In modern equipment used for information processing, rapid printers in which printing of the characters is accomplished without causing relief-printing type to strike a sheet of receptive paper are being used more and more. These printing machines, known as non-impact printers, generally include a recording medium, most frequently comprising a rotating drum or an endless belt, which is provided with a recording surface on which sensitized zones corresponding to characters to be printed can be created by electrostatic or magnetic means, these zones being capable of attracting the solid particles of a powdered developer product.
Various applicator devices can be used for applying the solid developer particles to the recording carrier of a printing machine of this type. For example, the device which is described and shown in U.S. Pat. No. 3,161,544 may be utilized. However, despite all the care taken in realizing this device, it is difficult to prevent the developer particles from being deposited not only to excess on the sensitized zones of the recording medium but also, although in very slight quantity, outside these zones. This phenomenon has been attributed to the fact that if the particles are charged with humidity or static electricity, or if they undergo melting, which, however slight, makes them more or less sticky, they adhere to the surface with which they have been placed in contact. An excessive deposit of developer particles on the sensitized zones of the recording medium is undesirable because when this developer is transferred to the sheet of receptive paper, there is a risk that the developer that has been deposited in accordance with the configuration of the image formed by these sensitized zones may smudge and blur the image. On the other hand, the deposit of particles outside the sensitized zones of the recording medium is also undesirable, because when these particles are transferred to the paper they form a base which reduces the contrast between the transferred image and the original base of the paper.
In the prior art, various retouching devices have been used to eliminate excess developer on the surface of the recording medium. One example of a device of this kind has been particularly described in U.S. Pat. No. 3,816,799, in which a mass of developer particles, placed in contact with the surface of the recording medium and downstream of a device for applying the particles, discharges the particles which adhere to the recording medium outside the sensitized zones, such that these discharged particles become detached from the recording medium and clump together with this mass. In any event a retouching device of this kind is not completely satisfactory in use, because it does not always assure a complete electrical discharge of the particles and as a consequence does not enable the reliable elimination of the developer particles which remain on the recording medium outside the sensitized zones.
Another known retouching device is described in U.S. Pat. No. 3,643,629 and includes a magnetic assembly rotating in the interior of a fixed cylindrical sleeve. The assembly itself is embodied by a plurality of magnetic elements, in the form of a sector, disposed side by side about a rotating shaft. These elements are magnetized in such a way as to have peripheral magnetized zones on their peripheral surface, the magnetic polarity of which remains constant along a direction parallel to the shaft, but alternating in passing from one peripheral zone to the next. Nevertheless, with this retouching device in which the magnetic element accommodated in the interior of the fixed sleeve rotated past the recording medium in the opposite direction from that in which the medium is displaced, uniform removal of the excess particles is not obtained, because in certain parts of the medium the removal is too pronounced and in other parts it is insufficient.
This disadvantage can be overcome by using the retouching device described in U.S. Pat. No. 4,067,018, which enables the elimination of the excess developer particles on an endless magnetic tape on which a latent magnetic image has been produced. This retouching device includes a guide roller of radius R, over which the endless magnetic tape travels. When this tape is driven at a constant speed V, the developer particles located on this tape are subjected as they travel past the guide roller to the action of a centrifugal force, the value of which, for each particle of mass M, is equal to MV.sup.2 /R. Under these conditions, the intensity of this centrifugal force can be adjusted by suitably selecting the speed at which the tape is driven and the radius of the guide roller such that the value of the centrifugal force is greater than that of the forces retaining the particles on the zones at the base of the tape, yet is not so great as to be able to detach all the particles located on the magnetized zones of this tape. The excess developer particles which have thus been detached from the tape are then eliminated by means of an aspirator disposed near the guide roller.
It has been found that to remove the excess developer particles on this magnetic tape the acceleration imparted by this centrifugal force must be at least equal to 10 g, g being the value of the acceleration of gravity. As a result, in the case where the guide roller used in this retouching device has a diameter of twenty millimeters, the endless magnetic tape must be driven at a speed of at least one meter per second. A displacement speed on this order, however, means that this retouching device cannot be used in modern magnetic printing machines, in which the drive speed of the recording medium in practice does not exceed 30 cm/s. To enable the use of this retouching device in a printing machine of this kind, the diameter of the guide roller must accordingly be reduced, but to do so means that a guide roller must be selected the diameter of which must not exceed two millimeters. A guide roller of that size would necessarily be fragile, and its use in a magnetic printing machine would not offer all the security that is desired.