Today, a part under commissioning exhibited ejector pin marks (ejector marks or white spots). White spots were visible on gate side. Below is an analysis of white spots and gate side:

 

Phenomenon: Ejector pin marks appear on part surface (especially on gate side), appearing as localized white spots (white spots), which can be slight depressions or bulges.
Effect: When ejecting part, ejector pin exerts excessive force (pressure) on part surface, exceeding yield strength or elastic limit of material at demolding temperature, resulting in permanent deformation (plastic deformation) or stress whitening.
Specialities of Gate Side: Gate area is typically:
Last area to cool and solidify (where holding pressure primarily acts).
Area with the most concentrated shrinkage stress (shrinkage toward gate).
Area with the highest material density and potentially the highest molecular orientation. Therefore, this area is typically harder and more brittle (relative to areas that are not fully cooled or have low molecular orientation), and also experiences greater shrinkage forces. This makes area near gate particularly sensitive to ejector pin pressure and more prone to ejection marks.

1. Insufficient Draft Angle and High Release Force:
Cause: Due to concentrated shrinkage stress near gate, part's grip on core may be greater than in other areas. If draft angle is insufficient, force required for demolding (ejection force) will increase significantly. Ejector pin must overcome this significant gripping force to eject part, naturally increasing pressure on part surface, which can easily cause indentation or stress whitening (ejection whitening) at ejector pin location.
Gate-Side Correlation: Gate area often has the greatest gripping force, making impact of an insufficient draft angle most significant here.
2. Ejector pin has a small cross-section, resulting in high ejection pressure on ejection surface of part:
Cause: Contact area between ejector pin and part is small (ejector pins are small in diameter, few in number, or poorly arranged). Ejection force is concentrated on a small area, resulting in extremely high local pressure (pressure/area). Even if the total ejection force is low, local pressure may exceed material's tolerance limit, causing yield deformation (depression) or stress whitening (bulges or whitening) at ejector pin contact point.
Gate-side correlation: Gate area is more brittle and more sensitive to high local pressure, making it more susceptible to irreversible deformation or whitening.
3. Rough mold surface in demolding direction, increasing demolding force:
Cause: Highly rough core surfaces (especially those near gate) or presence of scratches, wear, or rust increase friction between part and mold, significantly increasing demolding force. This increased demolding force requires greater ejection force to overcome, indirectly increasing pressure applied by ejector pin.
Gate-side correlation: Gate area inherently has strong holding force. If surface here is also rough, double effect will lead to a dramatic increase in demolding force.
4. Insufficient cooling time, resulting in insufficient strength during demolding:
Cause: Interior of part has not fully cooled and solidified, resulting in insufficient overall rigidity. Material is too soft at demolding temperature (above or near glass transition temperature). Ejection at this temperature can easily leave an indentation on part surface or cause significant elastic or plastic deformation at ejector pin location, resulting in whitening or denting. Gate area is typically last to solidify, and if cooling time is insufficient, this area becomes particularly soft.
Gate-side correlation: Gate area cools the slowest. If overall cooling time is insufficient, this area is the softest, weakest, and most susceptible to deformation or whitening caused by ejector pin.
5. Improper injection and holding parameters (especially those related to holding):
Cause:
Excessive holding pressure/time: This is a very common and critical cause of whitening on gate side! Excessively high holding pressure or excessively long holding time can lead to excessive compression and compaction in the area near gate, resulting in extremely high density and significant shrinkage stress (part feels as if it is being forcefully "pressed" against the core). This can also lead to increased molecular orientation and brittle material. This leads to extreme holding forces during demolding, requiring significant ejection force and easily causing whitening at ejector pin.
A too-late holding pressure switchover point (overfilling part): A too-late switchover point means holding phase is initiated too early, effectively initiating high-pressure compaction before cavity is fully filled. This also results in abnormally high pressure near gate, resulting in high density, shrinkage stress, and high holding forces.
Improper injection speed/pressure: This can lead to localized fill imbalance, abnormal molecular orientation, or internal stress concentration, indirectly affecting strength and demolding behavior of specific areas (such as near gate).
Gate-side correlation: Holding pressure is transmitted directly through gate and primarily affects the area near gate. Therefore, improper holding parameters have the most direct and significant impact on gate area, making it one of core process factors leading to whitening in this area.
6. Vacuum Formation on Ejector Side:
Cause: When a part has a deep structure or undercut (even with a slope), a local vacuum (negative pressure) may form between part and core during demolding, creating additional suction forces that require greater ejection force to overcome. This increases ejection force and local pressure required by ejector pin.
Gate Side Relationship: While vacuum can occur in any area, if structure near gate is prone to vacuum formation, this can compound demolding difficulties in that area.

1. Prioritize process parameters (holding pressure is paramount!):
Step 1: Immediately try reducing holding pressure (e.g., by 10-20%) and shortening holding time (e.g., by 1-3 seconds). Observe whether gate-side whitening decreases or disappears. This is the quickest and lowest-cost solution.
Step 2: Check and advance holding pressure switching point (starting with V/P switching point). Ensure mold cavity is switched when it is 95-99% full (depending on material and product) to avoid premature high pressure.
Step 3: Appropriately extend cooling time to ensure sufficient cooling and solidification in gate area. This can be determined by measuring temperature in this area or observing condition of part during ejection.
Record results of each parameter adjustment for comparison.
2. Inspect and address mold surface:
Focus on polishing core surface near gate! Ensure it achieves a high finish (mirror finish). This is an effective and relatively easy way to reduce friction and demolding force.
3. Evaluate ejector pin system:
Are ejector pins near gate thick enough? Are they sufficient in number? Is their layout appropriate?
If process adjustments and polishing have limited effect, and ejector pins are indeed too thin or too few, increasing ejector pin diameter or increasing number of ejector pins near gate is the most direct and effective mold modification, which can significantly reduce local pressure.
Check that ejector pins are moving smoothly and are not stuck.
4. Examine draft angle:
If the area near gate has a complex structure, a large depth, and a very small or zero draft, increasing draft angle is a fundamental solution. However, this typically involves significant mold modification costs and lead time. This should only be considered after process and ejector pin optimization have proven ineffective.
5. Consider vacuum issues:
If the part has a deep cavity or special structure near gate, a vacuum effect may be created during demolding. Installing an air blower is an effective solution.