CH636456A5 - ELECTROSTATIC PRINTING APPARATUS. - Google Patents

Description

The invention relates to an electrostatic printing device. The apparatus involves the formation of an electrostatic latent image on a dielectric surface carried by a cylinder and subsequent treatment.

To create charged particles and to extract them before applying them on another surface, one can use, for example, an ion production device which comprises a solid dielectric element, a first electrode on one of the sides of said solid dielectric element with an aerial breakdown zone in the vicinity of said electrode and of the surface not covered with said dielectric element, a second electrode on the other side of said solid dielectric element, means for applying an alternating potential having, for example, a sinusoidal, square or triangular shape with a frequency varying between 60 Hz and 4 MHz between the electrodes leading to an aerial breakdown.

Any of the elements of such a device, necessary for the formation of a latent electrostatic image on a dielectric surface, can be employed in the present electrostatic transfer printing apparatus according to the present invention.

The invention, in fact, proves to be particularly advantageous insofar as the same device can be used for a treatment subsequent to the formation of a latent electrostatic image.

According to the invention therefore, an electrostatic printing apparatus, combined with an ion production device, is characterized in that it comprises a cylindrical dielectric element, means for extracting the ions from said ion production device. ions to form a latent electrostatic image on the cylindrical dielectric element which is at least 25% apart. of the device for producing the ions, means for coloring the latent electrostatic image for obtaining a visible image, means for transferring the colored and visible image by pressing on a receiver which is engaged by contact between a roller transfer, coated with a plastic stress absorbing layer, and the cylindrical dielectric element, cleaning means for removing residual dye on the cylindrical dielectric element, and means for erasing any residual electrostatic image after printing by image transfer.

The electrostatic printing apparatus may, in addition to an ion production device, comprise a rotary drum composed of a conductive core carrying a dielectric layer, means for extracting the ions from said ion production device for the formation of a latent electrostatic image on said dielectric layer of thickness greater than 25 p., of resistivity greater than 1012 £ 2 / cm and an average roughness less than 0.025 p., means for coloring the latent electrostatic image, a rotary pressure roller in contact with the rotary drum for transferring and embedding the colored image on an image medium which passes between the drum and the roller along their tangential contact line, cleaning means such as two scraper blades adjacent to the drum and the roller for cleaning their surface after transfer of the colored image, and means for erasing the latent electrostatic image of the drum after transfer of the said image th image. These erasing means may, for example, include electrodes arranged on both sides of the dielectric layer and between which are caused high frequency alternating discharges, a conductor or a semiconductor to earth maintained in close contact with the dielectric surface, an earth conductor consisting of a highly charged blade scraper, an earth semiconductor consisting of a semiconductor roller.

Embodiments of the electrostatic printing apparatus according to the invention are described, by way of example, with reference to the appended drawings, in which:

fig. 1 is a schematic view of an electrostatic printing apparatus according to a preferred embodiment of the invention;

fig. 2 is a partial section view of a charge eraser for the printing apparatus according to the invention, and FIG. 3 is a partial section view of another construction of a charge eraser for the printing apparatus according to the invention. In FIG. 1, the dielectric layer 75 of the dielectric cylinder 73, the average roughness of which is less than 500 | i, must have sufficient electrical resistance to withstand a latent electrostatic image during the period between the formation of the latent image and the coloring. Consequently, the resistivity of layer 75 must be greater than 1012 Cl / cm. A desirable thickness of the dielectric layer 75 is between 0.025 and 0.075 mm. In addition, the surface of the layer 75 must have sufficient mechanical strength to resist abrasion and a low roughness, with an average finish better than 0.025 | i, so as to allow a total transfer of the color onto the receiving sheet. 81. The dielectric layer 75 also has a high elastic modulus so as not to be significantly deformed by the pressure of the transport clamp.

A number of organic and inorganic dielectric materials can be used for layer 75. Glass enamel, for example, can be deposited and liquefied on the surface of a steel or aluminum cylinder. A flame or plasma spray of high density aluminum oxide can also be used for

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place the glass enamel. Plastic materials, such as polyamides, polyimides and other rigid thermoplastic or thermosetting resins are also satisfactory. However, an advantageous dielectric coating is anodized, impregnated aluminum oxide. The dielectric cylinder 73 is placed at a distance of at least 25 n from the device 71 for producing and extracting ions. The latent electrostatic image formed on the dielectric surface 75 is transformed into an image visible at the coloring station 79.

Although any dye may be used, a preferred dye will be a single conductive magnetic component of the type described by JC Wilson in US Patent No. 2,846,333 issued August 5, 1958. This dye has the advantage of simplicity and cleanliness.

