Magnetic Developing Method Under A.C. Electrical Bias and Apparatus Therefor

1. Field of the Invention

This invention relates to a developing method for developing a latent image by the use of a developer and an apparatus therefor, and more particularly to a developing method using a one-component developer, especially a developing method which enables production of fogless visible images excellent in sharpness and tone reproduction, and an apparatus therefor.

2. Description of the Prior Art

Various types of developing methods using a one-component developer are heretofore known such as the powder cloud method which uses toner particles in cloud condition, the contact developing method in which a uniform toner layer formed on a toner supporting member comprising a web or a sheet is brought into contact with an electrostatic image bearing surface to effect development, and the magnedry method which uses a conductive magnetic toner formed into a magnetic brush which is brought into contact with the electrostatic image bearing surface to effect development.

Among the above-described various developing methods using one-component developer, the powder cloud method, the contact developing method and the magnedry method are such that the toner contacts both the image area (the area to which the toner should adhere) and the non-image area (the background area to which the toner should not adhere) and therefore, the toner more or less adheres to the non-image area as well, thus unavoidably creating the so-called fog.

To avoid such fog, there has been proposed the transfer development with a space between a toner donor and an image bearing member in which a toner layer and an electrostatic image bearing surface are disposed in opposed relationship with a clearance therebetween in a developing process so that the toner is caused to fly to the image area by the electrostatic field thereof and the toner does not contact the non-image area. Such development is disclosed, for example, in U.S. Pat. Nos. 2,803,177; 2,758,525; 2,838,997; 2,839,400; 2,862,816; 2,996,400; 3,232,190 and 3,703,157. This development is a highly effective method in preventing the fog. Nevertheless, the visible image obtained by this method generally suffers from the following disadvantages because it utilizes the flight of the toner resulting from the electric field of the electrostatic image during the development.

A first disadvantage is the problem that the sharpness of the image is reduced at the edges of the images. The state of the electric field of the electrostatic image at the edge thereof is such that if an electrically conductive member is used as the developer supporting member, the electric lines of force which emanate from the image area reach the toner supporting member so that the toner particles fly along these electric lines of force and adhere to the surface of the photosensitive medium, thus effecting development in the vicinity of the center of the image area. At the edges of the image area, however, the electric lines of force do not reach the toner supporting member due to the charge induced at the non-image area and therefore, the adherence of the flying toner particles is very unreliable and some of such toner particles barely adhere while some of the toner particles do not adhere. Thus, the resultant image is an unclear one lacking sharpness at the edges of the image area, and line images, when developed, give an impression of having become thinner than the original lines.

To avoid this in the above-described toner transfer development, the clearance between the electrostatic image bearing surface and the developer supporting member surface must be sufficiently small (e.g. smaller than 100 .mu.) and actually, asccidents such as pressure contact of the developer and mixed foreign substances are liable to occur between the two surfaces. Also, maintaining such a fine clearance often involves difficulties in designing of the apparatus.

A second problem is that images obtained by the above-described toner transfer development usually lack toner reproducibility. In the toner transfer development, the toner does not fly until the toner overcomes the binding power to the toner supporting member by the electric field of the electrostatic image. This power which binds the toner to the toner supporting member is the resultant force of the Van der Waals force between the toner and the toner supporting member, the force of adherence among the toner particles, and the reflection force between the toner and the toner supporting member resulting from the toner being charged. Therefore, flight of the toner takes place only when the potential of the electrostatic image has become greater than a predetermined value (hereinafter referred to as the transition threshold value of the toner) and the electric field resulting therefrom has exceeded the aforementioned binding force of the toner, whereby adherence of the toner to the electrostatic image bearing surface takes place. But the binding power of the toner to the supporting member differs in value from particle to particle or by the particle diameter of the toner even if the toner has been manufactured or prepared in accordance with a predetermined prescription, and therefore, it is considered to be distributed narrowly around a substantially constant value and correspondingly, the threshold value of the electrostatic image surface potential at which the flight of toner takes place also seems to be distributed narrowly around a certain constant value. Such presence of the threshold value during the flight of the toner from the supporting member causes adherence of the toner to that part of the image area which has a surface potential exceeding such threshold value, but causes little or no toner to adhere to that part of the image area which has a surface potential lower than the threshold value, with a result that there are only provided images which lack the tone gradation having steep .gamma. (the gradient of the characteristic curve of the image density with respect to the electrostatic image potential).

In view of such problems, a developing device in which a pulse bias of very high frequency is introduced across an air gap to ensure movement of charged toner particles flying through the air gap, whereby the charged toner particles are made more readily available to the charged image is disclosed in U.S. Pat. Nos. 3,866,574; 3,890,929 and 3,893,418.

Such high frequency pulse bias developing device may be said to be a developing system suitable for line copying in that a pulse bias of several KHz or higher is applied in the clearance between the toner donor member and the image retaining member to improve the vibratory characteristic of the toner and prevent the toner from reaching the non-image area in any pulse bias phase but cause the toner to transit only to the image area, thereby preventing fogging of the non-image area. However, the aforementioned U.S. Pat. No. 3,893,418 states that a very high frequency (18 KHz-22 KHz) is used for the applied pulse voltage in order to make the device suitable for the reproduction of tone gradation of the image.

U.S. Pat. No. 3,346,475 discloses a method which comprises immersing two electrodes in insulating liquid contained in a dielectrophoretic cell and applying thereto an AC voltage of very low frequency (lower than about 6 Hz) to thereby effect the development of a pattern corresponding to the conductivity variance.

Further, U.S. Pat. No. 4,014,291 dicloses a method in which dry, one component magnetic toner on the non-magnetic, non-conductive transfer cylinder which encloses a rotating cylindrical magnet is transferred to the deposit zone to develop an electrostatic latent image on coated paper, but this patent does not suggest that a bias is applied for the above-described purpose.

It is an object of the present invention to provide a developing method in which a latent image is developed by subjecting a magnetic developer to the action of an electric field and applying a low frequency alternating voltage to the developing clearance with a view to improve the tone reproduction in transfer development, and an apparatus therefor.

