Image forming apparatus

Abstract

An image forming apparatus includes a nip forming member and a transfer bias output device that outputs a transfer bias to form a transfer electric field in a transfer nip between the nip forming member and an intermediate transfer member. Upon transfer of a composite toner image including a particular toner image onto a recording medium in the transfer nip, the transfer bias output device outputs the transfer bias including a first superimposed bias in which a direct current (DC) component is superimposed on an alternating current (AC) component. Upon transfer of the composite toner image without the particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs one of the transfer bias including a second superimposed bias having a peak-to-peak value of the AC component smaller than that of the first superimposed bias and the transfer bias including only the DC component.

What is claimed is: 
     
       1. An image forming apparatus, comprising:
 a plurality of image bearing members to bear a toner image on a surface thereof; 
 an intermediate transfer member disposed facing the plurality of image bearing members, onto which toner images on the plurality of image bearing members are transferred such that they are superimposed one atop the other forming a composite toner image; and 
 a transfer device to transfer the composite toner image formed on the intermediate transfer member onto a recording medium, the transfer device including
 a nip forming member to contact the intermediate transfer member to form a transfer nip; and 
 a transfer bias output device to output a transfer bias to form a transfer electric field in the transfer nip, 
 
 wherein upon transfer of the composite toner image including a particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs the transfer bias including a first superimposed bias in which a direct current (DC) component is superimposed on an alternating current (AC) component, and 
 wherein upon transfer of the composite toner image without the particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs one of the transfer bias including a second superimposed bias having a peak-to-peak value of the AC component smaller than that of the first superimposed bias and the transfer bias including only the DC component. 
 
     
     
       2. The image forming apparatus according to  claim 1 , wherein the particular toner image comprises a toner image formed with a special color toner other than yellow, magenta, cyan, and black. 
     
     
       3. The image forming apparatus according to  claim 2 , wherein the special color toner is a transparent toner. 
     
     
       4. The image forming apparatus according to  claim 1 , wherein upon transfer of the composite toner image including the particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs the transfer bias having an absolute value of a peak-to-peak value of the AC component equal to or greater than four times the absolute value of the DC component. 
     
     
       5. The image forming apparatus according to  claim 1 , wherein upon transfer of the composite toner image including the particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs the transfer bias in which an average potential of the superimposed bias in every cycle of the AC component has a transfer polarity that causes the toner to move electrostatically from the intermediate transfer member side to the nip forming member side in the transfer nip, and the toner moves more easily from the intermediate transfer member side to the nip forming member side with the average potential than with a center value between a maximum value and a minimum value of the first superimposed bias. 
     
     
       6. The image forming apparatus according to  claim 1 , wherein upon transfer of the composite toner image including the particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs the transfer bias having a relation of
   f>2/(w/v), 
 where f is a frequency (Hz) of the AC component, w is a width of the transfer nip in the direction of movement of the intermediate transfer member, and v is a linear velocity (mm/sec) of the intermediate transfer member. 
 
     
     
       7. The image forming apparatus according to  claim 1 , wherein upon transfer of the composite toner image without the particular toner image onto the recording medium in the transfer nip, the transfer bias output device outputs the transfer bias having only the DC component. 
     
     
       8. The image forming apparatus according to  claim 1 , wherein in a case in which an area of the composite toner image at which the particular toner image is formed in the direction of movement of the intermediate transfer member is in the transfer nip, the transfer bias output device outputs the transfer bias including the first superimposed bias in which the DC component is superimposed on the AC component, and
 wherein in a case in which the area of the composite toner image at which the particular toner image is formed in the direction of movement of the intermediate transfer member is not present in the transfer nip, the transfer bias output device outputs the transfer bias including only the DC component. 
 
     
     
       9. The image forming apparatus according to  claim 1 , wherein an amount of toner adhesion of the particular toner image on the recording medium per unit area is equal to or greater than 0.5 mg/cm 2 . 
     
     
       10. The image forming apparatus according to  claim 1 , wherein the intermediate transfer member is a belt formed into an endless loop and has a tensile modulus of equal to or greater than 2 GPa. 
     
     
       11. An image forming apparatus, comprising:
 a plurality of image bearing members; 
 an intermediate transfer member onto which toner images on the plurality of image bearing members are transferred to form a composite toner image; and 
 a power source to output a transfer bias to transfer the composite toner image from the intermediate transfer member onto a sheet, 
 wherein the power source outputs a first bias including an AC component when the composite toner image includes a particular color toner, and the power source outputs a second bias including only a DC component when the composite toner image does not include the particular color toner. 
 
     
     
       12. The image forming apparatus according to  claim 11 , wherein the particular color toner is a transparent toner. 
     
     
       13. The image forming apparatus according to  claim 11 , wherein the transfer bias output from the power source is determined on every printing page. 
     
