Abstract:
A method for manufacturing an inkjet recording head includes an ejecting step, a measuring step, dividing step and applying step. The ejecting step ejects test ink droplets and print ink droplets from nozzles. The measuring step measures ejection results of the test ink droplets. The dividing step divides a plurality of nozzles into a plurality of groups based on the ejection results. The applying step applies a group-based polarizing voltage determined for each group to the piezoelectric elements belonging to a corresponding group to polarize the piezoelectric elements so that ejection results of the print ink droplets fall in a predetermined range.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ink jet recording head for recording a high-quality image at high speed with high reliability and an ink jet recording device equipped with the recording head, a manufacturing method of the recording head of the inkjet recording device. 
   2. Description of Related Art 
   For recording a high-quality image at high speed with high reliability by using a multi-nozzle on-demand ink jet recording head in which a lot of nozzles are integrated, it is important to reduce variations in the ink drop discharge speed or the ink drop weight among nozzles. 
   In an on-demand ink jet recording head according to push-type piezoelectric element system, a wall of an ink pressurizing chamber having nozzle apertures is formed of a diaphragm. By pushing the diaphragm with vertical vibrations of rod-like piezoelectric elements, the volume of the ink pressurizing chamber is decreased to discharge the ink drop. Conventionally, in the on-demand ink jet recording head according to push-type piezoelectric element system, in order to reduce variations in the ink drop discharge speed or weight among nozzles, the accuracy of components such as the piezoelectric elements and the ink pressurizing chamber is improved, or assembling accuracy of bonding of each part and the like is improved. 
   However, according to the above-mentioned method, there may cause disadvantages such as an increase in costs of parts and assembling time. On the contrary, Unexamined Patent Application Publication No. 2001-277525 discloses a method of reducing variations in the ink drop discharge speed or weight among nozzles by properly adjusting the polarization level of piezoelectric elements. According to this method, although it requires adjustment costs in a head manufacturing process, variations in the ink drop discharge speed and weight can be improved without adding any part or circuit. 
   However, according to the method disclosed in Unexamined Patent Publication No. 2001-277525, the ink discharge speed needs to be measured while varying a polarization level of each piezoelectric element in order to adjust the ink drop discharge speed of each nozzle of a recording head to a target speed. For this reason, since it takes time to measure the ink drop discharge speed and adjust the polarization level of the piezoelectric elements to a proper value, sufficient cost down and improvement in productivity cannot be achieved. 
   SUMMARY OF THE INVENTION 
   In view of the above-described drawbacks, it is an objective of the present invention to provide an on-demand ink jet recording head in which a lot of nozzles are integrated, a manufacturing method of the head and a recording device, which can record high-quality images at high speed at low costs. 
   In order to attain the above and other objects, the present invention provides a method for manufacturing an inkjet recording head including a plurality of nozzles and a plurality of piezoelectric elements provided in one-to-one correspondence with the plurality of nozzles. Each piezoelectric element expands and contracts in accordance with a driving voltage applied thereto and polarizes in accordance with a polarizing voltage applied to thereto. The method includes an ejecting step, a measuring step, dividing step and an applying step. The ejecting step ejects test ink droplets and print ink droplets from the nozzles. The measuring step measures ejection results of the test ink droplets. The dividing step divides the plurality of nozzles into a plurality of groups based on the ejection results. The applying step applies a group-based polarizing voltage determined for each group to the piezoelectric elements belonging to a corresponding group to polarize the piezoelectric elements so that ejection results of the print ink droplets fall in a predetermined range. 
   Another aspect of the present invention provides an inkjet recording head including a plurality of piezoelectric elements and a plurality of nozzles provided one-to-one correspondence with the plurality of piezoelectric elements. The plurality of piezoelectric elements expands and contracts based on a driving voltage applied thereto, and polarizes in accordance with a polarizing voltage applied to thereto. Each nozzle ejects test ink droplets and print ink droplets in accordance with the expansion and the contraction of the corresponding piezoelectric element. The plurality of nozzles are divided into a plurality of groups based on an ejection result of the test ink droplets. The group-based polarizing voltage determined for each group is applied to the piezoelectric elements belonging to a corresponding group to polarize the piezoelectric elements so that ejection results of the print ink droplets fall in a predetermined range. 
