A plastic injection mold slide is a movable side-action core that creates or releases an external undercut before the part is ejected. Slides let a tool mold side holes, windows, hooks, grooves, and other features that cannot leave along the primary mold-opening direction.
A slide solves a geometry problem, but it also adds moving steel, alignment, wear surfaces, lubrication, cooling, timing, and maintenance. A good design review asks whether the undercut is essential, whether it can be moved to the parting line, or whether a lifter, collapsible core, insert, or part redesign is more appropriate.
How a slide works
The mold opening stroke is converted into lateral motion through an angle pin, cam, hydraulic cylinder, rack, or another actuation method. The slide must retract far enough to clear the undercut before the ejectors move the part. When the mold closes, a locking or back-wedge surface must resist injection pressure and keep the slide in position.
- Identify the undercut and the direction in which it must clear.
- Calculate required slide travel, including a practical clearance margin.
- Choose an actuation and locking strategy that fits the tool envelope and cycle.
- Design guides, wear plates, stops, return components, and lubrication access.
- Check cooling, venting, machining, inspection, and replacement of the moving insert.
Slide versus lifter
| Feature | Slide | Lifter |
|---|---|---|
| Typical undercut | External side hole, hook, groove, or window | Internal hook, snap, rib, or pocket |
| Motion | Mostly lateral, driven separately from ejection | Combines ejection travel with angled lateral movement |
| Design priorities | Locking, guide, travel, wear, and timing | Angle, head shape, clearance, strength, and ejection alignment |
| Common alternative | Parting-line change, insert, or design relief | Core change, part relief, or collapsible/unscrewing solution |
The part geometry and ejection direction should decide the mechanism. Choosing a slide simply because the feature is called an undercut can create unnecessary cost and maintenance.
Slide design details
- Provide enough travel to clear the undercut and avoid scraping the molded part.
- Use a positive locking surface or back wedge to carry injection load; do not rely on the angle pin alone.
- Keep guide surfaces aligned, hardened or replaceable where appropriate, and accessible for inspection.
- Consider wear plates, lubrication channels, and debris management in the design.
- Keep the slide insert thick enough for machining, cooling, and strength at the shutoff.
- Vent the end of fill around the slide insert; a moving insert can otherwise trap gas or create a burn.
- Check that the cooling circuit does not weaken the slide body or interfere with the guide.
Large slides, long travel, or high injection pressure may require a hydraulic or mechanically assisted design. The selection should be documented with travel, load, and maintenance assumptions rather than described only by a catalog name.
Can the undercut be designed out?
Before accepting a side action, consider moving a hole to the parting line, adding a split line, opening a relief window, changing a snap direction, or separating a feature into an insert or assembly. These changes can reduce tool cost and cycle risk, but they may affect appearance, strength, sealing, assembly, or the visible parting line.
Slide validation and maintenance
- Run the tool slowly during first movement and confirm the slide reaches both stops without binding.
- Check flash, shutoff damage, parting-line mismatch, and witness marks after short-shot and full-fill trials.
- Inspect lubrication, wear plates, return components, and guide surfaces at a documented interval.
- Record slide travel and the condition of the critical insert in the mold maintenance file.
The [mold-making team](/mold-making/) and [engineering team](/engineering/) should review slide mechanics together with part requirements. [Contact Cavity Mold](/contact/) with the 3D model and the undercut dimensions for a side-action recommendation.
Technical reference
The Xometry injection molding design guide covers complex molded features and design-for-manufacturability considerations. Final slide dimensions and materials must still be engineered for the specific tool.