The big idea: Rapid prototyping makes a base model to be judged and thrown away. Additive manufacturing makes a part that goes into the product and stays there.
Often it is the same machine and the same process. What changes is what the part has to survive — and therefore the material, the tolerance, the finish and the inspection.
The same technology judged against two different sets of requirements, row by row.
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| Base model (rapid prototyping) | Production part (additive manufacturing) | |
|---|---|---|
| The question it answers | Is this the right shape? Does it fit the hand? Does it clear that part? | Will this part do its job for the life of the product? |
| How long it must last | One meeting, one test, one user session | Years of service, in the product's real environment |
| Material | Whatever is loaded — colour and speed matter more than properties | Chosen and specified for strength, temperature and chemical resistance |
| Accuracy | Close enough to hold and judge | Within tolerance, every part, repeatably |
| Finish | Layer lines are fine, and sometimes useful for showing it is a model | Specified, measured and usually post-processed |
| The cost that matters | Hours of a designer's time | Cost per part, at the volume required |
A base model earns its keep by being wrong: The point of an early model is to find the mistake now rather than after tooling.
So a model that reveals a problem in ten minutes has done its job better than one that looks beautiful and tells the designer nothing.
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| Additive production wins when… | Why |
|---|---|
| The volume is low | There is no tool to pay for, so the first part costs what the thousandth costs — below a few thousand parts that beats moulding |
| The shape is complicated | Internal channels, lattices and undercuts that no mould could release are no harder to print than a cube |
| Every part must differ | A hearing aid shell or a dental aligner is matched to one person, and a printer does not care that each file is different |
| The part is needed now | A spare can be printed on demand rather than stocked or tooled, which removes an inventory as well as a lead time |
And where it still loses: At high volume, because injection moulding produces a part in seconds and printing takes hours.
On large simple shapes, where forming is far cheaper. And anywhere the layer-to-layer weakness or the surface finish cannot be accepted.
How this is tested — distinguishing rapid prototyping for testing from additive techniques for production. It comes up two ways:
Paper 1 — multiple choice
- Identify whether a described part is a prototype or a production part.
- Choose the requirement that changes between the two.
Paper 2 — analysing a product
- Explain the difference between a base model and a production part.
- Justify using additive manufacturing for a production component.
The trap: Saying the difference is the machine. It is usually the same machine — what changes is the specification the part has to meet.
A company makes 800 custom hearing-aid shells a month, each matched to one ear. Justify additive manufacturing for production rather than moulding.
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