Improved method for the development of electrostatic images

Abstract

 In a method for the development of electrostatic images an apparatus as described in figure 1 is used, wherein said apparatus comprises a counter means (10) which functions automatically to register the number of copying cycles in which a batch of developer material (15) is used and to yield signals indicative of the registered number, and an electronic control means (11) which functions in dependence upon such signals automatically to control the voltage biasing of an applicator (14) so that in the course of the use of developer material (15) of said batch for developing a plurality of latent images such voltage biasing is varied in a way which at least partly compensates for variations in the developing capability of said developer material (15).

Description

[0001] The present invention relates to an improved method for the development of electrostatic images.

[0002] In electrophotography an electrostatic latent image is obtained with an electrophotographic material typically comprising a photoconductive insulating layer on a conductive support. Said layer is given a uniform surface charge in the dark, normally by corona-charging, and is then exposed to an image pattern of activating electromagnetic radiation such as light or X-rays. The charge on the photoconductive layer is dissipated in the irradiated area to form an electrostatic charge pattern which is then developed with an electrostatically attractable marking material also called toner. The marking material, whether carried in an insulating liquid or in the form of a dry powder deposits on the exposed surface in accordance with either the charge pattern or the discharge pattern as desired. If the photoconductive layer is of the re-usable type, e.g. a vacuum-deposited amorphous selenium-layer on a metal drum, the toner image is transferred to another surface such as paper and then fixed to provide a copy of the original.

[0003] A variety of development techniques is available e.g. cascade development, magnetic brush development, single component dry development and electrophoretic development, which development techniques are described in detail by Thomas L.Thourson in "Xerographic Develo_Dment Processes : A Review" - IEEE Transactions on Electron Devices, Vol. ED-19, No. 4, April 1972.

[0004] In the magnetic brush development technique a developing mixture composed of magnetically susceptible carrier particles carrying a number of smaller electrostatically adhering toner particles is brought by magnetic applicator means into contact with the information-wise charged photoconductive layer. The developer composition is maintained during the development cycle in a loose, brushlike orientation by a magnetic field surrounding the magnetic applicator, which applicator may, for example, comprise a rotatable non-magnetic cylinder having one or more magnets fixedly mounted inside. The magnetic carrier particles are attracted to the applicator by the aforesaid magnetic field, and the toner particles are held to said carrier particles by virtue of their opposite electrostatic polarity. Before and during development, the toner acquires an electrostatic charge of a sign opposite to that of the carrir_-r particles due to triboelectric charging of the different particles by their mutual frictional interaction. When this brushlike mass of magnetically susceptible carrier with adhering toner particles is drawn across the photoconductive surface bearing the latent electrostatic image, the toner particles are electrostatically attracted to an oppositely charged latent image area and form a visible toner image corresponding to the electrostatic image. In magnetic brush development, a bias voltage is usually applied to the magnetic applicator for restraining deposition of toner on the background areas surrounding the developed image and thus promoting clean development. United States Patent 4.314.755 describes a developing system wherein such a biasing voltage is applied and is varied to compensate for the effects of varying humidity conditions on the corona charging current which effects charging of the photoconductive layer preparatory to its information-wise exposure.

[0005] Instead of directly developing the electrostatic latent image in the non-exposed area as is required for example in the production of positive copies of positive originals, a reversal development technique can be applied. Reversal development is practised for example for obtaining positive reproductions of negative originals and for developing images formed by exposing the photoconductive layer to an information-wise modulated scanning laser beam or to points of light emitted from an exposure head comprising an array of selectively information-wise addressed light-emitting diodes in processes wherein the information to be reproduced is represented by the exposed areas of the photoconductive layer. Reversal development can be carried out by means of a magnetic brush development technique by applying to the magnetic toner applicator an appropriate bias voltage causing it to act as a development electrode which through the agency of the carrier particles in the developing mixture induces in the exposed areas of the photoconductive layer electrostatic charges of opposite polarity to the charges in the non-exposed areas (cfr. R.M.Schaffert "Electrophotography" The Focal Press - London, New York, enlarged and revised edition, 1975, and T.P. Maclean "Electronic Imaging" Academic Press - London 1979, p. 231).

