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.
Continuous reinforcing fibre — glass, carbon, or aramid, either drawn through a resin bath just before winding (wet winding) or supplied as pre-impregnated tow (prepreg) — is wound under controlled tension onto a rotating mandrel that defines the internal shape of the part. A computer-controlled winding head traverses along the mandrel's length as it rotates, laying down fibre at a precisely controlled angle (helical, hoop, or polar patterns) so that successive layers build up the wall thickness and fibre orientation needed for the part's expected loading. Once winding is complete the assembly is cured (at room temperature or in an oven/autoclave), and the mandrel is then removed — collapsible, soluble, or mechanically extracted, depending on the part's shape. The process is best suited to axisymmetric, hollow parts subject to internal pressure or axial/hoop tension, such as pipe, pressure vessels, and rocket-motor casings, because the fibre angle can be chosen to align directly with the dominant stress direction.
Sheet molding compound is a ready-to-mold composite sheet produced continuously on a dedicated machine: chopped (or sometimes continuous swirl-mat) reinforcing fibre is deposited into a thickened thermosetting resin paste — typically an unsaturated polyester with fillers and a maturation/thickening agent — and sandwiched between two carrier (release) films, then rolled to compact and de-air it. The resulting sheet is stored to mature (thicken to a leather-like, handleable consistency), then cut into charges of the size and shape needed for a given part and compression molded between matched, heated metal dies, which simultaneously shape the charge and cure the resin, typically within a few minutes. SMC's main appeal is that it combines the fast cycle time of compression molding with fibre-reinforced mechanical properties, and it is widely used for moderate-to-high-volume structural parts such as automotive body panels.
In spray-up, continuous fibre roving is fed through a chopper gun that cuts it into short lengths and sprays it, together with a stream of catalyzed resin (commonly polyester), directly onto an open mold surface. Because the fibre and resin are deposited simultaneously and somewhat randomly oriented, an operator then works over the freshly sprayed layer with a hand roller to compact it, drive out entrapped air, and thoroughly wet the fibres with resin before it cures (typically at room temperature). Spray-up is an open-mold process, so tooling cost is low and large parts (boat hulls, tub/shower enclosures, tank linings) are practical at low-to-moderate production volumes, but because fibre content and orientation are far less controlled than in filament winding or a prepreg process, spray-up parts have correspondingly lower and more variable mechanical properties.
