US3640617A

US3640617A - Toner doctor means

Info

Publication number: US3640617A
Application number: US780026A
Authority: US
Inventors: Donald M Fredrickson; Johannes H Bungener
Original assignee: Bell and Howell Co
Current assignee: Bell and Howell Co
Priority date: 1968-11-29
Filing date: 1968-11-29
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

Doctoring apparatus for use in the developing station of an electrostatic copying machine for removing excess liquid toner from the developed image surface of the sheet prior to squeegeeing of the sheet. Removal of excess toner eliminates image trailing, haloing and offsetting.

Description

[451 Feb. 8, 1972 United States Patent Fredrickson et al.

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TONER DOCTOR MEANS This invention relates to electrostatic copying machines. ln particular, this invention relates to electrostatic copying machines having development station apparatus utilizing liquid toner developing techniques and means for eliminating the deleterious effects of trailing, haloing and offsetting, from the developed image surface,

The advantages of electrostatic copying machines using liquid toners rather than dry toning particles are well known in the art. These advantages include rapid development and a greater degree of control over the developing process. ln contrast to its advantages, liquid toners present vexatious problems not always encountered in using dry toner particles, including trailing, haloing and offsetting of the developed image. These problems are most apparent when a copy sheet is passed through the nip of a pair of drying rollers, and are thought to be related to excess liquid toner on the developed image surface. Therefore, this invention has particular relevance to apparatus for removing excess liquid toner from the developed image surface of a copy sheet prior to passage of the sheet through a pair of drying rollers.

Knowledge of the prior art reveals a reluctance to contact the image surface immediately subsequent to development of a copy sheet, when using liquid toner developing techniques. This fear is evidently founded on the theory that the toner particles comprising the image are not thoroughly adhered or held suiciently close to the image surface to allow physical contact with the image surface by means other than those which have a velocity substantially equal to that of the moving sheet. This invention indicates that this reluctance is unfounded.

Accordingly, the apparatus of this invention provides stationary means for sweeping the image surface of a copy sheet substantially immediately after its development as it emerges from the liquid toner through which it has been transported in the developing station of an electrostatic copying machine. The stationary means physically engage the developed image surface of the copy sheet for removing excess toner thus obviating the problem of trailing, haloing and offsetting incident to subsequent drying of the copy sheet,

Accordingly, a primary object of this invention is to provide developing station apparatus for removing excess liquid toner from the developed image surface of a copy sheet image.

Another object of this invention is to provide developing station apparatus for removing excess liquid toner from the developed image surface of a copy sheet immediately upon emergence the sheet from the toner for alleviating the conditions causing trailing, haloing and offsetting incident to subsequent drying ofthe copy sheet.

Additional objects of this invention will become apparent to those versed in the art of electrostatic copying machines from an understanding of the following detailed description of the construction of the developing station apparatus taken in conjunction with the accompanying drawings, in which preferred embodiments of the apparatus is shown, and wherein:

F lG. l is an elevational cross-sectional view of a developing station for an electrostatic copying machine incorporating the apparatus of the invention;

F lG. 2 is an illustration of the effects of trailing, haloing and offsetting, which the apparatus of the invention eliminates; and

FlG. 3 is a three dimensional perspective view of a modified embodiment ofthe invention.

As best seen in FIG. l, a developing station l suitable for use in an electrostatic copying machine includes a cover l2 having an opening 14 for passage of a copy sheet 16 therethrough. From the developing station, the copy sheet may be transported toa drying station, not shown, whereat the copy sheet may be thoroughly dried by electric heating means.

Copy sheet 16 comprises a substrate 18 having a photoconductive surface 20 upon which a latent electrostatic image has heen impressed prior to transport of the copy sheet to the developing station. The latent electrostatic image is impressed on the photoconductive surface by well known techniques of uniformly electrostatically charging the photoconductive surface and exposing the charged surface to an image pattern comprising light and dark areas.

