Causes, Prevention, and Solutions for Ejector Pin Marks

Structural Concerns

Quality Inconsistency

Potential Damage Sites

What Causes Ejector Pin Marks In Injection Molding?

Product Design Issues:

• Insufficient wall thickness: Excessively thin wall thickness, especially for complex geometries with ribs/bosses (generally <2.5mm for simple parts, <2.8mm for complex parts).
• Lack of adequate draft angles or uniform wall thickness: Insufficient draft angles or non-uniform wall thickness can cause parts to stick to the mold, requiring a higher ejection force and potentially causing ejector pin marks.
• Presence of ribs, bosses, or complex geometries: Features such as ribs, bosses, or complex geometries may require higher ejection force to release the part from the mold, increasing the likelihood of ejector pin marks.

Mold Design Flaws:

• Improper ejector pin size, type, location, orientation, or number of pins: If the pins are too large or improperly positioned, they may exert excessive force on the part during ejection, resulting in indentations or surface scuffing. See more on injection molding ejector pins design.
• Inadequate venting: Poor venting can lead to vacuum formation in the cavity, making it difficult for the part to release from the mold and increasing the risk of ejector pin marks.
• Uneven cooling: Improper cooling channel design can result in uneven cooling of the mold, leading to differential shrinkage and potential deformation of the part upon ejection, causing ejector pin marks.
• Inappropriate gate design: Gate size, location, or type that is not properly matched to the part and mold design can result in improper filling and packing, leading to ejector pin marks.

Processing Conditions:

• Excessive injection pressure, speed, or holding pressure: High injection pressures or speeds can lead to overpacking of the cavity, resulting in increased ejection force requirements and potential ejector pin marks.
• Rapid cooling: Rapid cooling can cause internal stress build-up in the part, making it more difficult to eject and increasing the likelihood of ejector pin marks.
• Too high mold temperature: If the mold temperature is too high, it can lead to material sticking to the mold surface, making it more challenging to eject the part cleanly.
• Too low mold temperature: If the mold temperature is too low, it can result in uneven cooling and differential shrinkage, potentially causing distortion or deformation of the part upon ejection.
• High ejection force or speed required: If parts stick to the mold due to inadequate release agents or surface finishes, higher ejection forces or speeds may be required, increasing the risk of ejector pin marks.

Material Issues:

• Using materials without adequate lubrication or flow additives: Insufficient lubrication or flow additives in the material can increase friction between the part and the mold surface, making ejection more difficult and increasing the likelihood of ejector pin marks.
• Mismatch between ejector pin and mold steel conductivity: Mismatched conductivity between the ejector pin and mold steel can lead to uneven cooling and differential shrinkage, potentially causing ejector pin marks.

How to Remove Ejector Pin Marks?

1. For raised marks, a flat file or sanding stick can be used to carefully remove the protruding area until it is flush with the surrounding surface.
2. For recessed ejector pin marks, one option is to fill the indentation with putty, such as Tamiya Basic Type putty, let it dry, and then sand it smooth. Another approach is to cut a disc from a styrene sheet, glue it into the recessed mark, and sand it flush with the part surface. Thick gel super glue can also be used to fill the depression; once fully cured, the area can be sanded and polished smooth.
3. In some cases, applying heat and pressure using a heated tool can help remove marks, depending on the material. For clear parts, filling the recessed area with clear glue, allowing it to level and harden, and then buffing it with a rotary tool can restore a smooth, visually consistent surface.

How to Prevent Ejector Pin Marks in Injection Molding?

Proper Ejector Pin Design:

• Pin Size: Ensuring the appropriate size of ejector pins helps distribute the ejection force evenly across the part surface, reducing the risk of localized pressure points that can cause marks.
• Pin Shape: Opting for rounded or tapered pin tips can minimize the risk of sharp indentations or scratches on the part surface during ejection.
• Pin Placement: Avoid placing ejector pins on thin walls, curved surfaces, or near gates. Distributing ejector pins evenly and avoiding clustering them in one area can reduce the concentration of force and mitigate the risk of marks.

Optimize Mold Design

• Increase draft angles to facilitate easier part ejection and reduce vacuum formation.
• Incorporate proper venting to prevent vacuum entrapment.
• Design uniform cooling channels for even cooling and reduce internal stresses.

Adjust Processing Parameter

• Reduce injection pressure, speed, and dwell time to minimize internal stresses.
• Limit the amount of melt injected into the mold cavity.
• Optimize mold and barrel temperatures for uniform cooling.
• Increase cooling time to allow for gradual, even cooling of the part.
• Control ejection speed to reduce impact forces during ejection.

Improve Part Design

• Increase wall thickness, especially in areas with ejector pins.
• Incorporate reinforcing ribs or bosses in high ejection force areas.

Material and Surface Treatment

• Use materials with adequate lubrication and flow additives.
• Apply mold release agents properly and evenly.
• Increase cavity surface smoothness by polishing along the ejection direction.
• Use specialized mold coatings like heat/quick-cure coatings for better release.

Conclusion