It is another object of the present invention to provide a developing method based on the principle of development in which a low frequency alternate electric field having a phase of a particular polarity which causes the developer to one-sidedly reach both the image area and the non-image area of a latent image bearing member from a developer carrier and a phase of the opposite polarity to the particular polarity which applies a bias in a direction to cause the developer having reached at least the non-image area to return to the developer carrier side is applied in the developing clearance to thereby ensure transition of the developer to the non-image area and back transition of the developer to the developer carrier to be alternately repeated even in the clearance between the developer carrier and the non-image area in the developing station and enable a development very excellent in tone reproduction to be accomplished by such reciprocal movement of the developer, and an apparatus therefor.

It is still another object of the present invention to provide an electrostatic image developing method in which a magnetic developer is conveyed to the developing station while being restrained on a developer carrier by the action of a magnetic field and in the developing station, the developer carrier is disposed in opposed relationship with an electrostatic image bearing member with a clearance maintained therebetween, the clearance being greater than the thickness of the magnetic developer layer and development is effected by applying an alternate electric field of low frequency (preferably lower than 1.5 KHz) so that the bias field in the developing clearance alternates the magnetic developer particles both in the image area and the non-image area, and an apparatus therefor.

It is yet still another object of the present invention to provide a developing method in which the clearance between the electrostatic image bearing member and the developer carrier during the developing process is varied with time so as to vary the intensity of the alternate electric field affecting the developer, and an apparatus therefor.

It is a further object of the present invention to provide a developing method in which the electrostatic latent image bearing member and the developer carrier carrying thereon a developer layer are disposed in opposed relationship in the developing station with a clearance maintained therebetween and development is effected by applying an alternate voltage of low frequency below 1.5 KHz between the back electrode of the latent image bearing member and the developer carrier, the frequency and voltage value of the applied alternate voltage being selected to effect optimal visualization of the image in accordance with the kind of the image (for example, line image, half-tone image of a photograph or the like, colored image, etc.), and an apparatus therefor.

Other objects and features of the present invention will become apparent from the following description of some embodiments of the invention taken in conjunction with some accompanying drawings.

FIG. 1 illustrates the amount of transition of the toner and the characteristic of the degree of toner back transition for the potential of a latent image, as well as an example of the voltage waveform applied.

FIGS. 2A and 2B illustrate the process of the developing method according to the present invention, and FIG. 2C shows an example of the applied voltage waveform.

FIGS. 3A and 3B show the characteristic of the electrostatic image potential versus image density as the result of the experiment effected on the developing method according to the present invention, with the frequency of the applied alternate electric field varied.

FIGS. 4A and 4B show the characteristic of the electrostatic image potential versus image density as the result of the experiment effected on the developing method according to the present invention, with the amplitude of the applied alternate electric field varied.

FIG. 5 shows the characteristic of the electrostatic image potential versus image density as the result of the experiment effected on the developing method according to the present invention, with the frequency and amplitude of the applied alternate voltage varied.

FIG. 6 is a graph illustrating the range of selection of the amplitude versus frequency of the applied alternate electric field as the result of the experiment effected on the developing method according to the present invention.

FIG. 7 illustrates the electric lines of force produced from the electrostatic image in the developing method according to the prior art.

FIG. 8 illustrates the electric lines of force produced from the electrostatic image in the developing method according to the present invention.

FIGS. 9A and 9B illustrate the movement of the developer.

FIGS. 10 and 12 illustrate embodiments of the developing method according to the present invention.

FIG. 13A is a diagram of the output circuit of the alternating voltage applicable to the embodiment shown in FIG. 12, and FIG. 13B shows the output waveform thereof.

FIG. 14 illustrates a further embodiment of the developing method according to the present invention.

FIGS. 15A-15D to FIGS. 18A-18D illustrate the process of movement and vibration of the developer to the image area and the non-image area in the process of development.

The principle of the present invention will be described by reference to FIG. 1. In the lower portion of FIG. 1, there is shown a voltage waveform applied to a toner carrier. It is shown as a rectangular wave, whereas it is not restricted thereto. A bias voltage of the negative polarity having a magnitude of Vmin is applied at a time interval t1, and a bias voltage of the positive polarity having a magnitude of Vmax is applied at a time interval t2. When the image area charge formed on the image surface is positive and this is developed by negatively charged toner, the magnitudes of Vmin and Vmax are selected so as to satisfy the relation that

where V.sub.D is the image area potential and V.sub.L is the non-image area potential. If so selected, at the time interval t1, the bias voltage Vmin acts to impart a bias field with a tendency to expedite the contact of toner with the image area and non-image area of an electrostatic latent image bearing member and this is called the toner transition stage. At the time interval t2, the bias voltage Vmax acts to impart a bias field with a tendency to cause the toner which is transited to the latent image bearing surface in the time interval t1 to be returned to the toner carrier and this is called the back transition stage.

Vth.multidot.f and Vth.multidot.r in FIG. 1 are the potential threshold values at which the toner transits from the toner carrier to the latent image surface or from the latent imgage surface to the toner carrier, and may be considered potential values extrapolated by a straight line from the points of the greatest gradient of the curves shown in the drawing. In the upper portion of FIG. 1, the amount of toner transition at t1 and the degree of toner back transition at t2 are plotted with respect to the latent image potential.

The amount of toner transition from the toner carrier to the electrostatic image bearing member in the toner transition stage is such as curve 1 shown by broken line in FIG. 1. The gradient of this curve is substantially equal to the gradient of the curve when no bias alternate voltage is applied. This gradient is great and the amount of the toner transition tends to be saturated at a value intermediate V.sub.L and V.sub.D and accordingly, it is not suited for reproduction of half-tone images and provides poor tone gradation. Curve 2 indicated by another broken line in FIG. 1 represents the probability of toner back transition.

