Long-time injection pressure maintaining control loop of injection molding machine

The utility model discloses a long-time injection pressure maintaining control loop of an injection molding machine, which comprises an oil tank, a servo motor, an oil pump, a first cartridge valve serving as an injection valve, a second cartridge valve serving as an injection withdrawing valve, an electromagnetic directional valve serving as an injection valve and a proportional pressure valve serving as a material storage back pressure valve, the holding pressure set by the proportional pressure valve is larger than the injection pressure, an oil outlet of the oil pump is communicated with a cavity B of the first cartridge valve, a cavity A of the first cartridge valve is communicated with an injection cavity of the injection molding machine and a cavity A of the second cartridge valve, a cavity C of the first cartridge valve is communicated with a cavity A of the electromagnetic directional valve, and a cavity C of the second cartridge valve is communicated with a cavity A of the proportional pressure valv