7.1 Shell Feature
Key Takeaways
- Shell removes selected faces and hollows the solid to a uniform wall thickness (or multi-thickness), creating a thin-walled body common on advanced CSWA parts.
- Thickness can grow inward (into material) or outward; wrong direction and wrong face selection change volume and mass drastically.
- Multi-thickness Shell lets selected faces keep different wall values—use only when the drawing specifies more than one thickness.
- Feature order vs fillets/chamfers matters: shell before large fillets can fail or leave wrong inner edges; shell after may hollow rounded geometry differently.
- Shell fails when thickness is larger than available material, geometry is non-manifold, or faces cannot form continuous walls—fix selection and thickness before rebuilding.
7.1 Shell Feature
Quick Answer: Insert → Features → Shell hollows a solid. Select faces to remove (openings), set wall thickness, choose inward (default, into material) or outward, optionally assign multi-thickness faces, preview continuous walls, then OK. Shell is a high-impact mass tool on CSWA—wrong face, thickness, or direction leaves solid bulk or wrong openings.
On the Certified SOLIDWORKS Associate skill map, Shell appears among the intermediate/advanced part feature tools used to turn a solid prism into a thin-walled housing. Prep paths and sample models routinely pair shell with extrudes, cuts, fillets, and later mass checks. Treat shell as a volume killer: a solid 100 × 80 × 40 mm block of steel has one mass; the same envelope after a 2 mm shell with the top face removed has a fraction of that mass.
What Shell does geometrically
Shell offsets the remaining outer faces inward (or outward) by the thickness value and deletes the selected faces so the interior is empty and open at those faces.
| Element | Role |
|---|---|
| Faces to remove | Become openings; not shelled into walls |
| Remaining faces | Become outer walls of the hollow body |
| Thickness | Wall thickness of the shelled solid |
| Shell outward | Grows walls outside the original solid envelope |
| Multi-thickness | Overrides thickness on selected faces |
If you select no faces to remove, SOLIDWORKS can create a closed hollow solid (internal void, no opening). That is rare on CSWA drawings, which almost always show an open box, cover, or housing with at least one open face. Closed hollows also complicate manufacturing intent and can surprise mass checks if you meant an open shell.
Exam habit: Identify every open face on the drawing (top of a box, bottom of a cover, side window) before starting Shell. Those faces are your Faces to remove list.
Workflow (timed exam)
- Finish the solid that defines the outer envelope (extrudes, revolves, major cuts that define outer shape).
- Decide whether large outer fillets should exist before shell (see order section below).
- Shell → select face(s) to remove → enter thickness from the drawing (for example, 2 mm).
- Confirm Shell outward is off unless the drawing clearly grows walls outside the solid.
- If the sheet shows two wall values, enable multi-thickness and assign overrides.
- Preview: walls continuous, openings correct, no zero-thickness ribbons.
- OK → Mass Properties when you need a numeric answer or after a thickness modification.
Faces to remove: selection discipline
Selecting the wrong face is as bad as a wrong thickness.
| Drawing intent | Faces to remove |
|---|---|
| Open-top box | Top face only |
| Cover open on bottom | Bottom face |
| Tube-like open both ends | Two opposite end faces |
| Housing with side access | Top + one side (if both open) |
| Solid remains fully closed hollow | None (void only—rare on CSWA) |
Multiple openings: Hold Ctrl and multi-select every open face in one Shell feature. Creating a second shell is usually wrong and often fails because the body is already thin-walled.
Partial openings: Sometimes a drawing shows a solid with a large window cut first, then shell. Other times shell removes a whole face and a later cut trims a smaller opening. Match the sheet: if the open face is the full planar face of the envelope, remove that face in Shell; if the opening is a smaller cutout in a wall that should remain, do not remove the whole face—cut the window instead and shell with no (or different) remove faces as the geometry requires.
Face selection traps
| Mistake | Result |
|---|---|
| Remove a face that should remain a wall | Extra opening; missing wall mass |
| Forget to remove the open face | Closed void or solid-looking wrong interior |
| Remove both ends when only one end is open | Tunnel through; mass too low |
| Select a fillet face instead of the planar open face | Failed shell or odd opening boundary |
Zoom the preview. If you see daylight where a wall should be, clear the face from the remove list.
Thickness: inward vs outward
Default (inward): Walls form inside the original solid. Outer dimensions of the envelope stay the same; interior cavity shrinks by roughly the thickness on each wall.
Shell outward: Walls form outside the original solid. Outer dimensions grow; the original faces become closer to the inner cavity boundary. Use only when the solid you built is the inner volume and thickness must grow out—or when the PropertyManager preview clearly matches a drawing that shows growth outside a core shape.
| Mode | Outer envelope | Interior | Typical CSWA use |
|---|---|---|---|
| Inward (default) | Unchanged | Smaller cavity | Build outer size first, hollow to wall t |
| Outward | Grows by ~t | Cavity nearer original outer | Core solid represents inside; rare but intentional |
Mass impact: Inward shell of thickness t on a large block removes almost the entire core. Outward shell of the same t adds a skin around a smaller solid—mass is completely different. Always match direction to how you built the base solid relative to the drawing’s overall dimensions.
Sanity check: Overall outside length on the drawing should match your model’s outer measurement after shell if you built to outer size and shelled inward.
Multi-thickness shell (intro)
When one wall must be 3 mm and another 5 mm, use Multi-thickness settings (or the multi-thickness face list in the Shell PropertyManager):
- Set the default thickness (majority walls).
- Select the special face(s) in the multi-thickness list.
- Enter the override thickness for those faces.
