Image forming apparatus and method for controlling image forming apparatus

A method for controlling an image forming apparatus, the method comprising:

• controlling a fixed line head to eject ink from a nozzle to a conveyed paper sheet;
• controlling a first conveyor roller pair to convey the paper sheet to the line head;
• controlling a conveyor belt to convey the paper sheet in such a manner that the nozzle faces the paper sheet with an interval therebetween;
• controls a switchback roller pair to reverse front and back sides of the paper sheet and to reverse front and rear edges of the paper sheet for duplex printing;
• controls to convey the paper sheet printed by the line head to the switchback roller pair;
• controls to convey the paper sheet switched back by the switchback roller pair, so as to return the paper sheet to the first conveyor roller pair on upstream side of the line head;
• disposing a plurality of the line heads for individual ink colors, including at least cyan, magenta, yellow, and black;
• and
• when performing duplex printing, setting one of front and back sides as a first side and sets the other side as a second side, starting printing the first side first and printing the second side after the first side is printed, checking specific area ink quantity as ink quantity to be ejected to a specific area on the first side on the basis of image data of the first side used for printing the first side, determining an ink quantity ratio by dividing the specific area ink quantity by a predetermined solid printing ink quantity, and performing a black conversion process for pixels included in a conversion target range in the image data of the first side when the determined ink quantity ratio is larger than a predetermined reference ink quantity ratio, wherein
• the specific area is a strip-shaped area including a side edge on upstream side in the conveying direction of the paper sheet on the first side, out of side edges facing each other in a sub-scanning direction, and having a width in the sub-scanning direction, which is a predetermined distance,
• the conversion target range is a range corresponding to the specific area in the image data of the first side, and
• the black conversion process is a process of converting a pixel to which three color inks, i.e. cyan, magenta, and yellow inks are to be ejected into a pixel to which only black ink is to be ejected.

An image forming apparatus comprising:

• a line head including a nozzle for ejecting ink to a conveyed paper sheet, the line head being fixed;
• a first conveyor roller pair for conveying the paper sheet to the line head;
• a conveyor belt for conveying the paper sheet in such a manner that the nozzle faces the paper sheet with an interval therebetween;
• a switchback roller pair configured to reverse front and back sides of the paper sheet and to reverse front and rear edges of the paper sheet for duplex printing;
• a second conveyor roller pair for conveying the paper sheet printed by the line head to the switchback roller pair;
• a third conveyor roller pair for conveying the paper sheet switched back by the switchback roller pair so as to return the paper sheet to the first conveyor roller pair on upstream side of the line head;
• and
• a controller, wherein
• when performing duplex printing, the controller sets one of front and back sides as a first side and sets the other side as a second side, starts printing the first side first, determines first ink quantity as ink quantity to be ejected to a first area on the first side on the basis of image data of the first side used for printing the first side, determines second ink quantity as ink quantity to be ejected to a second area as a backside area of the first area on the second side, controls to perform jam prevention printing when the first ink quantity is more than the second ink quantity, and controls to perform normal duplex printing when the first ink quantity is the second ink quantity or less, and
• when performing the jam prevention printing, the controller determines an absolute value of a difference between the first ink quantity and the second ink quantity, controls the line head to print the first area by the same ink quantity as the absolute value in printing the first side first time, controls the switchback roller pair to switchback the paper sheet after printing the first side so as to return the switched-back paper sheet to the upstream side of the line head, controls next the line head to print the entire second side, controls the switchback roller pair to switchback the paper sheet after printing the second side so as to return the switched-back paper sheet to upstream side of the line head, and controls next the line head to print the remaining area on the first side, and wherein
• the first area is a strip-shaped area including a side edge on upstream side in the conveying direction of the paper sheet when printing the first side out of side edges facing each other in the sub-scanning direction, and having a width in the sub-scanning direction, which is a predetermined distance.