Shell, Draft & Rib
The plastic-part trio — hollow a solid to walls, taper faces for mould release, and add stiffening ribs.
Together these three turn a chunky solid into a mouldable plastic part.
Shell
Hollows the body to a constant wall:
- Start Shell, set the wall thickness.
- Pick the faces to remove (the openings) — none removed = a closed hollow body.
- Need a thicker floor? Per-face multi-thickness overrides let selected walls differ.
Shell late in the tree: features added after it are not hollowed, which is usually what you want for bosses and holes.
Draft
Tapers faces by an angle so the part releases from a mould:
- Pick a neutral plane (stays put) and the faces to draft; set the angle (1–3° typical).
- Applies to existing geometry — an alternative is the draft option built into Extrude while the boss is made.
Rib
A stiffener from a skeleton line:
- Sketch a line (open contour is the point — no closed profile needed) where the rib's spine should run.
- Start Rib: set thickness (spread evenly both sides), the fill direction, and optional draft for moulding.
- The rib grows until it meets the body — walls, floor, curves and all.
When it fails
- Shell fails on tight corners — the wall self-intersects at small radii; thin the wall or enlarge the Fillet & chamfer first.
- Rib "can't find the body" — the fill direction points away from material; flip it.
- Draft conflicts with a fillet — draft first, fillet after, is the reliable order.
Related: Extrude (boss & cut) · Fillet & chamfer · Holes, threads & coils
Rib thickens an open contour into a wall. A sketch may hold SEVERAL contours and each becomes its own wall in one feature — SolidWorks works the same way. Selected contours appears once there is more than one, and an empty selection means all of them.
The Draft box tapers every wall of the rib, with Draft outward to widen instead of narrow. It tapers the THICKNESS only: the ends stay where you drew them, so a rib drawn to meet a wall still touches it. To taper only certain faces, commit the rib and use Draft — a rib's own faces do not exist until it is built.