Prepare 3D printing files for print service

Lisa Ernst · 22.11.2025 · Technology · 7 min

You quickly export an STL file from CAD, upload it to the printing service – and soon after you get feedback: "File not watertight", "Wall thicknesses too thin", "Scale unclear". This costs time, shifts deadlines, and at the worst possible moment, it's precisely the component you urgently need that's affected. In short: it's annoying.

In our workshop at 33d.ch in Switzerland, we experience such situations regularly, especially when someone is working with a professional 3D printing service for the first time. Many models are structurally sound, but fail due to minor issues in file preparation.

With a few well-practiced routines, these stumbling blocks can usually be avoided. They help us in our daily work to calculate quotes faster, ask fewer questions, and deliver a printable result to you more reliably – whether you are in an SME, a development department, or an ambitious hobby maker.

In this article, we show from our practical experience what is important regarding format selection, geometry, and a sensible checklist before uploading, so that your 3D printing files work with the printing service right away.

Source: Own Representation

File Preparation Basics

Before we delve into details, it's worth taking a brief look at the formats that land on our server daily. Not every format is equally suitable for every task – and sometimes the combination of two formats makes the difference between "barely printable" and "cleanly documented and usable long-term".

Professional 3D printing services use neutral 3D formats like STL, 3MF, OBJ oder STEP, because they can be processed independently of the original CAD software. STL is an established standard accepted by almost all online services ( Instructables, Xometry Pro).

Increasingly, services also accept solid CAD formats like STEP/STP. These are better suited for precise machining, milling, and downstream processes ( onsite.helpjuice.com, Xometry's Manufacturing Community, weerg.com, SFS). ). The choice of format depends on whether the service should only print the model or also construct/adjust it. It is advisable to check the printing service's preferred formats on their website in advance, rather than uploading multiple formats without comment.

STL: the classic 3D printing format

The majority of files we receive for FDM or SLS parts are STL. This is perfectly fine – as long as the export is done consciously and not just with some default settings. This is where most avoidable errors occur in practice.

An STL file describes the surface of a 3D model as an unstructured triangular mesh. It saves neither units nor colors or material properties ( Wikipedia, iteration3d). ). The geometry is approximated by triangles, which with complex shapes either leads to large files with a fine mesh or to visible facets with a coarse mesh ( Xometry Pro, FacFox, matterhackers.com).

An export with very fine tolerance increases file size and processing time, while coarse tolerances create visible polygon edges or inaccurate radii ( Markforged, Protolabs, Protolabs Network, i.materialise.com). ). Send STL files when your model is fully designed and no longer needs parametric editing. Use a sensible ratio of tolerance to component size, for example, a chordal deviation of 0.05–0.1 mm for technical parts ( Markforged).

STL contains no feature history, radius information, or assembly structure, which makes later changes difficult ( 33d.ch). ). Since no units are saved, the import software must guess or ask for the unit of measurement (millimeters or inches) ( iteration3d, FacFox).

STEP: more precise CAD standard with more information

When customers from mechanical engineering or medical technology send us data, we almost always wish for a STEP file in addition to the STL. This allows us to slightly adjust boreholes, add chamfers, or derive variants if necessary, without the geometry having to be "repaired to death".

STEP (Standard for the Exchange of Product Data, ISO 10303) is an ISO-standardized CAD exchange format that can store complete solids, surfaces, and assemblies with high geometric accuracy ( Adobe, CertBetter, all3dp.com, Visao). ). It often includes additional product data such as assembly relationships or reference geometries and is therefore a preferred format in manufacturing for CNC machining and design ( Xometry Pro).

Send STEP files when the 3D printing service should scale parts, adjust boreholes, or derive variants, as the geometry remains cleanly editable ( 33d.ch). ). STEP is particularly recommended for complex assemblies and precise technical parts that will be milled or further processed later ( Xometry Pro).

STEP must be converted into a triangular mesh before printing, whereby texture or color information may be lost ( Xometry Pro). ). Some 3D printing portals aimed at end customers are optimized for STL uploads, so a pure STEP file can lead to inquiries ( i.materialise.com, Instructables).

