5.3 Contours, Thin Features & Common Extrude Traps

5.3 Contours, Thin Features & Common Extrude Traps

Quick Answer: Use Selected Contours / Contour Select Tool when one sketch contains multiple regions so you extrude or cut only the intended area. Prefer closed profiles for solid bosses and cuts; use Thin Feature for open profiles or controlled wall thickness. Avoid self-intersections and tiny gaps. On CSWA, a wrong contour or end condition changes volume—and mass—far more than a 0.1 mm cosmetic error.

By now you can pick Blind vs Through All and reverse a cut into the solid. This section is about which geometry participates in the extrude and the special Thin Feature path that turns a sketch path into a wall. These details separate clean exam models from “feature failed” rebuilds and mystery mass values.

Contours: what SOLIDWORKS is selecting

A sketch may define several closed regions (contours):

• Outer rectangle with an inner rectangle → two regions (outer loop island vs inner void depending on feature type).
• Two separate circles → two disjoint contours.
• A figure-eight self-touching path → invalid or ambiguous solid regions.

When you start Extruded Boss/Base or Extruded Cut, the PropertyManager Selected Contours box (and the Contour Select Tool) control which regions become solid or which regions are removed.

Contour Select Tool workflow

1. Create the full sketch with all intended loops (fully define).
2. Start Extrude Boss/Base or Extrude Cut.
3. If the auto-selection is wrong, activate contour selection and click the region (the face-like area of the sketch) you want—not only an edge.
4. Add multiple regions if one feature should process several islands (for example, three hole circles in one cut).
5. Preview: only selected regions should show the extrude/cut volume.

Exam payoff: One sketch + one feature for a pattern of holes is faster than three sketches. One sketch with outer profile and inner hole can define a plate with a cutout in a single boss if you select the region between outer and inner loops correctly (solid with a hole), rather than extruding the outer only and cutting later—both workflows are valid; pick the clearer one under time pressure.

Multi-contour strategy table

If you leave Selected Contours empty, SOLIDWORKS often auto-picks what it thinks is the outer region—verify. Auto-pick on a complex sketch is a classic “why is my boss missing a wing?” moment.

Open vs closed profiles

Closed means every endpoint meets with coincident relations (or continuous model edges). A 0.01 mm gap that looks closed on screen is still open—zoom corners, add coincident, or use Trim.

Open profiles are intentional for:

• Thin wall sweeps along a path (U-channel centerline, single-line rib path with thickness).
• Some thin tube-like shapes from a U-shaped sketch with Thin Feature.

Do not force an open profile into a normal solid boss; either close it or enable Thin Feature.

Thin Feature extrude option

In the Extrude PropertyManager, Thin Feature adds a thickness to the sketch path:

• One-Direction: thickness to one side of the sketch entities.
• Mid-Plane: half thickness each side of the path (symmetric wall about the sketch).
• Two-Direction: different thicknesses each side (less common on CSWA).

You also set wall thickness value and end conditions for how far the thin solid runs (Blind depth, etc.), same as a normal extrude’s length direction—do not confuse wall thickness (perpendicular to sketch entities in-plane sense for thin) with extrude depth (out-of-plane length).

When CSWA drawings suggest Thin Feature

• Sheet-metal-like channel shown as a centerline path with wall thickness callout (even if not a Sheet Metal feature).
• Simple tube from a circle with thin wall (sometimes easier as two circles solid boss; either works).
• Open multi-segment path with constant thickness walls.

Thin Feature traps

Self-intersecting and invalid sketches

Self-intersecting contours (lines crossing without intentional vertices/regions, bow-ties, overlapping duplicate lines) break extrudes or create unexpected regions. Before the exam clock burns:

1. Repair with Trim/Extend; delete duplicate stacked lines.
2. Ensure Tangent where arcs meet lines if the drawing is smooth—kinks can still extrude but may affect later fillets.
3. Run a mental “region test”: if you could not shade a single clean area, Contour Select will be ambiguous.

Zero-length entities and tiny segments from overzealous Convert Entities also cause headaches—delete debris construction you do not need.

Common extrude traps that change mass drastically

These are high-frequency CSWA failure modes. A 1% density mistake hurts; a wrong end condition can double mass.

1. Wrong end condition (the headline exam mistake)

Habit: After every extrude/cut, ask: “Is this open through, stopped at a number, or stopped on a face?” Match the sheet in those words before OK.

2. Wrong contour / partial selection

Only one of three hole circles selected → two holes missing → mass high. Outer loop extruded without subtracting inner island when you thought the hole was included → solid plug remains. Fix: re-edit feature, Selected Contours, click all intended regions; or use a separate Extruded Cut for holes for clarity.

3. Direction reverse

Boss on the wrong side of a face; cut away from solid. Preview is free—use it.

4. Open contour accidental gaps

Feature fails or Thin Feature engages unexpectedly if you toggled options while debugging. Close the profile for solid parts.

5. Units and decimal depth

Typing 10 in an inch template when the drawing is 10 mm (or the reverse) is an order-of-magnitude disaster. Confirm units in the status bar before the first extrude of every question.

6. Merge result off

Second body not included in the mass body you inspect, or cuts miss the body. Keep merged single body unless the problem is multibody by design.

7. Draft ignored

Section shows 5° draft; you extrude vertical walls. Volume error compounds on tall bosses.

8. Self-intersection after dimension change

A modification increases thickness of a thin feature or shrinks a hole until geometry crosses. Rebuild errors or silent bad geometry—inspect after every modification step before submitting mass.

Diagnostic sequence when mass is “impossibly” wrong

1. Material/density correct? (If density wrong, fix first—but also verify geometry.)
2. Section the model through holes/pockets: are they truly through? Is pocket depth visible?
3. Edit each Extrude/Cut: end condition name, depth value, direction arrows, Selected Contours list.
4. Measure a critical thickness with the Measure tool; compare to drawing.
5. Rollback bar: does mass jump when a particular cut is suppressed? That feature is the culprit.
6. Only then re-sketch if the profile region itself is wrong.

Practice checklist combining Chapters 3–5

1. Sketch entities + relations + fully defined (black).
2. Closed profile or intentional Thin Feature path.
3. Boss: correct end condition, direction, merge, draft if any.
4. Cuts: Through All vs Blind by intent; direction into solid; all hole contours selected.
5. Rebuild; material; mass/CoM to required decimals.
6. Apply modification; re-verify end conditions still match design intent (especially Through All vs Blind).

Worked trap scenario

Drawing: 20 mm plate, Ø10 through-hole, 8 mm deep rectangular pocket from the top.

• Correct: Base Mid Plane or Blind 20 mm; hole cut Through All; pocket cut Blind 8 mm.
• Trap A: Hole Blind 20 mm works until thickness becomes 25 mm → hole no longer through → mass high.
• Trap B: Pocket Through All → mass low; section shows daylight through the floor.
• Trap C: Multi-contour sketch of plate outer + pocket rectangle extruded as boss with only outer selected, then forgotten pocket cut → mass high.
• Trap D: Thin Feature accidentally enabled on the base rectangle with 1 mm wall → hollow shell, mass drastically low.

Each trap is visible in a 10-second section view. Build the habit of sectioning before you type the numeric answer.

Contours and Thin Feature give you control over what becomes solid; end conditions control how far. Together with fully defined sketches, they are the core of CSWA basic and advanced part creation. Master the traps in this section and mass-property questions stop feeling random—they become a checklist.