Prototyping and 3D printing

Why it matters

A prototype answers questions that CAD cannot: does it fit, does it feel right, does the mechanism move, will the customer understand it? Rapid prototyping makes those answers cheap and fast. Desktop 3D printers, laser cutters and online quoting services let an engineer hold a part days or even hours after modelling it. The skill is knowing what question each prototype should answer, and preparing the part so the print is right the first time.

Kinds of prototype

• Looks-like: appearance and ergonomics (often printed and painted).
• Works-like: mechanism and function, often ugly.
• Fit check: does it assemble with the real mating parts?
• Engineering or pre-production: made with (or close to) the production process and material for testing.

A 3D-printed prototype rarely matches a production part's strength. Printed parts are usually weaker between layers than along them, and plastics creep under load. Use printed parts to answer geometry and function questions; test production-intent parts for strength.

Common processes

• FDM / FFF (fused filament): melts plastic filament (PLA, PETG, ABS, nylon). Cheap and widely available; visible layer lines; parts are weakest between layers.
• SLA / resin: cures liquid resin with light. Fine detail and smooth surfaces; parts can be brittle; uncured resin is a skin and eye irritant, so handle it with gloves and follow the resin maker's safety data sheet.
• SLS / MJF (powder bed nylon): strong, no support structures needed, usually through a service bureau.
• Metal printing: powder-bed fusion for complex metal parts; expensive and usually a service.
• Laser cutting and CNC routing of flat sheet are also rapid prototyping, and often faster for flat parts.

Preparing a part for FDM printing

1. Export correctly. Export STL or 3MF from CAD. Check the units when you import into the slicer (a 50 mm part arriving as 50 inches or 0.05 mm is a classic error). 3MF stores units; STL does not.
2. Check the mesh is closed (watertight). Most slicers warn about errors and can repair simple ones.
3. Orient the part. Choose a large flat face on the bed; keep overhangs small; and, because layers are weakest in tension between them, orient so the main load runs along the layers, not across them.
4. Overhangs and supports. Most FDM printers handle overhangs up to about 45 degrees from vertical without support. Steeper overhangs and bridges longer than roughly 10 mm (it varies by printer) need supports, which leave rough surfaces.
5. Holes and fits. Printed holes usually come out undersize. Add clearance (often roughly 0.2 to 0.4 mm on diameter for a sliding fit, depending on the printer) or drill or ream to size afterwards. Better: print a small test coupon with a range of hole sizes first.
6. Walls and infill. Strength comes mostly from perimeters (walls), not infill. Three or four perimeters with 20 to 40 % infill is a common starting point for functional parts.
7. Slice and preview. Check the layer preview for gaps, unsupported islands and estimated time and material.

Worked example: a sensor bracket

A small PETG bracket must clip over a 20.0 mm aluminium tube. If we model the clip at exactly 20.0 mm, a printed hole will likely come out a little small and the tube will not fit. Print a coupon with openings of 20.0, 20.2, 20.4 and 20.6 mm first, find the one that slides on firmly (say 20.4 mm on this printer), then set that as a parametric variable (tube_clearance = 0.4 mm) in the CAD model. When the design moves to injection moulding later, change the variable back to the moulding clearance rather than redrawing.

Safety and good practice

• Printer nozzles (around 200 to 260 °C) and heated beds are hot enough to burn; keep hands clear while printing.
• Some filaments, especially ABS and ASA, release fumes and particles while printing. Print in a ventilated space or an enclosure with filtration, following the manufacturer's guidance.
• Never leave a printer running unattended unless your space's rules allow it.
• Follow your school, makerspace or employer's equipment rules and training requirements.

Free and low-cost tools

• PrusaSlicer, Cura and OrcaSlicer are free slicers. You can slice and preview a part, including time and material estimates, without owning a printer.
• Libraries, schools and makerspaces often have printers you can use after an induction.
• Online manufacturing services give instant quotes for printing, CNC and moulding by uploading a model.

Common mistakes

• Wrong units on import.
• Printing to answer a strength question a printed part cannot answer.
• Modelling fits at nominal size and being surprised they do not fit.
• Ignoring orientation, which changes both strength and surface quality.

Recap

• Decide what question each prototype answers.
• Export with the right units, check the mesh, orient for strength and minimal support.
• Plan clearance for fits; test with a coupon; capture the result as a parameter.
• Treat hot parts and fumes seriously and follow your space's rules.

Try it

On the Do tab, prepare a part for printing in a free slicer, from export to a checked layer preview.