How to Prepare a CAD File for 3D Printing (and Avoid Reprints)

A failed print usually begins long before the machine is switched on. Most of the parts that arrive unusable were already compromised by a CAD decision — and almost all of them are fixable in under ten minutes at the source.

1. STEP or STL?

Use STEP whenever it is available. STEP is exact solid geometry with real unit information, which means the volume your quote is based on is the volume you designed. STL is a mesh format intended for visualisation — every curved surface in your CAD model has been approximated by flat triangles, and the file carries no unit metadata, so scale errors are common.

If you only have STL, make sure your CAD software exported it in millimetres. An STL exported in inches will import 25.4 times too large and will be rejected.

2. Check the solid is manifold

A printable solid is a closed, connected volume with no gaps, no self-intersections and no internal walls. In practice, the three usual culprits are:

• ✓Tiny gaps left where two surfaces did not quite meet — usually under 0.01 mm.
• ✓Self-intersection where features overlap after a boolean operation.
• ✓A body and shell combined into one body with an internal void the slicer cannot resolve.

3. Respect wall thickness

Design walls at a minimum of twice the nozzle diameter, so 0.8 mm for a standard 0.4 mm nozzle, and 1.2 mm or more for parts that take impact. Below one nozzle width, walls print as intermittent gaps that look acceptable and fail under load.

4. Design for the process orientation

FDM layer lines are the weak axis of every printed part. If your part will be loaded in tension or impact, orient it so that load runs along the layers rather than pulling them apart. A bracket standing on end is dramatically stronger than the same bracket lying flat, and the choice costs nothing.

5. Add a small chamfer instead of a sharp edge

Sharp internal corners concentrate stress and are impossible to print cleanly. A 2–3 mm fillet on internal corners improves both strength and printability, and it is one of the highest-value changes you can make to a CAD model.

6. Avoid unsupported overhangs over 45°

Anything angling downward at more than 45° needs support. Support generation is included in our quote and our engineers place it, but features that cannot be supported at all — deep downward-opening cavities, suspended islands, bridging long gaps — need a design change.

7. Set tolerances deliberately, not nominally

Standard FDM holds about ±0.2 mm; fine-layer FDM reaches ±0.1 mm; DLP resin holds ±0.05 mm. If you need a press fit, design it against the process tolerance rather than the nominal CAD dimension. For a 6 mm hole in FDM, specify 6.3 mm rather than 6.0 mm.

8. Check it fits, and orient it sensibly

Confirm the part fits inside our available build volumes, and think about whether it should be split and joined. A part can often be printed in two sections and bonded, which is cheaper than scaling up the whole model.

9. Tell us what the part does

Two sentences in the notes prevent most material mistakes. Operating temperature, load direction, environment and whether it touches food, chemicals or electronics are all relevant, and our engineers use them to recommend the right material and flag any risk before production starts.

A clean STEP file with sensible wall thickness and chamfered corners gets quoted accurately, prints first time, and costs less — because you are not paying to reprint a failure.