Toner Distribution Process

Abstract

Toner is distributed and charged on an impression development surface by contact with at least one doctor blade electrically insulated relative to other parts of the apparatus and made of material remote from the toner in the triboelectric series and close to the impression development surface material in the triboelectric series. The toner particles are charged because of contact with the doctor blade.

Description

FIELD OF THE INVENTION

The present invention is concerned with the distribution of toner particles in impression development electrophotography.

As is well known to persons skilled in the art, electrostatic printing involves the production of electrostatic images on the surface of a dielectric member of the like, the application of charged minute electroscopic pigmented toner particles to the image bearing surface, and the fixing of the developed image before or after transfer to a print medium, such as paper. VArious techniques have been devised for applying the toner particles to the image bearing surface. Among these is the technique sometimes called impression development. The toner distribution process of the present invention particularly relates to use in impression development.

In impression development, the surface of a carrier member, such as a sheet or roller, is coated or impregnated with a quantity of minute electroscopic pigmented toner powder particles, i.e., toner, to form an image developing surface. The developing surface is then placed against the surface of the electrostatic image bearing member. The placing of the developing surface of the image bearing surface is performed so that virtually no relative peripheral speed exists during the contact between the developer and image-bearing surfaces. Thus, there is substantially no wiping, patting, or other motion during contact as is usually found in other techniques (e.g., brush or cascade development) for applying the toner to the image-bearing surface.

PRIOR ART

U.S. Pat. No. 3,152,012, issued Oct. 6, 1964 to R. M. Schaffert, and the references cited therein, summarize the prior art. In that patent, toner particles are carried to the impression development surface by means of a transfer surface. The patent teaches (column 7, line 50) that a doctor blade may be used to treat the transfer surface so as to distribute the toner. That procedure is in sharp distinction to the procedure of the present invention, according to which at least one doctor blade is used directly on the impression development surface. In this way the need for a transfer surface is avoided, and a considerable saving of machinery and space is obtained.

SUMMARY OF THE INVENTION

According to the process of the present invention toner particles are uniformly distributed and charged on the impression development surface by contacting the toner particles and that surface with at least one doctor blade having certain specified properties. The doctor blade simultaneously performs four functions:

1. It erases the residual complementary image from the previous development;

2. it removes excess toner;

3. it distributes the toner uniformly and reproducibly, and

4. it triboelectrically charges the toner to a polarity opposite to that of the image on the photoconductor.

The process of the present invention may be understood more readily by reference to the descriptive drawings.

DESCRIPTION OF THE DRAWINGS

The FIGURE shows diagrammatically (not according to scale) a preferred embodiment of an apparatus suitable for use in the process of the present invention.

Referring in more detail to the FIGURE, an apparatus is shown having the impression surface in a drum form. The system comprises an impression surface on a resilient backing with a toner reservoir adjacent to the drum surface. A series of toner distribution doctor blades are also provided. The blades rest directly on the impression drum and are held in contact by springs. They are electrically insulated from the remainder of the apparatus. Toner is drawn under the blades as the drum rotates. Contact among the drum surface, the blades and the toner results in a uniform layer of triboelectrically charged toner on the impression surface of the drum. By rotation of the drum, the charged toner is then brought into contact with the photoconductor, where toner is selectively transferred to the latent electrostatic image, which is also rotating on a drum. In the process of the present invention, a single pass is sufficient to provide enough toner to give adequate image density. It is important that zero relative peripheral speed exist between the toner surface and the photoconductor, as is always the case in impression development.

The toner distribution process of the present invention is suitable for use with many impression development surfaces. Acceptable results have been obtained with a surface of matte finish aluminum. It is, however, particularly suited for use with the surfaces described in U. S. application Ser. No. 100,980 in the name of Leo S. Chang, filed on the same date as the present application. That application describes impression development surfaces which are resilient, conductive, rough and remote from the toner in the triboelectric series. An example of such a surface is one comprising small graphite particles dispersed in a copolymer of vinyl chloride and vinyl acetate. The surface is made resilient by means of a resilient substrate to insure optimal contact with the photoconductor.

For the process of the present invention, the doctor blade edge is in contact with the impression developer surface and is held there by, for example, spring means. The blade, however, is otherwise electrically insulated from the remainder of the apparatus.

A single blade of the proper type is sufficient. It has unexpectedly been found advantageous, however, to use a plurality of blades, particularly three blades. There is apparently no advantage to using more than three blades.

For the process of the present invention, it is essential that the blade be made of material remote from the toner in the triboelectric series. The blade must be close to the impression development surface material in the triboelectric series, but it should not be identical to the impression development surface material. The triboelectric series has been long known to those skilled in the electrophotographic art. It is discussed, for example, in an article by V. E. Shashoua in the Journal of Polymer Science, Vol. 33, pages 65-85, (1958). That article also describes a simple test for determining where a particular material should be placed in the triboelectric series.

Most toners in commercial use today are based on polymers or copolymers of materials such as styrene and methacrylate esters. For use with such toner, excellent results have been obtained using blades made of polytetrafluoroethylene (available from DuPont under the trademark Teflon). Polyformaldehyde (available from DuPont under the trademark Delrin) is particularly suitable for reversal development. Blades may also be made of composite resin or resin and filler materials, or of polymer coated metal.

If desired, bias voltage may be employed in the process of the present invention. This is a known concept in the art, and involves applying to the impression development surface a voltage of approximately the same magnitude and polarity as that of the light exposed (background) areas of the photoconductor. As mentioned previously, the use of bias voltage is known in the art, and is not an essential feature in the present invention. It is often desirable, however, to use it in conjunction with the process of the present invention.

In like manner, it is sometimes helpful to add the step of D.C. corona charging, for example, before, between or after the treatment with the doctor blades. D.C. corona charging enhances the charge of the toner, and reduces background.

Excellent results have been obtained using doctor blades having circular tips in contact with the impression development surface. These circular tips have a radius of approximately three-thirty-seconds of an inch.

Claims

What is claimed is:

1. A process for distributing and charging toner particles on an impression development surface, said process comprising subjecting said toner particles and surface to contact with at least one doctor blade edge otherwise electrically insulated from the apparatus and made of material remote from the toner in the triboelectric series and close to the impression development surface material in the triboelectric series, said contact between the toner particles and the doctor blade charging said toner particles.

2. A process as claimed in claim 1 wherein a plurality of blades are used.

3. A process as claimed in claim 1 wherein the blade is held in contact with the impression development surface by spring means.

4. A process as claimed in claim 1 wherein the blade is made of polytetrafluoroethylene.

5. A process as claimed in claim 1 wherein the blade is made of polyformaldehyde.