Thicken and Solid From Surface

Overview#

There are three common ways to get a solid from surfaces:

1. Stitch a closed set of surfaces: the quilt becomes a solid automatically.
2. Thicken an open or closed surface: adds wall thickness in one or both directions.
3. Boundary Fill: selects a region bounded by surfaces, solids, and planes and fills it with a solid.

Choose by shape. A closed envelope is Stitch. A skin with a wall (a bottle, a shell, a grip) is Thicken. A cavity formed by many tool bodies (a mold cavity) is Boundary Fill.

Where to find it#

• Thicken: Solid tab > Create > Thicken.
• Stitch: Surface > Modify > Stitch.
• Boundary Fill: Solid tab > Create > Boundary Fill.

Step by step#

Thicken#

1. Start Thicken.
2. Select the surface face(s) or body. Chain Selection picks tangent-connected faces.
3. Enter a Thickness: a positive value thickens the exterior side, a negative value the interior side. You can also drag the manipulator.
4. Choose a Direction: One Side or Symmetric (both sides).
5. Choose a Thicken Type: Sharp Thicken or Rounded Thicken corners.
6. Choose the Operation: Join, Cut, Intersect, New Body, or New Component (Hybrid Design type only).
7. Click OK. Thicken fails if the offset collides with itself, usually at tight inside curves.

Stitch to a solid#

1. Make sure all enclosing surfaces share edges.
2. Stitch them. If closed, the result is a solid.
3. Confirm: select the body and check the Properties for a volume.

Boundary Fill#

1. Start Boundary Fill.
2. Select the bodies and surfaces (solids, surfaces or workplanes) that bound the region as the tools.
3. Select the cell or cells that make up the enclosed volume. Each enclosed region appears as a clickable cell.
4. Choose an Operation (Join, Cut, Intersect, New Body, or New Component in Hybrid Design) and click OK.

The complete workflow: skin to production solid#

1. Sketch key curves: profile, plan view, and any guide splines. Constrain them fully where possible.
2. Create primary surfaces with Loft, Sweep, or Extrude. Keep them simple: fewer, cleaner surfaces beat many patches.
3. Extend the surfaces past each other generously.
4. Trim them against each other so they meet at exact edges.
5. Patch any remaining holes, with tangency to match neighbors.
6. Stitch into a quilt. Fix free edges until the model is closed. If it turns into a solid, you are done; if you want a wall, remain a surface and continue.
7. Analyze: Zebra and Curvature Comb on key transitions. Rebuild surfaces that ripple.
8. Thicken the quilt to the wall thickness, or stitch a closed envelope.
9. Finish with solid features: Fillet small edges, add holes, bosses, ribs and a Shell if appropriate.
10. Check the result with Section Analysis and the Properties/Physical Material tools.

Options reference#

Thicken#

Boundary Fill#

Tips and pitfalls#

Tip: Thicken in the direction that matches the design intent: the visible class-A surface should stay exactly where you modeled it, so thicken toward the inside.

Tip: If Thicken fails, try a smaller thickness to confirm the cause is a tight radius, then fix the surface or fillet the inside corner.

Warning: Thickening a stitched quilt with many faces can leave seam artifacts. If a Thicken works only per-face, consider whether the quilt's faces are actually tangent and merged.

Warning: Stitch tolerance is not a repair tool. A solid created with large tolerance can carry hidden kinks and fail in later Shell or Fillet.

Tip: Keep the original surfaces (hide, don't delete). With history on, you can edit them and the solid updates.

Thicken in detail#

Autodesk's help places Thicken on the Solid tab under Create. It offsets one or more faces, surfaces or quilts an equal distance and adds sides to make a solid, with a Direction of One Side or Symmetric.

Rules from the help worth remembering: positive distances thicken the exterior side and negative ones the interior side, and Thicken cannot remove a face, so the maximum offset is limited by the distance at which a face would vanish. On a surface with a tight inside curve, the thickness may be limited to just below that radius.

Worked example: a 2 mm shell for a scoop#

A scoop surface has a bowl with a 25 mm inside radius. Thicken it 2 mm toward the inside: the inside radius becomes 23 mm, which is fine. Thicken it 30 mm inward and the limit is hit at 25 mm because the surface would pass through its center of curvature. So the maximum for that area is just under 25 mm.

If the surface has a tighter feature, such as a 3 mm fillet on the rim, a 4 mm inward Thicken fails at that fillet; a 2 mm one succeeds. Reduce the thickness, or rebuild the fillet larger than the thickness.

Choosing the direction#

Choose the side that preserves the surface that matters. A cosmetic shell designed to the outer skin should thicken inward so the visible shape is unchanged. For a product with a 2 mm wall, if the outer skin is exactly correct and you instead thicken to the exterior side, the finished dimension is 2 mm bigger on every side.

Boundary Fill#

Boundary Fill creates a solid from regions enclosed by surfaces, bodies or planes. Select the tool bodies, then the regions (cells) it highlights, and choose the operation. It is the tool when several surfaces intersect and you want the enclosed volume without stitching. Use it for mold cavities: a surface that wraps a part plus a surrounding box yields a cavity cell that you can keep or remove.

Failure modes and fixes#

Thicken versus Stitch versus Boundary Fill#

• If the surface is closed (watertight), Stitch converts it to a solid. No wall thickness is created; the solid is the enclosed volume.
• If the surface is open (a skin), use Thicken to give it wall thickness.
• If several surfaces cross each other, use Boundary Fill to pick the regions you want.

Workflow advice#

1. Decide the wall thickness early, since it limits minimum radii in the surface (radius at least as big as the thickness on the inside).
2. Thicken last, after Zebra and comb checks on the surface, because rebuilding a thickened solid is harder than editing a surface.
3. Verify the thickness with a section: measure the wall at three points.
4. Keep the source surface (hidden), so you can adjust thickness by editing the Thicken feature.

Checking wall thickness after Thicken#

After Thicken, verify the wall rather than trusting the dialog. Section through the part and use Measure at three locations; on a uniform wall they should match your input within 0.01 mm. A larger value on curved regions often means the wrong direction on a one-sided Thicken. For parts to be molded, also run Draft analysis on the new faces, since the inner and outer walls inherit the surface's draft.

Practice exercise#

1. Loft a surface between two rounded rectangles 80 mm apart to make a tube. Cap one end with Patch; leave the other open.
2. Thicken the quilt 2 mm inward. Result: a hollow container with an open end.
3. Try Thicken at 40 mm and note the failure (the thickness exceeds the local radius). Reduce until it succeeds.
4. Use Fillet on the open rim to complete it.

Expected: a cup-like solid with a 2 mm wall.

Stretch challenge#

Create a bowl-shaped surface by revolving a 25 mm radius quarter arc, then Thicken it inward by 2 mm and measure the wall at the rim and bottom with a section. Edit the thickness to 6, 12 and then 24 mm and note when it first fails (it should fail near 25 mm). Next, use Boundary Fill on a box tool and the bowl surface to create a mold cavity. Expected result: a 2 mm wall measured within 0.01 mm, a recorded failure threshold near 25 mm, and a cavity solid.

Related#

• Stitch and Unstitch
• Surface Repair
• Continuity G0 to G3