The big idea: Every manufacturing technique belongs to one of five categories: additive, subtractive (wasting), forming, joining and finishing.
Place an unfamiliar process: what happens to the **amount of material**?
- More of it — additive
- Less of it — subtractive
- The same amount in a new shape — forming
- Separate pieces becoming one — joining
- Only the surface changed — finishing
Each category drawn as what happens to the material — which is what the category name actually means.
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| Category | Processes | What it suits |
|---|---|---|
| Additive | 3D printing — FDM, SLA, SLS, powder bed fusion, laminated object manufacture | One-offs and low volumes, complicated internal shapes, and parts that could not be machined or moulded at all |
| Subtractive (wasting) | Cutting, sawing, drilling, milling, turning, laser and water-jet cutting, and abrading — filing, sanding, grinding | Accuracy and surface finish, in any material, with no tooling to make first — which is why prototypes and mould tools are machined |
| Forming | Bending, press-forming, casting, and moulding: injection, extrusion, rotational, blow and vacuum | Volume. The tool is expensive and every part after it is fast and cheap, with almost no waste |
| Joining | Adhering, fastening, stitching, weaving and welding | Anything made of more than one part — and it is where a designer decides whether the product can ever come apart again |
| Finishing | Anodising, electroplating, galvanising, powder coating, painting, polishing, and sealants such as oil, wax and silicone | Protecting the material, changing how it looks and feels, and extending the life of the whole product |
The material decides as much as the shape: A category is only available if the material allows it. Timber can be cut, bent with steam, joined and finished, but not cast or injection moulded.
A thermoset can be cast and moulded once, and can never be melted and reformed. A thermoplastic can.
Fusing is joining: Fusing melts two parts where they meet so they set as one, as in welding or heat-sealing a bag. It joins parts, so it belongs to joining, not forming.
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| Material | Typical processes | What rules a process out |
|---|---|---|
| Thermoplastic | Injection, blow, rotational and vacuum moulding; extrusion; machining; welding; adhering | It softens with heat, so it cannot be used hot, and it creeps under long steady loads |
| Metal | Casting, pressing, bending, machining, welding, plating and anodising | Casting needs a melting point a foundry can reach; welding needs compatible metals |
| Timber | Sawing, planing, routing, steam bending, gluing, screwing, oiling and lacquering | It cannot be cast or moulded, it moves with humidity, and it is far stronger along the grain than across it |
| Composite | Laying up in a mould, curing, trimming, bonding | It cannot be reshaped once cured, and it is very difficult to recycle for the same reason |
Answering "why this process": Every mark is a pairing: the technique, and the reason for it.
The reason is always one of a short list — a material property, a volume, a tolerance, a cost, or the environment the product lives in.
How this is tested — outlining additive, subtractive, forming, joining and finishing techniques. It comes up two ways:
Paper 1 — multiple choice
- Place a named process in its category.
- Choose the category that suits a stated requirement.
Paper 2 — analysing a product
- Outline the categories used to make a named product.
- Explain why a category was chosen for a given component.
The trap: Naming the category and stopping. The reason — a property, a volume, a tolerance, a cost or an environment — is where the mark is.
A stainless steel vacuum flask has a double-walled body, a moulded lid and a printed logo. Outline the manufacturing categories used and the reason for each.
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