The big idea: Design for process asks whether the shape suits the way it will be made.
Every process has rules the shape must obey
- Draft on a moulding.
- A constant section for an extrusion.
- A reachable feature for a cutter.
Break the rules and pay: 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 faces parallel to the mould-opening direction, 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 adds tooling complexity and 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where a tolerance is needed | Hot spots that shrink into voids, cracks at sharp corners, and a pattern that cannot be withdrawn |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
Uneven cooling
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.
Change the shape
Core out thick sections and add suitable ribs where needed. Check wall and rib dimensions for the material and moulding process.
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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 reduce uneven cooling, and suitable radii reduce stress concentration, 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
Match the manufacturing method
A part designed for machining is a different shape from the same part designed for moulding.
Check production feasibility
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 not yet proved that the final production process can make it.
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.
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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