Manufacturing processes and production techniques
Manufacturing questions test whether you can choose a process from material, volume, tolerance and cost - not from habit - and whether you can explain the consequences of that choice.
Scales of production
Job/one-off (skilled labour, high unit cost, no tooling outlay), batch (jigs and fixtures amortise setup costs), mass (dedicated lines, automation), continuous (film, sheet, paper - running 24/7). Total cost = fixed + (variable × quantity), so the process with the lowest total at your volume wins; expensive tooling only pays at volume.
Process families
- Casting: sand (ferrous, large parts), die (non-ferrous, high volume, costly tool), investment (fine detail, complex shapes).
- Polymer moulding: injection (cycle: clamp–inject–cool–eject; uniform wall thickness, 1–2° draft, 0.5–2% shrinkage), blow (bottles), rotational (tanks), vacuum forming (thin sheet, cheap tool, no undercuts), compression (thermosets, GRP).
- Forming: forging (grain flow, toughness), rolling, extrusion (constant section), press forming, bending.
- Machining: turning, milling, drilling, CNC. A jig locates and guides the tool; a fixture only locates and holds.
- Joining: welding (parent metal fused), brazing (filler above 450 °C, parent not melted), soldering (below 450 °C), adhesives, mechanical fasteners, knock-down fittings.
- Additive: FDM, SLA, SLS - no tooling, ideal for one-offs, weaker across layer boundaries.
Worked example
Choose a process for 20,000 polypropylene lids. Injection tooling ≈ €20,000 with €0.06 per part; vacuum forming tooling ≈ €1,500 with €0.14 per part plus a trimming operation. At 20,000: injection = 20,000 + 1,200 = €21,200; vacuum forming = 1,500 + 2,800 = €4,300. Break-even = (20,000 − 1,500) ÷ (0.14 − 0.06) ≈ 231,000 parts. Decision: vacuum form at this volume, but specify injection if the forecast exceeds roughly 230,000 or if tolerance and surface finish rule vacuum forming out. The reasoning - fixed cost divided by per-part saving - is what earns the marks.
Traps
- Choosing a process without stating volume, material or tolerance.
- Forgetting draft angle and shrinkage, so parts cannot be ejected or come out undersized.
- Swapping jig and fixture.
- Claiming brazing melts the parent metal.
- Confusing tolerance (permitted variation) with allowance (deliberate clearance or extra material).