Leaving a Mark – Considering Ejection in Injection Molding

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.

Ejection impacts part performance and appearance. Considering the type and positions of ejection systems during the design process can help ensure that issues do not crop up at the eleventh hour. You can read more about selecting the right plastic injection molding ejection system here.

You do not want to start molding and find you made the wrong ejection choices. During design, you want to consider both the positive and negative impacts on part aesthetics and functionality. A case study illustrates this process. 


A client had been manufacturing a medical device at another contract manufacturer. They were unhappy with the appearance and functionality of the parts. The ejection pins left marks on a surface that was both visible and used for sealing with an adhesive. Can you see the circular ejection pin marks in the image below? This caused aesthetic concerns and the risk of sample leakage. They came to the Natech Engineers to help address those problems in a new mold.


A possible solution could have been to relocate the pins to more discrete locations on the part. While this route would mitigate the aesthetic issue, the functional sealing risk would remain. 

Another option would have been to replace the ejection pins with a stripper plate. A stripper plate would also eject the part cleanly around its perimeter, reducing the aesthetic concerns. However, because the core side formed the part features, a plate could not stretch across the entire part to eject it.

The Natech engineer investigated a third option – ejector bars. Like 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 entire perimeter. This would still leave room for the core side features to be formed.


The Natech engineers decided to use a bar ejection system to eject the parts. The bar ejector offered the simplest and most cost-effective fix. This would meet the client’s aesthetic and functional needs while mitigating the open sealing risks.


The bar ejection system left a clean witness line along the edge of the parts. You can see in the image above that this removed the circular marks that were previously left by the ejector pins. The bar ejectors adjusted the ejection marks to more visually appealing and functionally sound locations. Moving the ejection marks to the edge of the part away from the flow path mitigated the leakage risk.

This did not fully eliminate the risk. You can still see a witness line along the edges. One option for future improvement would include adjusting the geometry of the sealing surface so the adhesive could avoid the ejection marks. This option would escalate cost and would complicate the assembly process.

A second option to eliminate the sealing risk entirely would mean switching the A and B sides of the part so that the ejection marks would appear on the non-sealing surface. This option would require a major tool redesign from the current mold.  

Ejection impacts both part aesthetic and functionality. Ejection systems can have various impacts on a part and introduce added costs and maintenance. Considering ejection during design helps ensure that remedial steps need not be taken after mold construction.

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