Collapsible Core Mold Undercut Solution: How It Works

• 1
  Why an Undercut Stops a Straight Pull
• 2
  Inside a Collapsible Core: Segments, Pin and Travel
• 3
  When a Collapsible Core Is the Wrong Choice
• 4
  Machining Precision That Decides Part Quality
• 5
  Cooling, Lubrication and Wear Surfaces
• 6
  How Core Quality Is Checked Before It Ships
• 7
  Collapsible Core vs Unscrewing vs Side Action

An undercut is any molded feature that sits behind a wall of material. A raised thread inside a cap, a snap ring groove, a lip that faces the parting line. When the mold opens and the core pulls straight back, steel has to pass through plastic that is in the way. That is the whole problem. It is geometric, not a mold quality issue.

The usual first answer is a lifter or a side-action slide. Both work, and both need space. A slide needs a cavity area to travel into, a wear plate, and a cam or hydraulic cylinder to drive it. On a small electrical connector with four internal latch windows, or a 30 mm cap with a full internal thread, there is no room for that hardware inside the cavity block.

A collapsible core solves the same problem from the inside. The core does not pull out as one solid pin. It breaks into segments that close inward on a central pin, shrinking the effective diameter until the molded part clears. Then the part ejects with no side movement at all.

The trade is accuracy. You replace a simple solid pin with a stack of ground segments, each with a sliding interface. Everything about the tool now depends on how well those interfaces fit and how evenly the segments wear.

That is the design conversation. If the undercut runs around the full circumference of a bore, the collapsible core mold undercut solution is usually the shortest path. If the undercut is a single localized hook on one side, a lifter is still cheaper.

• 1
  Full 360° undercutInternal thread, ring groove or recess around a bore — strong candidate.
• 2
  Localized undercutOne hook or window on one side — a lifter or slide often costs less.
• 3
  No room for slidesSmall cavity inserts with no travel space push you toward a collapsing core.

A typical core uses three to six segments arranged around a center axis. During injection, a central pin or wedge holds them expanded against each other. The segments carry the molded bore, so the joint lines land on the part surface. Six segments give a rounder bore but twice the joint lines of three. Threads usually run three or four segments so the thread form can be timed across the joint.

When the mold opens, the center pin retracts first. Each segment is guided inward along an angled face, either a dovetail, a T-slot or a straight angled slide. Collapse travel is small, often 0.5 mm to 2 mm radially, just enough to clear the undercut depth plus a safety margin. If the undercut is 0.8 mm deep, plan 1.2 mm to 1.5 mm of radial collapse.

Timing matters more than travel. The pin must clear before the segments move, and the segments must fully collapse before ejection starts. A hardened wear plate or a guided cage sets that sequence. Skip it and the segments drag on the part, marking the bore and wearing the angled faces.

Segment interfaces are where the tool lives or dies. A gap of 0.01 mm at the joint shows as flash on the molded part, and flash on an internal thread is a rework job, not a trim job. Mating faces are usually ground and matched in pairs, then numbered so they go back in the same orientation after service.

Face contact should be high, above 80% by bluing check. Low contact means point loading, and point loading means the segments peen and lose their fit within a few thousand cycles.

• 1
  Collapse travelUndercut depth plus 0.4-0.7 mm of clearance is a normal starting point.
• 2
  Segment count3-4 for threads, 4-6 for round bores with tight ovality limits.
• 3
  Joint contactTarget above 80% face contact; check with bluing at assembly.

Collapsing cores need a bore to collapse into. The molded part must be a closed or nearly closed loop around the core, and the core body must have room to shrink. An undercut on the outside of a part is a different problem; there the cavity side has to move, and a slide or a split cavity is the answer.

Wall thickness and material stiffness matter too. Glass-filled nylon and PBT fight the collapse because the molded ring grips the segments. If the part is stiff and the undercut is deep, ejection force climbs and the segments wear faster. In those cases a lower collapse angle and more segments spread the load.

Cycle time is the other limit. Each cycle adds a collapse and reset stroke, and the segments need to return to a repeatable position before the next shot. On high-cavity tools this sequence is handled by the machine or a sequence plate. A 16-cavity cap tool with collapsible cores needs a well-tuned ejector and return system, not just good segments.

Size caps the technology from both ends. Very small cores under 8 mm bore are hard to segment because the segments become thin and fragile. Very large cores, say above 150 mm bore, can be segmented, but the mass of the segments and the required travel push you toward hydraulic actuation and a heavier base.

