Pump with inlet and outlet simultaneously exposed to pump chamber and method of operating same

A pump unit comprising:

• a pump including:
• a cylindrical pump body,
• a first moving member and a second moving member slidably fitted in the pump body in a liquid-tight fashion and in an axially opposite arrangement, the first moving member and the second moving member forming a pump chamber therebetween, and
• a suction port and a discharge port formed in the pump body, said suction port and said discharge port being controllably opened and closed by the first moving member and the second moving member;
• and
• a driving mechanism individually moving the first moving member and the second moving member relative to the pump body such that the first moving member and the second moving member move at substantially equal speeds when the pump chamber is open to both the suction port and the discharge port.

A method of operating a pump including a pump body, at least two moving members movably fitted in the pump body in a liquid-tight fashion, the at least two moving members forming a pump chamber therebetween, and a suction port and a discharge port formed in the pump body, the method comprising:

• separating said two moving members to expand said pump chamber and to create a negative pressure in said pump chamber;
• blocking said discharge port during said separating, thereby suctioning ink through said suction port;
• shifting the separated two moving members at substantially equal speeds along substantially all of a distance between a position where the suction port communicates with the pump chamber to a position where the discharge port communicates with the pump chamber such that said suction port and said discharge port are open to the pump chamber at the same time;
• and
• converging said two moving members to contract said pump chamber while blocking said suction port, thereby discharging ink through said discharge port.