   Another aspect of the present invention provides an inkjet recording device including a body and an inkjet recording head provided on the body. The inkjet recording head includes a plurality of piezoelectric elements and a plurality of nozzles provided one-to-one correspondence with the plurality of piezoelectric elements. The plurality of piezoelectric elements expands and contracts based on a driving voltage applied thereto, and polarizes in accordance with a polarizing voltage applied to thereto. Each nozzle ejects test ink droplets and print ink droplets in accordance with the expansion and the contraction of the corresponding piezoelectric element. The plurality of nozzles are divided into a plurality of groups based on an ejection result of the test ink droplets. The group-based polarizing voltage determined for each group is applied to the piezoelectric elements belonging to a corresponding group to polarize the piezoelectric elements so that ejection results of the print ink droplets fall in a predetermined range. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
       FIG. 1  is a configuration view for describing the configuration and operation of a recording device in a first embodiment of the present invention; 
       FIG. 2  is a partial perspective enlarged view for describing the configuration and operation of a recording head in the first embodiment of the present invention; 
       FIG. 3  is a configuration view for describing the configuration and operation of a repolarizing device in the first embodiment of the present invention; 
       FIG. 4A  is a graph showing ink drop discharge speed characteristics with respect to repolarization voltage in the first embodiment of the present invention; 
       FIG. 4B  is a graph showing an example of variation characteristics of ink discharge speed among nozzles in the first embodiment of the present invention; 
       FIG. 5  is a graph showing adjustment characteristics of ink drop discharge speed through repolarization adjustment of the recording head in the first embodiment of the present invention; 
       FIG. 6  is a view for describing a modified example of repolarization adjustment of the recording head in a second embodiment of the present invention; 
       FIG. 7  is a graph showing adjustment characteristics of the ink drop discharge speed through repolarization adjustment of the recording head in the second embodiment of the present invention; and 
       FIG. 8  is another modified example of repolarization adjustment of the recording head in a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A recording device according to preferred embodiments of the present invention will be described while referring to  FIG. 1  to  FIG. 5  wherein like parts and components are designated by the same reference numerals to avoid duplicating description.  FIG. 1  is a view for describing the configuration and operation of a recording device  1  in accordance with this embodiment.  FIG. 2  is a partial perspective view for describing the configuration and operation of a recording head  10 . Note that the upper side in  FIG. 2  corresponds to the lower side in  FIG. 1 . 
   As shown in  FIG. 1 , the recording device  1  in this embodiment includes the recording head  10  and a recording head driving device  20 . The recording head  10  has an ink passage unit  101 , a head housing  102  and a piezoelectric element unit  103 . The head housing  102  holds the ink passage unit  101 . As shown in  FIG. 2 , the ink passage unit  101  includes an orifice plate  130 , an ink passage forming plate  142  and a diaphragm forming plate  122 . These plates are laminated in this order. The piezoelectric element unit  103  includes rod-like piezoelectric elements  110  (hereinafter referred to as piezoelectric elements) and a piezoelectric element supporting substrate  113 . As shown in  FIG. 2 , the piezoelectric elements  110  are fixed to the piezoelectric element supporting substrate  113  in a comb-like fashion. 
   A supporting substrate fixing part  114  ( FIG. 1 ) is provided at each end of the piezoelectric element supporting substrate  113  in the aligning direction of the piezoelectric elements  110  and the bottom face of the supporting substrate fixing part  114  is fixedly adhered to the ink passage unit  101 . The ink passage unit  101  is fixedly adhered to the head housing  102  in the vicinity of the above-mentioned adhered region. This results in that the bottom face of the piezoelectric element supporting substrate fixing part  114  is fixed to the head housing  102 . 
