Key Idea: Three strategies, three questions: design for process — can the chosen process make this shape? Design for assembly — can it be put together quickly? Design for disassembly — can it be taken apart at the end?\n\nAll three happen before the tooling is ordered. Afterwards they cost a new tool.\n\nDesign for process means matching geometry to the process: draft angles, uniform wall thickness, no undercuts, generous radii.\n\nDesign for assembly means fewer parts, self-locating features, one direction of assembly, no fasteners where a snap-fit will do.\n\nDesign for disassembly and assembly conflict, and the resolution is usually snap-fits that release with a tool — not glue, and not a compromise.
Paper 1 — multiple choice
- Identify a feature that prevents a tool opening
- Say what a draft angle is for
- Recognise a design-for-assembly improvement
- Name the conflict between assembly and disassembly
Paper 2 — analysing a product
- Redesign a part for a named process and justify each change
- Reduce the part count of an assembly and say what is gained
- Analyse a product that cannot be disassembled and explain the cost
- Evaluate a DfM change against its environmental effect
Carried into the design project
- Criterion E — state the process, then show the features that serve it
- Count your parts, then count them again after one revision
- Say how a user or recycler takes your product apart
The three are not alternatives — a well-made product does all three, and they are applied in this order because each one constrains the next.
| Strategy | The question it asks | What it changes on the drawing |
|---|---|---|
| Design for process | Can this process actually make this shape? | Draft angles so the tool releases, uniform wall thickness so it cools evenly, radii instead of sharp corners, no undercuts |
| Design for assembly | How fast and how reliably can it be put together? | Fewer parts, symmetrical or clearly asymmetric parts, self-locating features, one direction of assembly, snap-fits instead of screws |
| Design for disassembly | How does it come apart at the end of its life? | Accessible fasteners, few material types, no glued dissimilar materials, marked polymers, releasable snap-fits |
The same part redrawn three times — once for the process, once for the assembly, once for the end of its life.
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Important: Describing DfM as a cost-cutting exercise applied after the design is finished. It happens before the tooling is ordered or it does not happen at all.\n\nNaming a feature without its process reason — 'a draft angle' scores nothing; 'a draft angle so the moulding releases from the tool' scores.\n\nClaiming to design for disassembly while specifying adhesive between two different polymers.\n\nTreating fewer parts as automatically better when the part that was removed was the one being replaced under warranty.
| The conflict | Why it happens | The resolution |
|---|---|---|
| Assembly wants permanence; disassembly wants release | The fastest joint to make — welding, gluing, a one-way snap — is the hardest to undo | Snap-fits designed with a release feature: fast in, and openable with a simple tool |
| Fewer parts versus repairability | Integrating a wear part into a housing removes an assembly step and removes the repair | Integrate what does not wear; keep the wearing part separate and accessible |
| One material versus performance | Recyclers want one polymer; the product wants stiffness here and grip there | Two grades of the same polymer family, or a co-moulded grip, rather than two unrelated materials glued together |
| Thin walls versus stiffness | Uniform thin walls cool evenly and mould well but flex | Ribs and gussets rather than thick sections — stiffness from geometry, not from material |
Draft angle so the moulding releases; uniform wall so it cools without sinking.\n\nAn undercut stops the tool opening — it needs a side action, and that costs real money.\n\nFewer parts means fewer operations, fewer suppliers, fewer failures and less to get wrong.\n\nSnap-fits with a release feature are the answer to the assembly-versus-disassembly conflict.\n\nDfM is an environmental tool: less material, less scrap, less energy, and a product that can be taken apart.
A desk-lamp base is to be injection moulded. The current design has vertical walls, a sharp internal corner where the stem meets the base, a thick solid boss around the stem, and a lip that curls back underneath. Apply design for process to it.
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A cordless kettle is redesigned from 34 parts held by 12 screws to 19 parts held by snap-fits and ultrasonic welding. Evaluate the redesign.
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Explain why design for manufacture is also an environmental strategy.
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Name the three DfM strategies and their questions.
Why does a moulded part need a draft angle?
What is wrong with a thick section?
What does an undercut cost?
How do design for assembly and disassembly conflict?
When must DfM happen?
Exam tips
- Give the process reason with every feature — 'so the moulding releases', not just 'a draft angle'.
- Say before the tooling is ordered when you explain when DfM happens.
- Answer an undercut with a redesign, not with a side action, unless the shape truly requires it.
- Get stiffness from ribs and geometry, never from a thicker wall.
- Resolve the assembly-disassembly conflict with a releasable snap-fit.
- Link DfM to the environment through material, scrap, energy and recovery.