The big idea: Design for process asks whether the shape suits the way it will be made.
Every manufacturing process has rules the shape must obey — draft on a moulding, a constant section for an extrusion, a reachable feature for a cutter — and a design that breaks them either costs more or cannot be made at all.
Each process, and the tool whose rules the product must obey.
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| Process | The rules the shape must obey | What happens if it does not |
|---|---|---|
| Injection moulding | Draft on every face, uniform wall thickness, radii at internal corners, no undercuts, ribs instead of thick sections | The part sticks in the tool, sinks and warps as it cools, or needs a side-action that doubles the tool cost |
| Vacuum forming | Generous draft, one good face, no sharp internal corners, and a trimmed edge allowed for | The sheet thins to nothing in a deep corner, or the part will not lift off the mould |
| Casting | Thick even sections, generous radii, a draft angle on the pattern, and a machining allowance where a tolerance is needed | Hot spots that shrink into voids, cracks at sharp corners, and a pattern that cannot be withdrawn |
| Machining | Features a cutter can reach, internal corners no smaller than the cutter radius, and a way to hold the part | A shape that needs five set-ups instead of two, or an internal square corner that cannot be cut at all |
| Sheet metal | A minimum bend radius, enough flange for the press brake to hold, and holes kept away from bend lines | Cracked bends, distorted holes, and parts the machine physically cannot grip |
Uniform wall thickness is the one to know: A thick section in a moulding cools more slowly than the thin one beside it, so it shrinks after the surface has set — leaving a sink mark on the outside and internal stress inside.
The fix is never a thicker wall. It is a rib: the same strength, at the same wall thickness.
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| Advantage | Why it follows from the rules |
|---|---|
| A cheaper tool | No undercuts means no side-actions; simple geometry means less tool machining and fewer components in the mould |
| Fewer rejects | Uniform walls do not sink or warp, and generous radii do not crack, so more of what is made is sellable |
| Shorter cycle times | A part with an even wall cools evenly and can be ejected sooner — and cycle time is most of the cost per part at volume |
| Right first time | Fewer tool modifications after the first samples, which is where projects lose weeks and thousands |
| Less material | Ribs instead of thick sections use less polymer for the same stiffness, which is a cost saving and an environmental one |
Design for process is design for a SPECIFIC process: A part designed for machining is a different shape from the same part designed for moulding.
So the process has to be chosen before the detail design, not after it — and a prototype printed in a shape that could never be moulded has proved nothing about the product that will be sold.
How this is tested — outlining the advantages of design for process and applying it. It comes up two ways:
Paper 1 — multiple choice
- Identify the rule a described feature breaks.
- Choose the change that suits a stated process.
Paper 2 — analysing a product
- Explain how a product would be designed for a stated process.
- Outline the advantages of design for process for a named product.
The trap: Designing the shape first and choosing the process afterwards. The process decides the shape, so it has to be chosen before the detail design.
IB-style questionExplain[6 marks]
A designer submits a moulded housing with vertical walls, a solid 8 mm boss, sharp internal corners and a snap hook on an internal face. Explain the four problems and the changes needed.
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