Undercuts are features that prevent a part from releasing in the direction of mold opening. Every undercut requires a mechanical solution, and every solution adds cost, complexity, and potential failure points. Understanding undercut options helps designers make informed tradeoffs between part functionality and manufacturing economics. Sometimes the feature is worth the mold complexity; sometimes redesigning the feature costs less than accommodating it.
What Constitutes an Undercut
An undercut exists when any portion of the part geometry would be trapped by the mold if the mold simply opened in its primary direction. The mold cannot release the part without something moving out of the way.
External undercuts occur on the outside of the part. Common examples include side holes perpendicular to mold opening, snap features that protrude beyond adjacent surfaces, external threads, and horizontal grooves or slots.
Internal undercuts occur inside the part. Common examples include internal threads, side holes into hollow interiors, internal snap features, and horizontal internal grooves.
Identifying undercuts requires visualizing the mold opening direction (typically vertical) and asking whether any feature would trap the mold if it simply pulled straight away. Any horizontal hole, thread, snap latch, or protrusion that extends beyond adjacent surfaces likely creates an undercut.
Some apparent undercuts aren’t actually problems. A hole at an angle might align with the mold opening direction or parting line. A feature that flexes during ejection might clear without mold action. Careful geometry review sometimes eliminates perceived undercuts before engineering solutions.
Side Action Mechanisms
External undercuts most commonly use side actions: mold components that move perpendicular to the primary opening direction.
Cam-actuated slides move outward as the mold opens, pulled by angled pins (horn pins) that engage the slide. As the mold halves separate, the angled pin forces the slide sideways, clearing the undercut. On closing, the pin forces the slide back into molding position. This purely mechanical system requires no external power beyond normal mold operation.
Cam slides are the most common undercut solution for external features. They work reliably for features up to moderate depth, typically with stroke limited to approximately 1.5 times the horn pin length at typical angles.
Hydraulic cylinders power slides when cam action cannot provide adequate stroke, when timing must be controlled independently of mold opening, or when the action must occur in a specific sequence. Hydraulic slides add cost for the cylinders, valving, and integration with the machine’s hydraulic system. They also require safety interlocks to prevent mold closing while slides are extended.
Mechanical lifters handle undercuts at angles rather than pure perpendicular motion. As the ejector system advances, the lifter moves both forward (with ejection) and sideways (clearing the undercut). Lifter geometry provides the combined motion. Lifters suit modest undercuts that angle outward and occur on internal surfaces where ejector motion occurs anyway.
Cost comparison:
| Mechanism | Typical Added Cost | Complexity | Maintenance |
|---|---|---|---|
| Cam slide | $2,000-10,000 | Moderate | Wear surfaces, pins |
| Hydraulic slide | $5,000-15,000 | High | Seals, valves, timing |
| Lifter | $1,000-5,000 | Moderate | Wear surfaces, alignment |
Collapsing Cores
Internal undercuts, especially internal threads, often require collapsing cores that contract to smaller diameter during ejection.
The mechanism works through multiple segments that move inward when released, reducing the core’s effective diameter. Once collapsed, the core pulls out of the internal feature that would otherwise trap it. After part ejection, the core expands to its molding position.
Collapsing cores are mechanically complex. The segments must seal tightly during molding to prevent flash at segment joints, then release and collapse cleanly, then expand precisely back to position. Wear accumulates on the sliding segment surfaces, eventually causing flash or sticking problems.
Applications requiring collapsing cores include internal threads too large or coarse for stripping, internal recesses for O-rings or snap retention, and internal features that prevent straight ejection.
Alternative approaches sometimes avoid collapsing cores: redesigning to eliminate the internal undercut, using removable inserts that eject with the part and are stripped manually, or post-molding machining of the internal feature.
Unscrewing Mechanisms
Threaded features requiring rotation during ejection use unscrewing mechanisms. Rather than stripping (pulling threads over each other) or collapsing, these systems rotate the core while the mold opens, literally unscrewing the part from the thread form.
Motor-driven systems use electric or hydraulic motors to rotate threaded cores through belt, gear, or direct drive connections. Motor rotation synchronizes with mold opening, controlling speed and position precisely.
