In a novel electrographic copying method and apparatus, a latent electrostatic image is formed on an insulating layer of an electrophotographic element and, before, during or after the formation of the latent electrostatic image, there is created a plurality of charge islands, such as by an exposure through a screen. These charge islands are distinct and in addition to any charge islands which result from the image-forming step. After establishment of the charge islands and the latent electrostatic image, the resulting charge pattern is developed with a developer which either is at least partially conductive, i.e., has a maximum resistance of about 109 ohms, or is rendered so by the phenomenon of electrical breakdown.
|
1. An electrographic copying method for producing images having improved maximum density and tonal range comprising:
(a) forming a latent electrostatic image on an insulating layer; (b) before, during or after forming said image, creating a plurality of charge islands in the imaging area of said insulating layer in addition to any charge islands that result from the image forming step (a); and then (c) developing said image with a developer composition comprising a toner and a carrier, said developer having a maximum resistance of about 109 ohms.
13. In an electrographic apparatus which includes an imaging member having an image recording area and means for forming an electrostatic image on said area, the improvement which comprises means for forming on said area, before, during or after forming said image on said area, a plurality of charge islands in addition to any such islands already present, and development means including a supply of a developer comprising a toner and a carrier, said developer having a maximum resistance of about 109 ohms for applying such developer to the resulting electrostatic image.
3. An electrographic copying method for reproducing continuous tone images having improved maximum density and tonal range comprising:
(a) forming a latent electrostatic image of a continuous tone subject on a photoconductive insulating layer; (b) before, during or after forming said image, creating a plurality of charge islands in the imaging area of said photoconductive layer in addition to any charge islands that result from the image forming step (a); and then (c) developing said image with a developer composition having a maximum resistance of about 109 ohms.
14. In an electrographic apparatus which includes a photoconductive imaging member having an image recording area and means for forming an electrostatic image on said area, the improvement which comprises means for forming on said area, before, during or after forming said image on said area, a plurality of charge islands in addition to any such islands already present, and development means including a supply of a developer comprising a toner and a carrier, said developer having a maximum resistance of about 109 ohms for applying such developer to the resulting electrostatic image.
15. In an electrographic apparatus which includes a photoconductive imaging member having an image recording area and means for forming an electrostatic image on said area, the improvement which comprises the inclusion of a halftone screen in the imaging member for forming on said area, before, during or after forming said image on said area, a plurality of charge islands in addition to any such islands already present, and development means including a supply of a developer comprising a toner and a carrier, said developer having a maximum resistance of about 109 ohms for applying such developer to the resulting electrostatic image.
4. An electrographic copying method for reproducing continuous tone images having improved maximum density and tonal range comprising:
(a) forming a latent electrostatic image of a continuous tone subject on a photoconductive insulating layer; (b) before, during or after forming said image, uniformly exposing the photoconductive insulating layer through a halftone screen to create a plurality of charge islands on said layer in addition to any charge islands that result from the image forming step (a); and then (c) developing said image with a developer composition comprising a toner and a carrier, said developer having a maximum resistance of about 109 ohms.
5. A method as in
6. A method as in
7. A method as in
(a) moving an electrostatic image bearing member past a development zone; and (b) transporting such developer (i) through a first development zone in a direction generally counter-current to the moving image member; and (ii) through a second development zone in the same direction as the moving image member. 8. A method as in
9. A method as in
10. A method as in
11. A method as in
12. A method as in
|
|||||||||||||||||||||||||||||
Single component conducting developers may work. Other useful developer compositions are disclosed, for example, in U.S. Pat. No. 3,764,477 granted Oct. 23, 1973 to McCabe et al; U.S. Pat. No. 2,811,465; U.S. Pat. No. 2,904,000; U.S. Pat. No. 3,040,704; U.S. Pat. No. 3,098,765; U.S. Pat. No. 3,117,884; U.S. Pat. No. 3,246,629; U.S. Pat. No. 3,358,637; U.S. Pat. No. 3,402,698; U.S. Pat. No. 3,626,898; U.S. Pat. No. 3,640,248; U.S. Pat. No. 3,654,902; U.S. Pat. No. 3,665,891; U.S. Pat. No. 3,674,532 and U.S. Pat. No. 3,703,395.
Many developers having a resistance higher than about 109 Ω can be made at least partially conducting. Under controlled conditions, certain developer materials will undergo a phenomenon described as electrical breakdown. This phenomenon is described in U.S. Pat. No. 4,076,857 to Kasper et al on Feb. 28, 1978. U.S. Pat. No. 4,076,857 is incorporated herein by reference.