If the undercut is shallow, under about 0.3 mm, a flexible core or a bump-off sometimes works and costs far less. Try that first if the material is soft like PP or PE and the part tolerance is loose.

• 1
  External undercutCollapsible cores act inward; outside features need slides or splits.
• 2
  Bore under 8 mmSegments get too thin to hold tolerance and survive cycles.
• 3
  Shallow soft-material undercutBelow roughly 0.3 mm, try a flexible or bump-off core first.

Everything the part measures comes from the segment set. Bore diameter, ovality, thread pitch diameter, concentricity to the outside wall. If the segments are made to ±0.02 mm, the molded bore will drift more than that once you add shrink, wear and thermal growth. Tooling work is usually held tighter than the part print for that reason.

At GreatLight, core segments are machined on 16 simultaneous 5-axis machining centers, with a 4,000 mm maximum processing size when the core body is large. Angled mating faces, dovetail guides and thread forms on a curved segment are single-setup features on a 5-axis machine, which keeps the joint geometry consistent across the set. General tolerance is ±0.005 mm, with fine ground surfaces at Ra 0.2-0.8 μm on sliding faces.

Material choice follows the same logic. Hardened tool steel for long runs, and for the segments themselves, since they slide against each other every cycle. Copper alloys such as beryllium copper appear on core tips or cooling inserts where heat has to leave fast. Stainless grades like 420 or 440C show up on cores for medical and corrosive-resin jobs.

Surface finish on the molded bore is not just about the part. A rough sliding face wears a mating face. Polished guides at Ra 0.2-0.8 μm hold their fit longer than as-machined faces at Ra 1.6-3.2 μm.

• 1
  Segment set toleranceHold the set to ±0.005 mm where the part print is ±0.05 mm.
• 2
  Sliding facesGrind and polish to Ra 0.2-0.8 μm to control wear.
• 3
  Thread formCut or grind the thread form on the segment, then match the joint.

A collapsible core is a stack of sliding parts sitting in the hottest zone of the tool. Cooling has to reach the segment bodies, not just the cavity. Baffled water lines or a beryllium copper core tip pull heat out of the bore area. If the core runs hot, the segments grow, the fit tightens, and the collapse stroke binds.

Lubrication is a maintenance item, not an afterthought. The angled faces and the center pin need a high-temperature grease that survives the mold environment. Grease channels or grooves in the pin help carry lubricant to the sliding surfaces. On a well-run tool, the core is pulled and re-greased on a set cycle count, not when something breaks.

Wear shows up first as flash at the segment joints. A thin witness line on the molded bore is an early signal. If the line widens, the segments have lost face contact and need re-matching or replacement. Track it with a bore measurement at the same points each service interval.

Nitriding or a hard coating on the sliding faces is common for glass-filled resins. It raises surface hardness and cuts galling between similar steels. Coatings add cost, so they earn their place on long-run tools or abrasive materials, not on a 5,000-piece bridge tool.

• 1
  Cool the segmentsBaffled lines or a copper tip; keep the core near cavity temperature.
• 2
  Grease on a scheduleSet a cycle count for service, not a reaction to flash.
• 3
  Watch the joint lineA widening line on the bore means lost face contact.

A segment set is checked as an assembled unit, not as six separate parts. Each segment is measured on a CMM against the 3D model, then the set is assembled on its pin and the bore is measured again in the collapsed and expanded positions. Diameter, ovality and concentricity are recorded.

Function comes next. The core is cycled on a bench fixture or in the mold to confirm collapse travel, return position and repeatability. Return position is the one that bites. If the segments do not come back to the exact expanded diameter, the next shot is out of tolerance or flashes.

At GreatLight, inspection is 100% before shipment, covering raw material check, in-process monitoring and final inspection, with reports on request. The shop holds ISO 9001:2015, IATF 16949:2016 for automotive work, ISO 13485:2016 for medical devices and ISO 27001:2022 for information security.

For automotive connector and housing work, the tooling package usually includes first-article inspection reports and a dimensional layout tied to the part drawing. That record is what makes a tool transferable when the program moves to another plant or another molder.

• 1
  Measure assembledCMM the set on its pin, expanded and collapsed, not loose segments.
• 2
  Cycle the functionConfirm travel and return position on a bench fixture.
• 3
  Keep the recordLayout reports and material certs travel with the tool.