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Torsten’s Training Tips: Ejector Pin Sub-Gates

by Torsten Kruse

Gate design and placement are key factors in mold design. Because parts often need to be molded without surface marks or gate vestige, certain gate styles are more appropriate than others. Although many gate systems will leave behind a witness mark, selecting a gate that can be hidden inside the ejector pin with an automatically de-gated sub-gate is an option. An ejector pin sub-gate system is a robust automatic de-gating system that will prevent gate vestige.

Ejector pin sub-gated runners can have several different design styles. There are full-round and half-round runner systems, as well as a wedged version. The wedged ejector pin runner has a section that is thick at the intersection to the sub gate and then decrease towards the part.

Optimizing sub-gate and ejector pin gate sizes will lead to wider and more stable molding processes and better part quality.As with other gate options, ejector pin gate sizes and styles play a significant role in pressure development during the fill process. Smaller gates produce higher pressure spikes, overall injection pressures, internal cavity pressures, and clamp tonnages.

Ejector Pin Sub-Gate Styles


Full-Round Ejector Pin-Gate System: The full-round system has the lowest pressure drop of the three designs and therefore the widest possible process window. However, this style creates the largest witness mark on the part gate interface, potentially leading to small sink marks or surface blemishes.

Half-Round Ejector Pin-Gate System: The half-round system does not have as low a pressure drop as the full-round system but also has less possible surface appearance issues.

Wedged-Round Ejector Pin-Gate System: The wedged-round system has greater thickness at the gate and “wedges” into a smaller thickness at the intersection of the part. This design would have the highest pressure drop of the three systems and the fewest possible surface appearance defects.


Kruse Analysis, Inc. has been providing CAE simulation services to the plastic injection molding industry since 1995. By using advanced Moldex3D technology, Kruse helps customers troubleshoot from product design to development, optimize design patterns, shorten time-to-market, and maximize product return on investment (ROI). For more information, visit

Kruse Training, Inc. offers a state-of-the-art online knowledge and training solution that use 3D process simulation technology to demonstrate cause and effect of molding behavior. This interactive training program illustrates how “The Circle of Knowledge” between the part designer, mold designer, and process engineers works in the real world of plastic mold and design. For more information, visit

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