1) Introduction
Many engineers still drop injection molding or machined default tolerances onto 3D‑printed drawings and then wonder why suppliers balk or parts do not assemble. Additive tolerances are governed by the physics of the build: layer thickness and stacking, build orientation, local and global thermal behavior, powder or filament behavior, process control, and post‑processing. CAD precision does not force the process to comply.
Scope here: polymer Multi Jet Fusion (MJF), Selective Laser Sintering (SLS), and Fused Deposition Modeling (FDM), with a quick SLA comparator where helpful.
Deliverable: realistic, source‑backed tolerance expectations and concrete language to put on drawings so you get the parts you intend.
2) Why additive tolerances differ from machining and molding
Key factors that drive capability:
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Thermal shrinkage and gradients. Powder‑bed processes fuse polymer and then cool it. Shrinkage varies along X, Y, and Z and across the bed. HP’s MJF documentation explicitly models and compensates this with dimensional profiles because shrink is not uniform across the volume. 3DPrint
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Layer‑by‑layer stacking. Z is discretized by the layer height. HP MJF uses 80 μm layers, so surfaces built at shallow angles show step‑over and the Z dimension accumulates slice‑to‑slice variation. Webflow
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Powder spreading and fusion variability. In PBF, powder size distribution, layer recoating, and agent or laser energy density drive local fusion width and edge definition, which show up as dimensional bias and roughness. EOS specifies a typical 120 μm “Balance” SLS parameter set for PA12 to balance accuracy and surface quality, underscoring the link between layer settings and outcomes. EOS GmbH
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Machine calibration and process control. HP characterizes capability at Cpk targets and uses machine‑specific profiles to tighten results. That is fundamentally different from a fixed tool in a mold. 3DPrint
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Powder reuse and humidity. PA12 powder ages in SLS. Reuse, exposure time, and moisture shift melt behavior and can alter dimensions and surface quality if not controlled. The literature documents morphology change and property drift with reuse and moisture. MDPI+2SpringerLink+2
The punchline: specify to what the process can hold, not what you wish CAD could enforce.
3) Process capability overview
The table below consolidates credible, published numbers. Pay attention to the qualifiers: size bins, axis, and whether special calibration was enabled.
| Process | Typical dimensional capability (conditions) | Surface roughness Ra, as‑printed | Notes |
|---|---|---|---|
| HP MJF PA12 | General profile, Cpk 1.33, PA12, 20% refresh. 0–30 mm: XY ±0.25 mm, Z ±0.42 mm. 30–50 mm: XY ±0.30 mm, Z ±0.50 mm. 50–80 mm: XY ±0.37 mm, Z ±0.60 mm. Hardware‑specific profiles tighten further to XY ±0.17…0.23 mm, Z ±0.25…0.37 mm over the same size bins. 3DPrint | 8–12 μm typical for MJF PA12; independent tests report ~10–12 μm as‑printed. RapidMade+1 | Layer height 80 μm. Expect Z to be worse than XY. Webflow |
| SLS PA12 | ±0.3% with a lower limit of ±0.3 mm is a widely used service spec. Layer sets around 120 μm are common on industrial EOS processes. materialise.com+1 | As‑printed ranges reported ~10–30 μm depending on machine and orientation. A compact‑SLS PA12 example reports ~31.6 μm. ALEC+1 | Higher variability than MJF, more prone to curl on large flat spans. materialise.com |
| FDM ABS/PC (industrial Fortus/F900 class) | Stratasys publishes the greater of ±0.0035 in (0.089 mm) or ±0.0015 in/in (±0.0015 mm/mm, 0.15%), at ~95% dimensional yield. This is geometry dependent and Z adds + one layer height. Stratasys+2GoEngineer+2 | Literature reports ~9–40 μm depending on layer, path direction, and post‑work. Xometry shows ~22.5 μm on the roughest face for ASA. ResearchGate+1 | Open‑frame FFF and uncontrolled environments will not hit Fortus numbers. Many shops conservatively quote ±0.5 mm for general FDM. Raise3D |
| SLA resin (comparator) | Typical service specs ±0.05 to ±0.15 mm for small parts on tuned systems. 3D Systems positions SLA as the highest precision among common polymer AM. sgd.helpscoutdocs.com+1 | As‑printed ~1.5 μm for “industrial white ABS‑like” in one dataset. Xometry Pro | Materials can be brittle. Supports imprint local roughness. Xometry Pro |
Interpretation: your baseline “as‑printed” general tolerances for production MJF and SLS are in the ±0.25…0.6 mm band depending on size and axis, not in the ±0.05 mm machining band. Industrial FDM on enclosed, heated systems can be surprisingly tight in XY, but Z and thermal distortion still drive practical limits.