   The orifice plate  130 , the ink passage forming plate  142  and the diaphragm forming plate  122  form an ink pressurizing chamber  140 , an ink inflow port  145  for guiding ink into the ink pressurizing chamber  140  and a common ink chamber  150  for supplying ink to the ink inflow port  145 . Nozzle apertures  131  (hereinafter referred to as nozzle  131 ) are aligned on a face of the orifice plate  130  opposed to the ink pressurizing chamber  140  at a predetermined pitch. The nozzles each have the same configuration. One end of each piezoelectric element  110  is attached to a face of a diaphragm  120  on the opposite side of the ink pressurizing chamber  140  through an adhesive layer. 
   Each piezoelectric element  110 , as shown in  FIG. 2 , has a layer configuration in which a plurality of laminar piezoelectric elements  111  are laminated through laminar electrodes  112 . The laminar electrodes  112  are alternately connected to common electrodes  1121  and individual electrodes  1122  that are formed at side faces of the piezoelectric element  100 . The common electrodes  1121  and the individual electrodes  1122  are connected to common electrodes  1121 ′ and individual electrodes  1122 ′ which are formed on the piezoelectric element supporting substrate  113 , respectively. The common electrodes  1121 ′ and the individual electrodes  1122 ′ are connected to flexible cable terminals  161  of a flexible cable  160 . 
   The recording head  10  with such configuration is driven by a signal sent from the recording head driving device  20  through the flexible cables  160 . The recording head driving device  20  includes a timing signal generating circuit  301 , a recording signal generating circuit  302 , a driving signal generating circuit  303 , a switching circuit  304  and a driving voltage generating circuit  305 . 
   The recording signal generating circuit  302  generates a recording data signal according to recording signal input data sent from a host device not shown (for example, a personal computer). Based on the data signal and a timing signal sent from the timing signal generating circuit  301 , the driving signal generating circuit  303  generates a driving data signal. The driving data signal controls turning ON/OFF of switching elements  3041  of the switching circuit  304 . Since the switching elements  3041  are connected to the driving voltage generating circuit  305  that is a voltage source, a piezoelectric element driving pulse is applied to the piezoelectric elements  110  according to turning ON/OFF of the switching elements  3041 . Thereby, the piezoelectric elements  110  connected to the switching elements  3041  that is turned ON are charged or discharged and driven by the piezoelectric element driving pulse to discharge ink drops. 
   When a polarization voltage (for example, 45 to 100 V) larger than a driving voltage for discharging an ink drop (for example, about 25 V) is applied between the common electrodes  1121  and the individual electrodes  1122  and the application is stopped, as shown in  FIG. 2 , residual polarization  1123  occurs in each piezoelectric element  111 . In this embodiment, in an initial state, the residual polarization  1123  in each piezoelectric element  110  is assumed to be equal. By varying the magnitude of the residual polarization  1123 , the ink drop discharge speed can be varied. The magnitude of the residual polarization can be adjusted by varying, for example, the magnitude of the polarization voltage and temperature at polarization. In this embodiment, the poralization voltage is varied under a constant temperature (repolarization) to adjust the polarization level of the piezoelectric elements. 
   Specifically, in this embodiment, as shown next to the piezoelectric elements  110  in  FIG. 1 , the polarization level is set for each piezoelectric element  110  in a phased manner. That is, in the recording head or recording device, when variations in the ink drop discharge speed among the nozzles  131  fall within the range of “A”, the variations are defined to be allowable. In the case where the nozzles  131  are uniformly polarized at a certain polarization level, a group consisting of nozzles  131  having the ink drop discharge speed within the range “A” is defined as a group G 0  and the polarization level of the piezoelectric elements corresponding to the nozzles  131  in the group G 0  is collectively set as the same level b 0 =b60. Nozzles  131  having the ink drop discharge speed outside the range “A” are divided into a plurality of groups G +1-+n  and G −1-−n  depending on the magnitude of deviation of the ink drop discharge speed from the range “A”, and the polarization level of the piezoelectric elements corresponding to these nozzles  131  is adjusted to the same level b n  (b +1 =b50, b +2 =b45, b −1 =b75, b −2 =b100) for each group so that the ink drop discharge speed of the nozzles  131  in the group G n  may fall within the range “A”. 