[0006] Deviations from a required developed image quality are encountered when adopting a cyclical copying process wherein electrostatic latent images successively formed on a re-usable type photoconductive layer are developed by toner particles deriving from a given batch of developer composition comprising a mixture of toner particles and magnetically susceptible carrier particles. A cyclical copying process involves the repetitive performance of a copying cycle comprising the steps of electrostatically charging the photoconductive layer, information-wise photo-exposing such layer to electromagnetic radiation to which it is sensitive, applying the developer composition to effect development of the electrostatic charge pattern, information-wise transferring the applied toner to a receptor, and restoring the photoconductive layer to a rest potential preparatory to the next cycle. During researches which we have carried out, it has been discovered that in course of time the surfaces of the carrier particles in the supply of developer composition become smeared with toner material and that this is an important cause of variations in developed image quality. The smearing results in a change in the packing density of the toner-carrier mixture and in its triboelectric behaviour. As the toner in the mixture becomes used up the charge density on the residual toner particles decreases.

[0007] It is an object of the invention to provide a method wherein the quality of the developed images produced by the performance of a multiplicity of copying cycles, using a given batch of toner-carrier mixture, is less liable to deteriorate as a result of changes occurring in the said batch as its toner constituent becomes depleted.

[0008] According to the present invention, there is provided a method of producing developed electrostatic images involving the repetitive performance of a copying cycle comprising the steps of electrostatically charging a photoconductive layer, information-wise exposing said photoconductive layer to electromagnetic radiation to which it is sensitive, developing the resulting electrostatic charge pattern by means of electrostatically charged toner particles, information-wise transferring the applied toner to a receptor, and restoring the photoconductive layer to a rest potential preparatory to the next cycle, the toner used for the development step in the different copying cycles being derived from a common batch of developer material which comprises a toner-carrier mixture and which is carried to the photoconductive material by a magnetic brush while the latter is at a bias voltage with respect to an electrically conductive backing of the photoconductive layer, characterised in that the number of copying cycles performed from the commencement of use of said batch of developer material is automatically registered as the cycles are performed and the said bias voltage is automatically controlled in dependence on signals indicative of such number of performed copying cycles so as at least partly to compensate for a decrease in the charge density on the toner particles of said batch as its toner content decreases.

[0009] When employing this method, deterioration of image quality caused by change in the triboelectric properties of the batch of developer material during the progressive consumption of the toner particles is reduced and may in fact be nil.

[0010] In preferred embodiments of the invention, signals representing the number of performed copying cycles from the commencement of use of the batch of developer material are fed as input signals to electronic control means which, on the basis of experimental data quantifying the changes in the developing capability of an identical batch of developer material in funetion of the number of copying cycles in which it is used, has been programmed to yield output signals which are a measure of the bias voltage change necessary for at least partly compensating for the change in the development capability of the developer material resulting from the performance of the number of copying cycles represented by said input signals, and said output signals are used to control the bias voltage on the magnetic brush. The development capability of a batch of developer material is assessed in terms of the developed image density (at a maximum density image area) which is achieved when using that batch for developing a given electrostatic latent image under given conditions. The variation of the development capability of a given batch in function of the number of identical latent images which are developed by that batch under identical conditions, and the bias voltage changes which are necessary for keeping the actual development capability of the developer batch constant or nearly constant over a sequence of copying cycles, can be established by a series of tests.

[0011] Electronic circuitries for converting input signals into output signals whose value relationship to the input signals is determined in accordance with a stored function or programme are well known in the art of electronic control devices. For effecting the required signal conversion in carrying out the present invention, use is preferably made of a microprocessor which on the basis of experimental data as above referred to has been programmed to yield output signals which are a measure of the bias voltage changes which are required to compensate for the variations in the development capability of the developer material resulting from the performance of the number of copying cycles indicated by the input signals. A microprocessor is by definition an integrated-circuit computer, a computer on a chip called the central processing unit (CPU). The microprocessor has only a relatively small signal storage capacity (memory), and a small number of input/output lines. A microprocessor plus a few associated chips and some ROM (read-only-memory) can replace a complicated logic circuit of gates, flip-flops and analog/digital conversion functions. In carrying out the present invention use can be made of a microprocessor which includes a signal memory and a comparator circuit for determining which signals are equivalent. Examples of useful comparator circuits are given by Paul Horowitz and Winfield Hill in the book "The Art of Electronics" - Cambridge University Press - Cambridge (1980) p. 124-125, 337-338 and 390-392. The 8022 microprocessor illustrated in Section 8.27 of said book includes eight comparator gates on the same chip in the processor itself, in addition to an 8-bit analog-to-digital converter. Electronic circuits known as voltage regulators and power circuits are described in the same book at pages 172-222.