In the developing method according to the present invention, an alternating electric field is imparted so that such toner transition stage and toner back transition stage may be alternately repeated and in the biase phase t1 of the toner transition stage of that alternating electric field, toner is positively caused to temporally reach the non-image area of the electrostatic latent image bearing member from the toner carrier (of course, toner is also caused to reach the image area) and toner is sufficiently deposited also on the half-tone potential portion having a low potential approximate to the light region potential V.sub.L, whereafter in the bias phase t2 of the toner back transition stage, the bias is caused to act in the direction opposite to the direction of toner transition to cause the toner which has also reached the non-image portion as described to be returned to the toner carrier side. In this toner back transition stage, as will later be described, the non-image area does not substantially have the image potential originally and therefore, when a bias field of the opposite polarity is applied, the toner which has reached the non-image area as described tends to immdiately leave the non-image area and return to the toner carrier. On the other hand, the toner once deposited on the image area including the half-tone area is attracted by the image area charge and therefore, even if the opposite bias is applied in the direction opposite to this attracting force as described, the amount of toner which actually leaves the image area and returns to the toner carrier side is small. By so alternating the bias fields of different polarities at a preferred amplitude and frequency, the above-described transition and back transition of the toner are repeated a number of times at the developing station. Thus, the amount of toner transition to the latent image surface may be rendered to an amount of transition faithful to the potential of the electrostatic image. That is, there may be provided a developing action which may result in a variation in amount of toner transition having a small gradient and substantially uniform from V.sub.L to V.sub.D as shown by curve 3 in FIG. 1. Accordingly, practically no toner adheres to the non-image area while, on the other hand, the adherence of the toner to the half-tone image areas takes place corresponding to the surface potential thereof, with a result that there is provided an excellent visible image having a very good tone reproduction. This tendency may be made more pronounced by setting the clearance between the electrostatic latent image bearing member and the toner carrier so that it is greater toward the termination of the developing process and by decreasing and converging the intensity of the above-mentioned electric field in the developing clearance.

An example of such developing process according to the present invention is shown in FIGS. 2A and 2B. As shown in FIGS. 2A and 2B, the electrostatic image bearing member 4 is moved in the direction of arrow through developing regions (1) and (2) to a region (3). Designated by 5 is a toner carrier. Thus, the electrostatic image bearing surface and the toner carrier gradually widen the clearance therebetween from their most proximate position in the developing station. FIG. 2A shows the image area of the electrostatic image bearing member and FIG. 2B shows the non-image area thereof. The direction of arrows shows the direction of the electric fields and the length of the arrows indicates the intensity of the electric fields. It is important that the electric fields for the transition and back transition of the toner from the toner carrier are present also in the non-image area. FIG. 2C shows a rectangular wave which is an example of the waveform of the alternate current applied to the toner carrier, and schematically depicts, by arrows in the rectangular wave, the relation between the direction and intensity of the toner transition and back transition fields. The shown example refers to the case where the electrostatic image charge is positive, whereas the invention is not restricted to such case. When the electrostatic image charge is positive, the relations between the image area potential V.sub.D, the non-image area potential V.sub.L and the applied voltages Vmax and Vmin are set as follows: ##EQU1## In FIGS. 2A and 2B, a first process in the development occurs in the region (1) and a second process occurs in the region (2). In the case of the image area shown in FIG. 2A, in the region (1), both of the toner transition field a and the toner back transition field b are alternately applied correspondingly to the phase of the alternate field and the transition and back transition of the toner result therefrom. As the developing clearance becomes greater, the transition and back transition fields become weaker and the toner transition is possible in the region (2) while the back transition field sufficient to cause the back transition (below the threshold value .vertline.Vth.multidot.r.vertline.) becomes null. In the region (3), the transition neither takes place any longer and the development is finished.

In the case of the non-image area shown in FIG. 2B, in the region (1), both the toner transition field a' and the toner back transition field b' are alternately applied to create the transition and back transition of the toner. Thus, fog is created in this region (1). As the clearance is wider, the transition and the back transition field become weaker and when the region (2) is entered, the toner back transition is possible while the transition field sufficient to cause transition (below the threshold value) becomes null. Thus, in this region, fog is not substantially created and the fog created in the region (1) is also sufficiently removed in this stage. In the region (3), the back transition neither takes place any longer and the development is finished. As regards the half-tone image area, the amount of toner transition to the final latent image surface is determined by the magnitudes of the amount of toner transition and the amount of toner back transition corresponding to that potential, and after all, there is provided a visible image having a small gradient of curve between the potentials V.sub.l to V.sub.D, as shown by curve 3 in FIG. 1, and accordingly having a good tone gradation.

In this manner the toner is caused to fly over the developing clearance and is caused to temporally read the non-image area as well to improve the tone gradation, and in order that the toner having reached the non-image area may be chiefly stripped off toward the toner carrier, it is necessary to properly select the amplitude and alternating frequency of the alternate bias voltage applied. Results of the experiment in which the effect of the present invention has clearly appeared by such selection will be shown below.

FIGS. 3A and 3B show the plotted results of the measurement of the image relfection density D with respect to electrostatic image potential V, effected with the amplitude of the applied alternate voltage fixed and with the frequency thereof varied. These curves will hereinafter be called the V-D curves. The experiment was carried out under the following construction. A positive electrostatic charge latent image is formed on a cylindrical electrostatic image formation surface. The toner used is a magnetic toner to be described hereinafter (which contains 30% magnetite), and such toner is applied onto a non-magnetic sleeve to a thickness of about 60 .mu., the non-magnetic sleeve enveloping therein a magnet, and negative charge is imparted to the toner by the friction between the toner and the sleeve surface. The result when the minimum developing clearance between the electrostatic image formation surface and the magnetic sleeve is maintained at 100 .mu.is shown in FIG. 3A, and the result when such minimum developing clearance is maintained at 300 .mu. is shown in FIG. 3B. The magnetic flux density in the developing station resulting from the magnet surrounded by the sleeve is about 700 gausses. The cylindrical electrostatic image formation surface and the sleeve are rotated substantially at the same velocity which is about 110 mm/sec. Thus, after having passed through the minimum clearance in the developing station, the electrostatic image formation surface gradually goes away from the toner carrier. The alternate electric field applied to this sleeve comprises a sine wave of amplitude V.sub.p-p =800 V (peak-to-peak value) with a DC voltage of +200 V superimposed thereon. FIG. 3 shows the V-D curves when the alternating frequency of the applied voltage is 100 Hz, 400 Hz, 800 Hz, 1 KHz and 1.5 KHz (FIG. 3B only) and the V-D curve when no bias field is applied but conduction occurs through the back electrode of the electrostatic image formation surface and the sleeve.

