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An internal undercut refers to any feature within a molded part that obstructs its direct ejection from the core side. Common types of internal undercuts include:
•Internal threads (e.g., bottle caps, medical housings)
•Retention grooves/snap features inside tubes or housings
•Internal ribs, locking features, or bayonet-style geometries
•Recessed logos or internal sealing profiles
The core challenge is straightforward: the part mechanically "hooks" onto the core, making it impossible to eject the part without first removing or collapsing the geometry that created the undercut. This requirement adds cost and risk, as it forces mold designers to employ secondary mold actions, which often must be performed in confined spaces with strict tolerances.
While traditional methods can be effective in certain scenarios, they often come at the expense of other factors, introducing a new set of problems. Below are several common traditional solutions and their drawbacks:
1.Side Actions / Slides (Internal Lifters)
•Pros: Mature technology, widely understood.
•Cons: Increases mold size, complexity, wear points, and setup sensitivity. Internal undercut slides are often difficult to access, hard to vent, and prone to wear, especially when the undercut is deep, the resin is abrasive, or cycle counts are high.
2.Hand-Loaded Inserts
•Pros: Low initial mold cost.
•Cons: Requires significant labor, poses safety risks, leads to lower throughput, and inconsistent quality. Hand-loading is rarely a long-term answer for high-volume production programs or regulated industries.
3.Unscrewing Mechanisms (for Threaded Features)
•Pros: Excellent for threaded parts when designed correctly.
•Cons: Costly, high maintenance, and may increase cycle time. Unscrewing systems are highly effective for certain closures and threaded parts, but they are not ideal for every internal undercut and significantly add to mechanical complexity.
4.Stripping (Elastic Deformation)
•Pros: Lowest tooling complexity.
•Cons: Only works when geometry/material allow; carries risks of stress whitening, deformation, and rejects. Stripping relies on the part flexing off the undercut, which is often incompatible with tight dimensional requirements or brittle materials.
Summary: While traditional methods can solve the problem, they frequently come with a series of hidden costs, including increased maintenance burden, downtime, inconsistent ejection, and extended cycle times.
A collapsible core forms the internal feature at full size during injection and then retracts inward for ejection, allowing the part to release cleanly without the need for side actions or complex internal lifters. This translates to:
•Elimination of side actions in many internal undercut applications.
•Reduced mold footprint (fewer moving components around the cavity).
•Improved repeatability by simplifying the action sequence.
•Lower maintenance costs by reducing wear-intensive mechanisms.
•Shortened cycle times by enabling smoother, faster ejection routines (application-dependent).
Senlan collapsible cores are engineered for manufacturers who demand precision engineering, predictable performance, and long-run stability in challenging production environments.
Although designs vary by application, the underlying principle remains consistent:
1.Molding Phase: The core expands to accurately form the internal geometry.
2.Collapse Phase: The core segments retract inward to clear the undercut.
3.Ejection Phase: The part releases smoothly without dragging, scuffing, or locking.
This collapse action is particularly valuable when your internal undercut is:
•Deep or continuous (e.g., a full 360° retention groove)
•Located in a confined space
•Sensitive to surface damage
•Requiring consistent dimensional control
1.Packaging Industry: Caps, closures, dispensing components
•Packaging projects thrive on short cycle times and high uptime. Collapsible cores are an excellent fit for: internal retention rings, snap features inside caps, and complex internal sealing profiles. The result: fewer mold actions, fewer wear points, and more consistent ejection.
2.Medical Industry: Housings, diagnostic consumables, connectors
•Medical molding often demands: tight tolerances, controlled surfaces (no drag marks), and stable repeatability over long production runs. A collapsible core can significantly reduce the variability introduced by complicated slide timing and internal lifter tuning.
A collapsible core is not a "retrofit fix"; it is most effective when considered during DFM (Design for Manufacturability) and the mold design phase. Here are key items to validate:
1.Undercut Geometry & Depth: Is the undercut continuous (360°) or partial? How deep is it? Is there sufficient axial length for collapse and ejection?
2.Resin Behavior: Shrink rate and stiffness affect ejection force. Glass-filled or abrasive materials influence wear expectations. Medical-grade materials may require stricter surface and contamination controls.
3.Tolerance and Surface Finish Requirements: If the internal surface is a functional seal area or requires a precision fit, you will need: stable core alignment, consistent collapse behavior, and a surface finish appropriate to reduce drag/marking.
4.Venting and Gating Strategy: Internal features can easily trap air. Good venting and process stability are essential to prevent burns, shorts, and inconsistent filling.
5.Production Volume & Lifecycle Targets: High-volume production programs benefit most from: simplified mechanisms, reduced maintenance, and predictable performance over long production runs.
Senlan integrates collapsible core applications as part of broader custom tooling solutions and high-precision mold component manufacturing, ensuring that the core design aligns with your mold's overall performance needs.
•"The part sticks on the core and drags during ejection." Collapsible cores reduce interference by retracting away from the undercut before ejection, thereby lowering the chance of drag marks and scuffing.
•"We're battling flash or wear on internal slide actions." Eliminating internal slides can remove wear-heavy contact surfaces and timing sensitivity that often cause flash or mismatch.
•"Our cycle time is unstable due to inconsistent ejection." More predictable ejection can stabilize ejection force and timing, contributing to overall cycle stability (specific results depend on part geometry and process).
•"Mold maintenance is too frequent." Fewer complex actions generally mean fewer failure points, especially in high-cavitation or high-cycle environments.
When internal undercuts are involved, you need more than just a component; you need a precision system that performs reliably under real production conditions. Senlan's approach is aligned with what matters most to molders and OEMs:
•Precision Engineering: Ensuring dimensional stability.
•Advanced Manufacturing Technology: High-accuracy CNC machining processes to support repeatable fit and finish.
•Integration with broader mold component and tooling design needs.
•Engineering Support Mindset: Evaluating the undercut challenge, not just selling a part.
If you are already sourcing custom mold components or CNC machining parts, collapsible core solutions are a natural extension when internal geometry becomes the limiting factor in production.
Consider a collapsible core when:
•The internal undercut is continuous or difficult to access.
•Slide/lifter mechanisms are driving maintenance and downtime.
•Part cosmetic/functional surfaces are being damaged during release.
•You require tighter repeatability across long production runs.
•You want to reduce mold complexity or footprint.
Traditional methods may still be the best choice when:
•The feature is a straightforward thread that already runs well on an unscrewing tool.
•The undercut is minor, and the material reliably strips without deformation.
•Production volume is too low to justify a more engineered solution.
What types of internal features can a collapsible core form?
Common applications include internal grooves, retention rings, internal snap features, and certain internal thread-like or locking geometries, depending on design constraints.
Will a collapsible core improve cycle time?
Potentially – especially if it replaces slower or more finicky mechanisms – but the impact on cycle time depends on part design, cooling, ejection requirements, and overall mold layout.
Are collapsible cores suitable for high-volume production?
Yes, collapsible cores are often chosen specifically to support high-volume consistency by simplifying complex internal actions and improving ejection reliability.
What information should I provide for evaluation?
Share the part drawing/3D file, resin type, annual volume, cavity count targets, critical-to-quality dimensions, and details of the internal undercut (depth, location, functional surfaces).
Internal undercuts no longer need to necessitate complex side actions, fragile lifters, or labor-intensive workarounds. With Senlan Collapsible Cores, you can often achieve the required internal geometry while enhancing mold simplicity, repeatability, and production stability.
If you are planning a new mold – or troubleshooting an existing internal undercut part – Senlan can help evaluate the geometry and recommend a collapsible core approach aligned with your performance targets.