[0012] Our researches have also established that the developed image quality can also be affected by variations in the temperature of the photoconductive layer. An increase in the temperature of the layer, can, depending on the magnitude of the increase, result in a decrease in the chargeability of the said layer, assessed in terms of the charge level to which the layer will be raised by exposure to a given charging, e.g. corona-charging.

[0013] It has been found that variations in the temperature of the photoconductive layer can also be at least partly compensated for by appropriate modification of the voltage bias on the magnetic brush -, developer applicator. Accordingly, in certain embodiments of the present invention, signals representing changes in the temperature of the photoconductive layer are fed as input signals to electronic control means which, on the basis of experimental data, has been programmed to yield output signals which are a measure of the change of bias voltage on the magnetic brush required for at least partly compensating for the change in the chargeability of the photoconductive layer occasioned by a temperature change as represented by said input signals, and such output signals are used to influence the voltage biasing of the magnetic brush to effect such compensation.

[0014] The bias voltage changes required for compensating for temperature variations of the photoconductive layer can be established by tests. The data from such tests can be used in the programming of an electronic control means, preferably a microprocessor, so that its output signals are appropriately influenced by input signals from a temperature sensor indicative of changes in the temperature of the photoconductive layer.

[0015] Changes in the temperature of the photoconductive layer can be sensed by directly sensing changes in the temperature of the layer or by sensing the temperature of the atmosphere in the vicinity of such layer.

[0016] The information-wise photo-exposure of the photoconductive layer can involve simultaneous exposure of all parts of the layer to be irradiated, or a progressive exposure of the image area, e.g. by line-wise scanning. The method according to the invention can be employed for document copying. The method can also be employed for recording information transmitted as energising or triggering signals to the exposing radiation source or sources. The term "copying" where used herein is to be construed broadly to include such a translation of information signals into a developed visible record.

[0017] The restoration of the photoconductive layer to rest potential to complete a copying cycle is achieved by overall exposing the layer to light.

[0018] The invention includes apparatus for use in producing developed electrostatic images by a method according to the invention as hereinbefore defined.

[0019] Apparatus according to the invention for producing developed electrostatic images comprises a recording element comprising a photoconductive layer, corona discharge means for electrostatically charging such layer, means for information-wise exposing said layer to electromagnetic radiation to which it is sensitive thereby to form an electrostatic latent image, means for holding a batch of developer material comprising toner particles and larger carrier particles, magnetic brush applicator means for carrying developer material of said batch to said photoconductive layer to effect development of an electrostatic latent image, means for bias voltaging said applicator means with respect to an electrically conductive backing of said photoconductive layer, means for effecting information-wise transfer of applied toner to a receptor element, and means for restoring said photoconductive layer to a rest potential preparatory to another recording cycle, characterised in that the apparatus includes :

(i) counter means which functions automatically to register the number of copying cycles in which said batch of developer material is used and to yield signals indicative of the registered number; and

(ii) electronic control means which functions in dependence on such signals automatically to control the voltage biasing of said applicator so that in course of the use of developer material of said batch for developing a plurality of latent images such voltage biasing is varied in a way which at least partly compensates for variaties in the developing capability of said developer material.

[0020] An example of the present invention will now be described with reference to the accompanying drawings.

Fig. 1 is a block diagram of a development embodiment according to the present invention.

Fig. 2 is a cross-section drawing of a magnetic brush development unit including a controllable voltage source for biasing the magnetic brush with respect to the photoconductive layer.

Fig. 3 represents a cross-section drawing of a magnetic carrier particle with loosely adhering toner particles and a magnetic carrier particle having attached thereto firmly adhering smeared out toner particle material.

Fig. 4 and 5 represent graphs of bias voltage (V) versus copy number (n).

[0021] Referring now in detail to Fig. 1, element 1 represents a drum 1 comprising a photoconductive layer 2 on a conductive drum wall 3. While rotating the drum 1 in the indicated sense the photoconductive layer 2 is corona-charged with the corona device 4 comprising a grounded shield 5 and corona wires 6. The corona wires 6 are connected to the positive pole of a high voltage D.C. corona voltage source 7 having the other pole connected to the ground 8.

