A printed part is not a moulded part with a different lead time. It is built from 0.4 mm-wide lines stacked in layers, and almost every problem we see in a customer file comes from a geometry that ignores that. The eleven rules below decide whether a part survives its first week, whether it needs a second iteration, and whether the quote is small or large. They apply whether you draw in Fusion, SolidWorks, Onshape or Blender.
Key takeaways
- Wall thickness should be a multiple of the 0.42 mm extrusion width. 0.84 mm is the practical minimum; 1.26–1.68 mm is where functional parts live.
- Overhangs shallower than 45° from vertical print cleanly. Steeper than that needs support, a chamfer, or a different orientation.
- Holes print 0.2–0.4 mm undersized. Draw them oversize, or specify that they are reamed after printing.
- A printed part is strong across the layers and weak between them. Tell us how the part is loaded and we will orient it accordingly.
- Clearance for a sliding fit is about 0.2 mm per side; for a snug press fit, about 0.1 mm. Zero clearance welds the two parts together.
1. Match wall thickness to the nozzle
A 0.4 mm nozzle lays a line about 0.42 mm wide, and a wall is built from a whole number of those lines. A wall drawn at 1.0 mm therefore does not print at 1.0 mm: the slicer fits two lines and leaves a gap, or squashes them together, and either way the wall is weaker than your model suggests.
Draw walls as multiples of the extrusion width. 0.84 mm is the practical minimum; 1.26 mm to 1.68 mm is where functional parts belong: brackets, enclosures, fixtures, clips. Thicker is not automatically better: past roughly 2 mm the extra adds print time and mass rather than strength, because the load is carried by the perimeters. A rib is cheaper than material everywhere.
2. Keep overhangs above 45°
Each layer is laid on the one below. If a layer overhangs by more than about half a line width it has nothing to sit on and droops. The working rule is 45° from vertical: lean back less than that and the face prints cleanly, lean back more and it needs support material: a rougher surface and removal time you are paying for.
The fix is almost always a chamfer. A 45° chamfer under a boss, a lip or a horizontal edge costs nothing in material, removes the support entirely, and usually makes the part stronger by putting material exactly where the stress concentrates.
3. Know the bridging limit
A bridge is a horizontal span printed across open air between two supported points, and it works because the extruded line is stretched between the ends and cooled fast enough to hold. In PLA, with full cooling, spans up to about 50 mm are reliable. In PETG, which has to be cooled less so the layers weld, treat 25 to 30 mm as the comfortable limit.
Long spans sag slightly even when they succeed, so anything that has to be flat should not be bridged. Where a feature needs a horizontal roof, put a shallow arch or a pair of 45° faces on it instead of a flat ceiling.
4. Holes print undersized. Allow for it
This is the most common reason a part comes back not fitting. A circular hole is approximated by short straight segments laid on the inside of the circle, and the plastic then shrinks slightly as it cools. The result measures 0.2 to 0.4 mm smaller than the model, with the error largest on small holes and on vertical holes printed through the layers.
Three sensible responses. Draw the hole 0.3 mm oversize if it takes a fastener. Draw it at nominal and have it reamed afterwards, which is right where the fit is critical. Or, for a clearance hole, draw it a full millimetre over.
| Feature | Allow | Why |
|---|---|---|
| Minimum wall | 0.84 mm | Two extrusion widths |
| Functional wall | 1.26–1.68 mm | Three to four widths |
| Hole diameter | +0.2 to +0.4 mm | Prints undersized |
| Sliding or loose fit | 0.2 mm per side | Moves freely |
| Snug or press fit | 0.1 mm per side | Assembles with force |
| Maximum overhang | 45° from vertical | Beyond it, support |
| Bridge span, PLA | 50 mm or less | Full cooling |
| Bridge span, PETG | 30 mm or less | Reduced cooling |
| Embossed text | 0.8 mm stroke | Below it, unreadable |
| Part envelope | 256 mm cube | Larger is split |
5. Orientation is an engineering decision
Printed parts are anisotropic: strong within a layer, weaker between layers, because adjacent layers are joined by a partial weld rather than continuous material. Pull a part along the layers and it behaves much like solid plastic; pull it across them and it can fail at a fraction of that load. A hook printed flat on the plate snaps along the layer line at its root, while the same hook printed upright bends first. Same file, same material: the only difference is which way up it was built.
