Apparatus for printing images on both sides of an image printing medium by one process

An electrostatic both-side printing system, comprising:

• first image forming means having a photoreceptor for forming a first toner image;
• second image forming means having a photoreceptor for forming a second toner image;
• a medium transporting path for transporting an image printing medium, said first image forming means being disposed in opposition to said second image forming means at a same point along said medium transporting path;
• first and second before-transfer corotrons provided facing said photoreceptors of said first and second image forming means, respectively, said photoreceptors having the same charge polarity, said before-transfer corotrons setting up a state that the charge polarity of a toner image on the photoreceptor of said first image forming means before the toner image is transferred onto the image printing medium is opposite to that of a toner image on said photoreceptor of said second image forming means;
• and
• potential-difference forming means for forming a potential difference between said photoreceptors of said first and said second image forming means so that the first toner image is transferred onto one of the sides of the image printing medium passing between said first and said second image forming means, and the second toner image is formed on the other side of the image printing medium;
• wherein said first and said second image forming means and said potential-difference forming means cooperate to form the first and the second toner images on both sides of the image printing medium, the charge polarities of the first and the second toner images being different from each other.

An electrostatic both-side printing system, comprising:

• first image forming means provided upstream of an image printing medium when viewed in the transporting direction of the image printing medium, said first image forming means having a photoreceptor;
• second image forming means provided downstream of the image printing medium, said second image forming means having a photoreceptor, said photoreceptor of said first image forming means being the same in charge polarity as said photoreceptor of said second image forming means;
• a medium transporting path formed between said first and said second image forming means, for transporting the image printing medium;
• a first transferring unit located facing said first image forming means with said medium transporting path intervening therebetween, said first transferring unit transferring a toner image from said photoreceptor of said first image forming means onto the image printing medium by applying no charge onto the rear side of the image printing medium, which does not face said first image forming means;
• a second transferring unit for transferring a toner image from said photoreceptor of said second image forming means onto the image printing medium, said second transferring unit being located facing said second image forming means with the image printing medium with said medium transporting path intervening therebetween, in a state that said second transferring unit is apart from a first toner image formed on the image printing medium by said first image forming means;
• charge quantity adjusting means for setting the charge polarity of a first toner image to be the same as the voltage polarity of said second transferring unit, said charge quantity adjusting means is a before-transfer corotron provided facing said photoreceptor of said first imaging forming means, whereby said first and said second image forming means, and said first and said second transferring units cooperate to form first and second toner images on both sides of the image printing medium, the first and second toner images being opposite in polarity.