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In the world of injection molding, design engineers often say, "Where there is an undercut, there is a way." The "way" usually comes in the form of a mold slide, also commonly known as a slider or cam.
Mold slides are mechanical marvels that solve one of the biggest challenges in plastic part manufacturing: how to eject a part that has holes, clips, or recesses on its side.
If a part has a side hole or a retaining clip, you cannot simply push it out of the mold with ejector pins—it would get torn apart. Instead, we use slides that move sideways to clear the obstruction before the part is ejected.
Here is a breakdown of the most common types of mold slides and how to choose the right one for your project.
This is the workhorse of injection molds. If you look at a standard mold base, this is likely the mechanism you will see.
How it works: An angled pin (the horn pin) is fixed to the front half of the mold (A-side). As the mold opens, this pin pulls the slider body backward, away from the plastic part.
Best for: Standard external undercuts, side holes, and bosses.
Pros: Simple construction, reliable, and cost-effective for most medium-sized molds.
Sometimes, physics gets in the way. If the undercut is very deep or the core is very large, the friction on an angled pin becomes too high. That is when we bring in hydraulics.
How it works: A hydraulic cylinder (oil cylinder) is mounted to the mold. It pulls or pushes the slide core independently of the mold’s opening stroke.
Best for: Large sliders, long travel distances, or situations requiring "early return" (closing the slide before the mold closes to avoid damaging the part).
Pros: Infinite stroke length, immense force, and independent timing control.
When the mold is too compact for an angled pin, or when you need a slide on the front half of the mold (A-side), the T-slot design comes to the rescue.
How it works: Instead of a pin, a T-shaped block slides into a matching groove. The vertical movement of the mold plates is converted into horizontal slide movement.
Best for: Front mold slides and situations with limited space on the parting line.
Pros: Very compact, ideal for complex sequential actions.
Imagine a bottle cap or a cylindrical gear. You cannot put a standard slider on a round surface because it would leave a visible seam. Instead, we use "Harf" slides (named after the "half" shape they form).
How it works: Two (or more) slides come together in the middle to form a complete cavity. When the mold opens, they split apart like a clamshell.
Best for: Round parts, threaded closures, and parts with cosmetic requirements where a standard parting line is unacceptable.
Pros: Excellent for cylindrical geometry; allows for unscrewing or external threading.
Not all undercuts are on the outside of the part. Many are internal—like snap-fit hooks or ribs. While we often use Lifters (angled ejector pins) for this, there are also Internal Sliders.
How it works:
Lifters: Mounted on the ejector plate. As the ejector moves forward, the lifter angles inward, releasing the internal undercut.
Internal Sliders: A small slide mechanism built into the core of the mold that collapses inward before ejection.
Best for: Internal snap hooks, deep ribs, and internal features that would otherwise lock the part on the core.
Selecting the wrong slide mechanism is a common cause of mold maintenance issues. Here is a quick decision matrix:
Situation | Recommended Solution |
|---|---|
External side hole, short travel | Angled Lifter (Horn Pin) Slide |
Large core, long travel (>60mm) | Hydraulic (Oil Cylinder) Slide |
Under cut on the A-side (front) | T-Slot Slide or Hydraulic Slide with sequence control |
Round/cosmetic part (e.g., bottle cap) | Harf (Split) Slide |
Internal snap hook | Lifter (Angled Ejector) |
Hidden internal undercut | Internal Slider (Core Puller) |
Slides are often the most expensive components in an injection mold, but they are non-negotiable for complex geometries. A well-designed slide reduces cycle time, improves part quality, and ensures the longevity of the tool.
When designing your next plastic part, remember: Keep the slide direction simple. While complex mechanisms like "floating slides" or "double-angle sliders" exist, they significantly increase tooling costs. Whenever possible, design your part to pull in a single direction to allow for the simplest, most reliable slide mechanism.