     FIG. 3  is a view showing a polarizing device  400  for polarizing the piezoelectric elements  110 . The polarizing device  400  includes a polarization data generating circuit  401 , a polarization voltage generating circuit  402  and a polarizing switching circuit  403 . The polarization data generating circuit  401  receives polarization data from a host device not shown (for example, a personal computer) and controls the polarization voltage generating circuit  402  and the polarizing switching circuit  403  to turn ON polarizing switching elements  4031  connected to the nozzles  131  to be polarized. Thus, a predetermined magnitude of polarization voltage is applied to predetermined nozzles  131 . In this manner, the piezoelectric elements  110  corresponding to the nozzles  131  belonging to the same group can be collectively polarized. The polarizing device  400  may be provided integrally with or separately from the recording device  1 . 
   With reference to  FIG. 1  and  FIG. 4 , the polarization of the piezoelectric elements  110  in this embodiment will be described.  FIG. 4  is an explanation view of polarization of each piezoelectric element  110 . 
   In  FIG. 1 , a dotted line extending from each nozzle  131  downward represents a flying trajectory of the ink drop  30 . Circles located at the front ends of the dotted lines represent flying positions of the ink drops  30  after a lapse of a certain time period from the discharge from the nozzle  131 . White circles represent flying positions of the ink drops  30  before polarization adjustment of the piezoelectric elements  110 . Black circles represent flying positions of the ink drops  30  after polarization adjustment of the piezoelectric elements  110 . The flying position is represented by only black circle, which means that the flying position of the ink drop  30  remains unchanged before and after polarization adjustment. The dotted line connecting the circles in the horizontal direction is a reference line for clarification of variations in the flying positions of the ink drops  30  before polarization adjustment and the solid line is a reference line for clarification of variations in the flying positions of the ink drops  30  after polarization adjustment. 
     FIG. 4A  is a graph showing ink drop discharge speed characteristics with respect to repolarization voltage. A horizontal axis represents the repolarization voltage applied between the individual electrode  1122  and the common electrode  1121  of the piezoelectric element  110 , and a vertical axis represents the ink drop discharge speed. The driving voltage of the piezoelectric elements  110  is kept as a predetermined voltage (25V) so that an average value of the ink discharge speed may be about 8 m/s. 
     FIG. 4B  is a graph showing an example of variation characteristics of the ink discharge speed among the nozzles  131 . A horizontal axis represents nozzle numbers and a vertical axis represents the ink discharge speed. The nozzle numbers in  FIG. 4B  correspond to five nozzles  131  from the left end of the recording head in  FIG. 1 , respectively. A dotted line connecting speed data plots of the nozzles  131  to each other in the horizontal direction is a reference line for clarification of variations in the ink discharge speed before polarization adjustment and the solid line is a reference line for clarification of variations in the ink discharge speed after polarization adjustment. 
   As apparent from  FIGS. 4A and 4B , the ink discharge speed values before polarization adjustment vary centering on about 8 m/s. The discharge speed of the nozzle number  1  before polarization adjustment is about 7.5 m/s and the discharge speed of the nozzle number  3  before polarization adjustment is about 8.0 m/s. Since the values of the discharge speed are close to each other, the flying positions of the ink drops  30  in the discharging direction in  FIG. 1  also become close to each other. 
   On the other hand, the discharge speed of the nozzle numbers  2  and  4  before polarization adjustment is faster than 9 m/s. For this reason, the flying positions of the ink drops  30  discharged from these nozzles  131  are located closer to a recording medium  40  than the flying positions of the ink drops  30  discharged from the nozzle numbers  1  and  3 . On the contrary, the discharge speed of the nozzle numbers  5  and  6  is slower than 7.2 m/s. For this reason, the flying positions of the ink drops  30  discharged from these nozzles  131  are located closer to the nozzle  131  than the flying positions of the ink drops  30  discharged from the nozzle numbers  1  and  3 . 
