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 lower mold temperature widens the thermal gap between the hot melt (often 200–300°C for common thermoplastics) and the mold wall. Against that steep gradient, the polymer touching the wall solidifies almost instantly, while the interior remains molten and continues to cool and shrink over a much longer time. This creates a highly non-uniform solidification sequence: the already-rigid outer skin can no longer move to accommodate the core's ongoing shrinkage, so the shrinkage strain in the core is resisted by the skin and is locked in as internal (residual) stress rather than being relieved by free contraction, exactly as in the thickness-driven mechanism of Question 4 but here driven by the mold-temperature difference instead of section thickness.
A second mechanism compounds the first. During filling and packing, the flowing melt orients its long polymer chains (and any fibre reinforcement) along the flow direction; that orientation is itself a form of frozen-in stress unless the chains have time to relax back toward a random coil before the material solidifies. A cold mold quenches the melt so quickly that there is very little time available for this relaxation, so a larger fraction of the flow-induced molecular orientation is frozen in place — the faster the quench, the less relaxation occurs and the higher the resulting orientation-related residual stress. A colder mold surface can also raise the melt's local viscosity near the wall during fill, requiring somewhat higher shear/injection pressure, which further increases flow-induced orientation. Conversely, raising the mold temperature slows cooling, gives the polymer more time both to shrink more uniformly (smaller skin-to-core gradient) and to relax frozen-in chain orientation, and so directly lowers residual stress — at the cost of a longer cooling time and cycle.
