a valve pin actuator for a valve - gated injection molding system . the actuator has an annular piston mounted in and movable back and forth along an annular bore along a pin axis . the piston extends radially outwardly of any melt passages fluidly communicating with an injection molding nozzle . a tie bar extends substantially diametrically across the piston for securing an upper end of a valve pin extending into the injection molding nozzle to cause the valve pin to move axially with the piston thereby moving the valve pin between open and shut positions .