The big idea: A CAD model that looks right on screen may be unprintable.
Making one suitable for rapid prototyping is a set of decisions — orientation, wall thickness, clearances, supports and a watertight solid — not an export button.
Five checks, each with the rule and what happens if it is ignored — then the order to do them in.
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| Check | The rule | If you ignore it |
|---|---|---|
| Watertight solid | The model must enclose a volume — no gaps, no self-intersections, no stray surfaces | The slicer cannot tell inside from outside, so it prints a shell or refuses the file |
| Wall thickness | At least two or three times the nozzle or beam width — around 1 mm on a typical printer | A 0.3 mm wall on screen becomes a gap in the part, or a single fragile strand |
| Orientation | Lay the part so the load runs ALONG the layers, and the best surface faces up | Printed the wrong way up, a bracket snaps between layers at a fraction of its load |
| Overhangs | Anything overhanging beyond about 45° needs support that can be reached to remove | Support inside a closed cavity cannot be removed, so the part is scrap |
| Clearance | Mating parts need a designed gap, typically 0.2-0.4 mm per face | Modelled at the exact nominal size, a shaft and its hole fuse together |
Orientation first: Choose which way up the part prints before anything else.
It fixes which faces come out well, which surfaces overhang, and which direction the part is weak in — so deciding it last means redoing the other four checks.
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The three techniques by the property the part must have — finish, cost, or strength.
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| SLA | FDM | SLS | |
|---|---|---|---|
| Finish | Best — finest detail | Visible layers | Grainy and matt |
| Strength | Brittle, degrades in sunlight | Weak across the layers | Strong, near-isotropic |
| Supports | Needed, and they leave marks | Needed for overhangs | None — loose powder supports it |
| Choose it for | Appearance models | Cheap, fast, many prints | Parts that must actually work |
A printed part is not the production part: An FDM part is much weaker across its layers than an injection-moulded one is in any direction, and an SLA part yellows and embrittles in sunlight.
So a printed prototype surviving a test proves less than it appears to. Say which property your prototype could not represent.
How this is tested — constructing CAD models suitable for rapid prototyping. It comes up two ways:
Paper 1 — multiple choice
- Identify why a stated model will not print.
- Pick the correct orientation for a loaded part.
Paper 2 — analysing a product
- Explain how a CAD model is prepared for printing.
- Justify a rapid prototyping technique for a named part.
The trap: Treating printing as an export. Orientation, wall thickness and clearances are design decisions made in the CAD model, and getting them wrong wastes a print and a day.
A drill housing with a snap-fit battery latch is to be printed for user testing. Explain how the CAD model should be prepared, and justify the technique chosen.
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