23-Ind-B2 Manufacturing Processes · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 17-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. — engineering-material property overview, casting processes, polymer/composite processing, metal-forming theory, and metal-cutting theory.
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.
Warpage traces back to non-uniform shrinkage across the part as it cools from the melt. Plastic touching the (much cooler) mold-cavity wall solidifies first and continues to cool and shrink at a different rate than material still molten at the part's core, especially in sections of non-uniform wall thickness, where thick regions retain heat and shrink more, in total, than adjacent thin regions. On top of this thermal effect, the directional melt flow during filling orients polymer chains (and any reinforcing fibres) along the flow direction, and that orientation makes shrinkage anisotropic — different along the flow direction than across it. These non-uniform, direction-dependent shrinkage strains are locked in as internal residual stress as the part solidifies; once the part is ejected and free to move, those internal stresses partially relax, and the only way they can relax is by the part bowing or twisting out of its intended shape. Because real parts virtually always have some variation in wall thickness, flow length, and cooling-channel proximity, a perfectly uniform cooling/shrinkage field — the only condition that would eliminate warpage entirely — is not achievable in practice.
Molten thermoplastic is far more viscous than a molten metal at its own processing temperature, and although polymer melts shear-thin (viscosity drops as flow rate increases), a substantial pressure gradient is still needed to drive that viscous melt through a long, narrow flow path — sprue, runner, and gate — and then across a thin-walled, often geometrically complex cavity. The melt must fill the entire cavity before it cools enough at the gate to freeze and block further flow, so injection pressure must be high enough to complete filling quickly, overcoming both the friction/viscous pressure drop along the flow path and the rising resistance as the melt front cools and thickens near the cavity walls as it advances.
As the polymer inside a nominally full cavity continues to cool, it contracts — from ordinary thermal shrinkage and, for semicrystalline polymers, from the additional volume change of crystallization. If no further material were supplied during this contraction, the shrinking polymer would pull away from itself internally, showing up as sink marks on the surface, internal voids, and an undersized final part. The packing (holding) stage keeps pressure applied through the still-open gate after the cavity first fills, forcing a small additional amount of melt into the cavity to compensate for this ongoing shrinkage, right up until the gate itself freezes and can no longer pass material. This materially improves dimensional accuracy and reduces sink marks/voids compared with simply filling the cavity once and releasing pressure immediately.
