and mold life is finite, since repeated thermal cycling eventually erodes and cracks the cavity.
