Injection Mold Slides, Lifters, Collapsible Cores & Unscrewing Mechanisms Guide

Whenever a plastic part contains a feature perpendicular to the direction of mold opening—such as a side connector port, exterior latch recess, internal snap-fit hook, or internal bottle thread—the molded plastic is trapped behind steel (an undercut). If that undercut cannot be eliminated in CAD through a pass-through “telescope shut-off” window, the toolmaker must build a moving mechanical action inside the injection mold base that retracts the steel before the part is ejected.

Each moving action adds $1,200 to $25,000+ to upfront injection mold cost, increases required mold base footprint (requiring a larger press platen), and introduces sliding steel-on-steel wear points. Use this mechanism comparison and design audit guide to select the most reliable undercut action and verify that your toolmaker includes mandatory hardened wear plates and safety locks.

1. Comparison Matrix: Slides vs Lifters vs Hydraulic Core Pulls vs Collapsible Cores vs Unscrewing

Undercut Mechanism	Target Undercut Location	Maximum Practical Stroke ($S$)	Typical Tooling Cost Adder (Per Action)	Cycle Time Impact	Primary Engineering Advantage & Limitation
1. Mechanical Angle-Pin Slide (Cam Slide)	External Sidewalls (side holes, bosses, recessed grips, perimeter undercuts)	5 – 45 mm (limited by angle pin length and ≤ 22° bend angle)	+$1,200 – $3,500	Zero (actuated automatically during A/B mold opening)	Workhorse external action: Simple, 100% mechanical, positive locking heel block absorbs 100 MPa cavity pressure.
2. Hydraulic / Pneumatic Core-Pull Cylinder	Deep External Cores or Pre-Ejection Actions (long tubes, deep ports, cross-bores)	40 – 300+ mm (limited only by hydraulic cylinder stroke)	+$2,500 – $6,500 (cylinder + limit switches + lock wedge)	+1.0 to +3.0 s (if pulled before mold opens or sequenced)	Required when stroke exceeds ~40 mm (where a mechanical angle pin would be too long and snap) or when the core must pull before the mold opens.
3. Angled Lifter (Loose Core / Form Ejector)	Internal Undercuts (B-side snap hooks, internal bosses, inward-facing clips)	1.5 – 12 mm (lateral travel = $\text{Ejector Stroke} \times \tan\theta$)	+$1,000 – $2,800	Zero (rides upward with the standard ejector plate stroke)	Compact internal mechanism that fits inside tight B-side cores where an external slide cannot reach; requires clearance inside the part for lateral shift.
4. Collapsible Core (DME / Progressive)	Internal360° Threads, Snap Grooves & O-Ring Undercuts (Ø12 mm to Ø110 mm ID)	2% – 6% radial collapse (typically 0.8–3.5 mm thread depth per side)	+$4,500 – $12,000 (catalog precision segmented core unit)	Fast (+0.5 s) — collapses during standard ejector stroke	30% faster cycle and much simpler mold base than a rotary unscrewing rack; leaves 6 to 12 faint vertical witness lines from the collapsing segments.
5. Rack-and-Pinion / Servo Unscrewing System	Deep Precision Internal Threads & Helical Gears (closures, medical lures, pipe fittings)	Unlimited Thread Depth & Pitch (multi-turn full 360° rotation)	+$12,000 – $35,000+ (geartrain, hydraulic/servo drive, anti-rotation feature)	+2.0 to +6.0 s (for unscrewing & re-homing rotation)	Zero segment witness lines on threads (essential for high-pressure fluid sealing threads); highest maintenance and upfront capex.
6. Manual Hand-Loaded Insert (“Pick-Out”)	Prototype / Ultra-Low-Volume Complex Undercuts (SPI Class 104 / 105)	Unlimited	+$300 – $800 (2 duplicate loose inserts per cavity)	+15 to +45 s (adds $0.30–$1.20 labor per part!)	Lowest upfront mold cost for < 1,000–2,500 parts; operator ejects insert with part, pries it out on a fixture while a second insert molds the next shot.

2. Engineering Rules for Mechanical Slides (Angle Pin & Heel Block Physics)

When auditing a supplier’s 3D mold design for an external mechanical slide, verify five geometric rules that separate a 500,000-shot SPI Class 102 tool from a budget tool that flashes after 5,000 shots:

flowchart LR
    A["Mold Closes (A-Plate Moves Down)"] --> B["Angle Pin (15°–20°) Drives Slide Forward"]
    B --> C["Hardened Heel Block (17°–23°) Wedges Slide Tight"]
    C --> D["Cavity Injects at 80–140 MPa (Angle Pin Carries ZERO Load!)"]
    D --> E["Mold Opens -> Slide Pulls Clear (Stroke = Undercut + 3 mm) -> Part Ejects"]

