Valve-gated hot runners use a mechanical seal between the hot and cold areas by means of a pin called the valve stem. Usually around 3 mm (1/8 in.) to 5 mm (0.2 in.) in diameter, this pin moves back allowing melt flow into the cavity, stays back during hold time, then moves forward controlled by a timer that energized with the end of hold time. Gate vestigates made with valve-gated hot runners are on average shorter than with thermally gated ones and the cut-off is more precise. Properly operly operated they also tend to give less problems with stinging and gate crystallinity. Disadvantage of valve gates is the higher maintenance requirement for the pins and the air cyclinders driving them.
Article from Stretch Blow Molding Third edition, author O.Brandau.