How Are Injected Parts Ejected From Mold?

Ejecting parts from the mold is a critical step in the injection molding process. This step must be performed carefully to ensure that the parts are not damaged and that the production cycle remains efficient. Here’s a detailed look at how injected parts are ejected from the mold:

Steps of Ejecting Parts from the Mold

1. Cooling Phase

Before ejection can occur, the injected part must be sufficiently cooled and solidified. The cooling phase ensures that the part retains its shape and dimensional accuracy. Cooling is controlled through the mold’s cooling system, which typically consists of channels through which coolant circulates.

2. Mold Opening

Once the part has cooled and solidified, the clamping unit of the injection molding machine opens the mold. The mold consists of two halves – the cavity (stationary half) and the core (moving half). The opening mechanism involves:

• Retraction of the Clamping Unit: The clamping unit retracts, pulling the core half away from the cavity half.
• Separation of Mold Halves: The mold halves separate, exposing the part which is typically retained on the core side due to design features like undercuts or a higher surface roughness.

3. Ejection Mechanism

The ejection mechanism is activated to remove the part from the mold. This mechanism typically includes:

• Ejector Pins: These are metal pins strategically placed within the core half of the mold. When activated, they push the part out of the mold cavity.
• Ejector Plates: These plates hold multiple ejector pins and move them simultaneously.
• Ejector Rods: These rods are connected to the machine’s hydraulic or mechanical system, which actuates the ejector plates and pins.

4. Ejector Activation

Ejector pins and plates are actuated to push the part out of the mold cavity. The ejection process includes:

• Hydraulic or Mechanical Actuation: Most injection molding machines use hydraulic cylinders or mechanical cams to move the ejector system.
• Controlled Force and Speed: The force and speed of ejection must be carefully controlled to prevent damage to the part. Too much force or speed can deform or break the part.

5. Ejection Support Features

To assist with smooth ejection and minimize part damage, various design features and auxiliary components are used:

• Ejector Sleeves: These are cylindrical elements that provide additional support around ejector pins, especially for parts with significant surface area.
• Air Ejectors: Some molds include air channels that blow compressed air to assist in releasing the part.
• Stripper Plates: For parts with complex geometries, stripper plates can provide a more uniform push to eject the part without deformation.
• Sprue Pullers and Runners: These help remove the sprue (the channel through which the molten plastic enters the mold) and runners (paths that lead the plastic to the cavities) cleanly from the part.

6. Part Removal

Once the part is ejected from the mold, it typically falls onto a conveyor belt, slides into a collection bin, or is picked up by a robotic arm for further processing, inspection, or packaging.

7. Mold Reset

After the part is removed, the mold halves close, and the machine prepares for the next injection cycle. The ejector system resets to its original position, and the cycle begins anew.

Ejection Optimization and Troubleshooting

Optimizing Ejection

• Design Considerations: Part and mold design should facilitate easy ejection. This includes adequate draft angles (slight taper on the mold walls) and avoiding sharp corners.
• Material Selection: Different materials have different shrinkage rates and adhesion properties that can affect ejection. Selecting the right material can reduce ejection issues.
• Lubricants and Release Agents: Applying these can help reduce friction between the part and the mold, easing ejection.

Troubleshooting Common Ejection Issues

• Sticking: Parts may stick to the mold due to inadequate draft angles, improper cooling, or material properties. Adjusting these factors can help.
• Warping: Warpage can occur if the part is ejected before it has fully solidified. Ensuring sufficient cooling time is critical.
• Surface Defects: Excessive force during ejection can cause surface defects. Reducing ejection speed and force can mitigate this.

Conclusion

Ejecting parts from the mold is a vital part of the injection molding process that requires careful consideration of mold design, material properties, and ejection mechanisms. By optimizing these factors, manufacturers can ensure efficient production cycles and high-quality parts. Proper ejection techniques and troubleshooting methods are essential for maintaining the integrity of molded parts and achieving consistent production outcomes

Related Conten: Mold Design / Mold Manufacturing