   Since the recording device  1  performs recording by allowing the ink drops  30  to land while moving recording medium  40  with respect to the recording head  10 , recording quality deteriorates depending on variations in landing positions on the recording medium  40 . 
   In this embodiment, a range of 20% centering on 8 m/s (±10%) is specified as the allowable range “A” of variations in the ink drop discharge speed in order to ensure the recording quality of the recording device  1 . An allowable range “A′” of variations in the flying positions, that corresponds to the allowable range “A” of variations in the ink drop discharge speed ( FIG. 1 ), is also determined. 
   In the nozzle numbers  1  to  6  in this embodiment, the nozzle numbers  1  and  3  fall within the ranges “A” (“A′”), the nozzle numbers  2  and  4  fall outside the ranges “A” (“A′”) to the high speed side and the nozzle numbers  5  and  6  fall outside the range “A” (“A′”) to the low speed side. As shown in  FIG. 1 , some nozzles  131  subsequent to the nozzle number  7  fall outside the allowable range “A” (“A′”). 
   As shown in  FIG. 4 , for the nozzle number  2 , for example, if the piezoelectric element  110  is once released its polarization, repolarized at 45 V under a polarization ambient temperature of 80° C., and then driven at a driving voltage of 25 V, the ink drop  30  is discharged at about 8.3 m/s. If the piezoelectric element  110  is once released its polarization, repolarized at 100 V, and then driven at a driving voltage of 25 V, the ink drop  30  is discharged at about 11.2 m/s. 
   Thus, by varying the polarization voltage from 45 V to 100 V when the driving voltage is 25 V, the ink drop discharge speed can be varied from 8.3 to 11.2 m/s. The ink drop discharge speed (about 10.2 m/s) at the repolarization at 60 V is almost equal to that before the repolarization. In other words, the polarization level at initial polarization is nearly equal to that at the repolarization at 60 V. 
     FIG. 4A  also shows repolarization characteristics of the nozzle numbers  3  and  5 . This confirms that the ink drop discharge speed can be adjusted by adjusting the repolarization voltage and that the polarization level at initial polarization is nearly equal to that at the repolarization at 60 V. 
   In consideration with the above-mentioned characteristics, in this embodiment, polarization adjustment is carried out as follows. 
   The piezoelectric elements  110  corresponding to the nozzles  131  that fall within the allowable range “A” (within “A′” in  FIG. 1 ) (in this embodiment, the nozzles  1 ,  3 ,  7 ,  9 ,  12  and  13 ) are not repolarized. Alternatively, under a polarization ambient temperature of 80° C., for example, the piezoelectric elements  110  are polarized at a repolarization voltage 60V, which corresponds to the initial polarization state, and the polarization level b 0  is set as b 0 =b60. Here, providing that the recording head  10  is assembled with the same components and by the same manufacturing process, the initial polarization state of the piezoelectric element  110  is almost same as that of the other piezoelectric elements  110 . Therefore, in this embodiment, for example, if the repolarization voltage applied to the nozzle number  1  is 60 V, the nozzle numbers  3 ,  7 ,  9 ,  12  and  13  are also repolarized at 60 V. It is unnecessary to measure the repolarization voltage for each nozzle. 
   Next, adjustment of the piezoelectric elements  110  corresponding to the nozzles  131  that fall outside the allowable range “A” (“A′”) will be described. 
   First, the nozzles  131  that fall outside the allowable range “A” are divided into a plurality of groups depending on the magnitude of deviation from the allowable range “A” of variations in the ink drop discharge speed. In this embodiment, as shown in  FIG. 4 , the nozzles are grouped as follows: a group of nozzles that deviates by 10 to 20% from 8 m/s on the high-speed side is defined as G +1 , a group of nozzles that deviates by 20 to 30% from 8 m/s on the high-speed side is defined as G +2 , a group of nozzles that deviates by 10 to 20% from 8 m/s on the low-speed side is defined as G −1  and a group of nozzles that deviates by 20 to 30% from 8 m/s on the low-speed side is defined as G −2 . 
