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3D Print File Prep: Check Your STL Before You Order

By Dynafabs

Preparing a 3D print file? Check format, units, watertight mesh, thin walls and clearances before you order, so your STL prints right first time.

Why file prep matters before you order

A 3D printer can only print what's in the file. If a model is in the wrong units, has holes in its mesh or features thinner than the nozzle can lay down, the print will come out wrong no matter how good the material or settings are. The good news is that most problems are easy to spot and fix before you send anything over.

This guide walks through the checks worth doing on any 3D file before you ask us for a quote, whether you designed it yourself in CAD or downloaded it from a model site. A few minutes here saves a reprint later.

Pick a sensible file format

The most common formats for 3D printing are:

• STL: the classic. It describes the surface as a mesh of triangles. Simple and universally supported, but it doesn't store units, colours or materials.
• 3MF: a newer format that can carry units, multiple parts and colour or material information in one file. Handy for multi-part or multi-colour jobs.
• OBJ: another mesh format, sometimes used for models with colour or texture data.
• STEP: a CAD format that stores true geometry rather than triangles. Useful when exact dimensions matter or a part may need small tweaks.

If you've designed the part in CAD, export at a fine enough resolution that curves stay smooth. A coarse export turns round holes into visible polygons, which can affect fit. If you're unsure which format to send, mention it when you request a quote and we'll tell you what works best.

Check the units and scale

STL files don't record units. A part designed as 50 mm can open as 50 inches, or as a tiny 50 thousandths of a metre speck, depending on the software. Before sending:

• Note the real-world size of the part, at least one key dimension such as overall length.
• Include that dimension in your quote request so we can confirm the scale matches.
• For miniatures and scale models, state the scale you want (for example the base size or figure height), as many sculpts come at different sizes.

Make sure the mesh is watertight

A printable mesh needs to be a closed, "watertight" solid. Common faults include:

• Holes or gaps in the surface, so the slicer can't tell inside from outside.
• Flipped normals, where faces point the wrong way.
• Overlapping or intersecting shells, often from models built by stacking separate objects together.
• Non-manifold edges, where more than two faces share one edge.

Free tools such as Microsoft 3D Builder, Meshmixer and the repair functions in slicers like PrusaSlicer can find and often fix these automatically. If a repair changes the shape, check the result before you send it.

Watch out for thin walls and tiny details

Every printer has a smallest feature it can reliably make. On FDM printers that's tied to nozzle width; on resin printers details can be much finer, but very thin parts can still break when supports are removed. As a rule of thumb:

• Avoid walls thinner than about two nozzle widths on FDM parts, as they may print weak or not at all.
• Keep thin pins, spikes and spears on miniatures a little thicker than you might expect, or accept they'll be fragile.
• Embossed or engraved text needs enough depth and line width to show up, especially on FDM.

Our resin vs FDM guide explains which process suits fine detail and which suits tough functional parts.

Design in the clearances you need

If parts need to fit together, slide, snap or hold a bolt, the file should already include the right gaps. A hole modelled at exactly the bolt's diameter will usually be too tight once printed. Our tolerances and clearance guide covers fit types in detail, and the heat-set inserts and threads guide covers screws and fixings.

Think about orientation and supports

You don't have to choose the print orientation yourself, but it helps to tell us which faces matter most: the side that needs to look best, the surface that must be flat, or the direction the part will be loaded. Orientation affects strength, finish and where support marks end up. See our print orientation guide for why.

Tell us how the part will be used

The same file can be printed very differently depending on its job. Useful details to include:

• What the part does, and whether it takes load, heat, sunlight or bending.
• How many copies you need.
• Any colour or finish preferences.
• Whether you have a material in mind, or want a recommendation. Our materials page and material comparison guide are good starting points.

Respect the licence

If you downloaded the model, check its licence allows the use you have in mind, particularly for anything you plan to sell. For tabletop minis, stick to original designs or sculpts you have the right to print. If you'd like the finished minis painted, our friends at Some Miniatures offer painting commissions.

FAQ

Can you fix a broken file for me? Mention any problems you know about when you request a quote. Simple issues are often quick to sort, while badly damaged meshes may need rework at the design stage.

Do I need to add supports to my file? No. Supports are added during slicing, so send the clean model without them.

My file is huge. Is that a problem? Very high-polygon meshes can be slow to handle. If your file is large, a slightly lower-resolution export usually keeps the detail you need.

What if I only have a sketch or a broken part? Tell us what you have when you get in touch and we can talk through options.

Ready to send your file?

Run through the checks above, note the size, use and material you want, and get your Dynafabs quote. We'll check the file over and come back to you with the best way to print it.

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