From these results, it is seen that when no bias field is applied, the gradient or so-called value of the V-D curves is very great but by applying an alternate field of low frequency, the .gamma. value is made smaller to greatly enhance the tone gradation. As the frequency of the extraneous field is increased from 100 Hz, the .gamma. value becomes gradually greater to reduce the effect of enchancing the harmony and, when the clearance is 100 .mu. and when the frequency exceeds 1 KHz under the amplitude V.sub.p-p =800 V, that effect becomes weak; when the clearance is 300 .mu. and when the frequency reaches the order of 800 Hz, that effect is also reduced; and when the frequency exceeds 1 KHz, the effect of harmony becomes weak. This may be considered to be attributable to the following reason. In the developing process during which an alternate field is applied, when the toner repeats adherence and separation in the clearance between the sleeve surface and the latent image formation surface, finite time is necessary to positively effect the reciprocating movement thereof. Particularly, the toner which transits by being subjected to a weak electric field takes a relatively long time to positively effec the transition.

An electrostatic field exceeding a threshold value which will cause transition of the toner is produced from the half-tone image area, but the electrostatic field is relatively weak. To cause the toner to reach the half-tone image area, it is necessary that the toner particles moved relatively slowly by being subjected to the electrostatic field positively transit to the image area within one-half period of the applied alternate field. For this purpose, where the amplitude of the alternate field is constant, a lower frequency of the alternate field is more advantageous and accordingly, as shown by the results of the experiment, a particularly good tone gradation is provided for an alternate field of low frequency. This speculation is justified by the comparison between the results of the experiment shown in FIGS. 3A and 3B. The results shown in FIG. 3B have been obtained under the same conditions as those shown in FIG. 3A except that the clearance between the electrostatic image formation surface ad the sleeve surface is as great as 300 .mu.. The wider clearance results in a lower intensity of the electric field to which the toner is subjected. The wider clearance further results in a longer distance of jump and after all, longer time of transition. As is actually apparent from FIG. 3B, the .gamma. value becomes considerably great for the order of 800 Hz and when 1 KHz is exceeded, the .gamma. value becomes almost equal to that when no alternate voltage is applied. Therefore, in order to obtain the same effect of enhanced tone reproduction as that when the clearance is narrow, it is preferable to reduce the frequency as will later be described or to increase the intensity (amplitude) of the alternate voltage.

On the other hand, too low a frequency does not result in sufficient repetition of the reciprocating movement of the toner during the time the latent image formation surface passes through the developing station, and tends to cause irregular development to be created in the image by the alternate voltage. As the result of the foregoing experiment, generally good images have been provided down to the frequency of 40 Hz, and when the frequency is below 40 Hz, irregularity has been created in the visible image. It has been found that the lower limit of the frequency for which no irregularity is created in the visible image depends on the developing conditions, above all, the developing speed (also referred to as the process speed, V.sub.p mm/sec.). In the present experiment, the velocity of movement of the electrostatic image formation surface has been 110 mm/sec. and therefore, the lower limit of the frequency is 40/110.times.V.sub.p .apprxeq.0.3 .times.V.sub.p. As regards the waveform of the alternate voltage applied, it has been confirmed that any of sine wave, rectangular wave, sawtooth wave or asymmetric wave of these is effective.

Such application of the alternate bias, of lower frequency brings about remarkable enhancement of the tone gradation, but the voltage value thereof must be properly set. That is, too great a value for the .vertline.Vmin.vertline. of the alternate bias may result in an excessive amount of toner adhering to the non-image area during the toner transition stage and this may prevent sufficient removal of such toner in the developing process, which in turn may lead to fog or stain created in the image. Also, too great a value for .vertline.Vmax.vertline. would cause a great amount of toner to be returned from the image area, thus reducing the density of the so-called solid black portion. To prevent these phenomena and to sufficiently enhance the tone gradation, Vmax and Vmin may preferably and reasonably be selected to the following degrees:

Vth.multidot.f and Vth.multidot.r are the potential threshold values already described. If the voltage values of the alternate bias are so selected, the excess amount of toner adhering to the non-image area in the toner transition stage and the excessive amount of toner returned from the image area in the back transition stage would be prevented to ensure obtainment of proper development.

This is shown by the results of the experiment. FIGS. 4A and B show the V-D curves when the amplitude V.sub.p-p of the alternate field is varied with the frequency thereof fixed (200 Hz). FIG. 4A shows the result in the case where the developing clearance is set to 100 .mu., and FIG. 4B shows the result in the case where the developing clearance is set ot 300 .mu.. The other conditions are the same as those in FIGS. 3A and B. First, when the developing clearance is relatively small, and when the amplitude V.sub.p-p exceeds 400 V, the result of enchanced tone gradation appears as compared with the case where no electric field is applied. When the V.sub.p-p exceeds 1500 V, the tone gradation is good but fog beings to appear in the non-image area, and when the V.sub.p-p exceeds 2000 V, more fog appears. Prevention of such fog may be accomplished by increasing the alternating frequency to higher than 200 Hz.

A wider developing clearance of 300 .mu. has given rise to the effect of enhanced tone gradation from V.sub.p-p =400 V or higher and has given birth to visible images of good quality having good tone gradation and free of fog for the order of 800 V of the V.sub.p-p. If the V.sub.p-p exceeds 2000 V, the tone gradation is good but fog is created and therefore, in such a case, it is necessary to increase the alternating frequency.

When the developing clearance d is relatively great like this, it is advisable to provide a greater value of the V.sub.p-p of the applied voltage and providing a higher value for f than when the developing clearance d is small.

In order to provide enhanced tone gradation of the image, it is necessary to set the alternating frequency and amplitude value of the applied alternate voltage to proper ranges, and it has been found that, depending on the properties of the image, the relation between the frequency and amplitude value of the applied voltage may be selectively changed over within an appropriate range. That is, when the relation between the frequency and the voltage value of the alternate voltage are studied more strictly, it has become clear that the developing characteristic (V-D curves) can be selected arbitrarily by those values. An example of it is shown in FIG. 5.