The developing station further includes an arcuate receptacle 22 carrying a quantity of liquid toner or developer 24. The copy sheet generally conforms to the shape of the receptacle wall as it passes through the liquid and the liquid toner level is suitably maintained by an overflow barrier 23 and an adjacent liquid retention cavity 25. Liquid toner 24 comprises an organic liquid vehicle suspendingly carrying opaque developing particles having an electrical charge of a given polarity opposite the polarity forming the latent electrostatic image on photoconductive surface 20.

Any suitable copy sheet transport means may be used to move the copy sheet into and through the developing station. For example, a series of nip forming rollers not shown, may be used to transport the copy sheet into the liquid toner. As the copy sheet emerges from the toner it may be further transported by squeegee or drying rollers 26-28 for movement through passage 14. While the copy sheet passes through the liquid toner, the suspended, charged toner particles are electrically attracted to the oppositely charged latent electrostatic image on photoconductive surface 20 forming a developed image 30.

After passing through the liquid toner, the copy sheet emerges or breaks through the liquid surface along a line 3l. Toner removing means 32 are provided adjacent line 3l and adjacent the copy sheet for removing excess liquid toner from the developed image surface 20 almost immediately upon its emergence from the toner 24. The toner removing means comprise a thin doctor blade 34 having a length equal to the sheet width. The doctor blade is carried on a bracket 36 suitably secured to the walls of the copying machine, not shown, by fasteners such as fastener 38 and is held straight and rigid by an overlying elongated clamp or support 40. The blade is preferably fabricated of an electrically nonconductive material such as plastic so as not to electrically interfere with the previously impressed electrostatic image which may not have completely dissipated from photoconductive surface 20.

Bracket 36 and clamp 40 are positioned adjacent the image surface of the copy sheet and blade 34 extends from the bracket to the copy sheet so that an edge 35 thereof physically engages the developed image surface. As the copy sheet is transported along its path, engagement of blade 34 with the developed image surface removes, or sweeps away, excess or surplusage liquid toner carried on the photoconductive surface by surface adhesion.

Beyond the doctor blade, the copy sheet is engaged by drying rollers 26-28 which form a nip for squeeging the sheet member prior to its subsequent passage to a drying station. Squeeging the sheet through rollers 26-28 often causes trailing, haloing and offsetting, which the toner removing means 32 of the invention obviates.

Pictorial examples of the appearances of trailing, haloing and offsetting which the apparatus of the invention obviates, are shown in FIG. 2. Trailing is observed as a slight downstream continuation of the image pattern relative to the direction of movement of the copy sheet, while haloing is observed as a clean area adjacent the image pattern with an outer grayish area of misplaced toner particles. Haloing is believed to be due to electrical disturbances at the image surface as it passes through the squeegee rollers. Offsetting is a shadow like repeat of the image, typically the result of partially transferring an image pattern to the surface of a squeegee roller and transferring the pattern back to the copy sheet on the next roller revolution.

While the theory behind the elimination of trailing, haloing and offsetting by toner removing means 32 has not been firmly established, it is believed that the developed surface image consists of two layers, a first layer at the photoconductive surface comprising a closely held base of developed particles and vehicle, and a second, supetjacent layer comprising a loosely held, displaceable proximate image including excess toner vehicle. lt is believed that the doctor blade removes the displaceable, proximate image without affecting the closely held base image since the latter is apparently tightly adhered to the photoconductive surface such that contact thereof by the doctor blade produces no adverse effects, such as streaking. When the copy sheet is subsequently squeeged, only the closely held base image remains on the image surface and it is unaffected by passage through the rollers.

FIG. 3 illustrates an alternate embodiment of the doctor blade comprising a cylindrical member such as a rod 44. Rod 44 is also electrically nonconductive. The rod is carried between a pair of end supports 48 and 46 which may be clamped or otherwise secured to the machine walls in a position similar to that described with reference to the doctor blade, so that it makes substantially line contact with the developed image surface. The rod thus also functions to remove excess liquid toner from the developed image surface of the copy sheet prior to squeegee drying thereof thereby eliminating the conditions causing trailing, haloing and offsetting.