You do not need to decide this yourself. Tell us how the part is loaded and we orient it accordingly: the cheapest strength improvement available on any printed part.
6. Use the right clearance for the fit
Two printed parts drawn to touch will not assemble, because plastic squashes out slightly at every layer. About 0.2 mm per side gives a loose, sliding fit: a lid that comes off easily, a shaft that turns. About 0.1 mm per side gives a snug fit that assembles with deliberate force and stays put. Zero clearance gives you two parts fused together and a scalpel.
For threads, print a clearance hole and use a heat-set insert or a nut trap rather than printing the thread: printed threads below a few millimetres of pitch are rarely worth the trouble on a working part.
7. Put a fillet where the load turns a corner
Most printed parts fail in the same place: the sharp internal corner where a wall meets a base, or where a boss meets a face. That corner is a stress raiser, and a layer boundary running through it makes matters worse. A 1 to 2 mm fillet removes most of the problem and costs nothing.
It is also the right response to a broken part sent in for copying: reproducing the original faithfully reproduces the failure, so we normally propose thickening the cracked section and adding a fillet.
8. Emboss rather than engrave
Raised text prints far more reliably than recessed text, because a nozzle can lay a clean line on top of a surface but struggles to carve a narrow channel into one. Keep the stroke at 0.8 mm or more and the relief at 0.4 mm or more; below that, small type fills in and becomes unreadable. For part numbers and batch marks on production hardware, embossed lettering on a flat top face is the most legible option and survives handling.
9. Split large parts deliberately
Anything larger than the 256 mm envelope has to be split and bonded, and that is better done in CAD than by us in the slicer. A designed split puts the joint where it is hidden, takes no load, and can carry a locating feature: dowel pockets, a lap step, a keyed tongue.
Splitting can also be right well below the size limit. A tall thin part may need extensive support in one piece; split into two flat halves it needs none, prints faster and comes out stronger. We always show you where the seam would fall first.
10. Design supports out, do not add them
Support material is a cost, not a feature: it adds filament, print time and removal labour, and leaves a scarred surface where it was attached. Every support removed at the design stage is money off the quote.
The moves are simple. Chamfer the underside of overhanging features. Make horizontal holes teardrop-shaped so the top is self-supporting. Split the part so the difficult face sits on the plate. Move a boss outside a wall instead of hanging it inside one.
11. Export properly
A surprising share of quoting problems are export problems. Send STL or 3MF, and prefer 3MF where your CAD offers it: it carries units and orientation explicitly. STL is unitless, which is why a model occasionally arrives at a tenth or ten times its intended size.
Fusion: Save as Mesh with refinement High, Medium visibly facets a curved surface. SolidWorks: Save As, STL, Options, Fine. Onshape: Export, STL or 3MF, resolution Fine, units millimetres. Blender: apply every modifier and the object scale, set the scene unit to millimetres, and confirm the mesh is manifold: Blender will happily export a shell with holes in it that no slicer can fill.
One last check: is the model watertight, are the normals consistent, and is the part the size you think it is? Our instant quote page measures volume, surface area and bounding box in your own browser and tells you immediately when the numbers look wrong. Nothing is uploaded unless you send the file with an order.
Three of these matter more than the rest. Get wall thickness and orientation right and most parts work; get the hole allowance right and most assemblies go together. Everything else is refinement, which is what a second iteration is for, so prototype in cheap PLA before committing to a production material. That choice is covered in PLA against PETG, and the cost of any of it in the UK 3D printing cost guide.