FIG. 5 shows the developing characteristic when the clearance between a photosensitive drum which is the latent image bearing member and a sleeve which is the developer carrier is 300 .mu., the thickness of the developer layer on the sleeve is about 100 .mu., and the toner used comprises 100 parts of styrene acryl resin, 60 parts of ferrite, 2 parts of carbon black and 2 parts of auriferous dye as the charge controlling agent mixed together and ground and having extraneously added thereto 0.4% by weight of colloidal silica. The conditions of each curve shown there are the bias conditions (alternating frequency f (Hz), amplitude value (V.sub.p-p)) for visualizing the dark region potential (about 500 V) at the light region potential of about OV. The applied voltage waveform comprises a sine wave with a DC voltage superimposed thereon. (The slight difference of this graph from the foregoing graph is attributable to the difference between the developers used.)

As is apparent from FIGS. 3A and B and the graph of FIG. 5, when the frequency f is low, there is usually obtained a developing characteristic having high tone gradation and when the frequency f is slightly high, there is obtained a developing characteristic having a great value for .gamma.. By varying the amplitude of the alternate voltage in addition to such variation in frequency, it is possible to obtain any desired developing characteristic corresponding to the kind of the image. (The DC component is also varied slightly.)

The curve (a) shown in FIG. 5 is the VD curve when the frequency f is 200 Hz, V.sub.p-p =900 V and the superimposed DC component is 220 V, and it is seen therefrom that this bias condition has a good tone gradation. The curve (b) is the VD curve when the frequency and the amplitude value have been increased to f=400 Hz and V.sub.p-p =1600 V, respectively, with a DC component of 220 V, and it is some what greater in r than the curve (a) but still has a relatively high tone gradation.

If, with respect to the curve (b), the frequency is increased to 700 Hz and 900 Hz with the amplitude V.sub.p-p maintained constant (the superimposed DC voltage is decreased), the .gamma. becomes greater and greater as indicated by the curves (c) and (d), thus resulting in poor tone gradation. On the other hand, however, as shown by the curve (d), it can be seen that even if the electrostatic image potential is low, good development as that potential is possible. Further, although the tone gradation is poor, the so-called edge effect becomes great to provide good reproducibility of the line image and reduced fog.

By varying the bias conditions, it is possible to ensure all-round quality of image corresponding to the original or to the liking of the user.

A preferable range of combination between the alternating bias conditions (frequency f (Hz) and amplitude value V.sub.p-p (V)) on the basis of each experiment is shown in FIG. 6. FIG. 6, with the ordinate representing the amplitude V.sub.p-p (V) of the applied voltage and the abscissa representing the alternating frequency f (Hz) thereof, shows a referable range of combination between the two selectable in accordance with the image.

In FIG. 6, the solid-line curve p indicates the boundary at which fog relatively tends to appear when the developing clearance is 300 .mu., and the shaded area A indicates a range in which the fog tends to appear and which is not suited for the line copy. Also, the solid-line curve q indicates the boundary at which the quality of the tone gradation is judged when the developing clearance is 300 .mu., and the shaded area C indicates a range in which the effect thereof is low. Thus, the range B surrounded by the two curves p and q is a range in which fog is reduced and the image is excellent in definition and tone gradation.

Of course, the positions of these curves p and q may be more or less varied by a variation in size of the developing clearance d. When d is relatively small, the curves p and q become displaced to dot-and-dash line positions p' and q', respectively.

Particularly, in the area encircles by a broken line S, the overall effect of the bias by the alternate field of low frequency is pronounced. The lower limit value of the frequency in this area S is a value determined by the previously mentioned relation that f.gtoreq.0.3.times.V.sub.p, and the upper limit value thereof is determined with a view to well maintain the SN ratio. This SN ratio will now be described. When the frequency of the applied alternate field is increased as mentioned previously, it is necessary to make the amplitude V.sub.p-p of the applied voltage great to order to ensure the reciprocal movement of the developer (the movement of the developer which temporally reaches the non-image area, also) to take place between the developer carrier and the latent image bearing member. However, when such a voltage value high, it is much higher than the potential difference (V.sub.D) of the image area to be visualized and the transition phenomenon of the developer to the image area can hardly sense the potential difference V.sub.D. If so, the definition of the image becomes reduced so that the line reproducibility becomes poor and the fog becomes ready to appear. In addition, the use of a high voltage (higher than about 2500 V) in particular tends to cause the discharging phenomenon with respect to neighboring members and this leads to a problem in constructing an apparatus.

Therefore, under the above-described standard set conditions, the amplitude may preferably be V.sub.p-p .ltoreq.2500 V, and particularly preferably be V.sub.p-p .ltoreq.2000 V, and the frequency may particularly preferably be f.ltoreq.1 KHz. Depending on the combination with the amplitude, the frequency may practically be f.ltoreq.1.5 KHz to thereby obtain the intended effect.

As has hitherto been described, the application of an extraneous alternate voltage between the latent image formation surface and the toner carrier leads to remarkably enhanced tone gradation of the image and prevention of fog. Further, by using magnetic toner as the developer and a sleeve enclosing a permanent magnet as the developer carrier and by properly setting the extraneous alternate voltage value, as will hereinafter be described, it is possible to further enhance the reproducibility of line images at the same time.

Description will hereinafter be made with the electrostatic image formation charge as being positive, whereas the invention is not restricted thereto. In the so-called toner transfer developing method, the electric line of force produced from the end of the latent image goes around the back electrode of the latent image formation surface as shown in FIG. 7 and cannot reach the surface of toner carrier, and accordingly the toner which has started from the toner carrier can hardly adhere to the end of the image. Thus, the resultant image tends to suffer from thinning of lines and poor sharpness of the end, which in turn offers a problem in line copying.

Therefore, in this system, if an alternating bias is applied and if the Vmin thereof is selected to a sufficiently low value, the electric line of force in the developing station during the toner transition stage goes so little around the end of the electrostatic image, as shown in FIG. 8, that there are formed parallel electric fields. This enables the toner to positively adhere to the end of the electrostatic image. However, as already noted, too low a value for Vmin would usually cause fog or stain to be created in the non-image area.

