Ejection difficulty is one of the most common troubleshooting issues in injection molding production. After the mold opens, if the part remains stuck on the core or inside the cavity, it can easily cause part scratching, ejector pin marks, whitening, cracking and deformation. In more serious cases, it may lead to broken ejector pins, mold damage, reduced production yield, longer cycle time and increased unplanned downtime costs.

Essentially, ejection problems occur when the clamping force and frictional resistance generated by plastic shrinkage during cooling exceed the effective ejection force provided by the machine’s ejection system. The root causes often involve four dimensions: injection molding process, machine condition, mold design and manufacturing, and residual stress in the finished part.

A systematic troubleshooting approach should follow the principle of process and machine first, mold modification last. This helps avoid unnecessary mold changes and reduces production losses.

Konger’s K-TEC servo energy-saving series and CMS multi-color series machines are equipped with functions such as multi-stage ejection and precise pressure and speed control. These features support process optimization and help create more stable ejection conditions for mass production.

1. Main Causes and Improvement Strategies

1. Over-Packing

Symptom:Excessive injection pressure and holding pressure cause the part to be over-compacted. As a result, shrinkage is reduced, and the plastic grips the core more tightly, making ejection difficult.

Solution:Reduce injection pressure, optimize injection speed, control injection time, and lower holding pressure and holding time. It is recommended to adjust process parameters on the machine first before making mold modifications.

2. Insufficient Ejector Force or Insufficient Ejection Stroke

Symptom:Ejection pressure and speed may be set too low, or the ejection stroke may not be long enough to release the part completely from the core. Wear in the ejection hydraulic circuit or mechanical structure can also reduce actual ejection force.

Solution:Increase ejection pressure and ejection speed on the machine, and verify whether the ejection stroke is sufficient. Inspect the ejection mechanical structure and hydraulic circuit, and perform maintenance if necessary to ensure reliable ejection movement.

3. Low Mold Temperature

Symptom:If the mold temperature is too low, the plastic cools too quickly, resulting in uneven shrinkage and excessive localized clamping force. This increases frictional resistance during ejection.

Solution:Increase the mold temperature appropriately to achieve a more uniform cooling rate, improve plastic shrinkage behavior, and reduce mold sticking resistance. Use the machine’s temperature control system to stabilize mold temperature and minimize ejection fluctuations.

4. Insufficient Draft Angle

Symptom:If the side wall of the part has insufficient draft angle, the plastic will generate high friction against the mold wall during ejection. This commonly causes scratching, sticking and ejection problems, especially in deep cavity parts.

Solution:Modify the mold to increase the draft angle at the relevant positions. Draft angles should be considered during the early product design stage to prevent ejection issues from the beginning.

5. Undercuts or Unsafe Chamfer Interference

Symptom:Undercuts or improperly designed chamfers on the part can create mechanical interference in the direction of mold opening, directly causing the part to jam and preventing smooth ejection.

Solution:Modify the mold to remove undercut interference. For functional undercuts, add lifters, slides or side core-pulling mechanisms to eliminate obstruction in the ejection direction.

6. Unbalanced Ejection Force

Symptom:Poorly distributed ejector pins can cause ejection force to concentrate in local areas, while regions with high clamping force may lack sufficient support. This often results in localized sticking, ejector marks, and even part penetration.

Solution:Optimize the ejection system by adjusting or adding ejector pins. Distribute ejection force evenly around areas with high clamping force, such as ribs, bosses, and deep side walls, to achieve balanced ejection.

7. Insufficient Mold Surface Finish

Symptom:Rough cavity and core surfaces increase friction and adhesion between plastic and mold steel. This raises ejection resistance and may also scratch the part’s cosmetic surface.

Solution:Polish the cavity and core surfaces. The polishing direction should preferably follow the mold opening direction to reduce frictional resistance during ejection.

8. Vacuum Adsorption During Ejection

Symptom:In deep cavity or large shell parts, a vacuum may form between the part and the core during mold opening. The part can then be firmly sucked onto the core. Simply increasing ejection force may damage the part.

Solution:Use slow ejection at the beginning of mold opening. Install air valves in the mold to introduce compressed air during ejection and break the vacuum. This is a very effective solution for deep cavity parts that tend to stick.

9. Influence of Residual Stress in the Part

Symptom:Unreasonable molding conditions can cause high residual stress inside the part. After cooling, the part may continue to clamp tightly around the core. Even if the draft angle and surface finish are adequate, ejection can still be difficult, and deformation may occur later.

Solution:Optimize the overall mold design. At the same time, adjust the injection molding process by reducing holding pressure, controlling mold temperature properly, and minimizing residual stress.

2. Recommended On-Site Troubleshooting Sequence

For ejection difficulties, follow this priority sequence to reduce unnecessary mold costs:

1. Process Check:Confirm whether the cavity is over-packed. Reduce injection pressure, holding pressure and injection speed to eliminate excessive clamping force caused by over-filling.

2. Machine Inspection:Verify ejection pressure, ejection speed and ejection stroke. Check whether the mechanical ejection structure and hydraulic circuit are functioning properly.

3. Temperature Adjustment:Increase mold temperature moderately and observe whether ejection improves.

4. Mold Hardware Investigation:Polish mold surfaces, optimize draft angles, improve ejector pin layout, add vacuum break air valves, and resolve undercut interference.

Tip: Vacuum adsorption is a very common cause of sticking in deep cavity shell parts. Simply increasing ejection force often leads to part damage. A combination of slow initial ejection and air valve vacuum breaking is recommended.

3. Conclusion

Ejection difficulty is the result of multiple factors interacting together, including process settings, machine performance, mold design, and part structure.

Konger Machinery’s technical team has rich field experience in injection molding troubleshooting. For applications such as fruit crates, paint buckets, multi-color parts and PET preforms, we can provide equipment parameter suggestions, process debugging guidance and mold selection support. Our goal is to help customers reduce defect rates and achieve stable, efficient production.