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 thicker section has a larger cross-sectional moment of inertia, so it is stiffer and stronger in bending and better able to resist impact loads without adding ribs or other stiffening features — useful where the part must carry structural load directly. A thicker section also presents lower flow resistance to the advancing melt front (a thin section chills and its viscosity rises faster relative to its flow length), so long or geometrically complex flow paths that would otherwise "short-shot" (freeze before filling) are easier to fill completely, and the required injection pressure to complete the fill is generally lower for a thicker section than for an equivalent thin one.
Cost rises directly — more resin is used per part, so material cost per part increases roughly with thickness. Cooling time is the larger economic penalty: for a given polymer and mold-temperature difference, cooling time scales approximately with the SQUARE of wall thickness (heat must diffuse out to the mold wall from the thermal centre of the section), so doubling the thickness can roughly quadruple the cooling time; since cooling dominates the injection-molding cycle, this directly cuts the parts-per-hour production rate and raises the labour/machine-time cost per part. On the quality side, a thick section cools much more slowly at its core than at its skin, so the differential (skin-vs-core) shrinkage is larger, producing more pronounced sink marks on the surface directly opposite ribs/bosses and a greater risk of internal voids, because packing pressure struggles to reach and compensate the still-shrinking core once the (thinner) gate has already frozen. That same non-uniform, slower cooling also increases residual stress and warpage risk in the finished part, and it can require a longer, more expensive mold (more cooling-channel design effort) to control.