This breakdown phenomenon exhibited by developers manifests itself when measuring the resistance of the developer material as a function of the electrical field across the developer. The resistance is conveniently measured by (1) placing a metal electrode in the plane of the photoconductive element above an operating magnetic brush, (2) applying a known potential to the electrode, and (3) measuring the current passing through the magnetic brush. Resistance is calculated by dividing the voltage by the current.
At a certain level of the applied field, called the electrical breakdown value, for a small increase in field there is a large drop in the resistance of the developer material. The developer then acts as though it has a very low resistance. The breakdown value should be measured in the given process configuration under dynamic operating conditions (i.e., actual magnet configuration, actual toner concentration, relative humidity, support-brush spacing, support pressure on the developer, brush rpm. etc.) such as disclosed in U.S. Pat. No. 4,076,857.
Development by using the above-described electrical breakdown phenomenon can be carried out by (a) contacting a latent electrostatic image with a developer composition and (b) establishing across such developer an electrical field greater than the electrical breakdown value of the developer, thereby causing the developer to undergo electrical breakdown.
Development by the electrical breakdown mode can be influenced by a number of factors such as: the composition of the carrier particles; the concentration of toner particles in the developer; the strength of the electric field between the surface bearing the electrostatic charge pattern and the biasing electrode; and the thickness of the developer (i.e., the distance between the surface bearing the electrostatic charge pattern and the biasing electrode); initial photoconductor charge or charge on the support; bias voltage on the biasing electrode and varying the photoconductor thickness to alter surface potential per unit charge. Development is accomplished by selecting one or more of the aforementioned factors such that the electric field which forms across the developer during development is greater than the breakdown value of the developer material under the conditions of development.
The required field strength, in order to develop in the breakdown mode, can be obtained by properly selecting the development system parameters as discussed hereinabove. It is readily apparent, however, that physical limitations may prevent the designing of a development system which will enable field strengths to exceed the breakdown value for some particular developer compositions. Therefore, preferred developer compositions are those which have relatively low breakdown values. Less than 25 volts/mm is typical.
Also, in order to prevent discharging of the latent electrostatic image, preferred developer compositions are those which exhibit relatively high resistivity prior to breakdown, i.e., when subjected to a low strength electric field. A low field resistivity of at least 105 ohm-cm is preferred. By the terms "low field resistivity" and "measured under low fields" as used herein, we mean resistance measurements made in accordance with the procedure previously described.
Instead of forming the charge islands and developing the latent electrostatic image on the photoconductive layer as described above, the image could be transferred to another insulating support and developed as above.
FIG. 8 illustrates the transfer of the charge image and various alternative points in the process for forming the charge islands.
(The sequence of steps illustrated by FIG. 8 proceeds from left to right. The formation of charge islands, as shown in FIG. 8, can take place at any one of the indicated points in time of the overall process depicted; that is, charge islands can be formed before exposure or during exposure or after exposure, or they can be formed on a receiver element after transfer of the charge image to such receiver. FIG. 8 also indicates the alternative of transferring the charge image to a receiver in lieu of leaving the charge image on the original photoconductor for development in accordance with the invention.) The transfer can be made before or after the charge islands are formed in the image. In general, any of the methods for electrostatic image transfer described in the prior art will be useful. Representative useful methods for the transfer of electrostatic images (TESI) are disclosed in Electrophotography by R. M. Schaffert (1975) Focal Press, pages 167-176. If the image is transferred before the charge islands are formed, the charge islands may be formed on the insulated receiving element before, during or after the transfer. The techniques for forming the charge islands include many of the same methods described previously for formation of charge islands on photoconductive layers. Development is carried out on the transferred image in the same manner as described for development on photoconductive layers.
Development with conducting developers or via the electrical breakdown mode may be carried out utilizing any one of the conventional electrographic developing means such as cascade as described in for example U.S. Pat. No. 2,725,304 to Landrigan, or magnetic brush as for example disclosed in U.S. Pat. No. 2,874,063 or U.K. Patent specification No. 1,355,485.
A particularly useful magnetic brush technique is described in co-pending commonly assigned U.S. Patent Application Ser. No. 027,115 filed by Kroll et al on Apr. 4, 1979, now U.S. Pat. No. 4,292,921. This application discloses an improved electrographic development method for use with partially-conductive developers. The method comprises (1) moving an electrostatic image bearing member past a development zone and (2) transporting such developer (a) through a first development zone in a direction generally countercurrent to the moving image member and (b) through a second development zone in the same direction as the moving image member. The extent of image development within each such zone is controlled by the rate of developer transport and/or the magnitude of developer bias, so that overall development of the different portions of solid image areas (particularly leading and trailing portions of such areas) is equalized.
