Last updated: 2026-09-05 · 5 min read
Collapsible Cores in Injection Molding — How Do They Enable Undercut-Free Ejection of Internal Threads and Side Actions, and What Design Limits Their Use?
Collapsible cores are segmented core segments that collapse inward during ejection — they release internal undercuts (threads, beads, snap rings) without rotation or side actions. The segments retract 0.3–1.0mm radially, clearing the undercut as the part is pushed off. Used for caps, closures and containers where rotating cores add too much cycle time.
- A collapsible core consists of 3–8 hardened steel segments arranged around a central axis. During molding, the segments are held in the open position by a tapered sleeve — the outer surface of the segments forms the full core diameter. At ejection, the sleeve retracts and the segments are forced inward by springs or a tapered pin — each segment moves 0.3–1.0mm radially, clearing the internal undercut. The part is then pushed off the collapsed core by the ejector plate.
- Collapsible cores eliminate the rotation step required by unscrewing mechanisms — cycle time savings of 2–5 seconds per shot. For a 4-cavity cap mold: unscrewing adds 8–20 seconds (2–5s per cavity × 4 cavities). A collapsible core adds 0.5–1.0 seconds (just the collapse stroke). The trade-off: collapsible cores cannot produce full-depth continuous threads — they are limited to interrupted threads (bayonet-style), internal beads or shallow undercuts (max depth 1.5mm). For full continuous threads, a rotating core is still required. The gate vestige guide covers how gate placement interacts with core design.
- The core segments must be hardened to 52–56 HRC (H13 or S136 steel) to resist wear at the parting lines between segments. Each segment has a land (flat surface) that mates with adjacent segments — the gap between segments creates a parting line on the molded part. For PP and PE: the segment gap is 0.02mm (acceptable flash level). For engineering resins (PC, ABS): the gap must be 0.01mm or less — these resins flash at larger gaps. The segment lines are visible on the part surface — for cosmetic parts, position the segment lines at non-critical surfaces.
How Do Collapsible Cores Compare to Rotating Cores and Side Actions?
| Feature | Collapsible Core | Rotating Core (Unscrew) | Side Action (Slide) |
|---|---|---|---|
| Internal undercut release | Segments collapse inward | Core rotates out of thread | Slide retracts radially |
| Cycle time impact | +0.5–1.0s | +2–5s per thread | +0.5–1.5s |
| Thread type | Interrupted only | Continuous or interrupted | Not for threads |
| Max undercut depth | 1.5mm | Unlimited (thread depth) | 5–20mm |
| Mold cost | +$8,000–$20,000 | +$5,000–$15,000 | +$3,000–$8,000 |
| Part witness lines | 3–8 segment lines | None (single-piece core) | Slide parting line |
Choose collapsible cores when: the undercut is shallow (under 1.5mm), the thread is interrupted (bayonet or snap-in), and cycle time is critical (high-volume production). Choose rotating cores when: full continuous threads are required, the thread depth exceeds 1.5mm, or the part surface cannot tolerate segment witness lines.
Witness line — a visible line on the molded part where two mold segments meet. A collapsible core with 4 segments creates 4 witness lines at 90° intervals; 8 segments create 8 lines at 45°. Lines are 0.02–0.05mm wide — visible on glossy surfaces, acceptable on matte or textured. Position witness lines at non-cosmetic surfaces. The unscrewing mechanisms guide covers rotating cores that leave no witness lines.
What Three Design Rules Ensure Reliable Collapsible Core Operation?
- Limit the undercut depth to 1.5mm maximum — deeper undercuts require excessive segment travel and risk binding. The segment collapse distance is typically 0.3–1.0mm. If the undercut is deeper than the collapse distance, the segment cannot fully clear the undercut — the part sticks to the core or the segment is damaged during ejection. For a 1.0mm undercut: use 1.0mm collapse distance. For a 1.5mm undercut: use 1.5mm collapse distance with oversized segments. Above 1.5mm: switch to a rotating core.
- Position segment parting lines at non-cosmetic surfaces — the lines are visible on the molded part. A 4-segment core creates 4 witness lines at 90° intervals. An 8-segment core creates 8 lines at 45° intervals. The lines are 0.02–0.05mm wide and 0.01–0.03mm deep — visible on glossy surfaces, acceptable on matte or textured surfaces. Position the lines at the back or bottom of the part (non-visible areas). For cosmetic caps: use 8 segments (finer lines, less visible) and position them at the non-display surface.
