Most delays and reprints trace back to file issues caught after you've already submitted the order. Wall sections too thin to print, geometry with holes the slicer can't handle, or units set to inches when the bureau expects millimeters — these problems are easy to fix before you submit and expensive to fix after. This guide covers what a bureau actually needs and what to check before you send anything.
File Formats: STL vs STEP vs OBJ
Three formats cover almost everything you'll encounter at a US service bureau. Each has a specific role.
STL — the universal standard
STL (Standard Tessellation Language) is what every bureau accepts. It converts your CAD model into a mesh of triangles and saves just the surface geometry — no color, no material information, no parametric history. When you export STL, set tessellation resolution to "fine" or "high" in your CAD software. A low-resolution export creates visible faceting on curved surfaces that the printer faithfully reproduces. Most bureaus use millimeters, so export in mm unless you've confirmed the bureau works in inches.
STEP — for complex or modified parts
STEP (Standard for the Exchange of Product model data) preserves the solid geometry as mathematical curves rather than triangles. It's the right choice when your bureau needs to modify the orientation, add supports in their software, or check tolerances on specific features. Many professional bureaus prefer STEP for machined or metal parts. If you have the option, send both — STL for the slicer, STEP for reference.
OBJ and 3MF — for multicolor or multi-material
OBJ supports color information and is used for full-color powder printing (binder jetting). 3MF is the newer format that handles multi-material assignments cleanly and is increasingly supported across platforms. For standard single-material printing, neither format is necessary — use STL.
| Format | Best For | Preserves Color? | Universal Support? |
|---|---|---|---|
| STL | Standard single-material prints | No | Yes — every bureau |
| STEP | Metal, precision parts, bureau needs to modify | No | Most professional bureaus |
| OBJ | Full-color prints (binder jetting) | Yes | Varies |
| 3MF | Multi-material, modern workflows | Yes | Growing |
Wall Thickness Minimums by Technology
Walls that are too thin either fail to print or come out fragile enough to break during post-processing. These minimums are practical starting points — your bureau may have tighter or looser constraints depending on their machine and settings.
| Technology | Minimum Wall | Recommended Wall | Notes |
|---|---|---|---|
| FDM (0.4mm nozzle) | 0.8mm | 1.2–2mm | Walls must be a multiple of nozzle diameter for solid extrusion |
| FDM (0.6mm nozzle) | 1.2mm | 1.8–3mm | Faster print, coarser surface |
| SLA / MSLA | 0.5mm | 1–2mm | Thin walls are fragile post-cure; structural features need 1mm+ |
| SLS Nylon | 0.7mm | 1–2mm | No support structures needed; walls can be thinner |
| MJF PA12 | 0.6mm | 1–1.5mm | Similar to SLS; HP systems have slightly tighter capability |
| Metal (DMLS) | 0.3–0.5mm | 1.5–3mm | Thin walls warp during stress relief; functional features need more |
For FDM with a 0.4mm nozzle, make walls 0.8mm, 1.2mm, 1.6mm, or 2.0mm — multiples of 0.4mm. This lets the slicer fill each wall with a whole number of perimeter lines. A 1.0mm wall produces one partial-width line that's weaker than a proper two-line 0.8mm wall.
Tolerances and Fits
If your part needs to fit with another part — a lid that snaps onto a box, a shaft that turns in a hole — you need to design in clearance. 3D printers don't hit nominal dimensions exactly, and shrinkage varies by material and machine.
FDM tolerances
FDM is the loosest of the common technologies. Expect ±0.2–0.5mm on any given dimension, with more variation in the Z direction than XY. For a shaft-in-hole fit, add 0.3–0.5mm clearance per side (0.6–1.0mm total on diameter). For snap fits, design with 0.2–0.3mm clearance. If a part needs a press fit, prototype first — FDM shrinkage is too variable to nail press fits from the first print.
SLA / resin tolerances
MSLA and DLP are tighter than FDM: expect ±0.1–0.2mm on XY dimensions. Z accuracy depends on layer height — at 0.05mm layers, Z is also quite good. Resin parts shrink slightly during post-cure; some bureaus calibrate compensation into their slicers, others don't. For precision parts, request a 1:1 test feature (a 10mm cube or a calibration pin) with your order to verify actual shrinkage.
SLS and MJF tolerances
SLS and MJF typically hold ±0.1–0.25mm and have the best isotropic accuracy of any plastic process. Tolerances are consistent regardless of part orientation. For moving assemblies or pin-and-hole fits, design with 0.2–0.3mm clearance per side as a starting point.
Metal (DMLS) tolerances
As-printed metal tolerances run ±0.05–0.1mm, but post-processing (stress relief, support removal, bead blasting) can shift dimensions. For critical dimensions — bearing seats, mating surfaces, threaded holes — specify post-machining in your order. Most bureaus can machine specific features after printing to tight tolerances.