In the present embodiment of the invention, the advantage resulting from the use of the magnetic toner as the developer and the sleeve enclosing the permanent magnet as the developer carrier lies chiefly in solving this problem. By properly setting the content of the magnetic material in the developer and the intensity of the magnetic field of the permanent magnet, it is possible to uniformly enhance the restraining force of the toner onto the sleeve and accordingly select the value of .vertline.Vth.multidot.f.vertline. to a sufficiently great value. As the result, Vmin can be set to a low value with the amount of the toner adhering to the non-image area during the toner transition stage being minimized.

Thus, by applying an alternating bias in the toner transfer developing method using magnetic toner, it is possible to obtain images of good tone gradation which are clear at the end and free of fog and which are excellent also in line copying.

On the other hand, it is a very difficult problem to convey the developer to the developing station in the high resistance toner transfer development and to impart a charge. The method using magnetic toner as the developer and conveying the developer by means of a sleeve and imparting a charge by frictional charging between the sleeve surface or an applicator member and the toner is considered to be one very advantageous methods.

Also, application of the magnetic toner onto the sleeve may be effected by a method of urging a resilient member against the sleeve or a method of maintaining a magnetic member in opposed relationship with the magnetic pole of the permanent magnet within the sleeve and in non-contact with the sleeve surface and controlling the thickness of the magnetic toner by the magnetic force. In the conventional toner transfer development wherein development is effected with the sleeve opposed to the electrostatic image bearing member and with these two members being rotated in the same direction and at the same velocity, the state of the toner applied onto the sleeve directly affects the quality of image and when the application of the toner is effected by the former method, the status of application is relatively delicate and ensures a good quality of image. In this method of application, however, the toner strongly rubs against the sleeve surface and therefore the resin content of the toner adheres to the sleeve surface to remarkably prevent the toner from being charged.

On the other hand, if the latter method is used, the adherence of the toner to the sleeve surface is minimized but the status of the toner applied onto the sleeve surface presents scattered lumps of toner particles and is coarse and accordingly, the image after developed becomes coarse as shown in FIG. 9A.

In contrast, by applying an alternate bias in the developing station according to the present invention, toner particles are caused to effect reciprocal movement between the latent image and the sleeve surface and are separated into individual particles in that process, so that the toner can finely adhere to the image area of the electrostatic image surface as shown in FIG. 9B.

Some specific examples will be shown below in detail.

An example of the construction of the developing device for carrying out the developing method of the present invention is shown in FIG. 10.

Designated by 11 is a photosensitive drum having an insulating layer or a CdS layer. Denoted by 12 is a non-magnetic (stainless) sleeve. These two members 11 and 12 are rotated at the same peripheral velocity of 110 mm/sec. and in the same direction. The diameters of the members 11 and 12 are 80 mm and 30 mm, respectively, and the two members are maintained with a maximum clearance of 200 .mu. and form a developing station adjacent thereto. The two members are so configured that with their rotation, they necessarily pass through the most proximate position and then the clearance therebetween gradually becomes larger.

Designated by 13 is a permanent magnet fixed within the sleeve. Reference numeral 14 denotes a magnetic toner which will hereinafter be described, and 15 a magnetic (iron) blade for uniformly applying the toner onto the sleeve. The composition of the magnetic toner used in the present Example is as follows:

A member 15 is installed in opposed relationship with the magnetic poles of the member 13 with a clearance of 180 .mu. maintained between the end thereof and the member 12. The magnetic field at the end of the member 15 is about 1000 G. The magnetic toner 14 is controlled to a thickness of about 70 .mu. by the member 15 and conveyed to the developing station while being imparted a negative charge by the friction between the toner and the surface of the member 12. The members 12 and 15 are maintained electrically conductive to prevent discharging therebetween, and an alternate voltage is applied to the electrically conductive support member for the member 11 by a power source 16. The frequency of the alternate voltage is 200 Hz and applied in the form of a sine wave of amplitude V.sub.p-p =800 V with a DC voltage +200 V superimposed thereon. The electrostatic image potential is +500 V for the image area and OV for the non-image area. A member 17 is a toner container formed of plastics.

Under the above-described construction, there could be provided images of good tone gradation which were free of fog and clear.

The construction of the developing device for carrying out another developing method of the present invention is shown in FIG. 11.

Designated by 21 is a photosensitive drum having an insulating layer on a CdS layer. Denoted by 22 is an aluminum sleeve. The members 21 and 22 are rotated substantially at the same peripheral velocity of 400 mm/sec. and in the same direction. The diameters of the members 21 and 22 are 200 mm and 50 mm, respectively, and the two members are held with a minimum clearance of 300 .mu. and form a developing station adjacent thereto. The two members are so configured that with their rotation, they necessarily pass through the most proximate position and then the clearance therebetween gradually becomes larger.

Designated by 23 is an isotropical permanent magnet fixed within the sleeve, 24 a magnetic toner, and 25 an iron blade for uniformly applying the toner onto the sleeve.

The composition of the magnetic toner used in the present Example is as follows:

The member 25 is installed at a position opposed to the magnetic poles of the member 23 with a clearance of 250 .mu. maintained between the end thereof and the member 22. The magnetic field at the end of the member 25 is about 750 G. The magnetic toner 24 is controlled to a thickness of about 120 .mu. by the member 25 and is conveyed to the developing station while being imparted a negative charge by the friction between the toner and the surface of the member 22. The developing station is opposed to between the magnetic poles of the magnet within the sleeve. A member 27 is a toner container.

The member 22 and the member 25 are maintained electrically conductive to prevent discharging therebetween and an alternate voltage is applied to the conductive support member for the member 21 by a power source 26. The frequency of the alternate voltage is 400 Hz and the alternate voltage is applied in the form of a sine wave of amplitude V.sub.p-p =1200 V with a DC voltage +200 V superimposed thereon. The electrostatic image potential is +350 V for the image area and -20 V for the non-image area.

Under the above-described construction, there could be provided images of good tone gradation which were free of fog and clear.