One structural embodiment for practice of this development procedure is disclosed in FIG. 6. The development apparatus 30 there illustrated comprises two magnetic brushes 31, 32 mounted at a development station along the path of an electrographic image member 33. The image member can be of various types known in the art, e.g., including a photoconductive insulating layer 34, an electrically conductive backing layer 35 and a film support 36. Each of magnetic brushes 31, 32 respectively comprises an array of strip magnets, denoted N and S, arranged as shown around the periphery of inner cores 38 and 39, which are stationary within developer reservoir 40. Each brush also includes an electrically conductive outer cylinder 41 and 42 respectively, which is non-magnetic and rotatable around the core to transport developer mixture, attracted by the magnets N and S, from the reservoir 40 into contact with the image member 33 and back into the reservoir to be replenished. To facilitate uniform distribution of developer longitudinally across the brush surface, augers 48, 49 can be provided in the reservoir as shown. Preferably, the augers have a pitch which varies longitudinally to equalize the quantity of developer supplied. It is to be noted that the cylinders 41 and 42 of brushes 31 and 32 are rotated in different directions, as indicated, by drive means 43, 44 respectively, and that each cylinder has a separate electrical bias from respective potential sources Vb1 and Vb2.
In operation the image member 33 is moved as shown across the development apparatus as the magnetic brushes 31 and 32 are rotated in the directions described and shown.
Known electrographic apparatus, including electrophotographic apparatus, which include an imaging member having an image recording area and means for forming an electrostatic image on said area can be adapted to perform the method of the present invention. Such apparatus can be modified to also include means for forming, in the image recording areas of the imaging member, a plurality of charge islands and development means which includes a supply of conductive developer for applying the developer to the resulting electrostatic image. Image forming stations of a representative electrographic apparatus are presented schematically in FIG. 7. The electrographic apparatus, as presented, comprises a photoconductive imaging member 70 which includes the halftone screen described in FIG. 3. The apparatus also includes charging means 71, imaging exposure means 72, means 73 for uniformly exposing the imaging area of the photoconductive layer through the halftone screen included in the photoconductive imaging member 70. As shown, the uniform exposure can be made before, during or after formation of an electrostatic image. Finally, the figure shows development means 74 which includes a conductive developer composition as required by the present invention. Examples of apparatus which can be adapted for use herein include U.S. Pat. Nos. 3,876,106; 3,877,413; 3,535,036 and 3,203,394.
It is of course, readily apparent that the method of the present invention can be used to form both monochrome and polychrome images. Suitable colorants can be incorporated into toners according to known methods to render electrostatic images toned therewith colored. The colorants useful in suitable toners are preferably dyestuffs and pigments. In principle, virtually all of the compounds mentioned in the Color Index, Vols. I and II, second edition, 1956, can be used as colorants. Included among the vast number of suitable colorants would be such materials as carbon black, Nigrosine Spirit soluble (C.I. 50415), Hansa Yellow G (C.I. 11680), Chromogen Black ETOO (C.I. 14645), Rhodamine B (C.I. 45170), Solvent Black 3 (C.I. 26150), Fuchsin N (C.I. 42510), C.I. Basic Blue 9 (C.I. 52015), etc.
The unexpected results obtained with the method of this invention, are further illustrated by the following examples.
An aggregate photoconductive element of the type described in Example 1 of U.S. Pat. No. 3,615,414 granted to Light, Oct. 26, 1971, was charged and image-wise exposed to a step tablet having neutral density areas of 0.09, 0.41, 0.75 and 1.05.
The element was given a second uniform exposure from the front side through a halftone screen having a frequency of 60 dots/cm, and a percent tint of 50. This screen has opaque dots of uniform density over 50 percent of its area. The screened latent image was then developed with a magnetic brush and a developer composition described in detail in Examples 4-8. The resistance of the developer was 1.5×106 Ω measured as described hereinbefore. Developer resistances can be varied by controlling the degree of oxidation of metal carrier cores.
The reflection densities of the developed image of the step tablet were compared graphically with the expected densities resulting from the use of a 50 percent tint screen. The expected densities were calculated assuming that the dots on each step of the step tablet were faithfully reproduced. The calculation was carried out in the following manner.
The density (D) of a particular image area is given by the formula
D=log 1/R
R represents reflectance. To a first approximation reflectances are additive when viewed at a normal viewing distance. Hence, the total reflectance (Rt) of an area of several densities is given by
R1 =X1 R1 +X2 R2 +X3 R3 --+Xn Rn
in which Xn is the fraction of the total area covered by reflectance Rn.
