Preparing a 3D file for printing: the guide

In short: A print-ready file means a closed mesh exported at the right resolution (STL or STEP), walls of at least 0.8 to 1 mm, overhangs under control and, above all, tolerances thought through in advance. Allow 0.2 to 0.4 mm of clearance for a moving part, and slightly oversize your holes. A good file is already half of a good part.

A poorly prepared file is paid for in delays, extra costs or a failed part. A clean model, by contrast, prints first time and comes out to the expected dimensions. This guide covers, step by step, everything that matters in delivering a genuinely production-ready file: the format, the design rules and the point that trips people up most often — tolerances.

1. Choosing and exporting the right file format

Which format for which use

STL remains the standard for 3D printing. It describes only the surface of the part, as a mesh of triangles. That is enough in the vast majority of cases, but it loses two useful pieces of information: units and design history.

STEP (and IGES) come from CAD. They preserve the exact, parametric geometry, and the units. Preferable for technical parts, tight dimensions or subsequent machining.

3MF and OBJ are modern formats that carry colours, units and sometimes print settings. 3MF is gaining ground because it is more reliable than STL and much lighter.

Exporting the STL at the right resolution

This is the most frequent silent mistake: an STL exported with default settings, either too coarse (visible facets on curved surfaces) or needlessly heavy. Two parameters drive everything in your CAD software.

• Chord deviation (chord height, surface deviation): the maximum permitted gap between the real surface and the mesh. Set it between 0.01 and 0.05 mm, never going below 0.001 mm. A good rule: roughly 1/10 to 1/20 of the machine's layer height.
• Angular deviation (angle tolerance): the maximum angle between two neighbouring triangles. The default value of 15° is almost always fine; go down towards 5° only for highly curved surfaces.
  
  

Export as binary STL (far lighter than ASCII). There is no point aiming at extreme resolution: beyond a certain point the file balloons with no gain, because no printer reproduces those microscopic details.

The mesh must be closed (watertight)

A printable model is a closed, manifold volume: no hole in the surface, no inverted face, no edge shared by more than two faces. If the mesh is not watertight, the slicer cannot tell where the inside of the part is, and the print fails or goes astray. Tools such as Meshmixer, Netfabb or the repair function built into slicers correct most of these faults automatically.

2. Design rules (DFAM)

Designing for 3D printing takes a little more than drawing then starting the machine. A few simple rules avoid most failed parts.

Wall thickness

This is parameter number one. A wall that is too thin will not print, or breaks on removal. In FDM, the standard nozzle is 0.4 mm and lays down two perimeters, so about 0.8 mm of minimum wall. In resin you can go lower, but a wall that is too thin buckles under peel forces.

Overhangs, bridges and supports

Printing happens layer by layer: each layer has to rest on something. The 45° rule sums it up: an overhang leaning less than 45° from vertical prints without support. Beyond that, the material droops and supports are needed, which leave marks when removed.

A bridge (a horizontal surface spanning two supports) prints without support up to about 5 to 10 mm of span. Beyond that, plan a support or rethink the orientation.

A design tip: replace a sharp overhang with a 45° chamfer or fillet, and you remove the need for supports while strengthening the part.

Holes and drillings

Holes are a classic trap. In FDM, a hole printed vertically comes out slightly undersized (material shrinkage, about 0.1 to 0.2 mm). If the fit is critical, oversize the hole by around 0.2 mm on the drawing, or plan to ream it after printing.

A hole printed horizontally (axis parallel to the bed) deforms and becomes oval or teardrop-shaped. Two solutions: orient the part so the hole is vertical, or draw the hole as a teardrop to compensate.

Orientation and anisotropy

In FDM especially, a part does not have the same strength in every direction. Layers bond well to each other, but remain weaker along the Z axis (the part can split between two layers under load). Design and orient the part so the main forces act in the plane of the layers, not perpendicular to them. That orientation also affects surface finish, the supports needed and print time.

