How to Choose the Right Ejection System for Your Injection Molding Machine

The ejection system is a critical component of an injection molding machine. Its primary function is to remove molded parts from the mold reliably and accurately. Based on the power source, ejection systems are generally classified into three types: mechanical, hydraulic, and pneumatic. To ensure smooth demolding, the ejection system must provide sufficient stroke and force, operate with stability, and be capable of accurate and timely resetting.

Mechanical Ejection System

A mechanical ejection system uses an ejection rod mounted on the stationary rear platen or other non‑moving parts of the machine. During mold opening, as the moving platen retracts, the ejection rod passes through holes in the platen and pushes against the mold's ejection mechanism to eject the part.

With this system, both the ejection force and speed are determined by the clamping unit's opening force and moving speed. The ejection stroke can be adjusted via bolts based on mold thickness, while the ejection position depends on the clamping unit configuration and part size.

Advantages:

• Simple structure

• Widely used, especially on smaller machines

Limitations:

• Ejection can only occur after the fast opening phase transitions to slow speed, which may extend the cycle time.

• The mold's ejection mechanism cannot be reset until the mold is fully closed again, which is inconvenient for applications requiring insert placement before reset.

• For medium and large‑sized machines, mechanical ejection is usually not used alone — it is often combined with other ejection methods.

Best suited for: Small injection molding machines where simplicity and reliability are the primary concerns.

Hydraulic Ejection System

The core of the hydraulic ejection system lies in its use of a dedicated ejection cylinder to provide the power needed for part ejection.

Advantages:

• Ejection force, speed, position, stroke, and number of ejection strokes are all independently adjustable

• Highly flexible and easy to control

Disadvantages:

• More complex construction than mechanical systems

• Best suited for: Complex parts with difficult demolding requirements. In such cases, the hydraulic system is often used in combination with mechanical ejection to ensure reliable part removal.

Pneumatic Ejection System

A pneumatic ejection system uses compressed air to blow the part directly out of the mold cavity through air channels and tiny vent holes built into the mold.

Advantages:

• Simple structure

• Convenient operation

• Leaves no ejection pin marks on the part surface

Best suited for: Tray‑shaped and thin‑walled parts where appearance is critical and fast demolding is required.

Consideration: Requires a compressed air supply and additional air lines, which may increase installation complexity.