Practical recommendation: Prepare 3D printing files for the print service

If the service accepts STEP, it makes sense to upload both STEP as a reference and a controlled STL from your own export. This way, the printing service sees the desired surface and simultaneously has an editable solid body ( onsite.helpjuice.com). ). Avoid submitting only an "somehow" exported STL without information on units, target dimensions, and material.

At 33d.ch, it has proven beneficial for customers to send us both files for important projects: an STL, which we use unchanged for printing, and a STEP as a "single source of truth" for later adjustments. This allows us to clarify tolerances, implement minor corrections, and still print exactly the part that was originally intended.

Detailed Check

Before a file goes into the slicer at our end, we briefly check its "printability". Depending on the order volume, this is partly automated, but for critical or expensive parts, we still look at the layer view by hand. Some typical problem areas keep appearing.

For 3D printing, your model must be a closed volume, without holes, self-intersecting surfaces, or non-manifold edges ( simplify3d.com, i.materialise.com). ). Typical errors include open edges, internal superfluous surfaces, and inverted normals ( simplify3d.com, Wenext, 3d-gennady-yagupov.co.uk). ). Slicers often report such problems as "non-manifold" or "invalid mesh", which can lead to faulty layers or missing areas in the print ( Tom's Hardware).

Check STL files after export in a mesh tool (e.g., Meshmixer, netfabb) for holes, self-intersections, and inverted normals ( formlabs.com). ). Do not rely on the printing service using automatic repair tools, especially for critical parts.

Walls that are too thin and fine details

Especially with delicate geometries, we experience in practice how quickly a part breaks during depowdering, assembly, or even packaging if wall thicknesses were chosen too optimistically. It's better to design 0.3–0.5 mm thicker once than to have to reprint several parts later – it's almost always worth it.

The minimum wall thickness strongly depends on the process. For SLS plastics, it is often between 0.8–2.0 mm ( Protolabs Network). ). Many design guides recommend 2–3 nozzle diameters for FDM ( Sinterit 3D Drucker & Zubehör). ). Service providers often specify specific minimum wall thicknesses, e.g., 1 mm for MJF/MSLA walls and 3 mm for FDM with certain materials ( weerg.com). ). Walls that are too thin can break during handling or depowdering ( Shapeways).

Measure critical areas (webs, snap hooks, ribs, logos) before export and compare them with the service's design guidelines ( i.materialise.com). ). Avoid designing walls that are 0.4 mm thick over a large area, as they can warp or fail ( Sinterit 3D Drucker & Zubehör).

Units, scale, and tolerances

The topic of units is one of the classics. It also happened to us at the beginning that we suddenly had a model in inches on the screen instead of millimeters – it looks identical at first glance, but is dramatically too small. Since then, we have been extremely careful that construction, export, and slicer settings really fit together.

STL-Dateien save geometries without specifying the unit of measurement ( iteration3d, FacFox). ). CAM and slicer systems often ask for the unit upon import or make a standard assumption, which leads to scaled parts if selected incorrectly ( FacFox).

Deliberately set the export units in CAD to the unit expected by the printing service and explicitly state it in the order comment ( manual.eg.poly.edu). ). Do not design in inches and export silently to avoid scaling errors.

Practical Implementation

In our daily work, we use a simple checklist before files go into production. You can use it as a guide and adapt the points for your own workflow:

• Clarify format choice (STL, STEP, or combination)
• Consciously check units and scale
• Compare wall thicknesses and fine details with design guidelines
• Repair geometry and check for watertightness
• Document export settings
• Name and bundle files logically
• Create a short PDF checklist for future orders

Step 1 – Format Choice: STL, STEP, or both?

First, ask yourself: Should the service provider really only print, or are they allowed to adjust and contribute ideas? The answer determines which format you should provide.

If the part is finally designed and the service should only print, a cleanly exported STL is sufficient. For later changes or follow-up processes, an additional STEP file is useful as it contains parametric information ( 33d.ch, Xometry Pro). ). For technical components, if the service accepts both, you should provide both STEP (for machining) and STL (for the desired mesh) ( onsite.helpjuice.com).