In FIG. 12, reference numeral 31 designates an electrostatic latent image bearing member having an insulating layer on a CdS layer, and 32 a back electrode thereof. The members 31 and 32 form a drum shape. Designated by 33 is a non-magnetic stainless metal sleeve having a magnet roll 37 therewithin. The electrostatic latent image bearing member 31 and the sleeve 33 are held with the minimum clearance therebetween maintained at 300 .mu. by a well-known clearance maintaining means. Designated by 34 is a one-component magnetic developer in a developing container 39. The developer comprises 70% by weight of styrene maleic acid resin, 25% by weight of ferrite, 3% by weight of carbon black and 2% by weight of negative charge controlling agent mixed and ground and further has 0.2% by weight of colloidal silica extraneously added thereto to enhance the fluidity thereof. Designated by 36 is an iron blade opposed to the main pole 37a (850 gausses) of a magnet roll 37 enclosed in a sleeve 33. The iron blade controls the thickness of the magnetic developer 34 applied onto the sleeve 33 by a magnetic force. The clearance between the blade 36 and the sleeve 33 is maintained at about 240 .mu. and the thickness of the developer layer applied onto the sleeve 33 by the blade 36 is about 100 .mu.. Designated by 35 is a variable alternate voltage source and the voltage therefrom is applied to between the back electrode 32 and the conductive portion of the sleeve 33. The blade 36 and the sleeve 33 are at the same potential to prevent irregularity of application of the developer.

The average value of the electrostatic image potential is +500 V for the image area and 0V for the non-image area. The extraneous alternate voltage comprises a sine wave of frequency 400 Hz and peak-to-peak 1500 V rendered into a distorted sine wave having an amplitude ratio of about 1.9:1 between the positive phase and the negative phase (this will further be described). Again by this embodiment, it was possible to obtain visible images of good quality which were excellent in tone gradation and which were clear and free of fog.

An example of the circuit for providing such a distorted sine wave is shown in FIG. 13A. FIG. 13B illustrates the distorted output wave of such circuit.

The circuit of FIG. 13A produces the distorted sine wave as shown in FIG. 13B by reducing only the negative (-) side of the sine wave alternating voltage by means of a diode 43 and resistors 44, 45, and if the resistor 44 of the output terminal 0 is caused to slide, the negative (-) side voltage may be made variable. This circuit construction enables the circuit to be formed more easily than the DC superimposed type.

Again by this embodiment, there was provided a development which was free of fog and excellent in tone gradation.

In FIG. 14, 46 is an electrostatic latent image bearing member having an insulating layer on a CdS layer, and denoted by 47 is a back electrode thereof. The members 46 and 47 form a drum shape. Reference numeral 48 denotes a non-magnetic stainless metal sleeve having a magnet roll 52 therewithin. The electrostatic latent image bearing member 46 and the sleeve 48 are held with the minimum clearance therebetween maintained at 300 .mu. by well-known clearance maintaining means 55. Denoted by 49 is a one-component magnetic developer in a developing container 53. The developer comprises 70% by weight of styrene maleic acid resin, 25% by weight of ferrite, 3% by weight of carbon black and 2% by weight of negative charge controlling agent auriferous dye mixed and ground, and further has 0.2% by weight of colloidal silica extraneously added thereto to enhance the fluidity of the developer. Reference numeral 51 designates an iron blade which is opposed to the magnetic pole 52a (850 gausses) of the magnet roll 52 enclosed in the sleeve 48, and the blade 51 controls the thickness of the magnetic developer 49 applied onto the sleeve 48 by the magnetic force. The clearance between the blade 51 and the sleeve 48 is maintained at about 240 .mu., and the thickness of the developer layer applied onto the sleeve 48 by the blade 51 is about 100 .mu.. Designated by 50 is a variable alternate voltage source which applies an alternating bias voltage to between the back electrode 47 and the conductive portion of the sleeve 48. To prevent irregularity of application of the developer, the blade 51 and the sleeve 48 are at the same potential.

The average value of the electrostatic image potential was +500 V for the dark area and 0V for the light area. The variable alternate voltage source 50 is set to have respective oscillation sources so that the alternating voltages (a), (b) and (d) applied from the voltage source 50 may be selected from among the four types of voltage shown in FIG. 5. These individual power sources may be of the well-known type. Denoted by 54 is change-over means connected to the voltage source 50 for selecting the frequencies and amplitude values of the alternating voltages (a), (b) and (d). The changeover means may be a known electrical change-over means.

Thus, the operator can select a quality of image corresponding to his liking.

When the selection button A of the selective change-over means 54 shown in FIG. 14 is depressed, the bias condition is set to (a)f=200 Hz, V.sub.p-p =900 V (DC superimposed 220 V). At this time, the user can obtain a photographic image of delicate quality with a soft tone. When the selection button B is depressed, the bias condition is set to (b)f=400 Hz, V.sub.p-p =1600 V (DC superimposed 220 V). This condition is used when ordinary copies are to be obtained. When the selection button C is depressed, the bias condition is set to (d)f=900 Hz, V.sub.p-p =1600 V (DC superimposed 120 V). At this time, the user can reproduce an original which is thin in density and would tend to cause fog, or an original of colored image, or an original which consists chiefly of lines, without fog and at a good quality.

Of course, these selective combinations are only illustrative and if within the above-mentioned proper range, a combination of other frequency and voltage value can of course be adopted.

FIGS. 15A-D to FIGS. 18A-D schematically illustrate the reciprocal movement of the developer in the developing clearance under the low frequency condition which is applied to the developing method of the present invention, and the vibratory movement of the developer when the frequency f of the applied bias voltage is high (for example, 2 KHz or higher). In the results of the experiment shown in FIGS. 3A, B, 5 and 6, the range of frequency preferred for the enhancement of the tone gradation was indicated, and the reciprocal movement of the developer in the above-described embodiment, for example, is schematically illustrated in FIGS. 15A-B and FIGS. 17A-D.

