An undercut is any feature that prevents the formed part from being released from the mold by straight-pull demolding. Snap-fit hooks, return flanges, recessed labels, internal threads — all undercuts. They’re not impossible to thermoform, but they require specialized tooling and add cost. This guide explains the four solution paths and when each is right.
Why Undercuts Are Hard for Thermoforming
Standard vacuum forming uses a one-piece male or female mold. The heated sheet drapes over (male) or sucks into (female) the mold. After cooling, the part is lifted straight up and off — vertically. Any feature that “hooks” around the mold geometry blocks this vertical motion.
Examples of undercuts:
- External undercut — outward-facing flange or rim wider than the part below it. Once formed, the flange cannot pass back over the wider part above.
- Internal undercut — inward-pointing snap, recessed label area, or undercut groove on the inside of the part.
- Side-pull undercut — a horizontal feature (handle, hose port, mounting tab) that projects sideways from a vertical wall.
Solution 1: Slip-Ring / Loose Collar (Best for External Undercuts)
The mold’s outer perimeter is split into a fixed central core and a removable ring (the “slip ring”) that holds the undercut geometry. Parts form normally; after cooling, the slip ring is unclamped and the part lifts straight up while the ring stays around the part. The ring is then removed manually or pulled away on guides.
Cost impact: $400-2,500 added to tooling. Cycle time: +5-12 sec for ring handling.
Used for: External rims, lid flanges that overhang the body, integrated handles on box-shaped parts.
Solution 2: Side Action (Pneumatic Side-Pull)
A pneumatic cylinder mounted in or near the mold extends a side core into the cavity during forming, then retracts when the part is ready to demold. The retracted core leaves the formed undercut behind.
Cost impact: $1,500-7,000 per side action depending on size and travel. Cycle time: +3-8 sec for actuation.
Used for: Mounting flanges on enclosure sides, horizontal vent louvers, snap-fit hooks pointing outward.
For complex parts, multiple side actions on different mold faces. We’ve built thermoforming tools with up to 6 independent pneumatic side-pulls — used on a 1500mm-long EV charging station outer shell with mounting flanges on all 4 sides.
Solution 3: Removable Plug Insert (Best for Internal Undercuts)
A small mold insert (usually metal) is loaded into the cavity before forming and is left inside the part after demolding. Operator removes the insert manually after demolding. Insert is cleaned, returned to the mold cycle.
Cost impact: $200-1,500 per insert + custom handling tooling. Cycle time: +5-15 sec for insert handling per part.
Used for: Internal recessed snap features, internal mounting bosses, inside-pointing label recesses.
Drawback: requires reliable insert recovery process. Some insert designs let inserts ride along into post-trim where they’re automatically removed. We use this on high-volume medical device housings.
Solution 4: Design-Out (Always Preferred)
Sometimes the cheapest fix is to redesign the part to eliminate the undercut. Common approaches:
- Snap fits: Replace internal snap hooks with external flanges + clip-on retainers. Adds an assembly step but eliminates undercut tooling cost.
- Recessed labels: Use raised text/logo (positive feature) instead of recessed pocket. Achieves same visual goal without undercut.
- Mounting holes: Move mounting features to the outer flange (no undercut) instead of side walls (potential undercut). Often improves assembly access too.
- Handles: Use bonded-on or fastener-mounted handles instead of integrated formed handles. Cheaper for low-medium volume.
- Two-piece assembly: Split a complex part into two simpler thermoformings that snap or screw together. Eliminates side actions, simplifies tooling.
DitaiPlastic’s DFM team will always propose design-out options first when reviewing customer drawings. Customer makes the final call.
Decision Matrix: Which Undercut Solution Is Right?
| Undercut Type | Volume <500/yr | 500-5,000/yr | 5,000-50,000/yr | >50,000/yr |
|---|---|---|---|---|
| External rim/flange | Slip-ring | Slip-ring | Slip-ring (auto) | Side action |
| Internal snap | Design-out preferred | Removable insert | Removable insert | Side action / re-design |
| Side-pointing feature | Design-out / 2-piece | Side action | Side action | Side action |
| Recessed label area | Design-out (raised) | Design-out (raised) | Removable insert | Engraved post-form |
Real Project Examples
EV Charger Outer Shell with 4-Side Mounting Flanges (Side Actions)
- Part: 1500 × 600 × 800mm shell with mounting flanges on all 4 vertical faces
- Material: 6mm PC/ABS
- Solution: 4 pneumatic side actions, custom heavy-gauge tool
- Tooling cost: $34,000 (vs $14,000 for one-piece tool with no flanges)
- Volume: 8,000 units over 3 years — premium justified vs alternative redesign
Medical Diagnostic Housing with Internal Snap (Insert)
- Part: 450 × 350 × 200mm housing with internal snap retaining diagnostic cartridge
- Material: 3mm ABS
- Solution: Removable plastic insert with snap geometry; insert ejects during post-trim
- Tooling cost: $5,500 (mold) + $800 (12 insert backups for high-volume run)
- Cycle time impact: +6 sec/part
Industrial Machine Guard with External Lid Flange (Slip-Ring)
- Part: 1500 × 900 × 400mm machine guard, lid clips over body via external flange
- Material: 5mm ABS
- Solution: Slip-ring around outer rim of lid mold
- Tooling cost: $12,000 (mold) + $1,800 (slip-ring assembly)
- Cycle time impact: +8 sec/part
Got an Undercut? Send the CAD.
We’ll evaluate every undercut feature, recommend the lowest-cost solution path (often design-out), and quote tooling within 2 business days.
Undercut Design FAQ
How small an undercut still requires special tooling?
Anything more than 0.3mm of inward projection on a polished mold typically blocks demolding. On textured molds, even 0.2mm can hold the part. Soft draft (1-2°) on the undercut face can sometimes “snap” past during demolding for very small undercuts on flexible materials like HIPS or PP, but this is risky for production.
Can twin-sheet thermoforming form undercuts in the bond zone?
Yes — this is one of twin-sheet’s biggest advantages. The two halves of the part can have undercut features that would be impossible in single-sheet forming. Used heavily in fuel tanks, automotive air ducts, and structural panels.
Does pressure forming help with undercuts?
Pressure forming reproduces sharper features but doesn’t change the demolding problem. Undercuts in pressure forming still need slip-rings, side actions, or inserts.
What about CNC machining the undercut after forming?
Possible for accessible features. We CNC-mill or router-cut undercut grooves, mounting slots, and edge profiles in 25-40% of our parts. Cost: $0.30-3.00 per part depending on complexity. Not feasible for inside-pointing features inaccessible to a tool.
How long do side action cylinders last?
Industrial pneumatic cylinders rated for thermoforming environments last 500K-2M cycles. Replacement cost: $200-800 per cylinder. We pre-stock spares for high-volume runs.