   The piezoelectric elements  110  corresponding to the nozzle numbers  4  and  15  in the group G +1  are polarized at a repolarization voltage of 50 V to adjust the polarization level b +1  to b50. The piezoelectric elements  110  corresponding to the nozzle numbers  2  and  14  in the group G +2  are polarized at a repolarization voltage of 45 V to adjust the polarization level b +2  to b45. The piezoelectric elements  110  corresponding to the nozzle numbers  6 ,  8  and  10  in the group G −1  are polarized at a repolarization voltage of 75 V to adjust the polarization level b −1  to b75. The piezoelectric elements  110  corresponding to the nozzles  5 ,  11 ,  17  and  18  in the group G −2  are polarized at a repolarization voltage of 100 V to adjust the polarization level b −2  to b100. 
   For example, the nozzle number  2  belonging to the group G +2  is repolarized at 45 V. As apparent from  FIG. 4A , when the nozzle number  2  is repolarized at 45 V, the ink drop discharge speed from the nozzle number  2  can be decreased from about 10.2 m/s to about 8.3 m/s. That is, the ink drop discharge speed can be decreased by about 25% from 8 m/s. The nozzle number  5  belonging to the group G −2  is repolarized at 100 V. When the nozzle number  5  is repolarized at 100 V, the ink drop discharge speed from the nozzle number  5  can be increased from about 5.7 m/s to about 7.7 m/s. That is, the ink drop discharge speed can be increased by about 25% from 8 m/s. In this manner, the ink drop discharge speed from each nozzle  131  can be adjusted to fall between the allowable range “A”. 
     FIG. 5  is a graph showing relationship between the repolarization voltage and correction amount of the ink drop discharge speed. In the recording head  10  in this embodiment, the ink drop discharge speed with respect to the repolarization voltage can be decreased by about 25% at a repolarization voltage of 45 V and by about 15% at 50V from 8 m/s, and can be increased by about 15% at 75 V and by about 25% at 100 V from 8 m/s. As described above, when the piezoelectric element  110  is repolarized at a repolarization voltage of 60 V, the ink drop discharge speed from the nozzle  131  corresponding to the piezoelectric element  110  is almost the same as the speed in the initial state. Utilizing this feature, the ink drop discharge speed from the nozzle numbers  4  and  6  can be adjusted to be within the allowable range “A”. Polarization level adjustment values of the piezoelectric elements  110  determined on the basis of the above-mentioned polarization adjustment are described next to the piezoelectric elements  110  in  FIG. 1 . 
   However, even when the repolarization adjustment is performed by applying the same voltage to each piezoelectric element  110  of a plurality of nozzles  131  having the same discharge speed with the polarization level b 0 , the actual ink drop discharge speed varies from a target ink drop discharge speed. Variations a by which the ink drop discharge speed can vary from the target ink drop discharge speed are shown in  FIG. 4B . 
   It is found by experiments that the variation “α” caused by variations due to individuality of the recording head  10  and nozzle  131 , and reproducibility of repolarization, etc. falls within the range of about 8 m/s.+−.5%, if the recording head  10  with the same configuration and specification is assembled with the same components and by the same manufacturing process. In this embodiment, relationship between the variation “α” and the allowable range “A” is set so as to A&gt;α. Width “W” that indicates deviation of each group from the variation allowable range “A” is set so as to W≦(A−α). For example, in this embodiment, it is set as α=10% (.+−.5%), A=20% (.+−.10%) and W=10%. 
