Most prints that fail don't fail because of the printer. They fail because of the file. A wall that's too thin to survive being picked up, a bolt hole modelled at exactly the bolt's diameter, a bracket oriented so it snaps along the layer lines the first time it's stressed. The good news: every one of those is a five-minute fix at the design stage, and once you know what to look for you'll catch them yourself.
This is a practical guide for FDM printing, the melted-thermoplastic process we run at Azata, in materials like PLA and PETG. It's written for the people who actually send us files: product folks, engineers, students, and anyone with a part they need made. No theory you won't use. If you'd rather we just check your file, send it over and we'll flag anything before it prints.
Wall thickness: the one that breaks the most parts
Walls that are too thin are the number-one reason a part comes out flimsy or fails outright. On an FDM machine the nozzle lays down lines of plastic, so a wall has to be at least a couple of those lines wide to hold together.
The working numbers we design to:
- 0.8 mm is the practical minimum for a wall that just needs to exist (a shell, a cosmetic surface).
- 1.2, 2.0 mm is the comfortable range for most parts, strong enough to handle, easy to print cleanly.
- 1.5 mm and up for anything that takes load or gets handled a lot. When in doubt, go thicker here.
Going the other way matters too. Very thick solid walls don't make a part meaningfully stronger past a point, they just add print time, material, and heat that can warp the part. If you need stiffness, ribs beat a thick slab almost every time.
Tolerances and fit: leave room on purpose
FDM is accurate, not laser-accurate. On a well-calibrated machine, expect roughly ±0.3 mm on features under about 100 mm, and around ±0.3% of the dimension on bigger parts, so a 200 mm part might land half a millimetre off. That's normal for the process.
What it means for you: if two parts have to fit together, don't model them at the same size. Leave a gap.
- Loose / sliding fit: ~0.5 mm of clearance.
- Snug / press fit: ~0.2, 0.3 mm.
- Bolt holes: model them 0.4, 0.5 mm larger than the bolt, or plan to drill them out after.
Rule of thumb: if you designed a hole at the exact size of the thing going into it, it won't fit. Plastic shrinks a touch as it cools, and the first layers tend to squish slightly wide. Always give it clearance.
Overhangs and supports
Because the printer builds up in layers, each layer needs something underneath it. Steep overhangs print fine; flat ones sag. The line most FDM parts hold cleanly is about 45 degrees from vertical. Past that, the printer adds support material that you (or we) snap off afterward, which costs time and can leave a rougher surface.
You can often dodge supports entirely by rethinking the shape: chamfer a sharp overhang into a 45° slope, add a small fillet, or split the part into two pieces that each print support-free and glue together. If a clean surface matters on a particular face, tell us, orientation decides which face gets the supports.
Orientation and strength: parts are weakest along the layers
This is the one people miss most. A 3D printed part is not equally strong in every direction. The bond between layers is weaker than the plastic within a layer, so a part tends to snap along the layer lines, like splitting wood along the grain.
Design with that in mind. A hook, a clip, a bracket, anything that bends or pulls, should be oriented so the force runs along the layers, not across them. If you're not sure how a part will be stressed in use, tell us when you send it and we'll orient it for strength where it counts.
Holes, threads, text and small details
Holes
Vertical holes print rounder than horizontal ones (horizontal holes can come out slightly egg-shaped and may need reaming). For anything that needs to be precise, design it a hair undersized and drill to final size.
Threads
Printed threads work for low-stress, large-diameter cases. For anything that gets tightened repeatedly or carries load, design for a threaded insert (heat-set brass) or a captured nut, far more durable than plastic threads.
Text and fine features
Embossed (raised) text reads better than engraved on FDM. Keep lettering at least ~3 mm tall with a bit of depth, and keep any tiny standalone feature above ~1 mm or it may not survive printing and cleanup.
Infill: you usually need less than you think
Infill is the internal lattice that fills the inside of a part. More infill means more strength and more weight, time and cost, but the curve flattens fast. For most parts, 15, 25% is plenty. Go higher (40%+ or solid) only for parts under real mechanical load. If you don't specify, we'll pick a sensible infill for how the part is used; if strength is critical, say so.
How to send us a file
We work with STL, STEP and OBJ. STEP is our favourite because it keeps the exact geometry; STL is fine for most jobs. A few things that make a quote faster and the result better:
- Model in millimetres, real-world scale, and tell us the intended size if it's ambiguous.
- Make sure the model is "watertight" (no holes in the mesh). If you're not sure, send it anyway, we check.
- Tell us how the part is used and which surfaces or dimensions matter most.
No model yet? A dimensioned sketch or even a clear photo of the thing you're replacing is often enough for us to work from.
The 60-second pre-flight check
- Walls at least 1 mm, thicker where it takes load.
- Clearance left on every part that fits another (~0.5 mm).
- Overhangs near 45° or designed to avoid supports.
- Stress running along the layers, not across them.
- Inserts planned for any thread that matters.
- File in mm, watertight, saved as STEP or STL.
Frequently asked questions
What's the strongest material you print in?
For everyday parts PLA is stiff and easy; PETG handles heat, moisture and impact better, which makes it the usual pick for functional parts. Tell us where the part lives and what it does, and we'll recommend the material in your quote.
Can you make my part stronger without redesigning it?
Often, yes, by printing it in a tougher material, raising the infill, adding more wall perimeters, or simply orienting it so the load runs along the layers. We'll suggest the cheapest change that gets you the strength you need.
I don't have a CAD file. Can you still help?
Yes. Send a sketch, measurements or a photo and we can prepare a printable model for you. Light modelling and file fixes are part of the service.
Will you check my file before printing?
Always. Every file gets a quick design-for-print review, and we'll flag thin walls, risky overhangs or fit problems before anything goes on the machine, not after.
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