Understanding Ejector Marks in Injection Molded Parts

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Ejector marks are those visible imprints or blemishes on the molded parts surface which is as a result of contact with ejector pins during ejection in the injection molding process. They may be caused by improper pin placement and the use of extra force during ejection.

This is a very common defect in injection molding. These marks affect the surface quality, structural integrity, and functionality of the final product. In this article, we are going to look in detail at the causes of ejector marks and the strategies to prevent them to enhance product quality and functionality.

Understanding Ejector Marks

We have seen that ejector marks are visible indentations or blemishes caused by ejector mechanisms, such as ejector pins in injection molding, mainly found on the surface of finished products. They greatly impact the aesthetics requirement and structural integrity of the part.

Ejector pins are crucial components within the injection mold. These pins are strategically placed within the mold to ensure the smooth ejection of finished products from the mold. They are properly designed to push the molded part out of the mold cavity ensuring precision and no damage on the part. 

There are two common methods used during ejection; 

  • Manual ejector systems
  • Automated ejector systems

Primary Causes of Ejector Marks

Ejector marks are a headache for injection molding manufacturers. Let’s look at some of the common primary causes of ejector marks.

1. Injection and Holding Pressure Settings

Pressure plays a primary role in injection molding. Injection pressure is the pressure needed by the molten material to flow into the mold cavity. The holding pressure is the pressure maintained on the molten material after the mold cavity is filled. 

When the holding pressure is not kept within the optimum temperature and speed range it leads to excessive shrinkage, flashes, and varying dimensions. When the injection pressure is set too high it leads to the molten material being injected into the mold cavity with excessive force. 

During ejection, the high internal pressure can create indentation or marks on the part surface especially if the part is delicate and has intricate features. Overall, improper pressure setting in injection molding can create ejector marks causing excessive force to be exerted on the parts during ejection.

2. Mold and Pin Design

These two factors play a crucial role in the formation of ejector marks in injection molding. When ejector pins are too large in size or positioned improperly can exert uneven pressure on the molded part during ejection leading to marks. The surface of ejector pins also influences the likelihood of marks on the surface if it is rough. 

The mechanism used in ejector pins can also impact markings. Furthermore, any malfunction in the ejection mechanism can result in uneven ejection and marking. During mold design, inadequate venting can lead to air traps or pressure buildup that may lead to ejector marks. 

Improper gate design can lead to uneven filling and packing increasing the risk of ejector marks. Another factor contributing to ejector marks is misalignment along the parting line where the mold halves meet causing uneven ejection and marking.

3. Material and Process Parameters

The properties of the plastic material used in injection molding impact the likelihood of ejector marks. Viscosity is the measure of a fluid’s resistance to flow. Plastic materials with high viscosity need more force to properly fill the mold cavity. During ejection, the high resistance to flow may lead to increased friction between the part and mold surface resulting in ejector marks.

Some materials may not offer thermal stability properties. If the material’s thermal expansion coefficient is high there is a risk of the final part sticking to the mold surface as a result of uneven cooling rates. This causes ejector marks on the part. 

The presence of fillers and additives in the plastic material can affect the flow behavior and mechanical properties impacting the risk of ejector marks. The interaction between processing parameters and material properties is critical in the formation of ejector marks.

4. Cooling and Dwell Times

Non-uniform cooling of the mold can result in an increase in the internal stress of the material. The tension which builds up leads to dimensional instability and it becomes hard for the finished product to detach from its mold. Insufficient cooling time before ejection can cause premature interaction of ejector pins with the soft part, resulting in marks. 

When the parts have not adequately cooled, application of the ejector force by the ejector pins can lead to marks or indentation on the surface of the part. This may also lead to the part sticking to the mold surface increasing the risk of marking.

Dwell time is the duration the molten plastic remains in the mold cavity before ejection. This influences material cooling and solidification and material properties. Dwell time indirectly impacts ejection mark formation by affecting the part’s dimensional stability and mechanical behavior to deformation during ejection.

Ejector Marks 1

Ejector Pins In Injection Molding(Image Source: Pinterest)

Strategies to Prevent Ejector Marks

Manufacturers can prevent ejector marks by making use of a few techniques. Below are some of these techniques: 

1. Optimizing Mold Design

In order to prevent ejector marks on parts it is important to consider the ejector pin design and placement. To apply uniform ejection force across the part surface, distribute the ejector pins strategically within the mold cavity. For the production of desired requirement parts ensure the ejector pins are appropriately sized and shaped. 

Consider factors such as part geometry and material properties when determining pin size and shape. Design the mold with an optimized cooling channel to ensure uniform cooling across the part. Increase cooling channel coverage with the mold cavity to enhance efficient heat extraction.

Another consideration is proper overseeing of the flow rate and temperature of the coolant to achieve desired cooling conditions within the mold cavity. In addition, incorporate proper venting to facilitate the escape of air and gasses during injection and ejections. These measures greatly play a significant role in preventing ejector marks.

