STL files provide a convenient way to capture complex, organic shapes, but their faceted nature can make them frustrating to prepare for analysis. Because STLs store only triangles (no true solid bodies, analytic faces, or feature history) they frequently arrive with gaps, overlaps, or millions of tiny facets that stall meshing. Fortunately, Ansys Discovery’s Facets and Subdivision (SubD) toolsets let you transform even the roughest scan into a watertight solid ready for FEA or CFD.
This article dives deep into every cleanup and editing command, explaining how each works, which parameters matter, and (most important) when one tool is preferable to another.
Typical STL Pain Points—and Why They Matter
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Missing facets leave visible holes; any volume conversion will fail if a model isn’t watertight.
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Disconnected shells introduce unintended air gaps that corrupt fluid domains and inflate part counts.
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Excessive resolution means millions of triangles; viewport performance plummets and meshing slows to a crawl.
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Ragged surfaces create jagged boundary layers, yielding inaccurate stress or flow solutions.
The Facets Tab in Depth
Below, each tool is described in detail so you can see not only what it does, but how and why you might choose it for a given STL problem.
AutoFix – Your First Pass Heal‑All AutoFix is the one‑button triage doctor for imported STL files. The command runs a cascade of algorithms—checking manifold continuity, stitching open edges, flipping inverted normals, merging duplicate nodes, and deleting zero‑area triangles. Because the process is iterative, it revisits a repaired region if a downstream operation exposes a fresh gap.
- When to use: Immediately on import, before you spend time inspecting the mesh. If AutoFix reports success, you’ve eliminated 80 % of common STL problems. If it can’t fix everything, it still leaves far fewer issues for manual tools.
- Tip: After significant manual edits (especially Boolean cuts), run AutoFix again; new boundaries can re‑introduce non‑manifold edges.
Fill Holes – While AutoFix closes simple cracks, larger holes require Fill Holes. Discovery analyzes boundary loops, then creates a triangulated patch that can be either planar (flat fill) or curvature‑based (blended). The curvature option samples the surrounding facet normals so the new triangles transition smoothly onto curved regions.
- Parameters to know: Maximum hole diameter (ignore tiny gaps) and blend method.
- Good for: Missing caps on pipe ends; small gaps in anatomical scans; edge tears after a Boolean subtract.
- Avoid: Jagged, tooth‑like openings—Shrinkwrap yields better results because it re‑skins the area instead of forcing a thin patch.
Shrinkwrap – Re‑Skinning the entire model Shrinkwrap envelopes the original facets with a brand‑new surface, similar to stretching heat‑shrink film over electronics. You specify a target facet size and a tolerance for gap bridging; Discovery inflates a virtual balloon until it contacts the outermost triangles, then relaxes the mesh to produce a closed skin.
- Strengths: Solves thousands of micro‑holes in one step; merges floating fragments; smooths minor noise—ideal for MRI or CT scans.
- Trade‑off: Because it forms a new envelope, crisp edges blur slightly. If you need precise cylinders or datum planes, consider AutoSkin instead.
- Workflow: AutoFix → Shrinkwrap → Smooth. Only afterward should you reduce triangle count or convert to solid.
AutoSkin – Converting Facets to Hybrid Geometry, AutoSkin tries to preserve geometric intelligence. It segments the STL by curvature, fits analytic faces (planes, cylinders, cones) wherever possible, and fills remaining portions with SubD surfaces. The result is a hybrid body: sharp mechanical faces where tolerances matter, free‑form patches elsewhere.
- Ideal input: A mesh that is already watertight and not excessively noisy. For example, a topology‑optimized bracket or a scanned turbine blade cleaned by Shrinkwrap.
Why choose AutoSkin: Downstream CAD edits remain parametric—hole diameters stay true cylinders; mounting pads stay planes.
Preparation: Run Smooth and Reduce to even out facet density; AutoSkin’s curvature classifier works best on uniform triangles.
Boolean Operations – Combining and Carving STL Shells. Although Booleans are more common in solid modeling, Discovery lets you perform them on facet bodies.
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Combine (Add) merges multiple shells—perfect after a segmented scan where each lobe imported separately.
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Subtract cuts away supports or internal fixtures scanned with the part.
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Intersect isolates an ROI, useful when you only need a slice of a medical model.
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Split detaches islands based on seed selection, letting you delete floating debris quickly.
Run Booleans before converting to solid to avoid expensive solid‑to‑solid blends.
Smooth – Removing High‑Frequency Noise The Smooth command performs Laplacian or HC (Humphrey–Catmull) smoothing. You control iteration count and relaxation factor. Each iteration nudges vertices toward the centroid of their neighbors, averaging out spikes while respecting overall form.
- Use cases: CT scans often include staircase artefacts; Smooth can tame these without changing airway diameters significantly.
- Too much smoothing erodes corners—monitor curvature deviation.
Reduce – Reduce collapsed edges while minimizing shape deviation through a quadric error metric. You can target a specific triangle tally or a maximum geometric error.
- Why: GPUs struggle with ten‑million‑facet models, and meshing such data is slow. Reducing by 50 % typically has negligible visual impact but halves memory.
- Watch out: If your STL contains delicate struts, set a low error tolerance to protect thin walls.
Regularize – Evening Out Triangle Quality After Reduce, triangle shapes can be long and skinny. Regularize re‑distributes vertices to equalize edge lengths and aspect ratios, improving downstream finite‑volume surface meshes.