It is assumed that the developed density on a given dot for a specific voltage is the same as the developed density would be for that same voltage in a large solid area. The solid area output density (copy) as a function of input density (original) can be experimentally determined. Hence the expected output density for a halftone screen having a known percent dot area, can be calculated. For example:
Din (density input) of 1.0, for large solid areas results experimentally in Dout (density output) of 1.4 on a paper base of 0.1 density. Then for a halftone tint screen of 50 percent X=0.5 and: ##EQU1##
D1 represents the density of the solid area. D2 represents the density of the paper base. ##EQU2## Therefore the total expected density (Dt) is: ##EQU3## The calculation is then repeated for different Dout levels as found by developing in the absence of a halftone screen.
The graphic comparison of the actual reflection densities and the expected densities is shown in FIG. 4. The graph of FIG. 4 shows that the expected Dmax (0.38) is about 1/3 of the actual output Dmax (1.2). The tonal range of this example is also greater than that expected as can also be seen from the graph. More steps of the step tablet were faithfully reproduced than expected.
To further illustrate the unexpected improvement in maximum density and tonal range obtained in Example 1, prints were made as in Example 1 with two different developers: a dry partially conductive developer, with a resistance of 1.5×106 Ω, and a developer with a resistance of 4.4×109 Ω.
In FIG. 5, the input density versus output density of each developed image is shown. Dmax for the image developed with the more conducting developer is much higher than Dmax for the image developed with the more resistant developer. Also, the tonal range of the former is much greater than that of the latter.
Five halftone tint screens having a frequency of 33.5 dots/cm and tints of 67%, 52%, 42%, 40% and 30% respectively were prepared from 20 cm×25 cm sheets of Kodalith® Film available from Eastman Kodak Company. Each of these screens was cemented to a transparent base photoconductor film to form 5 separate elements. The photoconductor layer was of the type disclosed in U.S. Pat. No. 3,615,414.
Each element was imaged substantially as in Example 1 except that the uniform screen exposure was from the rear of the film through the screen pattern. A reflection original document was used as a test with areas having neutral densities of 0.09, 0.41, 0.75 and 1.05 respectively. The prints were developed as in Example 1 with developer having a resistance of 1.5×106 Ω. Density measurements were made and plotted as in Example 2.
The prints showed smooth, uniform, neutral tones with very little mottle and edge defects. The graphs describing density input vs. density output showed high Dmax, lowered image contrast and extended tonal range as in Example 1.
These examples were designed to illustrate the effect of developer resistance on copy density.
The developers used in Examples 4-8 contained toner particles comprising carbon black in a styreneacrylate polymeric matrix and magnetic carrier particles coated with a vinylidene fluoride-fluoroethylene-copolymer. Various carrier particle cores were used (see list below) to produce developers exhibiting a range of resistances that were measured as previously described.
| ______________________________________ |
| Example Carrier Core |
| ______________________________________ |
| 4 stainless steel |
| 5, 6, 7 EH oxidized iron (sold by Hoeganacs Co.) |
| Carrier cores for Ex. 5, 6 and 7 were |
| oxidized, as described in U.S. Pat. No. |
| 3,767,477, to three different levels of |
| oxidation to vary the developer |
| resistance. |
| 8 nickel plated EH iron (Hoeganacs Co.). |
| ______________________________________ |
In these examples an integral screen photoconductive element was prepared containing, in the following order: a transparent film support of poly(ethylene terephthalate), a magenta halftone dot screen of 60 dots/cm, 50% tint, printed by offset lithography onto the film support, an evaporated nickel conducting layer, and an aggregate photoconductive layer of the type described in U.S. Pat. No. 3,615,414.
Prints were made as in Example 3 with a reflection original document having neutral density area (referred to as input density of Din) as indicated in Table 1.
The results were consistent with those obtained in previous examples. Output densities (copy) vs. input densities (original) are reported in Table 1 for each developer at a different conductivity. Expected densities were calculated as in Example 1. This data shows that developers having a resistance of about 109 Ω or less produce higher Dmax than expected.