[0022] Element 9 represents an exposure unit which may be a lens type exposure device as in a camera or an electronically actuated exposure device e.g. laser beam or an array of light-emitting diodes information-wise operated for printing of digital data.

[0023] Element 10 is a copy counter generating an input signal for the microprocessor 11. Element 17 is a temperature sensing means also producing an electrical signal for the microprocessor 11. The microprocessor 11 provides through a comparator circuit contained therein a control signal 12 in response to the correspondence of the actual input signal of element 10 with a same copy number signal of a stored signal series of a test development. The microprocessor 11 also provides through the comparator circuit contained therein a control signal added to signal 12 in response to the correspondence of the actual input signal of the temperature of the photoconductive layer with a same signal of a , series of stored signals obtained by temperature measurement in the test development. Said control signal 12 is fed into the bias voltage generator 13 which is connected to the magnetic brush means 14, rotating in a tray 15 filled with magnetic carrier-toner mixture 16.

[0024] The photoconductive layer is after transfer of the toner pattern exposed overall to light of a sufficient strength to bring the rest potential to a minimum level before starting a new corona-charging.

[0025] Figure 2 represents a cross-section drawing of a magnetic brush developing unit in operative position with respect to a rotatable photoconductive drum 20.

[0026] The vacuum-deposited amorphous Se-As alloy photoconductor layer 21 is sufficiently sensitive to red light for the recording of information-wise modulated light-emitting-diode (LED) light of 665 nm.

[0027] The photoconductive layer 21 is applied to the conductive aluminium substrate 22 of the drum which is maintained to ground potential. A magnetic carrier-toner mixture 23 held in a hopper 24 is picked up by the force of the external magnetic field between the north (N) and south (S) poles of the magnets 25 which are radially arranged inside an aluminium cylinder 26. The thickness of the toner layer is controlled by a doctor blade 27. The conductive cylinder 26 is connected to the controllable voltage source 28 for biasing the magnetic brush at a desired potential for reversal development. The controllable voltage source 28 receives a control signal from the already mentioned microprocessor.

[0028] Figure 3 represents (1) a fresh negatively charged carrier particle C having electrostatically adhering positively charged toner particles T₁ and (2) such carrier particle C after repeated friction with the toner particles whereby toner material T₂ is smeared out onto the carrier particle C.

[0029] For a fresh developer the charge exchange between the carrier particle C and the toner particles T is high and as a consequence thereof the developed density is low. The toner particles have a high charge to mass ratio in the fresh developer which ratio gradually decreases as a function of the number of copying cycles, i.e. toner depletion.

[0030] In order to compensate for the relatively low developed density the bias voltage (V) is decreased for the first five thousand copies (n = 5,000).

[0031] Figure 4 illustrates the change of bias voltage during the first 5,000 copies when using a developer prepared as described in Example 1 of European Patent Application 83 200 514.4.

[0032] In the embodiment illustrated in Figure 4 the initial bias voltage was 270 V and the bias voltage after five thousand copies was 220 V.

[0033] After five thousand copies the smearing of the toner particles is such that a loss in conductivity in the developer mass is obtained and as a consequence thereof a loss in developed density. In order to compensate for this effect the bias voltage (V) was increased for the next forty five thousand copies (n finally = 50,000) as represented in Figure 5.

[0034] In a particular embodiment using the already mentioned developer it was established by experiment that the temperature coefficient for a proper bias voltage change as a function of temperature of the photoconductive layer was -3 V per centigrade in the temperature range from 20°C to 40°C.

[0035] In an embodiment of reversal development the exposure of the photoconductive layer proceeds with a dot-wise exposing laser beam modulated by digital data supplied by a computer or word-processor. Hereby the printed information is built up by dots. A good image reproduction requires a development of these dots to the same optical density. Such is obtained not only by biasing the magnetic brush but also by timely replenishing the toner.

[0036] The present method may be combined with any method of toner-replenishment with the proviso that in the actual work-load conditions of development the toner-replenishment proceeds as in the test development that stands model for the actual development. Suitable toner-replenishing methods are described in the European Patent Applications 83 200 134.1, 83 200 801.5, 83 200 802.3 and 83 201 564.8.

