Engineers who use 3D printing tend to learn the hard way. The first print works. The second one warps. The third one fails to fit into the assembly. The fourth one breaks in the field. After a year of reprints, the engineer has accumulated enough tribal knowledge to spec a 3D printed part correctly on a drawing. This guide is a shortcut to that knowledge — everything we wish every engineer knew before they uploaded their first file.
Material mechanical properties at a glance
| Material | Process | Tensile (MPa) | Elongation (%) | HDT (°C) | Best for |
|---|---|---|---|---|---|
| PLA | FDM | 60 | 6 | 55 | Concept models, indoor |
| PETG | FDM | 50 | 20 | 70 | Functional, chemical resistance |
| ABS | FDM | 40 | 15 | 98 | Injection mold matching |
| ASA | FDM | 44 | 20 | 95 | Outdoor, UV exposure |
| PC CF | FDM | 80 | 3 | 150 | Engineering structural |
| Standard Resin | SLA | 38 | 8 | 55 | Visual models |
| Tough 2K | SLA | 46 | 24 | 62 | Snap-fit validation |
| Rigid 10K | SLA | 65 | 2 | 218 | Engineering validation |
| High Temp | SLA | 58 | 3 | 238 | Thermal test fixtures |
| Nylon PA12 | MJF | 48 | 20 | 170 | Production parts |
| Nylon PA11 | MJF | 48 | 45 | 170 | High-impact production |
| Nylon PA12 GF | MJF | 53 | 6 | 180 | Structural brackets |
Tolerance expectations
| Process | Small features | 100mm dim | 200mm+ dim |
|---|---|---|---|
| FDM | ±0.2mm | ±0.3mm | ±0.5mm |
| SLA | ±0.1mm | ±0.15mm | ±0.2mm |
| Industrial SLA | ±0.2mm | ±0.3mm | ±0.5mm |
| MJF | ±0.3mm | ±0.4mm | ±0.5mm |
Design rules
Wall thickness minimums:- FDM: 1.2mm (two perimeters minimum)
- SLA: 0.6mm load-bearing, 0.4mm cosmetic-only
- MJF: 0.8mm load-bearing, 0.5mm cosmetic-only
- Industrial SLA: 1.0mm
- Horizontal holes: oversize by 0.3–0.5mm (overhang droop)
- Vertical holes: print to nominal (drill if tight)
- Snap-fit clearance: 0.2–0.4mm between mating surfaces
- Sliding fits: 0.3–0.5mm clearance
- Minimum text height: 3mm with 1mm raised/recessed depth
- Minimum feature size: 1.5mm (FDM), 0.6mm (SLA)
- Minimum hole diameter: 1.5mm (FDM), 0.8mm (SLA)
- Maximum unsupported overhang: 45° from horizontal without supports
How to spec a 3D printed part on a drawing
A drawing that tells a 3D printing service exactly what you need:
- Material callout: Specify the exact material name ("Nylon PA12" not "plastic")
- Process callout (if you care): Specify "MJF," "FDM," or "SLA" if the process matters to the part's function
- Critical dimensions: Mark which dimensions must hold tolerance (not all of them)
- Tolerance callouts: Use per-feature tolerance on critical features, allow looser process-default on non-critical
- Finish callout: "As-built" vs "sanded" vs "vapor-smoothed" vs "painted"
- Assembly notes: Any features that need post-processing (drilled holes, heat-set inserts, threaded inserts)
- Orientation note (optional): If you have a specific build orientation in mind, mark it. Otherwise leave it to the service.
The engineer's quick decision tree
- Is the part going into the hands of an end customer? → MJF or Industrial SLA
- Is the part for engineering validation (functional testing)? → MJF (if load-bearing) or SLA Tough 2K (if snap-fit)
- Is the part a visual prototype for client review? → SLA Standard or Grey Pro
- Is the part a jig, fixture, or production tool? → FDM PETG or PC CF
- Does the part live outdoors? → FDM ASA or Industrial SLA
- Is volume under 50 parts? → Any 3D printing process
- Is volume 50–2,000 parts? → MJF preferred
- Is volume 2,000+ of the same part? → Injection molding (not 3D printing)
