Parts with internal undercuts are difficult to eject safely. If the part is forced out before it releases, the result is scratched surfaces, sticking, stress whitening, or a rejected part. The cause is rarely ejection force alone; it is the absence of controlled clearance at the moment the part separates from the tool.
How a Collapsing Core Works
A collapsing core retracts before ejection, giving the part room to release without being dragged across the mold surface. The design uses a segmented structure, often six segments made of three large and three small pieces, arranged so the core expands during injection to form the undercut and contracts before the part is pushed off. The movement is driven by a center pin or wedge, timed with the ejector sequence.
Segment Design and Fit
Segment interfaces need a precise angle so the core collapses smoothly without binding. The angle trades travel against force: a shallow angle reduces friction but needs longer stroke. Guiding surfaces should be hardened and lubricated for the expected cycle count, and the center pin must be rigid enough to resist injection pressure without deflection.
Material and Heat Treatment
Core segments see sliding wear plus injection pressure, so through-hardened tool steel or a nitrided surface is common. Where the resin is abrasive, coatings extend life. The center pin should be made from a tough grade that resists bending, since a bent pin causes uneven collapse and part damage.
Cooling the Core
Cooling inside a segmented core is the hardest part of the design. Baffles or bubbler tubes in each segment remove heat near the undercut, where thick sections otherwise stay molten. Uneven cooling causes the part to shrink onto the core, which increases ejection force and accelerates wear. Thermal simulation before machining is cheaper than reworking a finished core.
Timing and Ejection Sequence
The collapse must complete before the ejector plate advances. Mechanical interlocks or sensors confirm position, and the sequence should be documented in the process sheet. A part ejected early shows drag marks on the undercut surface, which is the first symptom to check when a new run misbehaves.
Maintenance and Life
Inspect segment faces for galling and check the center pin for straightness at planned intervals. Lubrication type matters: some greases migrate into the cavity and cause cosmetic defects. Spare segment sets reduce downtime, since replacing one segment is faster than rebuilding the core.
Frequently Asked Questions
When is a collapsing core better than a slider?
When the undercut is internal and wraps around the part axis, a collapsing core avoids the parting line a slider would leave.
How many cycles does a core last?
It depends on resin abrasiveness and maintenance; hardened segments with proper lubrication run for hundreds of thousands of cycles.
Can an existing mold be converted?
Sometimes, if plate space and ejector stroke allow. The conversion usually needs a new core and a revised ejection sequence.
Conclusion
A collapsing core turns an impossible undercut into a repeatable part when segments, cooling, and timing are designed together. For related reading, see injection moulding reference, our collapsible core molds guide, and defect troubleshooting. Contact TCHD to review your undercut part.