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1. A full-plastic liquid pump, comprising a plastic molding pump body, a pump head, a piston, a spring, a threaded cover, a one-way valve, and a suction pipe, two ends of the pump head respectively defining a liquid outlet and providing a slidable pressing rod, the slidable pressing rod internally providing with a liquid outlet channel in communication with the liquid outlet, a middle portion of the slidable pressing rod movably passing through a cover hole formed in a center of an upper end of the threaded cover, an air inlet gap being formed between the peripheral side of slidable pressing rod and the cover hole, the interior of the upper end of the threaded cover coaxially and fixed with an upper end of the pump body, a peripheral side of the piston is in sealed sliding connection with the inner wall of the pump body, the piston dividing the inner space of the pump body into an air pressure cavity and a liquid storage cavity arranged from up to down, the air pressure cavity communicated with the outside atmosphere via the air inlet gap, a vent hole being defined in the side wall of the upper portion of the pump body, and the vent hole being communicated with the air pressure cavity under a control of the piston; an axial through hole being defined in a center of the piston, an inner wall surrounding the axial through hole hermetically and slidably connected with a peripheral side of a lower end of the slidable pressing rod, a liquid outlet cavity formed between the lower end of the slidable pressing rod and the axial through hole, the liquid outlet cavity communicating with the liquid outlet channel, a lower end of the spring being fixed in a lower end of the liquid storage cavity, a liquid passing gap being formed between a peripheral side of the spring and the inner wall of the liquid storage cavity, the lower end of the slidable pressing rod connected with an upper end of the spring, the slidable pressing rod driving the upper end of the spring and the piston up and down to control the liquid passing gap to communicate or un-communicate with the liquid outlet cavity, a bottom of the liquid storage cavity communicating with an upper end of the suction pipe, the one-way valve being fixed in the liquid storage cavity to correspond to the suction pipe to control liquid to flow unidirectionally from the suction pipe to the liquid storage cavity.
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