[0037] The present method may be further combined with the method described in co-pending Patent Application filed on even date herewith by the present applicant under the title : "Improvements relating to the production of developed electrostatic images", wherein the corona-charging voltage is varied automatically to compensate for fatigue and dark recovery phenomena affecting the chargeability of the photoconductor.

Claims

1. A method of producing developed electrostatic images involving the repetitive performance of a copying cycle comprising the steps of electrostatically charging a photoconductive layer, information-wise exposing said photoconductive layer to electromagnetic radiation to which it is sensitive, developing the resulting electrostatic charge pattern by means of electrostatically charged toner particles, information-wise transferring the applied toner to a receptor, and restoring the photoconductive layer to a rest potential preparatory to the next cycle, the toner used for the development step in the different copying cycles being derived from a common batch of developer material which comprises a toner-carrier mixture and which is carried to the photoconductive material by a magnetic brush while the latter is at a bias voltage with respect to an electrically conductive backing of the photoconductive layer, characterised in that the number of copying cycles performed from the commencement of use of said batch of developer material is automatically registered as the cycles are performed and the said bias voltage is automatically controlled in dependence on signals indicative of such number of performed copying cycles so as at least partly to compensate for a decrease in the charge density on the toner particles of said batch as its toner content decreases.

2. A method according to claim 1, wherein signals representing the number of performed copying cycles from the commencement of use of the batch of developer material are fed as input signals to electronic control means which, on the basis of experimental data quantifying the changes in the developing capability of an identical batch of developer material in function of the number of copying cycles in which it is used, has been programmed to yield biassing voltage control signals for effecting at least a partial compensation for the change in the development capability of the developer material resulting from the performance of the number of copying cycles represented by said input signals, and said control signals are used in said control of the bias voltage on the magnetic brush.

3. A method according claim 1 or 2, wherein signals representing changes in the temperature of the photoconductive layer are fed as input signals to electronic control means which, on the basis of experimental data, has been programmed to yield output signals which are a measure of the change of bias voltage on the magnetic brush required for at least partly compensating for the change in the chargeability of the photoconductive layer occasioned by a temperature change as represented by said input signals, and such output signals are used to influence the voltage biasing of the magnetic brush to effect such compensation.

4. A method according to claim 2 or 3, wherein a microprocessor is used as said electronic control means.

5. A method according to any of claims 1 to 4, wherein the development is a reversal development.

6. Apparatus for producing developed electrostatic images comprising a recording element comprising a photoconductive layer, corona discharge means for electrostatically charging such layer, means for information-wise exposing said layer to electromagnetic radiation to which it is sensitive thereby to form an electrostatic latent image, means for holding a batch of developer material comprising toner particles and larger carrier particles, magnetic brush applicator means for carrying developer material of said batch to said photoconductive layer to effect development of an electrostatic latent image, means for bias voltaging said applicator means with respect to an electrically conductive backing of said photoconductive layer, means for effecting information-wise transfer of applied toner to a receptor element, and means for restoring said photoconductive layer to a rest potential preparatory to another recording cycle, characterised in that the apparatus includes :

(i) counter means which functions automatically to register the number of copying cycles in which said batch of developer material is used and to yield signals indicative of the registered number; and

(ii) electronic control means which functions in dependence on such signals automatically to control the voltage biasing of said applicator so that in course of the use of developer material of said batch for developing a plurality of latent images such voltage biasing is varied in a way which at least partly compensates for variations in the developing capability of said developer material.

7. Apparatus according to claim 6, wherein said electronic control means comprises a microprocessor which, on the basis of experimental data indicative of variations in the charging capability of an identical batch of developer material in function of the number of copying cycles in which such batch is used, is programmed to yield voltage biasing control signals which effect at least a partial compensation for the changes in charging capability resulting from the performance of the number of copying cycles registered by said counter means.

8. Apparatus according to claim 6 or 7, wherein the apparatus also includes means which senses variations in the temperature of the photoconductive layer and supplies signals indicative of such temperature variations to said microprocessor, and said processor is programmed so that said voltage biasing control signals also effect at least a partial compensation for the change in the chargeability of the photoconductive layer resulting from the temperature changes represented by said temperature change signals.

Drawing