1.  Field of the Invention
The present invention relates to an image forming apparatus, and more specifically relates to an electrophotographic image forming apparatus used in laser printers and copying machines which form images by developing a toner image on an image bearing member and transferring said toner image to a transfer medium. 
2.  Description of the Related Art
In electrophotographic type image forming devices an electrostatic latent image is formed on a photoconductive member [a] and said electrostatic latent image is developed with toner so as to form a toner image which is then transferred to a transfer sheet using a transfer charger [b], as shown in FIG.  I. The transfer sheet is guided by a pair of top and bottom sheet guides [d] and [e] prior to the aforesaid transfer so as to properly transit the transfer portion. 
Transfer charger [b] is charged with the opposite polarity of the toner image by a corona discharge by a corona wire, and the toner image on the image bearing medium [a] is electrostatically attracted onto the transfer sheet so as to be transferred thereby.  Ozone and nitrogen oxides (NO.sub.x) are produced during the corona discharges of transfer charger [b] and the separation charger [c] disposed immediately anterior to said transfer charger [b], and because said ozone and nitrogen oxides (NO.sub.x) emissions produce detrimental effects during image formation the aforesaid transfer charger [b] and separation charger [c] are provided with a shield member [f] having an exhaust orifice [g] through which the aforesaid ozone and nitrogen oxides (NO.sub.x) are expelled by means of a suction fan [h]. 
On the other hand, developing devices [i], which develop the electrostatic latent image formed on the surface of photoconductive member [a], are provided between transfer portion [k] and exposure portion [j] where the aforesaid electrostatic latent image is formed on the surface of photoconductive member [a], and as spacing is a critical factor around the periphery of photoconductive member [a] said developing devices [i] are positioned immediately above pretransfer sheet guides [d] and [e] in the upstream direction of transfer portion [k]. 
The toner used in the aforesaid developing devices [i] forms an airborne powder cloud which is dispersed outside said developing devices [i].  The previously mentioned sheet guides [d] and [e] are soiled by the escaping airborne powder toner, and the residual toner remaining on said sheet guides soils the transfer sheet.  The suction action of the previously described suction fan [h] produces an associated airflow containing the dispersed toner escaping from the developing devices [i] around photoconductive member [a] and around chargers [b] and [c] and the like in the transfer portion.  Further, the aforesaid associated airflow draws the atmosphere containing airborne powder toner and the like from around developing devices [i] and draws said atmosphere through the space between photoconductive member [a] and sheet guides [d] and [e]. 
The previously described draft air is suctioned past chargers [b] and [c] in the transfer portion, and the toner dispersed in said draft air adheres to and soils the corona wires of chargers [b] and [c] in the transfer portion.  The aforesaid toner adhesion on the corona wires causing soiling reduces the output of chargers [b] and [c] in the transfer portion only at the soiled parts of said corona wires, thereby reducing transfer efficiency and adversely affecting separation characteristics. 
On the other hand, although the suction of the airborne powder toner and dispersed toner can be eliminated from the side of developing devices [i], when the number of developing devices is increased for color coding and/or colorization, it becomes difficult to install the suction means with proper spacing around the developing devices in a compact image forming device, thereby making it difficult to use a suction means.