To illustrate the dot enlargement effect of the invention, photomicrographs were taken of output density (Dout) neutral areas corresponding to Din values: 1.58; 0.78; 0.40 and 0.21 of the print made in Example 8. The dot enlargement effect was observed as increased toner fill-in between the halftone dots at a given Din as developer resistance decreased. For comparison, a photomicrograph is included of a halftone dot pattern on a photoconductive layer which has been developed with a non-conductive liquid developer. The photomicrograph corresponds to an area having a Din value of 1.58. FIG. 2 is a representative photomicrograph of the dot enlargement effect achieved in Examples 7 and 8. FIG. 1 is a photomicrograph of the results obtained with the non-conductive liquid developer composition. No dot enlargement can be observed in FIG. 1.
| TABLE 1 |
| __________________________________________________________________________ |
| Step of |
| Neutral Dout |
| Density Ex. 4 |
| Ex. 5 Ex. 6 Ex. 7 Ex. 8 |
| Wedge |
| Din |
| >1010 Ω |
| 1.4 × 109 Ω |
| 1.4 × 108 Ω |
| 1.8 × 107 Ω |
| 1.2 × 106 Ω |
| Expected |
| __________________________________________________________________________ |
| 1 1.58 |
| .22 .60 .69 .95 1.32 .38 |
| 2 1.12 |
| .20 .53 .60 .88 1.26 .38 |
| 3 .87 |
| .19 .44 .52 .75 1.20 .37 |
| 4 .78 |
| .17 .42 .49 .65 1.08 .37 |
| 5 .53 |
| .13 .30 .36 .38 .60 .36 |
| 6 .40 |
| .11 .20 .25 .24 .34 .32 |
| 7 .30 |
| .10 .13 .16 .19 .19 .19 |
| 8 .21 |
| .08 .08 .08 .10 .08 .10 |
| 9 .14 |
| .07 .06 .06 .07 .05 .10 |
| 10 .09 |
| .06 .06 .06 .06 .05 .10 |
| __________________________________________________________________________ |
This example illustrated the use of a developer that is made conductive by the breakdown development mode. The developer was similar to the developer described in Example 8 with the exception that the mean particle size of the toner was smaller (6.8 millimicrons). The toner concentration was 3.1%.
An integral screen photoconductive element was used similar to the element described in Examples 4-8 with the exception that the halftone screen had a frequency of 52 dots/cm. and a 40% tint.
The developer was run in a two roller magnetic brush development station for 1 hour to allow the developer to come to equilibrium.
In operation, the photoconductor film was charged to -500 volts, exposed such that the film voltage corresponding to a 0.15 neutral density grey scale step was -150 volts, uniformly rear exposed through the screen and developed in a breakdown development mode in a two roller magnetic brush development device with 7.6 cm diameter rollers operating at brush speeds of 160 and 180 RPM. The film velocity was 25 cm/second and the magnetic brush spacing from the film surface was 1.91 millimeters with a development brush bias of -140 volts. The breakdown value for this developer, as measured according to the procedure described in U.S. Pat. No. 4,076,857 was 13.6 volts per millimeter.
The resultant prints exhibited high Dmax, smoothness and extended tonal scale.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
For example the present invention is useful in forming reversal images. Image tone reversal can be obtained by developing the discharged areas of the images, instead of the charge islands, with a highly biased magnetic brush and developing with a toner having the same polarity as the brush bias.
Kroll, Arthur S., Kasper, George P., Mosehauer, Michael
| Patent | Priority | Assignee | Title |
| Patent | Priority | Assignee | Title |
| 3121010, | |||
| 3234017, | |||
| 3337339, | |||
| 3932034, | Jun 20 1973 | Canon Kabushiki Kaisha | Developer concentration detecting and replenishment device |
| 4076857, | Jun 28 1976 | Eastman Kodak Company | Process for developing electrographic images by causing electrical breakdown in the developer |
| 4081571, | Aug 01 1974 | Mita Industrial Co. Ltd. | Method for developing electrostatic latent images |
| GB1072557, |
| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Jan 28 1985 | Eastman Kodak Company | (assignment on the face of the patent) | / |
| Date | Maintenance Fee Events |
| Date | Maintenance Schedule |
| Oct 07 1989 | 4 years fee payment window open |
| Apr 07 1990 | 6 months grace period start (w surcharge) |
| Oct 07 1990 | patent expiry (for year 4) |
| Oct 07 1992 | 2 years to revive unintentionally abandoned end. (for year 4) |
| Oct 07 1993 | 8 years fee payment window open |
| Apr 07 1994 | 6 months grace period start (w surcharge) |
| Oct 07 1994 | patent expiry (for year 8) |
| Oct 07 1996 | 2 years to revive unintentionally abandoned end. (for year 8) |
| Oct 07 1997 | 12 years fee payment window open |
| Apr 07 1998 | 6 months grace period start (w surcharge) |
| Oct 07 1998 | patent expiry (for year 12) |
| Oct 07 2000 | 2 years to revive unintentionally abandoned end. (for year 12) |