     
       14. The image forming apparatus according to  claim 11 , wherein the first bias is a superimposed bias in which a DC component is superimposed on the AC component. 
     
     
       15. An image forming apparatus, comprising:
 a plurality of image bearing members; 
 an intermediate transfer member onto which toner images on the plurality of image bearing members are transferred to form a composite toner image; and 
 a power source to output a transfer bias to transfer the composite toner image from the intermediate transfer member onto a sheet, 
 wherein the power source outputs a first bias including a first AC component when the composite toner image includes a particular color toner, and the power source outputs a second bias including a second AC component having a peak-to-peak value smaller than that of the first AC component when the composite toner image does not include the particular color toner. 
 
     
     
       16. The image forming apparatus according to  claim 15 , wherein the particular color toner is a transparent toner. 
     
     
       17. The image forming apparatus according to  claim 15 , wherein the transfer bias output from the power source is determined on every printing page. 
     
     
       18. The image forming apparatus according to  claim 15 , wherein the first bias is a first superimposed bias in which a first DC component is superimposed on the first AC component, and the second bias is a second superimposed bias in which a second DC component is superimposed on the second AC component. 
     
     
       19. An image forming apparatus, comprising:
 a plurality of image bearing members respectively bearing a yellow toner, a magenta toner, a cyan toner, a black toner and a metal color toner; 
 an intermediate transfer member onto which toner images on the plurality of image bearing members are transferred to form a composite toner image; and 
 a power source to output a transfer bias to transfer the composite toner image from the intermediate transfer member onto a sheet, 
 wherein the power source outputs a the transfer bias including includes a DC component in superimposition with an AC component when the composite toner image includes a transparent the metal color toner, and 
 wherein the transfer bias includes the DC component and does not include the AC component when the composite toner image does not include the metal color toner. 
 
     
     
       20. The image forming apparatus according to  claim 19 , wherein an amount of toner adhesion of the transparent metal color toner on the sheet per unit area is equal to or greater than 0.5 mg/cm 2 . 
     
     
       21. The image forming apparatus according to  claim 19 , wherein the bias is a superimposed bias in which a DC component is superimposed on the AC component. 
     
     
       22. The image forming apparatus according to claim  21  19, the transfer bias is a superimposed that bias has an absolute value of a peak-to-peak value of the AC component equal to or greater than four times the absolute value of the DC component. 
     
     
       23. The image forming apparatus according to  claim 19 , wherein the power source outputs the bias when the composite toner image includes one of a line image or a character image of the transparent metal color toner that is formed on a solid background image of toner of a single color or a composite of at least two colors among yellow, magenta, cyan, and black. 
     
     
       24. The image forming apparatus according to claim 11, wherein the particular color toner is a metal color toner. 
     
     
       25. The image forming apparatus according to claim 15, wherein the particular color toner is a metal color toner. 
     
     
       26. The image forming apparatus according to claim 19, wherein the metal color toner is polyester-based toner. 
     
     
       27. The image forming apparatus according to claim 23, wherein the metal color toner forms a solid background image of toner of a single color or a composite of at least two colors among yellow, magenta, cyan and black. 
     
     
       28. The image forming apparatus according to claim 19, wherein the plurality of image bearing members are arranged side by side. 
     
     
       29. The image forming apparatus according to claim 19, wherein a polarity of the transfer bias alternates between a positive polarity and a negative polarity in the superimposition of the AC component and the DC component. 
     
     
       30. The image forming apparatus according to claim 19, wherein upon transfer of the toner image onto the sheet, the power source outputs the transfer bias in which an average potential of the superimposed bias in every cycle of the AC component has a transfer polarity that causes toner to move electrostatically from an intermediate transfer member side to a nip forming member side in a transfer nip. 
     
     
       31. The image forming apparatus according to claim 30, wherein the toner moves from the intermediate transfer member side to the nip forming member side with the average potential than with a center value between a maximum value and a minimum value of the superimposed bias. 
     
     
       32. An image forming apparatus, comprising:
 an image bearing member; and   a power source to output a transfer bias to transfer a toner image onto a sheet,   wherein the power source outputs a bias including an AC component when the toner image includes a metal color toner,   wherein the bias is a superimposed bias in which a DC component is superimposed on the AC component, and   wherein upon transfer of the toner image onto the sheet, the power source outputs the transfer bias in which an average potential of the superimposed bias in every cycle of the AC component has a transfer polarity that causes toner to move electrostatically from an intermediate transfer member side to a nip forming member side in a transfer nip, and the toner moves from the intermediate transfer member side to the nip forming member side with the average potential than with a center value between a maximum value and a minimum value of the superimposed bias.