How Collapse Core Molds Create Internal Undercuts
Internal undercuts allow plastic components to incorporate retaining grooves, locking features, and other functional details inside a molded opening. However, these features also prevent a conventional rigid core from being withdrawn directly after injection molding.
Collapse core molds solve this problem using a segmented core that moves inward before the finished component is ejected. This allows certain complex internal geometries to be molded without damaging the part during release.
WIT Mold manufactures custom injection tooling in China, including collapse core molds for specialized plastic part designs. We evaluate undercut geometry, material requirements, and core movement to develop suitable tooling structures for OEM projects.
1. Why Internal Undercuts Require Special Mold Mechanisms
An internal undercut is a feature that prevents a molded component from sliding directly off its forming core.
Common examples include internal retaining rings, circumferential grooves, locking shoulders, and selected threaded features.
With a fixed core, the undercut can mechanically trap the plastic component. Attempting to eject it without sufficient clearance may cause cracking, distortion, or damage to the molded feature.
A collapsible core provides clearance by reducing its effective diameter before ejection.
2. How the Collapse Core Mechanism Works
A typical collapsible core consists of movable segments supported by a central actuating mechanism. The segments remain expanded during injection to form the internal geometry.
After the plastic solidifies, the mechanism allows the segments to retract inward.
| Molding Stage | Core Movement | Purpose |
|---|---|---|
| Mold Closing | Segments move into molding position | Form the internal cavity and undercut |
| Plastic Injection | Core remains expanded | Maintain the required internal geometry |
| Cooling | Core supports the molded features | Allow sufficient solidification |
| Core Retraction | Segments collapse inward | Clear internal undercuts |
| Part Ejection | Collapsed core provides release clearance | Remove the component |
The exact movement sequence depends on the collapsible core design and mold actuation system.
3. How Segmented Cores Form Internal Grooves
Consider a plastic component containing an internal circumferential retaining groove.
During injection, the expanded core has a corresponding raised feature that forms the groove inside the plastic part.
Once cooling is complete, the core cannot simply move out because the raised feature is locked behind the molded groove.
The collapsible core mechanism retracts its segments toward the center, reducing the effective core diameter until the groove is cleared.
Three dimensions are especially important:
- Undercut Depth: Determines the radial clearance needed for release.
- Internal Diameter: Limits the space available for core collapse.
- Release Stroke: Must provide enough movement to avoid interference during ejection.
If the required radial movement exceeds the mechanism’s available collapse distance, a different mold structure or part redesign may be necessary.
4. Internal Undercut Design Requirements
Not every internal undercut can be produced using collapsible core tooling.
We evaluate the actual geometry before finalizing the mold design.
| Design Factor | Engineering Requirement |
|---|---|
| Undercut Depth | Must remain within available collapse travel |
| Feature Location | Must allow clearance throughout retraction |
| Internal Opening | Must accommodate the collapsed core |
| Part Wall Thickness | Must support molding and ejection loads |
| Plastic Material | Shrinkage and rigidity affect release |
| Dimensional Tolerances | Critical features require suitable core accuracy |
| Core Segment Joints | Witness lines must be acceptable on functional surfaces |
Parts with very small internal openings, deep undercuts, or complicated internal profiles may require alternative tooling.
For threaded components requiring rotational release, WIT Mold also supplies unscrewing molds.
5. Preventing Damage During Core Collapse and Ejection
A collapse core mold must release the internal features without transferring excessive force to the finished plastic component.
Common engineering concerns include:
- Premature Retraction: Moving the core before sufficient cooling can deform internal features.
- Insufficient Clearance: Incomplete collapse may damage the undercut during ejection.
- Segment Misalignment: Poor alignment can affect dimensions and surface consistency.
- Uneven Ejection: Unbalanced forces may distort or crack the molded component.
- Core Wear: Repeated segment movement requires suitable fitting, lubrication, and maintenance.
WIT Mold’s design and engineering services include DFM analysis, mold flow analysis, and structural design review to support complex injection mold development.
6. Verifying Internal Undercuts During Mold Trials
Before tooling approval, the molded part should be checked for both dimensional accuracy and reliable release.
| Inspection Item | Acceptance Focus |
|---|---|
| Undercut Dimensions | Depth, profile, and position |
| Internal Diameter | Fit with mating components |
| Segment Witness Lines | Acceptability on functional surfaces |
| Ejection Quality | No cracking or deformation |
| Mechanism Operation | Complete and repeatable core movement |
| Assembly Performance | Proper engagement of locking or retaining features |
Inspection requirements should follow the customer’s approved drawings and functional specifications.
WIT Mold’s quality system includes mold assembly inspections, mold trials, and manufacturing quality controls.
Frequently Asked Questions
How do collapse core molds release internal undercuts?
The core segments retract inward after molding, creating enough clearance to release the internal undercut before the part is ejected.
Can a collapsible core form internal threads?
Certain thread geometries are suitable, provided the core can collapse far enough to clear the thread profile. Other thread designs require an unscrewing mechanism.
What limits the maximum internal undercut depth?
The available radial collapse distance, internal opening, segment geometry, and required release clearance determine the practical limit.
Custom Collapse Core Mold Manufacturing With WIT Mold
WIT Mold manufactures custom injection molds for plastic components with internal undercuts, grooves, and complex release requirements.
We support mold engineering, precision machining, tooling assembly, and trial evaluation based on customer drawings and production specifications.
Contact WIT Mold to submit your 3D CAD models, internal undercut dimensions, resin specifications, and estimated production quantities for a custom collapse core mold quotation.
