Inkjet printer and image recording method

1. An inkjet printer, comprising:

a head part for ejecting droplets of ink from a plurality of outlets toward an object and including piezoelectric elements provided for respective outlets of the plurality of outlets;

a scanning mechanism for moving said object in a predetermined scanning direction relative to said head part; and

a controller for repeatedly inputting an output value and a driving signal to said head part to form a solid area having an edge on said object, said output value and said driving signal being input in parallel with relative movement of said object to said head part, said output value indicating whether to form a dot or not, and said driving signal being applied for ejection of droplet; wherein

the driving signal includes a plurality of ejection pulses each operative to make a respective one of the piezoelectric elements perform a series of operations,

a leading droplet and a following droplet are ejected from an outlet by inputting said output value indicating to form a dot and said driving signal, wherein said leading droplet is a main droplet ejected by an ejection pulse included in said driving signal and said following droplet is a main droplet ejected by the next ejection pulse in said driving signal, and said leading droplet and said following droplet land onto said object,

an average distance from the center of a leading dot element formed on said object by said leading droplet to the farthest point in a group of dot elements formed on said object by said leading droplet and said following droplet to form the solid area is equal to or more than 1.1 times an average radius of said leading dot element and equal to or less than 3.0 times said average radius, said leading dot element and a dot element formed by said following droplet partially overlap with each other, and said leading dot element and said dot element formed by said following droplet are included in a dot corresponding to a pixel,

another type of driving signal having a waveform different from said driving signal can be selectively inputted to said head part, said another type of driving signal including a plurality of ejection pulses, and

in the case where said another type of driving signal is inputted to said head part:

said leading droplet and said following droplet are ejected from said outlet so that an average distance from the center of said leading dot element by said leading droplet to the farthest point in said group of dot elements by said leading droplet and said following droplet is less than 1.1 times an average radius of said leading dot element, or

said average distance in the case of said another type of driving signal is different from said average distance in the case of said driving signal, while staying within a range which is equal to or more than 1.1 times an average radius of said leading dot element and equal to or less than 3.0 times said average radius.

2. The inkjet printer according to claim 1 , wherein

a plurality of dots having different sizes can be formed on said object by ejecting droplets of ink from said outlet toward said object,

said leading droplet and said following droplet are ejected from said outlet by inputting said driving signal when a dot having the maximum size or a dot used for representing the maximum gray level area is formed.

3. The inkjet printer according to claim 1 , wherein

said driving signal includes first, second, and third ejection pulses operative to cause the ejection of first, second, and third main droplets, respectively, and

an average distance relationship between the first and second main droplets and an average distance relationship between the second and third main droplets each are the same as the average distance relationship between the leading droplet and the following droplet where the average distance from the center of the leading dot element formed by the leading droplet to the farthest point in the group of dot elements formed by the leading droplet and the following droplet is equal to or more than 1.1 times the average radius of the leading dot element and equal to or less than 3.0 times the average radius.

4. An image recording method performed in an inkjet printer, wherein

said inkjet printer comprises a head part for ejecting droplets of ink from a plurality of outlets toward an object, the head part including piezoelectric elements provided for respective outlets of the plurality of outlets,

said image recording method comprising the steps of:

a) moving said object in a predetermined scanning direction relative to said head part; and

b) repeatedly inputting an output value and a driving signal to said head part to form a solid area having an edge on said object, said output value and said driving signal being input in parallel with said step a), said output value indicating whether to form a dot or not, and said driving signal being applied for ejection of droplet; wherein

the driving signal includes a plurality of ejection pulses each operative to make a respective one of the piezoelectric elements perform a series of operations,

a leading droplet and a following droplet are ejected from an outlet by inputting said output value indicating to form a dot and said driving signal, wherein said leading droplet is a main droplet ejected by an ejection pulse included in said driving signal and said following droplet is a main droplet ejected by the next ejection pulse in said driving signal, and said leading droplet and said following droplet land onto said object, and

an average distance from the center of a leading dot element formed on said object by said leading droplet to the farthest point in a group of dot elements formed on said object by said leading droplet and said following droplet to form the solid area is equal to or more than 1.1 times an average radius of said leading dot element and equal to or less than 3.0 times said average radius, said leading dot element and a dot element formed by said following droplet partially overlap with each other, and said leading dot element and said dot element formed by said following droplet are included in a dot corresponding to a pixel,

another type of driving signal having a waveform different from said driving signal can be selectively inputted to said head part, said another type of driving signal including a plurality of ejection pulses, and

in the case where said another type of driving signal is inputted to said head part:

said leading droplet and said following droplet are ejected from said outlet so that an average distance from the center of said leading dot element by said leading droplet to the farthest point in said group of dot elements by said leading droplet and said following droplet is less than 1.1 times an average radius of said leading dot element, or

said average distance in the case of said another type of driving signal is different from said average distance in the case of said driving signal, while staying within a range which is equal to or more than 1.1 times an average radius of said leading dot element and equal to or less than 3.0 times said average radius.

5. The image recording method according to claim 4 , wherein

a plurality of dots having different sizes can be formed on said object by ejecting droplets of ink from said outlet toward said object,

said leading droplet and said following droplet are ejected from said outlet by inputting said driving signal when a dot having the maximum size or a dot used for representing the maximum gray level area is formed.

6. The image recording method according to claim 4 , wherein

said driving signal includes first, second, and third ejection pulses operative to cause the ejection of first, second, and third main droplets, respectively, and

an average distance relationship between the first and second main droplets and an average distance relationship between the second and third main droplets each are the same as the average distance relationship between the leading droplet and the following droplet where the average distance from the center of the leading dot element formed by the leading droplet to the farthest point in the group of dot elements formed by the leading droplet and the following droplet is equal to or more than 1.1 times the average radius of the leading dot element and equal to or less than 3.0 times the average radius.