A collapsible core (also called a collapsing core) is a specialized mechanical device in an injection mold that retracts radially inward to release an internal undercut — typically a threaded bore, a barbed internal feature, or a grooved socket. Unlike unscrewing molds that rotate the core, collapsible cores use segmented wedge action to collapse inward during ejection.
This guide covers collapsible core types, actuation methods, design constraints, and the practical cost and maintenance implications for buyers.
A collapsible core is a multi-segment assembly. In the extended (molding) position, the segments form a complete, solid core surface. During ejection, the center wedge pin retracts (pulled by the ejector plate), allowing the segments to collapse inward, releasing the undercut. The collapsed diameter is typically 0.5-2mm smaller than the extended diameter — enough to clear most internal threads and barbs.
Three identical wedge segments arranged around a central puller pin. Each segment has about 120° of arc coverage. The segments have precision-ground mating surfaces with a 3° wedge angle. Advantages: lowest cost per core, easiest to maintain, wide interchangeability. Disadvantage: visible witness lines at each segment boundary — three witness lines on the part surface.
Six segments provide smoother collapse and finer feature detail. The higher segment count reduces witness line visibility and accommodates more complex undercut shapes. Used for medical components, precision caps, and custom profiles. Cost: approximately 2x premium over 3-segment.
For non-circular undercuts (oval, D-shaped, or irregular cross-sections), a custom collapsible core is designed and machined. The segment geometry is specific to the part, so spare cores must be ordered from the same manufacturer. Lead time for custom cores: 4-8 weeks.
The collapsible core is actuated by the mold's ejector plate. As the ejector plate moves forward, a center puller pin retracts, pulling the wedge inward. The segments collapse radially. Key design parameters:
| Parameter | Standard Range | Limit / Constraint |
|---|---|---|
| Core diameter | 8 – 80 mm | Under 8mm, segments are too thin; over 80mm, collapse mechanism becomes bulky |
| Maximum depth | 3 × core diameter | Deeper cores require longer puller pins and risk segment misalignment |
| Undercut depth | 0.3 – 2.0 mm per side | Deeper undercuts require more collapse and longer stroke |
| Thread pitch | 0.5 – 3.0 mm | Fine threads (<0.5mm) may not release cleanly; coarse threads risk stripping |
| Segment surface finish | SPI-C1 min. | Rougher finishes cause part sticking to segments, preventing collapse |
Collapsible cores are the highest-maintenance components in any mold. The segment-to-wedge interface operates under high pressure and must be lubricated every 5,000-10,000 cycles. Without lubrication, the segments gall against the wedge pin, causing the core to "stick" in the extended position, preventing ejection.
Typical maintenance schedule:
Replacement cost: $800-2,500 per core unit (segments + puller pin). For a mold with 4 collapsible cores, budget $3,200-10,000 for replacement over the mold's production life.