- CFD benefit: Wall‑function y"+" values rely on smooth normals; Regularize reduces random normal flips.
Converting to Solid
Once your facets are watertight, you can convert it to a solid directly from the Model tree: right‑click the facet body → Convert → Solid Body. Discovery offers two modes:
- Mostly Tessellated – The software groups adjacent coplanar or smoothly connected triangles into analytic patches wherever possible. You keep a lighter face count (better performance) and gain planar or cylindrical surfaces that are easier to select, dimension, or mate. Choose this for mechanical parts or scans where large flat/round areas exist.
- Fully Tessellated – Each triangle becomes an individual face in the resulting solid. Geometry fidelity is maximized—every bump and ripple survives—but the model may contain hundreds of thousands of faces. Use this only when micro‑scale surface detail is critical (for example, roughness studies or highly intricate lattice structures) and you can tolerate larger file size and slower meshing.
Selecting the right option balances accuracy with usability: start with Mostly Tessellated for most engineering workflows; switch to Fully Tessellated only if you notice loss of essential geometric detail after conversion.
Subdivision (SubD) Workflows – Organic Sculpting
Subdivision surfaces offer a sculpt‑friendly alternative to traditional Non-Uniform Rational B-Splines (NURBS) modeling. Rather than relying on precise sketches and trims, SubD represents a smooth limit surface controlled by a lightweight polygonal “cage.” Moving or subdividing this cage reshapes the entire surface intuitively (much like pushing or pulling clay). In Discovery, you can start with basic SubD primitives, or convert cleaned STL facets into SubD bodies for free‑form refinement. The big advantage is speed: rounded biological or topology‑optimized shapes that would require dozens of fillets in parametric CAD can be tweaked in seconds with SubD tools. The trade‑off is dimensional precision—SubD excels at organic continuity, not tight tolerance machining. That’s why Discovery supports hybrid models, letting you keep analytic cylinders or planes where accuracy matters while using SubD for the complex, flowing regions.
Subdivision surfaces turn a coarse “control cage” into a mathematically smooth limit surface. Discovery integrates SubD with traditional CAD, letting you convert between facets, SubD, and solids. The "SubD" tab is not active by default. To make it appear in the ribbon, you need to go to File --> Settings --> Customize --> Ribbon Tabs, then toggle the eyeball icon on as shown.
Conversion Modes
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Facets → SubD: Select the cleaned STL, pick a fit tolerance, and Discovery builds a control cage. Lower tolerance yields a closer fit but more faces.
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Region → SubD: Box‑select a curved area (the bifurcation zone of an airway) and convert only that section, preserving cylinders on inlet/outlet planes.
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SubD → Solid: Once sculpting is done, convert the geometry to a solid for meshing or export.
Core Editing Commands
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Pull extrudes or offsets faces. With “Proportional” active, neighboring faces follow in a falloff radius—ideal for bulking branch walls without hard steps.
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Move translates vertices; the proportional option lets you push large surface patches like clay.
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Subdivide adds control‑cage density, giving finer edit control where detail is needed.
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Crease/Sharpen introduces controlled hard edges—handy if an airway mouthpiece attaches to a planar flange.
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Snap drapes a sculpted SubD back onto the reference STL, maintaining accuracy after freeform edits.
Pros and Cons
SubD shines for sculpting and rapid iteration, but because it approximates rather than exactly replicates dimensions, you may still need analytic CAD for precision machined features. Using hybrid models (analytic faces + SubD patches) often provides the best of both worlds.
Choosing the Right Tool for the Job – Industry Examples
The tools outlined above are flexible enough to solve vastly different STL challenges across industries. Below are three representative cases that illustrate which Discovery tools typically come into play—without enumerating every mouse‑click.
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Medical Scan of a Lung Airway
High‑resolution CT data captures delicate bronchial tubes but arrives riddled with pinholes, stair‑step artefacts, and tiny floating fragments.
Use‑case emphasis: automated healing to restore watertightness, global wrapping to seal micro‑defects, and gentle smoothing to preserve airway diameters before volume conversion for internal‐flow CFD studies.
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Topology‑Optimized Lightweight Bracket
A lattice‑like structure exported from optimization software often carries jagged facets and inconsistent curvature.
Use‑case emphasis: localized hole filling for small gaps, facet reduction to keep file size manageable, AutoSkin to promote analytic bolt‑hole cylinders, and selective SubD conversion to free‑form ribs prior to structural validation.
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Additively Manufactured Lattice with Support Debris
Reverse‑engineered scans of lattices contain overscan material and disjoint support remnants.
Use‑case emphasis: region splitting and Boolean subtraction to discard extraneous shells, targeted remeshing for uniform triangles, and a final shrinkwrap to produce a clean envelope suitable for modal or thermal simulation.
Across these scenarios, the sequence may differ in detail, but the guiding principle remains: start with broad automatic fixes, address structural integrity (holes and islands), optimize facet quality (reduce, regularize, remesh), then convert to hybrid or solid geometry as analysis demands.
Conclusion
Cleaning STL geometry can feel like surgery. Every model is different, but Discovery’s toolbox covers every step from emergency triage (AutoFix) to sophisticated reconstructive work (SubD sculpting). Mastering when to deploy each command—Shrinkwrap for pervasive leakage, AutoSkin for hybrid precision, Smooth and Regularize for mesh quality—lets you transform raw scan data or topology‑optimized shells into robust solids quickly, freeing you to focus on simulation insights rather than geometry headaches.