- Preview both wall values.
| Approach | When |
|---|---|
| Single thickness | All walls same t (most CSWA items) |
| Multi-thickness | Drawing calls out different wall thicknesses on different faces |
| Second shell | Almost never—prefer multi-thickness or redesign order |
Do not invent multi-thickness when the drawing shows one wall callout. Extra complexity invites selection errors under time pressure.
Shell order vs fillets (and chamfers)
Feature order is a classic advanced-part trap.
Fillet (or chamfer) first, then Shell
Outer edges are rounded, then shell offsets the filleted outer shape. Inner corners appear as the shell creates inner faces. Large outer fillets with small shell thickness can produce remaining wall that is thinner than t near the rounds or can fail if the offset cannot maintain thickness through the fillet region.
Shell first, then Fillet
Shell creates thin walls and sharp inner/outer edges. Fillets applied afterward act on those edges. A fillet radius larger than wall thickness or edge length often fails. Internal fillets on shelled parts must fit the remaining geometry.
| Order | Benefit | Risk |
|---|---|---|
| Large outer fillets → Shell | Outer rounds exist before hollowing | Shell fail near thick rounds; thickness not constant through fillets |
| Shell → fillets on thin edges | Clean walls first | Fillet R too large for wall/edge; inner edge failures |
| Small edge breaks after shell | Matches many molded-part drawings | Must use R that fits thickness |
| Cuts that open walls after shell | Windows in thin walls | May need to reselect shell faces if parents change |
Exam scenario: Drawing shows 2 mm shell and R3 outer fillets. If shell fails after R3, try shell before fillet only if the drawing still allows correct outer rounds on the shelled body—or reduce competing geometry. Prefer the order that rebuilds green and matches outer silhouette and wall thickness on the sheet. Never “fix” mass by skipping required fillets.
Chamfers follow the same logic: a large chamfer before shell changes faces available for shell; chamfer after shell needs enough face width on the thin wall.
Thin-wall mass impact (why shell wins or loses points)
CSWA answers are often mass to two decimals. Shell is one of the largest single mass deltas on a part.
| Change | Mass effect (typical) |
|---|---|
| Apply shell (open faces + thickness t) | Mass drops sharply (core removed) |
| Increase thickness t | Mass increases (thicker walls) |
| Decrease t | Mass decreases |
| Add another face to remove | Mass decreases (more opening, less wall) |
| Shell outward vs inward | Completely different outer volume |
| Material density after shell | Same density × much smaller volume |
Modification questions: “Change shell thickness from 2 mm to 3 mm” is a one-edit FeatureManager change—do not rebuild the whole part. Edit Shell → thickness → rebuild → Mass Properties. Forgetting to re-apply or verify material after rebuilds still yields wrong mass even if geometry is perfect.
Order of magnitude check: A steel shelled box a few hundred millimeters on a side with 2 mm walls should not weigh the same as the solid block. If mass barely changed, you may have failed to remove faces, used a tiny thickness incorrectly, or shelled the wrong body.
Failures: non-manifold, too thick, and zero thickness
| Symptom | Likely cause | Fix |
|---|---|---|
| Shell fails; thickness too large | t greater than half-width or local feature size | Reduce t only if drawing wrong—usually fix model size/selection; verify units (2 vs 20) |
| Non-manifold or invalid geometry | Zero-thickness faces, knife edges, self-intersecting prior features | Repair parent solid; remove bad fillets/cuts; check multibody merge |
| Cannot offset face | Complex curvature, tiny faces, gaps | Simplify; shell earlier; remove tiny decorative faces |
| Zero-thickness face warning | Walls meet with no material | Change thickness, remove faces, or fix sharp re-entrant geometry |
| Works until a fillet is added | Fillet conflicts with thin wall | Reorder; smaller R; different edge set |
| Multi-thickness fails on one face | Override t impossible on that face | Adjust override; exclude face from multi list |
Too thick: On a 10 mm tall rib, a 6 mm shell thickness cannot leave valid walls everywhere. Local thin features in the solid before shell are frequent culprits—cut them after shell, or design the solid without knife features that shell cannot offset.
Non-manifold: Shared edges belonging to more than two faces, or zero-thickness sheets, break shell. Ensure Merge result kept a single clean solid before shelling. Multibody accidents from unchecked merge can make shell act on the wrong body or fail.
Shell vs Thin Feature extrude
| Tool | Best for |
|---|---|
| Shell | Hollow an existing solid envelope; open box/housing from a block |
| Thin Feature extrude | Build thin walls from an open sketch path without a solid core first |
| Extruded cut of large cavity | Simple pocket when only one cavity face set matters—not a full wall offset |
If the drawing shows constant wall thickness all around an outer shape, Shell after a solid base is usually faster and more stable than modeling every wall as separate thin extrudes.
CSWA shell checklist
- Outer solid matches envelope dimensions (units MMGS when required).
- List open faces → select as faces to remove.
- Thickness and multi-thickness match the sheet.
- Inward vs outward matches how outer size was built.
- Coordinate fillet/chamfer order so shell and rounds both succeed.
- Preview continuous walls; no accidental extra openings.
- Material applied → Mass Properties after shell and after any t edit.
Shell mastery is high leverage on advanced part items: one correct face list and thickness can swing mass by orders of magnitude and still accept clean modification steps when thickness is a single driving value in the Shell feature.
You build a solid rectangular housing to the outer dimensions on a CSWA drawing, then apply Shell with the top face removed. Which thickness direction keeps the outer envelope size while creating interior walls?
A CSWA shell feature fails with an indication that the thickness is too large relative to the model. What is the most likely geometric cause?
When should you use multi-thickness settings in Shell on a CSWA part?
A part shows R5 outer edge fillets and a 2 mm shell. After filleting all outer edges, Shell fails near the rounds. Which response best reflects good CSWA practice?