   Thus, even when the slowest ink drop discharge speed among the group G +1  is decreased at a maximum, that is, the slowest ink drop discharge speed (+10%) is decreased by 15% through polarization adjustment and by 5% through the variation “α”, the ink drop discharge speed is decreased by 10(−10=10−15−5)% from the reference speed 8 m/s and falls within the allowable range “A”=20% (±10%). Even when the fastest ink drop discharge speed among the group G +1  is decreased at a minimum, that is, the fastest ink drop discharge speed (+20%) is decreased by 15% through polarization adjustment and increased by 5% through the variation “α”, the ink drop discharge speed is increased by 10(10=20−15+5)% from the reference value 8 m/s and falls within the allowable range “A”=20% (±10%). Similarly, all nozzles  131  in the other groups fall within the allowable range “A”=20% (+10%).  FIG. 1  shows the state where the flying positions of ink drops discharged from all nozzles  131  fall within the range “A′” through this adjustment and that variations in the ink discharge speed are greatly improved. 
   When the recording head  10  in this embodiment is manufactured in this manner, time and effort necessary for polarization adjustment can be greatly reduced for the following reasons. First, since the piezoelectric elements  110  corresponding to nozzles  131  forming an arbitrary group can be polarized at one time by collectively applying the same repolarization voltage thereto, polarization adjustment can be finished for a short time. 
   For example, when the nozzles  6 ,  8 ,  10  and  16  forming the group G −1  are collectively polarized as shown in  FIG. 3 , polarizing switching elements  4031  connected to the nozzles  6 ,  8 ,  10  and  16  are closed and a polarization voltage of 75 V is applied to the nozzles  6 ,  8 ,  10  and  16 . Thereby, since the piezoelectric elements  110  corresponding to the nozzles  6 ,  8 ,  10   16  can be collectively polarized, repolarization processing is finished much faster than the case where polarization voltage is individually applied to each piezoelectric element  110  as conventional. Furthermore, it is possible to omit repolarization of the nozzles in the group G 0  within the allowable range “A” of the ink drop discharge speed. Thus, the number of nozzles to be subjected to polarization adjustment can be greatly reduced and thus, time and effort for polarization adjustment of the whole head can be greatly reduced. 
   Second, time and effort necessary for determining appropriate adjustment voltage value for repolarization can be greatly reduced. In other words, if one recording head has the same configuration and specification as another recording head, the discharge speed characteristic of one recording head is same as that of another recording head that has been determined for one recording head. Thus, since data on discharge speed characteristics with respect to the polarization level as shown in  FIG. 5  is collected in advance, it is not necessary to determine the discharge speed characteristic with respect to another recording heads. Since an appropriate adjustment voltage value for repolarization is determined based on the data, time and effort can be greatly reduced. 
   In the above-mentioned embodiment of the present invention, the nozzles  131  are divided into five groups. However, the number of groups is not limited to five. As the width of the group is smaller and the number of groups is larger, adjustment accuracy can be improved. As the width of the group is larger and the number of groups is smaller, time and effort for polarization adjustment can be reduced more. 
   Next, a second embodiment of the present invention will be described with reference to  FIG. 6  and  FIG. 7 .  FIG. 6  is a view for describing a repolarization adjustment in the second embodiment. The present embodiment is different from the first embodiment in the directions of accelerating and decelerating the ink drop discharge speed by repolarization adjustment. The piezoelectric elements  110  in the present embodiment are adjusted so that the ink drop discharge speed only decelerates, while the piezoelectric elements  110  are adjusted in the first embodiment so that the ink drop discharge speed both accelerates and decelerates. In other words, the target ink discharge speed is set at an ink discharge speed of the nozzle  131  whose ink discharge speed is the slowest in all of the nozzles  131 . Hereinafter, polarization adjustment of the piezoelectric elements  110  will be described in the case where the target ink discharge speed is 6.8 m/s when the piezoelectric elements  110  are driven at 23 V. 
     FIG. 7  shows measurement results of polarization adjustment, that is, deceleration and acceleration level of ink drop discharge speed with respect to repolarization voltage at normal temperatures. When a repolarization voltage is 100 V, the speed adjustment amount becomes 0, which corresponds to the polarization level at initial polarization. When the recording head  10  can be subjected to repolarization adjustment at normal temperatures, application of a voltage of 100 V or more may cause a problem in terms of the withstand voltage of the piezoelectric elements  110 . For this reason, in the present embodiment, only deceleration adjustment at 100 V or less is performed. 