4) Why the Z axis is less accurate
Three mechanisms dominate:
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Stacked error from each discrete slice. Even with repeatable layer height control, any small bias accumulates with part height. Webflow
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Anisotropic shrinkage and bow as layers cool. HP explicitly compensates directionally because shrinkage is not uniform across X, Y, and Z or across the bed. 3DPrint
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Reference plane uncertainty from powder depth and post steps. In powder‑bed systems the Z datum is defined through many powder layers. In FDM, support removal and surface bead geometry shift where “top of Z” really lands.
A simple reality check from HP’s own PA12 data: for small features, XY ±0.25 mm vs Z ±0.42 mm at Cpk 1.33. The gap widens with height. 3DPrint
5) Geometry and orientation effects
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Keep critical dimensions in XY where possible. That places them in the process plane of best repeatability. 3DPrint
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Avoid long, thin, flat spans and abrupt section changes. Those geometries concentrate thermal gradients and warp in powder‑bed systems. HP’s own MJF handbook advises thickening, smoothing section transitions, and using lattices to cut stored heat. Webflow
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Use orientation tradeoffs consciously. XY typically gives best dimensional control. Z can give a cosmetically consistent “top” surface but at the cost of stair‑step and Z accumulation. HP calls out 80 μm layers and recommends keeping upward‑facing shallow angles above ~20 degrees if you care about stepping visibility. Webflow
6) Practical drawing callouts that prevent grief
Do not copy ISO 2768 fine machining defaults onto printed plastics. ISO 2768 was written around machined parts and general tolerances for linear and angular dimensions in subtractive workflows. State the additive process and its capability explicitly. DEK
Recommended baseline notes and practices:
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For MJF PA12, as‑printed general tolerance: cite the HP size bins and axis. Example language:
“Unless otherwise specified, as‑printed dimensional capability for MJF PA12 shall follow HP 5200 characterization at Cpk 1.33: 0–30 mm features XY ±0.25 mm, Z ±0.42 mm; 30–50 mm XY ±0.30 mm, Z ±0.50 mm; 50–80 mm XY ±0.37 mm, Z ±0.60 mm.” 3DPrint -
For SLS PA12, as‑printed general tolerance: “±0.3% with a lower limit of ±0.3 mm unless otherwise specified.” materialise.com
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For Industrial FDM (Fortus/F900 class): “The greater of ±0.127 mm or ±0.0015 mm per mm, Z adds + one layer height, geometry dependent.” Stratasys+1
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Put critical‑to‑fit features in a separate block. Only tighten where needed. Call out post‑machining where required.
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Datum strategy: choose printed reference planes that are actually controllable in the build, not arbitrary CAD faces.
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Inspection: prefer CMM or structured‑light scanning over calipers for whole‑part verification. Add a note such as “Inspection method: CMM or structured light with best‑fit to datums A/B/C.”
If your quality system demands a single default, reference a published process guide instead of ISO 2768. Example for MJF: “General dimensions apply to as‑printed condition per HP MJF 5200 dimensional capability at Cpk 1.33.” Then list exceptions that will be post‑machined.
7) When and how to tighten tolerances
You can hit ±0.10 mm and tighter on polymer AM parts, but usually not directly off the machine for every feature. Tighten only where the function requires it:
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Sealing surfaces, bearing bores, dowel holes, press fits: print with machining stock and finish after print. For MJF, plan ~0.3–0.5 mm stock for facing or boring so the cutter “cleans up” consistently. Ajatec
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Threads and inserts: use heat‑set or molded‑in inserts for repeatable threads and precise alignment pins rather than trying to print fine threads in PA12. HP’s guide shows practical clearances for printed assemblies and minimum feature rules. ThomasNet
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Holes: pilot features are fine, but drill or ream to final size if location and clearance matter.
Surface finish can also be tightened: post‑processing like chemical smoothing can push MJF and SLS surfaces well under 5 μm Ra, at the expense of slight dimensional shift. A finishing study on MJF showed <4 μm Ra after a few hours of automated smoothing, and near 2 μm at longer cycles. Note this is a finish operation, not the native print. PostProcess Technologies
8) Design and process recommendations that help you hold spec
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Uniform walls and lighter sections. Break up large masses, add fillets, and consider lattices or shelling to reduce warpage risk and variability. HP’s handbook and Fast Radius’s MJF DFM guide both emphasize this. Webflow+1
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Space parts appropriately in the build to reduce cross‑heating and part‑to‑part interaction. Follow your provider’s packing rules.
