The big idea: A production system does not just make a design — it limits it.
Draft angles, uniform wall thickness, radii instead of sharp corners, split lines where a mould opens, and a shape a cutter can reach: all of them are visible on the product, and all of them exist because of how it is made.
Design for process, assembly and disassembly — the three ways a production system reaches back into the design.
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| Process | What it forces | How you can see it on the product |
|---|---|---|
| Injection moulding | Draft angles, uniform wall thickness, no undercuts, radii at corners | Slightly tapered sides, ribs instead of thick sections, a split line round the part, and small ejector-pin circles inside |
| Vacuum forming | Generous draft, one good face, rounded corners, a trimmed edge | A shape with an open back, a rolled or trimmed rim, and thinner material in the deepest corners |
| Casting | Thick even sections, generous radii, and a machining allowance where a tolerance is needed | A rough as-cast surface with machined faces where it matters, and a parting line round the middle |
| Extrusion | A constant cross-section along the whole length | Every window frame, curtain track and aluminium section you have ever seen |
| Assembly line | Parts that can only fit one way round, and assembly from one direction | Asymmetric mouldings, keyed connectors, and screws all of one size on one face |
The split line is a design decision: A moulded product has a line round it where the two halves of the tool meet, and there is no way to avoid it.
So a designer chooses where it goes — along an edge, under a rim, hidden in a change of form — and a good product makes it look deliberate. That is design responding to manufacture.
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| Limitation | Effect on function | Effect on aesthetics |
|---|---|---|
| Uniform wall thickness | Ribs must carry the load instead of thick sections, so the inside looks structural | The outside can stay smooth and thin-looking, which is why moulded products look lighter than they are |
| Draft angles | No surface is truly vertical, so tolerances have to allow for it | A subtle taper that most people never notice — and that looks wrong when it is absent |
| Assembly from one direction | Faster and less error-prone, with fewer awkward reaches | Fasteners end up on one face, which is why so many products have all their screws underneath |
| Standard bought-in parts | Proven, available and cheaper than designing your own | The product is shaped around a motor or a battery somebody else designed |
| Batch-sized colour runs | Fewer changeovers and less waste | Colours come in ranges rather than being freely chosen, which is why product families share a palette |
Designing WITH the process, not against it: A designer who understands moulding hides the split line, puts the ejector pins where nobody looks, and uses the draft angle as a visible taper.
A designer who does not gets the same features anyway — in the wrong places, looking like faults.
How this is tested — discussing how production methods influence the function and aesthetics of a product. It comes up two ways:
Paper 1 — multiple choice
- Identify the process from a feature visible on a product.
- Choose the design change a stated process requires.
Paper 2 — analysing a product
- Explain how the production method has shaped a named product.
- Evaluate a manufacturing limitation on function and appearance.
The trap: Treating manufacturing limits as compromises only. A good designer uses them: the split line becomes a line of the form, and the draft angle becomes a taper.
IB-style questionAnalyse[6 marks]
Analyse how injection moulding has shaped the function and appearance of a plastic kettle body.
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