Prototype Mold Collapsible Core: How It Works and When to Use It

• 1
  What a Prototype Mold Collapsible Core Actually Does
• 2
  Where a Collapsible Core Works and Where It Fails
• 3
  Why Segment Fit Decides Whether the Core Runs
• 4
  How We Build and Prove a Prototype Collapsible Core
• 5
  Common Failure Modes and How They Show Up
• 6
  Collapsible Core vs Solid Core vs Side Action

A prototype mold collapsible core is a segmented steel assembly that forms an internal feature, then shrinks inward so the part can be ejected. The segments sit on a central pin and are held outward by the pin during injection. When the pin retracts, the segments slide down an angled wedge and collapse toward the axis by 0.5 to 2 mm. That small movement is what frees a thread or an undercut that a solid core would tear apart.

The problem it solves is geometric. If you mold a 20 mm internal thread on a solid core, you either unscrew the part or you destroy it. A collapsible core lets you pull the part straight off. The same logic applies to snap-fit grooves, O-ring seats and internal barbs.

Prototype work changes the economics. A segmented core costs more than a plain core because every segment is a precision fit. In a prototype you accept that cost to prove the geometry, not to run a million parts. The prototype core validates wall thickness, thread depth and release angle before you cut a hardened production tool.

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  FormSegments expand to the molded internal shape.
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  CoolPart shrinks onto the segments while they hold position.
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  CollapseCenter pin retracts; segments move inward.
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  EjectPart slides off with no unscrewing motion.

The mechanism has hard boundaries. Each segment must slide without binding, so the number of segments is limited by the internal diameter. Below roughly 12 mm internal diameter, the segments get too thin to survive injection pressure. Above 80 mm, segment deflection starts to dominate and the core needs a different support strategy.

Undercut depth is the second limit. A typical collapsible core handles an undercut of 0.5 to 3 mm per side. Deeper than that, the collapse travel becomes long, the wedge angle gets shallow, and the segments can gall or stick. If your design needs a 6 mm internal groove, a collapsing core is usually the wrong answer. A side-action or a lost-core process fits better.

Material matters too. Glass-filled nylon and PEEK push hard on the segments because the melt is stiff and abrasive. We run those jobs with hardened segment tips and more generous draft. Soft ABS or PP is far more forgiving.

Threads are a special case. A molded internal thread with a 1.5 mm pitch and 60° flank angle releases cleanly if the core collapses 1.2 to 1.5 mm. A buttress thread or a square thread needs more travel and a different segment layout. Send the thread profile with the RFQ, not just the nominal diameter.

• 1
  Good fitInternal thread, snap ring, O-ring groove, shallow barb.
• 2
  Poor fitDeep groove over 3 mm, tiny bore under 12 mm.
• 3
  Wrong processParting line through a sealing face.

A collapsible core is a stack of sliding fits. The gap between adjacent segments is typically 0.01 to 0.03 mm. Open it up and plastic flashes into the seam, leaving a visible line on the molded thread. Close it and the segments seize after a few thousand cycles. That window is why the core is machined on 5-axis centers and finished by grinding, not turned on a manual lathe.

The wedge angle on the center pin controls collapse force and return speed. A 15° wedge collapses smoothly but needs a longer pin stroke. A 30° wedge is compact but can self-lock under high injection pressure. For prototype tools we usually land between 18° and 22°, then verify by hand-cycling the core before it ever sees a molding machine.

Heat treatment comes after rough machining. Segments are usually 1.2343 or 1.2344 tool steel, hardened to 48 to 52 HRC, then ground to final size. Grinding after hardening is the only way to hold ±0.005 mm on the sliding faces without distortion.

We machine the core body, the segments and the center pin in one shop so the fits are measured against each other, not against a drawing from a different vendor. That single-source approach removes the most common failure mode in prototype core work.

The work starts with a DFM review, not a quote. We check the undercut depth, the draft on the internal wall, and whether the part can be ejected without a parting line crossing a seal. If the geometry does not suit a collapsible core, we say so before any material is cut.

From there the sequence is predictable. Rough machine the segments and body, heat treat, grind the sliding faces, assemble and hand-cycle. Then we measure the collapsed envelope and the expanded envelope on a CMM and record both. A core that expands to the wrong dimension will produce an out-of-round thread, and you will not see it until the first shot.

Prototype cores are often run in a soft tool for 50 to 500 shots. That is enough to validate the geometry and the release. If the part moves to production, the same segment layout carries over into a hardened tool, so the data you collect on the prototype is not wasted.

We keep the core and the molded samples together in one inspection report. When a thread measures out of tolerance, the report shows whether the core moved or the process drifted.

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  DFM firstUndercut depth and draft checked before quoting.
• 2
  Single sourceBody, segments and pin machined in one shop.
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  MeasuredCollapsed and expanded envelopes recorded.

Flash on the internal thread is the most common complaint. It almost always traces to segment gap, not to injection pressure. If the seam line is heavier on one side, the segments are not closing evenly and the wedge or the guide pins need attention.

Sticking on ejection points to insufficient collapse travel or a shallow wedge angle. The part hangs on the segments for a fraction of a second, then tears. Increasing collapse travel by 0.3 mm often clears it, but that change has to be designed in, not added later.

Segment breakage happens when the core is run outside its bore range or when glass-filled material is molded without hardened tips. A segment that cracks at the tip usually means the undercut was deeper than the design allowed.

Out-of-round internal dimensions come from uneven cooling or from a center pin that deflects under injection pressure. On a prototype core, check the pin support before you blame the plastic.