When designing a part for injection molding, most molds end up having some kind of movement taking place. Meaning the two mold halves pull apart. If the mold cannot be a straight pull, i.e. there are undercuts or threads, some mold movement will be required to facilitate opening the mold fully.
Whether slides have to be moved, threads have to be unscrewed before pulling, core pulls are retracted from open areas, ejection plates moved, etc., a decision has to be made on how to accomplish this movement.
One option is mechanical actuation, where pins, springs or inserts accomplish the actuation or movement.
Another option is hydraulic actuation, where hydraulic cylinders can be used to pull slides, unscrew cores from threads, remove cores from open areas, etc.
The primary decision point between one concept and the other, is cost. Incorporating hydraulics into the mold will make the mold cost more, also possibly increase long term maintenance costs.
The data typically feeding into that decision is the designed part’s cost and volume.
Hydraulics lend themselves to a more automated process and are typically geared toward higher part quantity programs. But even if the quantities are not that high, a molder/mold designer may incorporate the hydraulics because it is a better fit for their operation, i.e. labor, processing time, higher revenues.
When receiving mold quotes for your newly designed part, understand the theory of what features the mold being proposed has, and are they the correct fit/application for your particular program regarding cost and volume.