What has been described is a developing station for use with an electrostatic copying machine having apparatus for removing surplus liquid toner from a developed image surface prior to squeegee drying the copy sheet for obviating problems of training haloing and offsetting.

lt is obvious that upon study by those skilled in the art the disclosed invention may be altered or modified both in its physical appearance and construction without depriving from its inventive concept. Therefore, the scope of protection to be given this invention should not be limited by the embodiment described above, but should be determined by the essential descriptions thereof which appear in the appended claims.

What we claim is:

1. Developing station apparatus including liquid toner for developing a sheet member bearing a latent electrostatic image on one surface, comprising: means transporting said sheet member along a path through said developing station; means for applying said liquid toner to the image surface for developing the latent image, and stationary means in physical engagement with said developed image surface of the sheet for removing excess toner therefrom.

2. The developing apparatus as set forth in claim l wherein said stationary means is electrically nonconductive and is secured adjacent said path for sweeping excess toner from said developed image surface as the sheet member is transported thcrepast.

3. The developing apparatus as set forth in claim 2 wherein said stationary means comprise a thin doctor blade having a length substantially equal to the width of said sheet member and having an edge in physical engagement with said developed image surface.

4. The developing apparatus as set forth in claim 2 wherein said stationary means comprise a cylindrical member having a length substantially equal to the width of said sheet for physi cally engaging the developed image surface with substantially line contact.

5. The developing apparatus as set forth in claim 3 wherein said liquid toner comprises a vehicle carrying a suspension of opaque developer particles; and wherein said doctor blade removes only a loosely held proximate image without affecting closely held base image, and including additionally: roller squeegee means for drying said sheet member after removal of the proximate image.

6. An electrostatic copying machine having a developing station for developing a sheet member bearing a latent electrostatic image, comprising: a receptacle containing liquid toner; means transporting said sheet member along a path through said toner for developing the latent image, and stationary means disposed along said path closely adjacent the emergence of the sheet from the toner, said last named means physically engaging with the developed image surface of said sheet for removing excess toner therefrom.

7. The electrostatic copying machine as set forth in claim 6 wherein said last named means comprise an electrically nonconductive doctor blade having an edge engaging said image surface for sweeping only a portion of the excess toner from said image surface, and including additionally: roller squeegee means for drying said sheet member after sweeping said excess toner thereby obviating image defects of trailing, haloing and of setting.

8. The electrostatic copying machine as set forth in claim 6 wherein said last named means comprise an electrically nonconductive cylindrical member disposed with its axis substantially parallel to the plane of the image surface and having a surface portion in generally line engagement with said developed image surface for removing only a portion of the excess liquid toner therefrom, and including additionally: roller squeegee means for drying said sheet member after removing said excess liquid toner thereby eliminating image defects of trailing, haloing and offsetting incidental to said drying.

Claims

2. The developing apparatus as set forth in claim 1 wherein said stationary means is electrically nonconductive and is secured adjacent said path for sweeping excess toner from said developed image surface as the sheet member is transported therepast.

4. The developing apparatus as set forth in claim 2 wherein said stationary means comprise a cylindrical member having a length substantially equal to the width of said sheet for physically engaging the developed image surface with substantially line contact.

5. The developing apparatus as set forth in claim 3 wherein said liquid toner comprises a vehicle carrying a suspension of opaque developer particles; and wherein said doctor blade removes only a loosely held proximate image without affecting a closely held base image, and including additionally: roller squeegee means for drying said sheet member after removal of the proximate image.

7. The electrostatic copying machine as set forth in claim 6 wherein said last named means comprise an electrically nonconductive doctor blade having an edge engaging said image surface for sweeping only a portion of the excess toner from said image surface, and including additionally: roller squeegee means for drying said sheet member after sweeping said excess toner thereby obviating image defects of trailing, haloing and offsetting.