A velocity controlled process uses a velocity setpoint to control the rate at which the polymer is injected into the mold. Usually this is represented on the control panel as in/s or mm/s. 

On a hydraulic machine, the hydraulic pressure will adjust to maintain the desired screw velocity. On an electric machine, the power applied to the injection servo-motor will be adjusted to maintain the desired velocity.

This is much better than injection using pressure control where a specific pressure is maintained, and the screw slows down as the mold fills.

For more information on this, please visit: Open Loop vs. Closed Loop Controls

Andy