Step 2 – Clarify Units and Scale

When a part appears much too large or tiny to us in the viewer, the wrong unit is almost always the first suspicion. You can save yourself and us this check with a quick look in CAD and in the upload portal.

Before exporting in CAD, check if the model is scaled in the intended unit and if the export dialog uses the same unit. This is particularly critical for STL, as the units are not stored in the file ( iteration3d, FacFox). ). Remember a characteristic dimension and check in the upload portal if it is displayed correctly before submitting the order ( i.materialise.com).

Step 3 – Check Wall Thicknesses and Details

A typical example from our daily work: A customer from mechanical engineering designs a housing with very thin walls because everything looks stable in CAD. In real printing, the part warps or cracks when screwed. With a little reserve in wall thickness, this would not have happened.

Use a function in CAD or a mesh tool to measure all thin areas and compare them with the design guidelines of the chosen material ( Protolabs Network, weerg.com). ). It's better to design functional parts a bit thicker, especially if post-processing is planned, as material removal reduces wall thickness ( Sinterit 3D Drucker & Zubehör).

Step 4 – Geometry Repair and Watertightness

While we rely on automatic repair functions, for safety-critical, expensive, or time-critical parts, we always look at the layer view manually. A missing layer in the wrong place can mean a non-functional component.

Before uploading, check the mesh with a repair tool for holes, self-intersections, duplicate surfaces, and non-manifold edges ( simplify3d.com). ). Many tools offer automatic repair functions, but a visual check is advisable ( formlabs.com). ). Open the repaired STL export in a slicer and check the layer view before handing over the file ( Protolabs Network).

Source: youtube.com

Slicer software like Bambu Studio allows detailed checking and adjustment of print settings before sending to the print service.

Step 5 – Document Export Settings

Especially for recurring components, we create project-specific export templates: same tolerance value, same units, same mesh quality. This takes some time for the first order, but noticeably saves effort for subsequent projects.

Chordal tolerance, angular resolution, and binary/ASCII affect file size and surface quality. Many manufacturers recommend a chordal tolerance around 0.1 mm and binary STL ( Markforged, digitalengineering247.com). ). Note the export parameters used and add them in the comment to the printing service to be able to trace problems ( Protolabs).

For typical FDM production parts, for example, a chordal tolerance of around 0.1 mm has proven effective in our workshop. For very small or high-precision parts, we go finer; for large, robust components, we deliberately set the resolution slightly coarser to keep file sizes and slicing times within limits.

Step 6 – Bundle Files Logically

When everything reaches us in a single, merged file, the risk of misunderstandings increases: What belongs together? What should be permanently glued, what should remain movable later? It's better to have clearly separated components with understandable file names – this noticeably speeds up quoting and manufacturing.

Many services require individual parts as separate files or as clearly separated bodies in an assembly ( i.materialise.com, Xometry). ). Model parts that should move or be assembled separately later with a defined gap and name them clearly (e.g., "housing_top_STEP") instead of uploading them as one merged body ( weerg.com).

Step 7 – Incorporate Your PDF Checklist

A simple, one-page PDF checklist with the mentioned points (format, units, wall thicknesses, geometry repair, export settings, file naming, and comments) is helpful in everyday life ( i.materialise.com).

). Our own checklist is actually printed and hanging on the workshop wall. A quick glance at it before we push data into the system prevents many of the queries that we used to have to clarify laboriously by email.

Mini-Conclusion: Fewer queries, better printed parts

Good 3D printing results depend on cleanly prepared files: the right format (STL or STEP), correct units, sufficient wall thicknesses, and watertight geometries are the basis ( Xometry Pro, simplify3d.com, Protolabs Network). ). A consistently used checklist reduces inquiries, rework, and misprints.

• Consciously choose the appropriate format: STL for direct printing, STEP for editing and variants – both if in doubt.
• Check units, scale, wall thicknesses, and fine details before exporting, rather than only after the first misprint.
• Use repair tools and a look at the layer view to find holes, non-manifold geometries, and other problem areas early.
• Document export settings and name files clearly so the print service understands your setup without questions.
• Keep your personal PDF checklist up to date – it costs a few minutes but saves time and money on every order.