Question 4 of 7: Extrusion and Injection Molding of Plastics, Typical Products, and Reinforced-Plastic Design Characteristics
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams — December 2014 — 98-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. — material selection, casting, metal-cutting theory and machinability, polymer processing, welding, grinding/finishing, and automation/numerical control; Montgomery, Introduction to Statistical Quality Control, 8th ed. — statistical process control and X-charts.
Question 4: Extrusion and Injection Molding of Plastics, Typical Products, and Reinforced-Plastic Design Characteristics (20 marks: 7/6/7)
(i) Characteristics of Extrusion and Injection Molding
(a) Extrusion. Polymer pellets are gravity-fed into a heated barrel and melted, then conveyed and further heated by a rotating screw, and forced continuously through a shaping die that imparts a constant cross-section; the emerging profile is cooled (air or water bath), pulled by a haul-off at a controlled rate, and cut to length or coiled. It is a continuous process, well suited to long constant-cross-section shapes, with relatively low tooling cost (only the die need be changed between products) and high production rate; it cannot produce parts whose cross-section varies along their length.
(b) Injection molding. Polymer pellets are melted in a heated barrel and injected under high pressure through a nozzle into a closed, cooled, matched-metal mold cavity; the part solidifies under continued packing pressure, the mold opens, and the part is ejected. It is a cyclic (discrete-part) process capable of complex three-dimensional geometry, fine surface detail, and tight dimensional tolerance in a single step, with high production rate per cycle once running, but the mold itself is expensive to design and machine, so the process is economical only at moderate-to-high production volumes that amortize the tooling cost.
(ii) Typical Products of Extrusion and Injection Molding
Extrusion: pipe and tubing, sheet and film, wire and cable insulation coating (wire drawn through the die centre as the polymer is extruded around it), weatherstripping and other continuous gaskets, siding and window profiles, and drinking straws; blown-film extrusion (a variant) produces plastic bags and packaging film.
Injection molding: bottle caps and closures, food and consumer-product containers and housings, gears and small mechanical components, automotive interior trim and dashboard components, toys, and electronic device enclosures — essentially any discrete part with moderate-to-complex geometry produced in moderate-to-high volume.
(iii) Design Characteristics/Properties of Reinforced Plastics and Composites
A reinforced plastic (composite) combines a polymer matrix with a reinforcing fibre (glass, carbon, or aramid) and its properties/design considerations differ fundamentally from an unreinforced (bulk) plastic:
Anisotropy. Strength and stiffness are highly direction-dependent, governed by fibre orientation and layup sequence — unlike an isotropic bulk plastic, the designer must specify fibre orientation to align strength with the principal load path.
High specific strength/stiffness. Strength- and stiffness-to-weight ratios can exceed those of many metals, which is the primary reason composites are chosen over unreinforced plastic or metal in weight-critical applications.
Matrix vs. reinforcement roles. The resin (matrix) governs maximum service temperature, chemical/environmental resistance, and transfers load between fibres, while the fibres carry the bulk of the mechanical load — so matrix and reinforcement must be selected and evaluated as separate design decisions, not as one material.
Joint and fastening design. Mechanical fasteners (bolted/riveted joints) create stress concentrations and can cause interlaminar delamination around the hole; adhesive bonding is generally preferred because it distributes load over a larger area and avoids cutting the reinforcing fibres.
Failure modes. Composites are prone to delamination (separation between plies) and matrix cracking rather than the ductile yielding typical of metals, so factors of safety and inspection methods (ultrasonic/NDT for internal delamination) differ from metal design practice.
Recyclability. Thermoset-matrix composites in particular are difficult to recycle because the cured resin cannot be re-melted, which must be weighed against the weight-saving benefit at the design stage.