Shrinkage and distortion

Every plastic cools and shrinks. Shrinkage runs from 0.2 to 1 % depending on the material (see our guide to choosing the right 3D printing material), and more for ABS/ASA, which tends to warp on large flat surfaces. These distortions are limited with fillets at the corners, a wide base, and by avoiding large solid flat surfaces.

3. Tolerances: the point that decides everything

This is where most projects go wrong. A part can be perfectly modelled and still fail to assemble, because the designer worked in "exact" dimensions while 3D printing works with a margin. Here is how to think about it correctly.

Dimensional accuracy depends on the technology

Each process holds a different accuracy. Desktop FDM is the least precise, resin and SLS considerably tighter. If you are still unsure about the process, start with our guide to which 3D printing technology to choose.

The practical consequence: do not ask for a dimension to the hundredth if the process holds to the tenth. Reserve tight dimensions for the surfaces that genuinely need them (bearing faces, assemblies), and leave the rest on a loose tolerance.

Assembly clearances (the table to keep to hand)

As soon as two parts have to fit, slide or rotate one inside the other, clearance is decisive. Too little and the parts fuse or jam; too much and the assembly is sloppy. Here are the reference values in FDM, expressed as total clearance on diameter (the gap between shaft and hole), with an example on a 10 mm shaft.

Indicative values in FDM. Resin being more precise, clearances can be tightened by about 0.1 mm, while keeping at least 0.4 to 0.5 mm between two separate parts printed together so they do not fuse.

A few habits that make the difference:

• Test before running a batch. Print a small test part with several clearances (0.1 / 0.2 / 0.3 / 0.4 mm) and keep the one that works. Every machine and every material behaves slightly differently.
• TPU and flexibles compress: add another 0.1 to 0.2 mm of clearance.
• ABS/ASA shrinks more: pre-enlarge the part by 0.5 to 1 %, and allow 0.4 to 0.5 mm of clearance for a sliding fit.
• "Elephant's foot" (the first layers spreading as they are squashed onto the bed) eats into the clearance at the bottom of parts: add 0.1 mm or design a small starting chamfer.

Threads and inserts

Printing a fine thread directly in FDM rarely gives a good result. Two more reliable approaches: model a coarse thread (≥ M6) with 0.25 to 0.5 mm of clearance and a lead-in chamfer, or better, plan for a heat-set threaded insert fitted after printing, far stronger and reusable. In resin, fine threads work better thanks to the higher accuracy.

4. The checklist before sending your file

• Closed, error-free mesh (checked in a repair tool)
• Correct scale and units (millimetres, almost always)
• Correct export resolution (deviation 0.01 to 0.05 mm, angle 15°)
• Walls ≥ 0.8 mm (FDM) or ≥ 0.6 mm (resin)
• Overhangs < 45° or supports accepted
• Holes oversized if the fit is critical
• Assembly clearances chosen according to the type of fit
• Orientation thought through for strength and surface finish
  
  

At Symbio3D, you upload your file straight into the online configurator, and we check printability before starting.

FAQ

Which format should I send: STL or STEP?
STL is enough for most prints. STEP is preferable for very precise parts, tight dimensions or subsequent machining, because it preserves the exact geometry and the units.

I have no CAD software — can I still order?
Yes. Our online designer lets you configure parts without a technical drawing.

What is the minimum wall thickness?
About 0.8 to 1 mm in FDM and 0.6 mm in resin. The thinner the wall, the more fragile the part or the greater the risk of warping.

What clearance should I leave between two parts that have to move?
Allow 0.2 to 0.4 mm of total clearance in FDM for a sliding fit, slightly less in resin. For a tight assembly, aim rather for 0 to 0.1 mm, and test on a small part before a batch.

Why do my holes come out too small?
That is normal in FDM: the material shrinks and a vertical hole comes out undersized by about 0.1 to 0.2 mm. Oversize it on the drawing, or plan to ream it after printing.

Is your file ready? Upload it and get an instant quote. We check printability and tolerances and alert you to any issue before starting production.