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pin's speed and stroke, unlike conventional hydraulic and pneumatic actuators. Hydraulic actuators are most commonly used in larger applications where large pins need to be moved against high injection pressures, and therefore high forces are required to move the pin quickly. These actuators are very responsive as long as there is no air in the hydraulic circuit. As a result, it is necessary to completely bleed the air out of the hydraulic lines and the actuator piston itself in order to realize maximum performance. Bleeding is obviously necessary when a new system is assembled, but just the act of discon- necting and reconnecting the hydraulic hoses during the life of the tool can introduce air into the system, requiring periodic bleeding for maximum responsiveness. Many newer hydraulic actuators have built-in self-bleeding mechanisms that completely elimi- nate the need to bleed the system after initial installation or at any time during the life of the tool. These self-bleeding actuators save tooling personnel time over the life of the tool and eliminate scrap caused by sluggish valve pins. BENEFITS OF POSITION SENSORS Probably the most significant of the recent advances to both hydraulic and pneumatic hot-runner actuators is the option of adding a position sensor. Simple position sensors can now be added to these actuators, which can later be combined with flow-control technologies yielding all the same control benefits of an electric actuator. Systems that have actuators with position sensors can be hooked to simple monitoring devices. These devices can give feedback on the current position of the pin as well as the amount of time the pin takes to complete a movement. This is a great diagnostic tool to ensure that all connections are made properly after a mold is installed in the press. With large molds, especially those with deep cavities, it is impos- sible to see the pins move from outside the press, even when the mold is open. It is not uncommon for an operator to stand inside the press in order to confirm that the pins are moving correctly and that they correspond to the appropriate There are many types of flow- control technolo- gies available today that allow the molder to manipu- late the movement of the valve pins. Instead of adjusting the valve-pin position with shims, the effective pin length can be changed with a simple turn, using a tool through a hole in the top clamp plate. The light blue line shows the temperature of an actuator with a water- cooled isolation plate when the cooling water is turned off at the same time as the hot-runner heats. The darker blue line shows the temperature of an actuator with a passive cooling device under the same circumstance. The actuator that depends on the water-cooled plate overheats while the actuator with the passive cooling device stays cooler. Temperature Time @plastechmag 51 Plastics Technology V A LV E - G AT E D H O T R U N N E R S