Optimizing Injection Molding: The Importance of Considering Ejection Strategies

Leaving a Mark – How Ejection Impacts Part Aesthetics and Functionality

Does anything cause more frustration than developing an entire process only to discover issues at the very last step? In the world of injection molding, plastics engineers can relate to this. Injection molding doesn’t end with the fill phase. Ejection is the last step of the injection molding process.

Injection Molding Process

Ejection is a critical aspect of the injection molding process that impacts part aesthetics, functionality, and cost. Poor ejection can lead to visible marks and defects on the surface of the part, compromising its aesthetic appeal. It can also affect the functionality of the part, especially if the ejection system leaves marks on surfaces that require sealing with adhesives or gaskets. Additionally, the wrong ejection system can introduce added costs and maintenance requirements.

You do not want to start molding and discover you made the wrong ejection choices. To avoid these issues, you must consider ejection during the design process.

You should carefully choose the type of the ejection system to ensure that the part meets the required aesthetic and functional standards. This involves weighing the positive and negative impacts of each ejection strategy and choosing the most cost-effective option that meets your requirements. A case study illustrates the importance of considering ejection during design.


A client was manufacturing a medical device with another contract manufacturer but was unhappy with the appearance and functionality of the parts. The ejection pins left marks on a visible surface used for sealing with an adhesive, causing aesthetic concerns and the risk of sample leakage. Can you see the circular ejection pin marks in the image below?

The Natech engineers investigated three different options: relocating the pins to more discrete locations, replacing the pins with a stripper plate, or using a bar ejector.


Option 1: Relocate the Pins

A possible solution was to relocate the pins to more discrete locations on the part. However, while this would mitigate the aesthetic issue, the functional sealing risk would remain.

Option 2: Replace the Pins with Stripper Plate

Another option was to replace the ejection pins with a stripper plate. A stripper plate would cleanly eject the part around its perimeter, which would reduce the aesthetic concerns. However, since the core side formed the part features, a plate could not stretch across the entire part to eject it.

Option 3: Use Ejector Bars

Similar to a stripper plate, a bar ejector would leave clean lines along the edges of the part, but it would not need to encircle the part’s perimeter. This allows sufficient space to form the core side features of the part without interference.


After careful consideration, our engineers decided to use a bar ejection system to eject the parts. This solution offered the simplest and most cost-effective fix, meeting the client’s aesthetic and functional needs while mitigating the open sealing risks.


The bar ejection system resulted in a distinct witness line along the edge of the parts, as shown in the above image. This witness line effectively removed the circular marks previously left by the ejector pins. In addition, the bar ejectors adjusted the ejection marks to more visually appealing and functionally sound locations. We mitigated the risk of leakage by moving the ejection marks away from the flow path and towards the edge of the part.

However, it should be noted that the risk of sealing is not completely eliminated. While the witness line along the edges is still visible, it is far less pronounced than the previous circular marks. To address this, one potential solution could be to adjust the geometry of the sealing surface, allowing the adhesive to avoid the ejection marks. However, this option would increase costs and complicate the assembly process.

Another possible option to completely eliminate the sealing risk would be to switch the A and B sides of the part. This would result in the ejection marks appearing on the non-sealing surface, requiring a major tool redesign from the current mold.

It is important to consider the impacts of ejection on both the aesthetic and functional aspects of the part. Ejection systems can introduce additional costs and maintenance requirements. Therefore, considering ejection during the design phase and before injection molding helps to ensure that remedial steps do not need to be taken after mold construction.

Speak with a Natech Engineer to improve the ejection strategy of your part so you can advance your program to the manufacturing stage.