Ensuring Your Part Releases Cleanly from the Mold

Two design features determine whether a vacuum formed part can be easily removed from the mold: draft angle and undercuts. Neglecting either one leads to sticking, distortion, damaged parts, and costly mold modifications.

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Draft Angle: Why It Matters

Draft angle is the taper on sidewalls that allows the part to slide off the mold. Without it, the plastic cools and grips the mold surface tightly, making release difficult or impossible.

Factors That Affect Required Draft

The minimum draft angle isn't a single number — it depends on:

Draft Angle Guidelines by Material

Draft Angle Design Rules

1. Always apply draft to every vertical surface — even if the drawing doesn't call for it. The thermoforming mold is not an injection mold; the draft is essential for release.
2. Increase draft with draw depth — for shallow draws (under 50mm), 2° may be sufficient. For deep draws (over 200mm), use 5° or more.
3. Account for texture — if you want a textured surface on the part, the mold texture will increase friction. Add 1–2° to your draft angle.
4. Apply draft in the direction of mold travel — the part slides off in one direction. Ensure all surfaces taper in that direction.
5. Variable draft is acceptable — different parts of the mold can have different draft angles based on local geometry. The key is that every vertical surface must have some draft.

Common Draft Angle Mistakes

• Designing with zero draft because "the injection mold didn't need it" — injection molds release differently than thermoforming molds
• Applying draft in the wrong direction — the draft must be in the direction of part removal
• Forgetting draft on internal features — internal ribs, bosses, and ribs also need draft
• Using too much draft — excessive draft causes visible wall thickness variation and can affect part aesthetics

Undercuts: The Dealbreaker

An undercut is any feature that prevents the part from sliding off the mold in a straight direction. It creates a "lock" between the part and the mold.

Common Undercut Features

How to Handle Undercuts

Option 1: Eliminate the Undercut (Best)

Redesign the feature to avoid the undercut:

• Replace an internal thread with a through-hole + metal nut
• Replace a snap hook with a screw-mounted boss
• Replace a return lip with a straight flange

Option 2: Split Mold (Acceptable)

If the undercut is essential, use a mold that opens in two or more pieces. The part releases when the mold halves separate.

Trade-offs:

• Higher mold cost (30–50% more than a single-piece mold)
• More complex mold alignment
• Longer cycle time (mold open/close sequence)
• Potential parting line marks on the surface

Option 3: Removable Tool Section (Compromise)

A portion of the mold that can be removed after forming, allowing the part to release.

Trade-offs:

• Adds cycle time (manual removal step)
• Risk of misalignment between cycles
• Limited to simple undercut geometries

Option 4: Post-Forming Machining (Last Resort)

Form the part without the undercut, then machine the undercut feature after forming.

Trade-offs:

• Adds a secondary operation (CNC machining, drilling)
• Increases part cost and lead time
• Only practical for low-volume or critical features

Undercut Decision Guide