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Orient text, holes, pins, and fine details in XY where possible for best definition and size control. Webflow
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Clearance design for assemblies: for co‑printed SLS/MJF assemblies, start at 0.4–0.7 mm gap, then tune by coupon. Webflow+1
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Validate with a coupon: before committing, print a coupon that exercises your critical features at their orientations and sizes. It is standard practice in HP’s own characterization to sample many features across the bed to build machine‑specific profiles. You can do a smaller version of that for your job. 3DPrint
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Control powder and environment if you are running in‑house. Track refresh ratios and moisture for SLS powders to avoid drift over time. MDPI
9) Summary table: practical tolerance ranges you can plan around
These are realistic starting points for drawings, based on the cited data and the reality that Z is worse than XY. Tighten only where a function truly needs it, and state the method.
| Process | Recommended general tolerance | Tight tolerance with secondary ops | Notes |
|---|---|---|---|
| MJF PA12 | Use HP bins by size and axis. For ≤30 mm features, plan XY ±0.25 mm, Z ±0.42 mm. Loosen per HP table for taller features. 3DPrint | ±0.10 mm typical on machined faces and bores if you add stock. Surface Ra <5 μm with chemical smoothing is achievable. Ajatec+1 | Best isotropy among polymer PBF in practice when profiled. 3DPrint |
| SLS PA12 | ±0.3% with a 0.3 mm lower limit. Expect more warp on large, flat plates. materialise.com | ±0.15 mm after machining is common on localized features. | Layer ~120 μm typical on EOS “Balance.” EOS GmbH |
| FDM ABS/PC (industrial) | Fortus‑class systems: greater of ±0.127 mm or ±0.0015 mm/mm. Add + one layer height in Z. Stratasys+1 | ±0.2 mm after machining on pads and bores is typical. | Open FFF or poor thermal control: plan closer to ±0.5 mm unless proven otherwise. Raise3D |
10) Key takeaways you can put to work
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Match your tolerance asks to the documented capability of the process, not to machining defaults. Publish the process note right on the drawing. 3DPrint+1
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Orient critical geometry in XY. Put tight bores, pins, and sealing lands in the plane where the process is strongest. Webflow
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Plan secondary machining where you truly need ±0.10 mm‑class features. Add stock and call it out. Ajatec
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Avoid blanket ISO 2768 machining notes for additive. If you must have a single “unless noted” statement, point it to an additive capability reference for the process you are buying. DEK
Appendix: Source details you can hand to QA or buyers
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HP Jet Fusion 5200 PA12 Dimensional Capability White Paper. Contains the XY and Z tolerance tables at Cpk 1.33 and 1.00 and explains hardware‑specific profiling. 3DPrint
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HP MJF Handbook. Confirms 80 μm layers, stair‑step considerations, and orientation guidance for features and assemblies. Webflow
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EOS PA2200 Balance. Confirms common SLS layer thickness choice at 120 μm to balance accuracy and finish. EOS GmbH
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Materialise SLS PA12 design guidelines. Service spec of ±0.3% with 0.3 mm lower limit and general cautions on warpage. materialise.com
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Stratasys FDM accuracy references. Fortus/F900 accuracy spec and study showing the greater of ±0.0035 in or ±0.0015 in/in at ~95% yield. Stratasys+1
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Surface roughness references. MJF ~8–12 μm as printed, SLS examples, and SLA ~1.5 μm as printed. RapidMade+2Xometry Pro+2
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Powder reuse and humidity. Reviews and studies on PA12 reuse effects in SLS that impact quality over time. MDPI+1
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ISO 2768 background and ISO/ASTM 52910 design for AM for your internal standards library. DEK+1
Suggested drawing boilerplate you can copy and adapt
General
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Process: [HP MJF 5200 + PA12] unless noted.
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As‑printed dimensional capability per HP 5200 Cpk 1.33 tables:
0–30 mm features: XY ±0.25, Z ±0.42.
30–50 mm: XY ±0.30, Z ±0.50.
50–80 mm: XY ±0.37, Z ±0.60. 3DPrint
Critical features
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Datum scheme A/B/C oriented to printed reference planes.
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[Hole IDs X, Y, Z] to be drilled or reamed after print to ±0.05. Add 0.3–0.5 stock on pads for facing. Ajatec
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Inspection: CMM or structured‑light scan to CAD with best‑fit to A/B/C.
Finish
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As‑printed MJF surface. If cosmetic requirement below Ra 8 μm, apply [chemical smoothing] and re‑verify critical features. RapidMade+1
If you need a version of the summary tables with your house specs prefilled, say the word and I will generate a clean one‑pager you can drop into your drawing template set.
