Sliders and lifters are moving components in an injection mold that create features (undercuts) which cannot be formed by the straight opening and closing action of the mold halves. Every slider or lifter adds cost, cycle time, and a potential failure point — so minimizing their use is a primary design goal.
This guide explains when sliders versus lifters are used, design constraints, wear considerations, and what buyers should ask during mold design review.
| Feature | Slider (Cam Action) | Lifter (Angle Pin Action) |
|---|---|---|
| Movement direction | Perpendicular to mold opening | At angle (typical 10-15°) combining vertical + lateral |
| Actuation | Angled cam pin (horn pin) pushes slider; spring or mechanical lock returns it | Mounts on ejector plate; moves with ejector stroke |
| Undercut type | External (hole, window, side protrusion) | Internal (rib, snap-fit, boss on inner wall) |
| Travel range | Unlimited (within mold base size) | Limited by ejector stroke (typically 5-30mm) |
| Relative cost | Higher (wear plates, locking mechanisms) | Lower (simpler design, fewer components) |
| Typical max angle | 25° on cam pin; 15° preferred | 12-15° (steeper angles cause binding) |
The angled cam pin (horn pin) drives the slider laterally as the mold opens. A 15° cam angle produces the best balance of slider travel vs. pin wear. Angles above 20° generate excessive lateral force that accelerates wear on the pin and bushing. The heel block (locking face) must have a steeper angle than the cam pin — typically 20-25° — to lock the slider in the closed position during injection. If the heel angle equals the cam angle, the injection pressure can push the slider backwards and cause flash.
Every slider needs wear plates or hardened gibs on the sliding surfaces. Standard in quality molds: hardened tool steel (≥60 HRC) wear plates on both the slider bottom and sides. Budget molds often omit bottom wear plates, letting the slider ride directly on the mild steel of the mold plate. The result: rapid galling, increasing friction, and eventually seizure. After 100,000 cycles, an unlubricated slider on mild steel shows 0.5-1mm of vertical play.
During injection, the slider experiences injection pressure from the plastic. Without a positive lock, the slider moves backward, producing flash. Locking mechanisms include:
The lifter body is a steel block that moves at an angle relative to the ejector plate. The angle is typically 8-12° — steeper angles generate high side loads that cause the lifter to gall against its guide bushing. Lifter stroke = ejector stroke × tan(lifter angle). For a 100mm ejector stroke at 10°, the lifter moves laterally 17.6mm — sufficient for most internal undercuts.
The head of the lifter forms the undercut surface. Key considerations:
Lifters require a guide bushing in the B-plate to prevent rotation and lateral movement. Without a guide bushing, the lifter deflects during return stroke, eventually wearing its through-hole into an oval. Replacement cost: $300-800 plus dismantling time.
| Failure Mode | Slider | Lifter | Prevention |
|---|---|---|---|
| Gall / Seizure | Common (wear plates omitted) | Common (no guide bushing) | Hardened wear plates + oil grooves; guide bushings |
| Flash at moving interface | Heel block insufficiently steep | Clearance too large between lifter and core | Lock angle > cam angle; 0.01-0.02mm clearance |
| Part hangs on lifter | N/A | Insufficient back-taper on head | Add 1-2° back-taper; polish head surface |
| Broken cam pin | Pin too thin; angle too steep | N/A | Minimum 12mm pin dia; max 20° angle |