Inkjet printing apparatus and a purging method therefor

1. An inkjet printing apparatus for carrying out printing by moving an inkjet head and printing paper relative to each other, the inkjet head having a plurality of nozzles for discharging ink droplets, the apparatus comprising:

a feed pipe connected to each nozzle of the inkjet head for feeding ink thereto;

an ink feed control valve mounted on the feed pipe for controlling circulation of the ink;

a cap for blocking the plurality of nozzles of the inkjet head;

a decompression device for decompressing an inside of the cap;

decompression piping for connecting the cap and the decompression device;

a switching valve mounted on the decompression piping for controlling decompression by switching to and from communication between the cap and the decompression device and communication between the cap and atmosphere, the switching valve including a leak valve for returning the inside of the cap to atmospheric pressure at low speed when switching is made to the communication between the cap and atmosphere; and

a control device configured to control the ink feed control valve, the cap, the decompression device and the switching valve;

wherein the control device is configured to move the cap into contact with the inkjet head to block all the nozzles of the inkjet head, then close the ink feed control valve, switch the switching valve to communication with atmosphere, operate the decompression device, thereafter switch the switching valve to communication with the cap to decompress the inside of the cap to a first pressure in a first predetermined time, open the ink feed control valve after the first pressure is reached, and further, when the inside of the cap has reached a second pressure which is between the first pressure and atmospheric pressure, upon lapse, after the ink feed control valve is opened, of a second predetermined time which is a time for enabling discharge of bubbles along with ink in view of flow paths and characteristics of the ink, the control device is configured to close the ink feed control valve, switch the switching valve to the communication with atmosphere for returning the inside of the cap to atmospheric pressure at low speed, thereafter stop the decompression device, and separate the cap from the inkjet head.

2. The inkjet printing apparatus according to claim 1 , wherein:

the controller is configured to close the ink feed control valve upon lapse of a second predetermined time after the first predetermined time, and

the second predetermined time is set, by taking characteristics of the ink and flow paths into account, to a time for allowing bubbles to be discharged along with the ink.

3. The inkjet printing apparatus according to claim 2 , wherein the switching valve comprises a three-way valve.

4. The inkjet printing apparatus according to claim 1 , wherein the inkjet head and the cap, respectively, are provided plurally, and the decompression piping has a plurality of purge branch pipes each connected at one end thereof to one of the caps, and an accumulator connected to the other end of each purge branch pipe and communicating with the decompression piping.

5. The inkjet printing apparatus according to claim 2 , wherein the inkjet head and the cap, respectively, are provided plurally, and the decompression piping has a plurality of purge branch pipes each connected at one end thereof to one of the caps, and an accumulator connected to the other end of each purge branch pipe and communicating with the decompression piping.

6. The inkjet printing apparatus according to claim 3 , wherein the inkjet head and the cap, respectively, are provided plurally, and the decompression piping has a plurality of purge branch pipes each connected at one end thereof to one of the caps, and an accumulator connected to the other end of each purge branch pipe and communicating with the decompression piping.