- Use hardened steel (H13 at 52–56 HRC) for segments — the segment edges wear quickly from resin friction during collapse. Each ejection cycle, the segment edges slide against the molded part and the sleeve bore. Soft steel (P20 at 28 HRC) wears within 50,000 shots — the segment gap widens, flash increases. Hardened steel (H13 at 52–56 HRC) lasts 300,000–500,000 shots before needing replacement. For glass-filled resins: use tungsten carbide inserts at the segment edges — glass fiber accelerates wear by 5–10× compared to unfilled resins.
Collapse distance — the radial distance each segment moves inward when the core collapses (typically 0.3–1.0mm). The collapse distance must equal or exceed the undercut depth — if the undercut is 1.0mm, the collapse distance must be at least 1.0mm. Beyond 1.5mm, segments require excessive travel and risk binding. The screw boss design guide covers how internal features affect core design.
Collapsible cores: 3–8 segments collapse inward (0.3–1.0mm) to release internal undercuts. Cycle +0.5–1.0s (vs. +2–5s for unscrewing). Limited to interrupted threads, max 1.5mm undercut depth. Segment witness lines visible. Hardened H13 steel 52–56 HRC.
- Undercut depth ≤1.5mm — deeper requires rotating core; collapse distance max 1.0–1.5mm.
- Interrupted threads only — continuous threads cannot release by collapse.
- Segment lines at non-cosmetic surfaces — 4 segments = 4 lines; 8 = finer, less visible.
- Hardened H13 at 52–56 HRC — soft steel wears in 50k shots; hardened lasts 300–500k.
- Segment gap within spec — 0.02mm PP, 0.01mm PC; wider gap = flash.
Frequently Asked Questions
Can collapsible cores mold internal threads?
Only interrupted threads (bayonet-style, with flat sections between thread segments). Continuous threads (full 360° helix) cannot be released by collapsing — the thread profile wraps around the core and cannot clear the segment gaps. For bayonet threads: the flat sections align with the segment gaps, and the collapse motion clears the thread peaks. For continuous threads: use a rotating core (unscrewing mechanism). Some collapsible cores use a combined motion (collapse + partial rotation) to release shallow continuous threads — these are custom mechanisms with higher mold cost.
How do I prevent flash at the segment parting lines?
Flash at segment lines is caused by excessive gap between segments or insufficient clamp force holding the segments together. Check: is the segment gap within spec (0.02mm for PP, 0.01mm for PC)? If wider, the segments have worn — replace them. Check: is the sleeve applying sufficient force to keep segments closed during injection? The sleeve must hold segments with 5–10 tonnes of force — if the sleeve spring is weak, segments separate during injection and resin enters the gap. Replace worn springs or increase the sleeve pre-load.
What maintenance does a collapsible core require?
Every 50,000 shots: inspect segment edges for wear, clean the sleeve bore, lubricate moving surfaces. Every 100,000 shots: measure segment gap — if wider than 0.03mm, replace segments. Every 200,000 shots: inspect the tapered sleeve bore for wear, replace if the taper surface shows grooves. For glass-filled resins: maintenance intervals are halved (glass fiber accelerates wear). Keep spare segment sets on hand — replacement takes 2–4 hours (disassemble core, replace segments, reassemble and test).
Key Takeaways
- Collapsible cores: segments collapse inward to release undercuts.
- Cycle +0.5–1.0s. Interrupted threads only. Max 1.5mm undercut.
- Segment witness lines visible — position at non-cosmetic surfaces.
- Hardened H13 52–56 HRC. Maintenance every 50k–200k shots.
Send the part drawing with internal undercut dimensions, thread type and resin grade — the review checks whether a collapsible core can release the undercut or whether a rotating core is needed, and specifies the segment count, collapse distance and steel hardness for reliable production.
Sources: Plastics Technology — Collapsible Cores for Internal Undercuts · Plastics Technology Online — Collapsible Core Design and Maintenance · Society of Plastics Engineers (SPE) — Collapsible Core Mechanisms Technical Papers · HASCO — Collapsible Core Units and Components · Meusburger — Collapsible Core Systems for Closure Molds
— Arthur, Tooling Engineer at aCatMould