Rack-and-pinion systems convert linear motion (from hydraulic cylinders or mold opening) into rotation. They’re simpler than motor drives but provide less control and may limit thread engagement.
Cycle time impact is significant. Unscrewing adds 2 to 5 seconds to each cycle (sometimes more for long threads), directly reducing production rate. For high-volume applications, this penalty represents substantial capacity cost.
Applications requiring unscrewing:
Threaded closures with continuous thread engagement.
Coarse threads too aggressive for stripping.
Fine threads at risk of damage during stripping.
Functional threads requiring full engagement for sealing.
Some threaded features strip during ejection without unscrewing. The part deforms slightly, allowing threads to pass over each other. Stripping works for some thread profiles (rounded, fine pitch) and some materials (flexible, resilient) but damages others. Testing determines whether stripping is acceptable.
Design Alternatives to Avoid Undercuts
Often the lowest-cost approach eliminates the undercut through design changes rather than accommodating it mechanically.
Pass-through holes replace side holes. A hole that goes completely through the part, aligned with the mold opening direction, requires no side action. Core pins from each mold half meet in the middle. Where partial depth holes seem necessary, evaluate whether pass-through would serve the function.
Witness line at undercut places the parting line through the undercut feature. If a side groove could split at its deepest point, each mold half forms its portion without any side action needed. Witness lines must be acceptable cosmetically and functionally.
Split features divide an undercut into multiple components that assemble after molding. A snap feature might become two parts that assemble to create the snap function. Assembly cost trades against mold complexity.
Bump-offs for flexible features use part deflection during ejection rather than mold action. Small snaps or shallow undercuts in resilient materials (PP, TPE) may flex past the mold steel during ejection. Feature geometry and material stiffness determine feasibility. Testing confirms whether the feature survives ejection without damage or permanent deformation.
Secondary operations machine undercut features after molding. When volume is low or undercut geometry is complex, drilling, tapping, or machining after molding may cost less than mold complexity. The calculation compares one-time mold cost increase against per-part secondary operation cost times expected volume.
Cost Impact Analysis
Undercut mechanisms add costs throughout the mold’s life, not just initial construction.
Initial mold cost additions vary by mechanism complexity:
Simple cam slides: $2,000 to $5,000 each.
Complex slides with cooling or wear plates: $5,000 to $10,000 each.
Hydraulic slides: $5,000 to $15,000 each including cylinders and valving.
Collapsing cores: $5,000 to $15,000 depending on size and segments.
Unscrewing mechanisms: $8,000 to $20,000 depending on drive system and threads.
Multiple mechanisms multiply costs and interactions.
Ongoing maintenance costs accumulate over production life:
Slide wear surfaces require periodic replacement or refurbishment.
Lifter alignment needs adjustment as wear develops.
Hydraulic seals and fittings require service.
Unscrewing mechanisms need lubrication and wear part replacement.
A slide that requires $500 in annual maintenance over a 10-year mold life adds $5,000 to total cost of ownership.
Cycle time impacts compound for high-volume production:
Unscrewing adds 2-5 seconds per cycle directly.
Complex mechanisms may require slower mold movement for safety.
Maintenance downtime reduces effective production time.
Decision framework:
- Can the undercut be eliminated through design change? If design change cost is less than mechanism cost, redesign wins.
- Is the feature critical to function? Non-critical undercuts may not justify accommodation cost.
- What is production volume? High volumes justify mold investment; low volumes favor secondary operations or design simplification.
- What is the tolerance requirement? Tight tolerances on undercut features may exceed what mold mechanisms reliably achieve.
Undercut handling is often the largest driver of mold complexity and cost. The design question isn’t whether a feature can be molded, but whether its value justifies the manufacturing complexity. Early evaluation of undercuts, before design commitment, enables informed tradeoffs that balance function against cost.
Sources
- Rees, Herbert. “Mold Engineering.” Hanser, 2002.
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.
- Malloy, Robert A. “Plastic Part Design for Injection Molding.” Hanser, 2010.
- Proto Labs. “Undercut Design Guidelines.”
- Society of Plastics Engineers. “Mold Design Best Practices.”