Rhino 3D Tip: Cap Planar Openings to Create Closed Solids in Rhino

When a modeled object has a flat, open boundary, Rhino’s Cap command can quickly convert it into a closed solid—provided the opening is truly planar and its edges form a valid closed loop.

Use Cap whenever you have an open polysurface or extrusion with planar end openings. It is one of the fastest ways to prepare geometry for Boolean operations, volume calculations, 3D printing, CAM, or export to downstream applications.

• Select the open object and run Cap.
• Rhino analyzes the naked-edge loops on the selected polysurface.
• If an opening lies on a plane and is bounded by a closed loop, Rhino creates a planar face to seal it.

A common example is an open tube created with ExtrudeCrv or Sweep1. If both ends are flat, selecting the result and using Cap creates end faces immediately. For simple closed planar curves, enable the Solid=Yes option during ExtrudeCrv to create capped extrusions from the start. However, Cap remains essential when you receive imported geometry, modify an existing model, or intentionally build surfaces before closing the object.

Before capping, inspect the object with ShowEdges and choose the Naked edges option. Naked edges identify boundaries that are not joined to another surface. A successful cap removes the planar naked-edge loop and helps turn the model into a closed polysurface.

• Use What after capping. Rhino should report Closed polysurface or Closed solid polysurface.
• Use SelOpenPolysrf to locate polysurfaces that remain open.
• Use Check if a model appears closed but later fails in a Boolean, export, or fabrication workflow.
• Use Volume as an additional confirmation: a valid closed solid can return a volume value.

If Cap fails, do not force the issue by adding arbitrary patches. First identify why the opening is not eligible. The most frequent cause is that the boundary is not planar. Even a very slight deviation can prevent Rhino from generating a planar cap. Use PlanarSrf only after confirming that the boundary curves are coplanar, or revise the surrounding surfaces so their edges terminate on the intended plane.

Other common causes include tiny gaps, overlapping edges, self-intersecting boundaries, and unjoined surface edges. Use Join to join compatible surfaces, then inspect the remaining naked edges. For problematic imported models, zoom in closely, use DupBorder or DupEdge to extract the boundary as curves, and evaluate whether those curves form a clean closed planar loop. If necessary, repair the boundary curves before creating a replacement planar surface.

Keep in mind that Cap is designed for planar openings—not curved holes. For non-planar openings, consider commands such as EdgeSrf, NetworkSrf, Patch, or FillSrf, selecting the method that best supports the required shape and surface continuity.

Developing the habit of capping and validating solids early prevents many downstream issues. Closed, clean geometry is more dependable for Boolean modeling, STL export, render preparation, and manufacturing workflows. For Rhino software, training resources, and workflow tools, visit NOVEDGE’s Rhino collection.

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