FIGS. 15A-B show the movement of the developer in the clearance between the image area of the latent image bearing member 4 to be visualized and the toner carrier 5, and FIGS. 17A-D show the movement of the developer in the clearance between the non-image area of the latent image bearing member 4 and the toner carrier 5. (A) in these respective Figures shows the initial state in which the bias field is not yet applied. In the toner transition stage shown in (B) of these Figures, more developer transits from the toner carrier 5 to the image area 4a due to the electrostatic attraction thereof than in the non-image area. It should be noted that the developer also transits to and reaches the non-image area 4b from the toner carrier 5. Arrows indicate the direction of movement of the developer. Next, in the toner back transition stage wherein the applied field assumes the opposite phase as shown in (C) of these Figures, a relatively small amount of developer returns to the toner carrier from the image area, but in the non-image area there is no charge which attracts the toner, so that almost all of the developer which has transited at the toner transition stage is returned to the toner carrier in accordance with the reverse bias. Next, when the phase of the bias is again changed, the toner transition stage as shown in (D) of these Figures takes place, and such reciprocal movement of the developer is thereafter repeated in the described manner. Thus, a number of reciprocal movements takes place and in the meantime, the developer is once caused to reach the non-image area also, whereby from the half-tone image area near the light region of relatively low potential to the solid black image area, there is obtained an effect of image visualization faithful to the potential held thereby.

As shown in the embodiments, the latent image bearing member is in the form of a drum and the toner carrier is a sleeve, so that with the rotation of these two members in the same direction, the opposed portions of the two members provide a gradually widening clearance from their most proximate position and the intensity of the bias alternate field acting on this clearance is gradually reduced and converged to complete the development. Therefore, in this converging stage, the tone gradation is very excellent and substantially no developer adheres to the non-image area.

On the other hand, when the alternating frequency is increased to a high frequency, say, 2 KHz or higher, the tone gradation is reduced. This phenomenon will be explained by reference to FIGS. 16A-D and FIGS. 18A-D. (A) in these Figures shows the states of the latent image bearing member 4 and the toner carrier 5 before the application of the bias. When the bias for toner transition is applied to the image area, the toner is liberated from the toner carrier toward the image area 4a as shown in FIG. 16B, but there is more or less irregularity in the degree of transition because of the force acting on the individual toner particles and the alternating frequency of the bias is high before this irregularity is converged, so that the reverse bias is applied to the toner which has reached the image area and to the toner which is still suspended in the developing clearance, and it is believed that most of the suspended toner returns to the toner carrier side as shown in FIG. 16C. When the bias phase is again reversed before this return movement of the toner is terminated, the toner is again subjected to the bias force directed toward the image area. Accordingly, not the reciprocal movement but the vibration of the toner is taking place in the clearance between the image area and the toner carrier.

Such vibratory movement of the toner is pronounced in the clearance between the non-image area in which no latent image charge is present and the toner carrier. This state is shown in FIGS. 18A-D. From the initial state shown in FIG. 18A, a bias phase for toner transition is applied. In this case, when a bias exceeding the transition threshold value is applied, the toner is liberated from the toner carrier but since the alternating frequency of the bias is high as shown in FIG. 18B, the phase of the bias is reversed before the toner reaches the non-image area 4b, and the toner returns to the toner carrier (FIG. 18C). Next, when the toner transition bias is applied, the toner is again liberated from the toner carrier but during the time the toner is being suspended in the aforementioned clearance, the reverse bias is again applied so that the toner goes toward the toner carrier as shown in FIG. 18D. In this manner, the toner vibrates in the clearance and substantially does not reach the non-image area 4a, so that there is no toner adhering to the non-image area even when the development has been terminated, thus avoiding formation of the so-called fog. However, on the other hand, it is believed that the adherence of the toner to a region having a half-tone image potential which is approximate to the light region (the non-image area) does not sufficiently take place, thus reducing the tone gradation. This phenomenon is theoretically considered to take place until a certain degree of high frequency exceeding 2 KHz is reached, and this would raise a difficulty in the reproduction of the tone gradation as intended by the present invention.

The foregoing description has been made with respect to the case where the image area potential VD is positive, whereas the present invention is not restricted thereto but it is also applicable to a case where the image area potential is negative and in this latter case, if the positive of the potential is small and the negative of the potential is great, the present invention is equally applicable. Therefore, when such image area charge is negative, the aforementioned formulas (1)-(4) are represented as the following formulas (1')-(4'). ##EQU2##

The present invention, as has hitherto been described in detail, provides a method which comprises disposing in opposed relationship with each other a latent image bearing member and a non-magnetic developer carrier carrying thereon a layer of magnetic developer and enclosing a magnet therein, with a clearance larger than the thickness of the developer layer maintained in the developing station, and applying an alternate electric field having a phase of a particular polarity which causes the developer to one-sidedly reach both the image area and the non-image area of the latent image bearing member from the developer carrier in the developing clearance and a phase of the opposite polarity from said particular polarity which applies a bias in a direction to cause the developer having reached at least the non-image area to return toward the developer carrier side, thereby effecting development, and an apparatus for carrying out such method.

Thus, the developing method according to the present invention which uses a magnetic developer and in which the transition and back transition of the developer are effected has enabled obtainment of fogless beautiful images having good tone reproduction and clear at the image end portions by the application of an alternate bias field of low frequency.

The present invention is not restricted to the illustrated embodiments, but is applicable to the development of latent images formed by an electrophotographic method, an electrostatic recording method or other image formation method.

The present invention provides a developing method which is characterized by effecting development while applying an alternate electric field in a range satisfying the relation that ##EQU3## where V.sub.p-p represents the amplitude of a preferable low frequency alternating field and f represents the alternating frequency thereof, and an apparatus for carrying out such method. Therefore, by the application of a low frequency alternating field within such range, the transition of the developer to the non-image area and the back transition of the developer to the developer carrier are alternately and positively repeated in the clearance between the developer carrier and the non-image area in the developing station, and such reciprocal movement of the developer may accomplish a development which is highly excellent in reproduction of tone gradation. Further, a layer of magnetic developer is carried on a non-magnetic sleeve enclosing a magnet therein and therefore, the magnetic developer uniformly enhances its restraining force onto the sleeve by the action of the magnetic field, whereby the value of Vth.f which is the potential threshold of the developer transition may be selected to a sufficiently great value, thereby reducing the amount of developer adhering to the non-image area and minimizing the fog.