Inkjet printing apparatus

An inkjet printing apparatus that discharges ink onto a printing medium to perform printing, comprising:

• an ink circulation path including a discharge head configured to discharge ink, a supply tank in which ink supplied to the discharge head is stored, a collecting tank in which ink collected from the discharge head is stored, a supply pipe connecting the supply tank and the discharge head, and a feedback pipe connecting the collecting tank and the supply tank;
• a circulation pump configured to deliver ink from the collecting tank to the supply tank via the feedback pipe;
• an external tank in which ink is stored outside the circulation path;
• a branch pipe connecting a branch point on the feedback pipe and the external tank;
• an external pump configured to deliver ink from the external tank to the circulation path via the branch pipe;
• a switch valve configured to switch between a state in which the external tank communicates with the branch point via the external pump and the branch pipe and a state in which the external tank communicates with the branch point not via the external pump, but via the bypass pipe connecting the branch pipe and the external tank;
• a control unit configured to control the discharge head, the circulation pump, the external pump, and the switch valve;
• a first liquid-level sensor configured to detect that a liquid level of ink stored in the supply tank is at a height of a first reference value;
• and
• a second liquid-level sensor configured to detect that a liquid level of ink stored in the collecting tank is at a height of a second reference value, wherein
• the control unit is capable of selectively performing a replenishment mode in which ink in the external tank is delivered to the circulation path via the branch pipe and a backflow mode in which ink in the circulation path is delivered to the external tank via the bypass pipe, and
• when the control unit detects that the liquid level of ink stored in the supply tank is at a height exceeding the first reference value, from a result of detection performed by the first liquid-level sensor, and detects that the liquid level of ink stored in the collecting tank is at a height exceeding the second reference value, from a result of detection performed by the second liquid-level sensor, the control unit performs the backflow mode and performs the replenishment mode after the backflow mode.