Method and apparatus for controlling retracting speeds of injection molding machines

Claim 1

Apparatus for controlling a charging speed of injection molding machine wherein a molten resin contained in a heated barrel is injected into a metal mold, said apparatus comprising: a screw contained in said heated barrel; means for rotating said screw; piston-cylinder means for reciprocating said screw in said heated barrel; pressurized oil transfer valve means for admitting and discharging pressurized oil into and from said cylinder; means operated by said piston for generating a signal representing a screw retraction speed; means for setting a reference screw retraction speed; and means responsive to a difference between said signal representing the screw retraction speed and said reference screw retraction speed for controlling a back pressure of said pressurized oil acting upon a rear surface of said piston, said means comprising a back pressure setting relief valve connected between a chamber in said cylinder on a back side of said piston and a drain, said back pressure setting relief valve having a pilot port also connected to said chamber and a vent port, and an electromagnetic control valve having a port connected between the vent port of said back pressure setting relief valve and said drain and a vent port controlled by an output of a driver actuated by a difference between said signal representing said screw retraction speed and said reference screw retraction speed.