23-Ind-B2 Manufacturing Processes · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examinations, December 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, 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.
Given. The three feed-system passages of an injection mold (sprue, runner, gate); shrinkage behaviour of metals in casting and of polystyrene in injection molding; the category of fiber-reinforced composite processes.
Find. (i) freeze order of sprue/runner/gate. (ii)–(iii) typical linear shrinkage values. (iv) a continuous-fiber composite technology.
| Part | Answer | Reasoning |
|---|---|---|
| (i) | The gate freezes first | The gate has by far the smallest cross-section of the three passages, giving it the highest surface-area-to-volume ratio and therefore the fastest heat loss to the mold; the runner (medium cross-section) freezes next, and the sprue (largest cross-section) freezes last. Gate freeze-off is what ends the packing/holding stage, since no more melt can be pushed into the cavity once it seals. |
| (ii) | ≈ 1–2% linear shrinkage | Typical solidification shrinkage allowance for common casting metals: gray cast iron ≈ 0.8–1%, aluminum alloys ≈ 1.3%, carbon steel ≈ 2%. Pattern makers historically build this allowance directly into the pattern dimensions. |
| (iii) | ≈ 0.4–0.7% linear mold shrinkage | Polystyrene is amorphous (no crystalline phase-change shrinkage spike), so its mold shrinkage is low and predictable compared with semicrystalline polymers such as polyethylene or nylon (≈ 1.5–3%). |
| (iv) | Filament winding | Continuous reinforcing fiber (glass, carbon, or aramid) is pulled from a spool, drawn through a resin bath, and wound under tension onto a rotating mandrel — the fiber runs the full length of the part with no chopped/discontinuous segments. Pultrusion is an equally valid alternate answer (continuous fiber pulled through a resin bath and forming die). |