The colored image is transferred and embedded in a receiving sheet 81 by the high pressure obtained between the cylinder 73 and the roller) 5 83. The lower roller 83 consists of a metal core 87 which can receive a coating of industrial plastic 85. The pressure necessary for good overlay on untreated paper depends on factors such as, for example, the diameter of the rollers, the dye used and the presence of a coating on the surface of the paper. Usual pressures are of the order of 43.5 to 304.5 kg per linear centimeter of contact. The function of the plastic coating 85 is to absorb the extreme tensions appearing in the clamp in the event of a pile or crumpling of the paper, which avoids deterioration of the cylinder 73 coated with a dielectric layer. The covering 85 can be produced with a nylon or polyester sleeve having a thickness of between 0.3 and 1.25 cm. This coating is not necessary if, for example, a roll of paper is used for which heaps or creases are unlikely. 30

Blade scrapers 89 and 91 can be provided to remove any paper dust, accidentally dyed dye on the roller, dust or dirt from the dielectric pressure cylinder and the back pressure roller. Since normally the entire colored image is transferred to the sheet 81, the blade scrapers are not necessary, but they remain useful in allowing reliable use over a longer period.

The residual latent electrostatic image on the dielectric layer 75 after transfer of the colored image can be neutralized at the latent electrostatic image discharge station 93. The coloring and transport of a colored image on reduced untreated paper l importance of the electrostatic image, typically of several hundred volts to several tens of volts. In some cases, if the coloring threshold is too low, the presence of a residual latent electrostatic image would determine ghost images on the copy sheet, but they are eliminated at the discharge station 93.

In fig. 2, the dielectric cylinder 73 with a dielectric layer 75 is kept in contact with or at a short distance from a woven screen with open meshes 95, raised to a potential substantially identical to that of the cylinder 77. The screen is mounted on a bracket and a ring of 97 alternating current wire is placed behind the screen at a distance of approximately 0.6 to 1.25 cm. A high alternating potential, for example at 60 Hz, is applied to the wire 97. The screen 95 determines a reference plane to earth near the dielectric layer and the ring of wire 97 emits positive and negative ions. A local field on the screen 95, due to a latent electrostatic image on the dielectric layer 75, attracts ions emitted by the wire crown 97 on the dielectric layer, and thus neutralizes most of the residual electrostatic images. If the linear speed of the dielectric layer 75 is very high, the residual charge can lead to ghost images. In this case, several discharge stations make it possible to reduce the residual charge below the coloring threshold.

The residual latent electrostatic image can be erased by contact discharge. The dielectric surface can be kept in close contact with a grounded conductor or semiconductor so as to effectively remove any residual charge from the surface of the dielectric layer 75; for example, a heavily loaded metal scraper will be used. The charge can also be removed by a semiconductor roller maintained in close contact with the dielectric layer. Fig. 3 shows a partial sectional view of a semiconductor roller 98 in rolling contact with a dielectric layer 75. The roller 98 advantageously has an external elastomer surface.

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1 sheet of drawings

Claims (8)

636,456 1. An electrostatic printing apparatus combined with an ion production device, characterized in that it comprises a cylindrical dielectric element, means for extracting the ions from said ion production device in order to form an electrostatic image latent on the cylindrical dielectric element which is at least 25 µ from the ion generating device, means for coloring the latent electrostatic image to obtain a visible image, means for transferring the colored and visible image by pressing on a receiver which is engaged by contact between a transfer roller, coated with a plastic layer for absorbing stresses, and the cylindrical dielectric element, cleaning means for removing the residual dye on the dielectric cylindrical element, and means for erasing any residual electrostatic image after the image transfer printing. 2. Apparatus according to claim 1, characterized in that the dielectric cylindrical element has an average roughness less than 500 | i and a resistivity greater than 10 12 Si / cm. 2 3. Apparatus according to claim 1, characterized in that the cylindrical dielectric element is composed of a conductive core carrying a dielectric layer of thickness greater than 25 n, of resistivity greater than 1012 Cl / cm and of an average roughness of less than 0.025 (X. 4. Apparatus according to one of claims 1 to 3, characterized in that the cleaning means for removing the residual dye on the dielectric cylindrical element are two scraper blades. 5. Apparatus according to one of claims 1 to 3, characterized in that the means for erasing the latent electrostatic image after its transfer comprise electrodes disposed on both sides of the dielectric layer and between which are caused alternative high frequency discharges. 6. Apparatus according to one of claims 1 to 3, characterized in that the means for erasing the latent electrostatic image consist of a conductor or a semiconductor to the earth kept in contact with the surface of the dielectric layer. 7. Apparatus according to one of claims 1 to 3, characterized in that the means for erasing the latent electrostatic image are a ground conductor constituted by a scraper with charged blade. 8. Apparatus according to one of claims 1 to 3, characterized in that the means for erasing the latent electrostatic image are a semiconductor to the ground consisting of a semiconductor roller.