2. Material Selection and Handling

It is essential to consider material properties for the injection molding process to minimize the risk of ejection marks. Choose material with low viscosity to facilitate smooth flow and filling, and are less likely to resist ejection causing marking. 

Materials with high thermal stability maintain their integrity and dimensional stability throughout the molding process minimizing the risk of surface defect. Consider low shrinkage materials because they maintain dimensional accuracy and reduce the need for excessive ejection force that can cause marking. 

Ensuring proper consistency in material handling and preparation is important. Store materials in a controlled environment to prevent moisture absorption, degradation, or contamination. You can use sealed containers to protect material from exposure.

Use dedicated handling equipment to automate the material handling process reducing the need for manual intervention and avoiding the risk of errors which may result in ejection marks. Also, develop standardized procedures for material inspection and quality control to identify any irregularities and address them promptly.

3. Process Optimization

Fine-tuning injection parameters to harmonize with material characteristics and mold design is essential for optimizing the injection molding process. In addition, it leads to the consistent production of high-quality parts. Regular evaluation and refinement of parameters based on performance feedback are crucial for continuous improvement.

You also need to set up proper cooling times and monitor product temperature throughout the cycle of the injection molding process. This is essential for ensuring dimensional stability where proper cooling times ensure that the molded part cools uniformly and reaches sufficient rigidity before ejection. 

Proper cooling and monitoring prevent the premature ejection of the part reducing the risk of surface defect and allowing for the detection of temperature irregularity within the part. Optimizing cooling time and monitoring contribute to reducing the production cycle times and improving overall production efficiency. 

Another benefit is these two factors promote consistency and quality assurance of parts meeting the desired design requirements. 

4. Regular Maintenance and Inspections

To ensure reliable operations of injection molding components such as ejector pins regular maintenance and inspection are essential. Preventive maintenance such as lubrication, cleaning, and component inspection can help identify and address any issue prolonging the lifespan of the ejector pin.

Well-maintained ejector pins and mold cavity operate at optimum performance ensuring consistent part quality and enhancing production efficiency. Scheduled inspection and maintenance help prevent sudden equipment failures and minimize downtime. This prevents production interruptions. 

Furthermore, it ensures that equipment meets safety standards and regulatory requirements reducing accidents. Investing in regular maintenance and inspection yields long-term cost savings through extending the life cycle of the ejector pins and mold cavity, avoiding costly emergency repairs. 

Regular inspection and maintenance also provide opportunities for process optimization equipment upgrades based on feedback and observation facilitating continuous improvement. 

Through conducting regular inspections and replacing worn-out or damaged ejector pins manufacturers ensure continued reliability of their molds. This minimizes part defects and enhances part quality over time.

Technological Solutions and Innovations

The introduction of advanced technologies like sensor-based process control and automated adjustments are significant adjustments in injection molding processes. Let’s have a detailed look below!

Sensor-Based Process Control

These are systems that continuously monitor key process parameters such as pressure and temperature in real-time. These sensors provide immediate feedback enabling manufacturers to identify any deviation from desired requirements and adjust appropriately to avoid part defects. 

Malfunction in equipment or process can also be detected using these sensors such as material degradation and mold misalignment. Data collected from these sensors over time can be analyzed to identify trends and patterns between process parameters and part quality. Insight from this data can be utilized in optimizing the production process.

Automated Adjustments

These are based on the sensor data allowing for precise control of process parameters enhancing consistency and quality of part production. Automated adjustment reduces the need for operators to manually make adjustments minimizing the risk of human error. Algorithms used in automated adjustments can optimize process parameters based on sensor feedback.

3D printing

Production of mold components using 3D has a significant impact on reducing ejector marks in the injection molding process. Additive manufacturing enables the integration of complex venting features directly into mold components facilitating air and gas evacuation during injection and ejection. 

3D printing is used for the production of complex geometry parts with intricate features. Custom design ejector components minimize contact pressure and reduce the risk of ejector marks on the part surface. Through simulation and 3D models, manufacturers can refine design requirements to optimize design and process parameters to meet the desired part requirements specifications.

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Ejector Pin Marks (Image source: Plastopia)


Ejector marks in injection molding can impact the quality and aesthetics of molded parts. By understanding the causes of ejector marks and adopting preventive measures, manufacturers can reduce defects, improve part quality, and enhance overall productivity in the injection molding process. Some of the causes are improper mold venting and inaccurate ejector pin position.

Continuous process evaluation and the adoption of new technology are essential for improving product quality and maintaining competitive advantage in the industry. Through this customer satisfaction is guaranteed promoting the brand of the organization.


Steven Paul

Steven Paul

Steven Paul is the Technical Director in TDL, he has more than 15 years experience in Injection molding design.

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