The Magic of Movement: What is a "Slider" in Injection Molding?

What is an Undercut? An Obstacle to Ejection

Imagine a simple plastic cup. It has a straight profile, so when the mold opens, the cup can be pushed out easily. Now, imagine a bucket with a handle. The handle creates an indentation or a "hook" that locks the part into the mold. If the mold tried to open straight up, the bucket would be trapped by this undercut.

An undercut is any protrusion, indentation, or lateral feature on a part that prevents its direct removal from the mold along the main opening direction. It's the geometric obstacle that makes a part "stuck."

This is where the slider comes in as the hero.

How Does a Slider Work? The "Slide and Eject" Dance

The magic of the slider lies in its synchronized movement with the mold's opening. Its most common driving partner is an Angled Pin (or Cam Pin).

Here’s the step-by-step process:

1. Mold Closes & Injection: The slider is held in its "forward" position by the angled pin, forming the precise shape of the undercut in the mold cavity. Molten plastic is injected.

2. Mold Opens & Slider Retracts: As the two halves of the mold begin to separate, the fixed angled pin moves through a corresponding hole in the slider. Because the pin is at an angle, it forces the slider to move sideways on its guide rails, retracting from the undercut.

3. Part Ejection: Once the slider has fully retracted and cleared the undercut, the path is clear. The ejector system (ejector pins) then pushes the finished plastic part out of the mold, completely free.

4. Mold Closes & Slider Resets: As the mold closes again, the angled pin guides the slider back to its forward position, ready for the next cycle.

Common Types of Sliders

Sliders are incredibly versatile and aren't limited to simple side actions. They can be designed for various complex scenarios:

• External Side Sliders: The most common type, used for side holes, clips, and grooves.

• Internal Sliders: Used to form undercuts on the inside of a part, like an internal latch or groove.

• Angled Lifters: For undercuts that are not perfectly horizontal, combining a lifting and sliding motion.

• Hydraulic/Pneumatic Sliders: For very long travel distances or complex sequences, a hydraulic or pneumatic cylinder is used to drive the slider instead of an angled pin.

Why Are Sliders So Important?

Without sliders, product design would be severely limited. They enable the creation of complex, functional, and user-friendly parts in a single, efficient molding cycle. Almost every consumer product you see with a snap-fit, a side button, a threaded feature, or a lateral vent owes its existence to a well-designed slider.

In a nutshell: A slider is the moving part of a mold that makes the impossible possible. It slides sideways to release complex features, ensuring that what you design is what you can actually manufacture.