Geometry Issues That Cause Print Failures
Non-manifold geometry
A manifold (watertight) mesh is a closed 3D surface with no holes, no overlapping faces, and no edges shared by more than two faces. Non-manifold geometry confuses slicers and causes missing sections, failed prints, or incorrect wall generation. If you're exporting from professional CAD software (SolidWorks, Fusion 360, Onshape, CATIA), your exports are almost always manifold. Problems usually come from polygon modeling tools (Blender, ZBrush, SketchUp) where geometry can have gaps or internal faces. Use Meshmixer, Netfabb, or PrusaSlicer's repair tools to check and fix before submitting.
Intersecting bodies
If you have two separate solid bodies that overlap in your CAD assembly, the export may include both volumes independently rather than merging them into one. Slicers handle this inconsistently — some merge, some print both shells independently, producing weak internal boundaries. Boolean-unite intersecting bodies before export.
Zero-thickness faces
A face that exists in the model but has no thickness (a surface rather than a solid) can't be physically printed. This usually happens when modeling in a surface-based workflow rather than a solid modeling workflow. Make sure every surface in your export is part of a closed solid, not floating geometry.
Parts too small for the technology
Features that are below the technology's minimum resolution simply won't print. Text smaller than about 3–4mm height disappears on FDM. Pin features thinner than 1mm break off during post-processing on most technologies. If fine features are critical, verify they meet the bureau's minimum feature size before ordering.
Orientation Matters More Than Most People Think
For FDM and SLA, the orientation of your part on the build plate directly affects strength, surface quality, and support placement. Most bureaus orient parts for you, but if you have a specific requirement — a smooth surface on one face, a strong layer direction along a load axis — include that information with your order.
The practical rule: FDM parts are weakest at layer boundaries in the Z direction. If your part will be stressed along one axis, orient it so that axis runs horizontally (in the XY plane) during printing. For SLA, the top surface (furthest from the build plate) typically has the best finish — orient display faces accordingly.
Pre-Submission Checklist
Before you upload your file to a bureau, work through this list:
- File format confirmed. STL for standard orders. STEP or both if the bureau requested it for precision work.
- Units verified. Export in millimeters unless you've confirmed the bureau expects inches. Open the STL in a viewer and check that the bounding box matches your expected dimensions.
- Wall thickness checked. Run a wall thickness check in your CAD tool or Meshmixer. Every wall meets the minimum for your technology.
- Mesh is manifold. No open edges, no non-manifold geometry. Check with Meshmixer, Netfabb, or PrusaSlicer's repair function.
- No intersecting bodies. Boolean-unite any overlapping solids before export.
- Resolution is adequate. For STL export, set tessellation to "fine" — visible faceting on curves usually means the export tolerance was too coarse.
- Tolerances designed in. Mating features have appropriate clearance for the technology. Snap fits and press fits have been prototyped or are explicitly flagged for the bureau's advice.
- Orientation note included. If a specific face needs to be smooth or a load axis matters, note it in your order comments.
- Quantity specified. Single prototype vs. a batch of 50 should be stated upfront — batch pricing is different and the bureau may recommend a different technology.
- Delivery deadline noted. If you need parts by a specific date, state it when you order — don't assume standard lead time will get you there.
What to Include When Contacting a Bureau
The fastest way to get an accurate quote is to send the bureau everything they need in the first message. That means: the file (STL or STEP), the material you want (or ask for a recommendation), the quantity, the target delivery date, any surface finish requirements, and any critical dimensions or tolerances. Bureaus deal with incomplete inquiries all day — a complete brief gets you a faster, more accurate quote and starts the relationship on the right foot.
If you're not sure which technology or material fits your application, describe the part's function, the environment it will be used in (temperature, chemical exposure, load), and your budget. A good bureau will tell you the right process for the job.
Frequently Asked Questions
STL is accepted universally. Every professional service bureau handles STL files. STEP is accepted by most professional shops and is preferred for metal and precision parts. OBJ is used for full-color prints. When unsure, send STL.
Yes, with caveats. Both can export STL files that print fine for simple geometry. Tinkercad is beginner-friendly but limited in precision. Blender produces meshes that often need repair (non-manifold issues are common). Run your Blender export through Meshmixer or PrusaSlicer's repair function before submitting. For mechanical or precision parts, purpose-built CAD tools (Fusion 360, Onshape, FreeCAD) are more reliable.
Most professional CAD tools have a wall thickness analysis tool. Fusion 360 has it under Inspect > Section Analysis. Meshmixer (free) can run a thickness analysis on STL files — highlight thin sections in red. As a quick sanity check: zoom in on thin sections and verify they meet the minimums above for your target technology.
No — service bureaus add supports themselves in their slicing software. Don't include supports in your design file. If you have geometry you're worried about (steep overhangs, very thin bridges), note it in your order and the bureau will orient and support accordingly. For SLS and MJF, no supports are needed at all — parts are self-supporting in the powder bed.
For a prototype, you generally want clearance — design a bit loose so parts go together easily and you can assess fit and function. For production or functional assemblies, tighten up based on what you learned from the prototype. Many engineers design prototypes with 0.5mm clearance on all mating features, then adjust to 0.2–0.3mm for the production version once the geometry is validated.
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