   If one recording head has the same configuration and specification as another recording head  10 , both of the recording heads have almost the same characteristics as shown in  FIG. 7 . In this embodiment, the variation “α” is set as 8.8% (.+−.4.4%), the allowable range “A” is set from 6.3 to 7.3 m/s, that is, as 14.8% (.+−.7.4%), and the width “W” is set as 6% so as to meet A&gt;α and W≦(A−α). As shown in  FIG. 6 , the nozzles  131  of the recording head  10  are divided into groups G 0  to G 6 . 
   Subsequently, an adjustment deceleration value V +n  for the group G +n  required in order to fall the fastest ink drop discharge speed among the group G +n  within the allowable range “A” is acquired according to the following equation: V +n =(the highest speed in G +n )−A/2+α/2. Accordingly, for example, “V +1 ” becomes 10.2 (13.2−14.8/2+8.8/2)%. Since a repolarization voltage for decreasing the speed by 10.2% is found to be 80 V as shown in  FIG. 7 , piezoelectric elements  110  corresponding to the nozzles  131  in G +1  are polarized at a repolarization voltage of 80V. Similarly for the other groups G +2  to G +6 , the repolarization voltage and the polarization level are set as shown in  FIG. 6 . Thereby, the ink drop discharge speeds from all nozzles  131  can fall within the allowable range 6.3 to 7.3 m/s. Further, since the recording head  10  can be subjected to repolarization adjustment at normal temperatures, manufacturing is facilitated and productivity is also improved. 
   Next, a third embodiment of the present invention will be described with reference to  FIG. 8 .  FIG. 8  is a view for describing the third embodiment. The present embodiment is different from the above-mentioned embodiments in that the ink drop speed deviation width “W” varies depending on the group of nozzles  131 . It is possible that the variations “α” becomes larger as the polarization adjustment amount is increased. Thus, the width “W” becomes smaller as the groups deviate from the allowable range “A” in the present embodiment. Given that the variations in the ink drop discharge speed adjustment amount with respect to G n  is defined as α n  (in this embodiment, α +1 , α +2 , α +3 , and α +4  are set at 5.8%, 10.3%, 13.1% and 14.7% respectively), since the width “W n ” must be equal to or smaller than (α−α n ), maximum values of “W +1 ” of G +1 , “W +2 ” of G +2 , “W +3 ” of G +3  and “W +4 ” of G +4  become 11.8 (=17.6−5.8)%, 7.3 (=17.6−10.3)%, 4.5 (=17.6−13.1)% and 2.9 (=17.6−14.7)% respectively. 
   By assigning the width “W n ” and the variation “α n ” to the following equation: V +n =(the highest speed in G +n )−A/2+α/2, a required adjustment deceleration value “V +n ” is obtained. The repolarization voltage for each group is obtained from  FIG. 7 . As shown in  FIG. 8 , the repolarization voltage and the polarization level are set. Thereby, all nozzles  131  can fall within the allowable range 6.2 to 7.4 m/s of variations in the ink drop discharge speed of the nozzles in the recording head. 
   In this embodiment, since a lot of nozzles can be simultaneously subjected to repolarization adjustment by reducing the number of groups while ensuring adjustment accuracy, the productivity of the recording head can be improved. In the above-mentioned embodiments, the on-demand ink jet recording head according to so-called push-type piezoelectric element system is used. However, an on-demand ink jet recording head having the configuration in which plate-like piezoelectric elements are formed on a diaphragm face, that is, according to so-called bend-type piezoelectric element system, may be used. 
   In the above-mentioned embodiments, the ink drop discharge speed is adjusted through polarization adjustment. However, it is well-known that the ink drop discharge amount can be also adjusted by adjusting the repolarization voltage. Therefore, in an embodiment in which the ink drop discharge speed in the above-mentioned embodiments is replaced with the ink drop discharge weight, similarly, the ink drop discharge weight can be adjusted with less time and effort and a recording head with small variations in the ink drop discharge weight can be manufactured with high productivity. 
   While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.