Views: 0 Author: Site Editor Publish Time: 2026-10-20 Origin: Site
Plastic parts with undercuts cannot eject directly along the mold opening direction. Mold designers have three mainstream mechanical solutions: slides (side actions), lifters (angled ejectors), and collapsible cores. Many engineers confuse these three structures. This article explains their working principles, use cases, pros & cons, and selection logic for your DFM review and quoting.
A slide is the most common side 抽芯 mechanism, mainly for external undercuts, side holes and outer grooves. It can also be designed as an internal slide for large single-direction internal undercuts.
Slides are usually driven by angle pins or hydraulic cylinders. When the mold opens, the slide moves horizontally first to retract from the undercut. Ejection starts only after the undercut is fully cleared.
High rigidity, strong locking force from wedge blocks; stable under high injection pressure, low flash risk.
Large available travel, suitable for deep undercuts.
Controlled parting line, good for cosmetic surfaces.
Requires extra space on mold sides, increases mold base size and tooling cost.
More components: angle pin, wedge block, wear plates, retaining ball. Machining and fitting take longer hours.
Outer snap hooks, side openings, deep side undercuts, large one-sided internal recesses (internal slide).
Lifters are designed for small local internal undercuts, such as inner snap clips and small inner bosses inside plastic housings.
The lifter is mounted to the ejector plate. Ejection and undercut retraction happen at the same time. As the ejector plate moves upward, the angled lifter slides along the inclined guide, lifting the part while moving sideways inward to release the internal undercut. Typical lifter angle ranges from 8° to 15°, and lateral travel is limited, normally ≤ 5–10 mm.
Simple structure, low mold cost. It fits inside mold core without widening the mold base.
No extra hydraulic or angle pin system required.
Limited stroke and stiffness. Long thin lifters bend easily and may break during mass production.
No heavy wedge locking. High injection pressure may cause core expansion and flash.
It creates shear force on plastic during ejection; soft or thin-wall parts tend to have whitening or scratching marks.
Small local internal undercuts, inner snap fits, limited core space where a slide cannot fit.
Collapsible core is a precision segmented mechanism for full 360° circular internal undercuts, such as cap internal threads and continuous annular inner grooves. Neither a regular slide nor a lifter can handle full-circle undercuts.
It consists of a central tapered pin and multiple petal-shaped core segments.
Mold closing: the central pin pushes outward and expands all segments to form the full diameter and internal undercut.
Demolding: pull back the central taper pin first. Segments lose support and move radially toward the center, shrinking the overall diameter.
After contraction, the part can eject axially, no rotation required.
Handles continuous full-circle internal undercuts.
Faster cycle time than unscrewing mold because no rotation movement is needed.
Highest manufacturing cost among the three. Wire EDM is required for the precise petal segments.
Segments leave parting seams on the inner part surface.
Thin steel tips wear quickly in mass production, easy to generate burrs and flash.
Strict tolerance requirement for tapered mating surfaces; poor fitting leads to stuck core.
Internal threads of caps, cylindrical parts with full circumferential inner undercuts.
Mechanism | Primary Use | Driving Source | Motion Sequence | Travel | Strength & Cost |
|---|---|---|---|---|---|
Slide | External undercuts, side holes, large single-side internal undercut | Angle pin / hydraulic cylinder | Retract first, then eject | Large | High rigidity, medium-high cost |
Lifter | Small local internal undercut / inner snap | Ejector plate | Eject & retract simultaneously | Small ≤ 5~10 mm | Low rigidity, low cost |
Collapsible Core | 360° circular internal undercut / inner thread | Hydraulic pull central taper pin | Shrink radially first, then eject | Radial shrinkage limited | High precision, highest cost |
Outer undercut, side hole, deep one-sided inner recess → Slide
Small local inner snap, tight space inside core → Lifter
Full-circle annular inner undercut / internal thread → Collapsible Core
Lifters: Do not assign deep undercuts. Thin or flexible materials may suffer scuffing and whitening.
Slides: Reserve enough mold base width for side action space. Verify wedge locking capacity for high-pressure thick-wall molding.
Collapsible Cores: Evaluate seam marks for cosmetic parts. Plan regular maintenance for mass production.
Slides, lifters and collapsible cores solve undercut problems with totally different movement logic. The right selection starts at DFM phase. Wrong mechanism choice will cause high mold cost, unstable mass production, cycle loss or cosmetic defects.
If you have a part drawing with undercut features, we can review and recommend the best mold mechanism before mold kick-off.