1. The 2° to 3° Angle-Pin vs Heel-Block Rule:
   • The Angle Pin (Horn Pin) mounted in the stationary A-plate has only one job: sliding the unloaded carrier block back and forth while the mold is open. Its angle ($\alpha$) must be kept between 15° and 22° (25° absolute maximum); steeper angles create excessive bending torque that snaps the pin.
   • The Locking Heel Block (Wedge) absorbs 100% of the 80–140 MPa cavity injection pressure when the mold is closed. Its locking angle ($\beta$) must be 2° to 3° steeper than the angle pin (i.e., $\beta = \alpha + 2^\circ\text{ to }3^\circ$, typically 17° to 25°). That 2° angular clearance ensures the angle pin disengages cleanly before the heel block wedges tight, and prevents the slide from crashing into the angle pin during clamp-up.
2. Slide Stroke Formula ($S = U + 2\text{ to }3\text{ mm}$): The lateral slide stroke ($S$) must equal the total undercut depth ($U$) plus 2.0 to 3.0 mm safety clearance so the ejected part never scrapes the slide face.
3. Mandatory Hardened Wear Plates (54–58 HRC / Ampco Bronze): Under SPI Class 101 and 102 standards, a slide must never ride directly against soft P20 (30 HRC) pocket steel. Require replaceable hardened tool-steel wear plates (O1/D2/H13 at 54–58 HRC with lubrication grooves) or graphite-plugged Ampco aluminum-bronze gibs underneath the slide base and on the heel block face.
4. Positive Slide Retainer (Spring Plunger + Safety Limit Switch): While the mold is fully open during ejection, gravity or vibration can cause an upper or side slide to creep inward—crushing the angle pin when the mold closes. Require a mechanical ball/spring detent retainer (Progressive or DME slide retainer) plus a redundant backup compression spring (and an electrical proximity switch on large slides or slides situated beneath ejector pins!).
5. “Slide-Under-Ejector-Pin” Crash Protection: If an ejector pin sits underneath the travel footprint of a slide, and the ejector plate fails to retract before the mold closes, the closing slide will shear the ejector pin off. Require accelerated ejector return (early return mechanism) or hydraulic sequence interlocks whenever a slide crosses over an ejector pin.

3. Engineering Rules for Internal Angled Lifters (5° to 15°)

An angled lifter forms an internal undercut on the core side (B-side) and is attached at an angle ($\theta$) to the ejector plate via a T-slot or guided coupling. As the ejector plate strokes forward by distance $E$ (e.g., 30 mm), the lifter moves both forward with the part and sideways inward toward the center of the core by lateral distance $S = E \times \tan(\theta)$:

• Safe Lifter Angle Window (5° to 12°, 15° max): At a 10° lifter angle over a 30 mm ejector stroke, the lifter clears a 5.29 mm lateral undercut (minus 1.5 mm safety clearance = 3.8 mm max undercut). Exceeding 15° places severe lateral bending force on the slender lifter rod (Ø8–16 mm), causing it to gall in its guide bushing and snap.
• Lifter Travel Clearance Inside the Part: Because a lifter moves inward as it rises, verify in 3D CAD that there is no internal central boss, rib, or partition wall blocking the lifter head’s inward travel path—and ensure the part does not shrink so tightly onto the lifter head that it gets dragged sideways during ejection!

Buyer FAQs

What is the difference between a slide and a lifter in an injection mold?

An injection mold slide moves horizontally along the parting line (driven by an angled horn pin or hydraulic cylinder during mold opening) to release external sidewall undercuts such as side holes, connector ports, and exterior recesses. An angled lifter moves diagonally upward and inward from the moving core half (B-side) during the ejector stroke to release internal undercuts such as inward-facing snap-fit hooks and interior bosses where an external slide cannot fit.

How much does a slide or lifter add to the cost of an injection mold?

In a standard production mold (SPI Class 102/103), each mechanical cam slide adds $1,200 to $3,500 and each angled lifter adds $1,000 to $2,800 to the tooling cost, depending on part contour complexity and cavity count. Hydraulic core-pull slides add $2,500 to $6,500, catalog collapsible cores add $4,500 to $12,000 per cavity, and automated rotary unscrewing mechanisms add $12,000 to $35,000+.

Why must the slide locking heel angle be 2°–3° steeper than the angle pin?

The locking heel block angle must be 2° to 3° larger than the angle pin (e.g., 20° heel block with an 18° angle pin) so that the heavy heel wedgeabsorbs 100% of the 80–140 MPa cavity injection pressure while leaving 0.5–1.0 mm of clearance around the slender angle pin when the mold is closed. Additionally, during mold opening, that 2°–3° difference allows the heel wedge to separate by 1–2 mm before the angle pin engages to pull the slide back, preventing steel-on-steel binding.

When should I use a collapsible core instead of an unscrewing mold for internal threads?

Use a collapsible core (DME or Progressive) when molding shallow-to-medium internal threads, O-ring grooves, or 360° snap rings (Ø12 mm to Ø110 mm diameter, 2%–6% radial undercut depth) where 6 to 12 faint vertical segment witness lines (0.01–0.02 mm) on the inner wall are acceptable; it cuts mold cost by 30%–50% and shaves 2–4 seconds off every cycle. Specify a rotary unscrewing mold when the thread depth exceeds the collapsible core’s radial stroke limit, when thread diameter is < Ø10–12 mm, or when a continuous, witness-free helical thread seal is required for high-pressure fluid or medical luer fittings.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, design tooling, or certify suppliers. Slide and lifter stroke calculations, angle-pin bending loads, and hydraulic limit-switch sequences must be verified in 3D tool kinematics simulation before releasing mold steel.