Preparing 3D printing files for printing service

Avatar
Lisa Ernst · 22.11.2025 · Technology · 7 min

You quickly export an STL file from CAD, upload it to the printing service – and shortly after, the feedback arrives: "File not watertight", "Wall thicknesses too thin", "Scale unclear". This costs time, delays deadlines, and in the worst-case scenario, it's precisely the part 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 okay, 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 more reliably to you – whether you're working in an SME, an R&D department, or as an ambitious hobby maker.

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

File Preparation Basics

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

Format Typical Use Advantages What to look out for
STL Direct 3D printing (esp. FDM, SLS) Very common, accepted by almost all services Units are not saved; deliberately choose mesh quality (tolerance)
STEP Technical parts, assemblies, later modifications Clean, parametric geometry, easily editable A mesh is still generated before printing; colors/textures are usually lost
3MF / OBJ Multi-color prints, textures, special workflows Supports colors and partly materials Not every service processes all additional information equally

Professional 3D printing services use neutral 3D formats such as STL, 3MF, OBJ oder STEP, as these 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 is only supposed to print the model or also engineer/modify it. It is advisable to check the printer's preferred formats on its website beforehand, 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 deliberately and not just with any default settings. This is precisely where most avoidable errors occur in practice.

An STL file describes the surface of a 3D model as an unstructured triangular mesh. It stores 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 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 part 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, making later modifications difficult ( (33d.ch). Since no units are stored, the import software must guess or ask for the unit of measurement (millimeters or inches) ( (iteration3d, FacFox).

STEP: 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 adjust bores slightly, add chamfers, or derive variants if necessary, without the geometry having to be "repaired" in a broken way.

STEP (Standard for the Exchange of Product Data, ISO 10303) is an ISO-standardized CAD exchange format that can store complete bodies, 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 needs to scale parts, adjust bores, or derive variants, as the geometry remains cleanly editable ( (33d.ch). STEP is particularly recommended for complex assemblies and precise technical parts that will later be milled or further processed ( (Xometry Pro).

STEP needs to be converted into a triangular mesh before printing, and 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 printing 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 at the same time has an editable solid body ( (onsite.helpjuice.com). Avoid submitting only an STL exported "somehow" 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 that we use unchanged for printing, and a STEP as the "single source of truth" for later adjustments. This way, we can clarify tolerances, implement minor corrections, and still print exactly the part that was originally intended.

Detailed Check

Before a file goes into our slicer, we briefly check it for "printability". Depending on the order volume, this is partly automated, but for critical or expensive parts, we still manually inspect the layer view. Some typical problem areas keep reappearing. (simplify3d.com, i.materialise.com). For 3D printing, your model must be a closed volume, without holes, self-intersecting surfaces, or non-manifold edges ( (simplify3d.com, Wenext, 3d-gennady-yagupov.co.uk). Common errors include open edges, internal superfluous surfaces, and inverted normals ( (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 filigree 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 it 0.3-0.5 mm thicker once rather than having to reprint several parts later – this is almost always worth it.

The minimum wall thickness depends heavily on the process. For SLS plastics, it's often between 0.8-2.0 mm ( (Protolabs Network). Many design guides recommend 2-3x nozzle diameter for FDM ( (Sinterit 3D Drucker & Zubehör). Service providers often specify 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 large areas with 0.4 mm thick walls, as they can warp or fail ( (Sinterit 3D Drucker & Zubehör).

Units, scale, and tolerances

The topic of units is a classic. We also made the mistake at the beginning of suddenly having a model in inches on screen instead of millimeters – it looks identical at first glance, but is dramatically too small. Since then, we've been extremely careful to ensure that design, export, and slicer settings really match.

STL files store geometry without specifying the unit of measurement ( (iteration3d, FacFox). CAM and slicer systems often ask for the unit upon import or make a default assumption, which leads to scaled parts if selected incorrectly ( (FacFox).

Consciously 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 practice, we use a simple checklist before files go into production. You can use it as a guide and adapt the points to your own workflow:

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

First, ask yourself: Should the service provider only print, or is it allowed to adapt 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 modifications or subsequent processes, an additional STEP file is useful as it contains parametric information ( (33d.ch, Xometry Pro). For technical parts, if the service accepts both, you should provide both STEP (for modification) 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 and we can save this check with a quick look in CAD and the upload portal.

Before exporting from 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 units are not included in the file ( (iteration3d, FacFox). Memorize 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 the actual print, the part warps or cracks during screwing. With a little buffer in the wall thickness, this would not have happened.

Measure all thin areas with a function in CAD or a mesh tool and reconcile them with the design guidelines of the chosen material ( (Protolabs Network, weerg.com). Prefer to make 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

We rely on automatic repair functions, but for safety-critical, expensive, or time-critical parts, we always manually inspect the layer view. A missing layer in the wrong place can mean a non-functional part.

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 recommended ( (formlabs.com). Open the repaired STL export in a slicer and check the layer view before handing over the file ( (Protolabs Network).

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

Quelle: youtube.com

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

Step 5 – Document Export Settings

Especially for recurring parts, we create project-specific export templates: same tolerance value, same units, same mesh quality. This takes some time for the first order, but significantly 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 used export parameters 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 use finer settings, and for large, robust components, we deliberately set the resolution slightly coarser to keep file sizes and slicing times within limits.

Step 6 – Bundle Files Meaningfully

When everything reaches us in a single, merged file, the risk of misunderstandings increases: What belongs together? What should be permanently bonded, and what should remain movable later? Separate components with understandable file names are better – this significantly 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., "Case_top_STEP") instead of uploading them as a merged body ( (weerg.com).

Step 7 – Incorporate Your PDF Checklist

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

Our own checklist is actually printed and hanging on the workshop wall. A quick glance at it before we put data into the system prevents many of the queries 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 appropriate 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 queries, rework, and failed prints.

Fits well with (internal link ideas)

Teilen Sie doch unseren Beitrag!