DE68912889T2 - Image forming apparatus with control means for the electrostatic attraction of transmission material. - Google Patents

Description

FIELD OF THE INVENTION AND RELATED ART

The invention relates generally to an image forming apparatus for forming an image on an image receiving material in which the image receiving material is electrostatically attracted to a carrier device by the action of an attraction device. Such an image forming apparatus is known from DE-A-2027008. In particular, the invention relates to a single-color or multi-color image forming apparatus such as an electrophotographic copier or a single-color or multi-color printer equipped with an image transfer device in which a transfer material is electrostatically attracted to and conveyed on a transfer material carrier device and an electric field is established on the transfer material in order to transfer a visualized image formed with a developer on an image carrier member such as an electrophotographic photosensitive member to the transfer material.

A typical image forming apparatus of this type has, for example, a structure shown in Figure 19. In the image forming apparatus shown in Figure 19, a transfer material conveyor belt and a photosensitive drum 1 are provided. Around the photosensitive drum 1, a cleaning device 9, a pre-exposure lamp 10, a primary charger 2, a developing device 4, a transfer charger 8 and a metal rollers 13, 14 and 15 tensioned transfer material conveyor belt 5 as main components. The structure will be described in detail: The primary charger 2 and the developing device 4 form a space therebetween through which image exposure light 3 is projected from an image exposure device onto the outer periphery of the photosensitive drum 1. The transfer material conveyor belt 5 is stretched generally in the shape of a triangle around the metal rollers 13, 14 and 15. The metal rollers 13, 14 and 15 are electrically grounded. The transfer material conveyor belt 5 is rotatable in the direction shown by an arrow in Fig. 19 (counterclockwise direction) by a drive motor (not shown) which is operatively coupled to the metal roller 15. Around the transfer material conveyor belt 5, there are arranged an attraction charger 6 for attracting the transfer material P which is a material for receiving the image onto the transfer material conveyor belt 5, an opposing roller 7, a discharger 11 for removing charge, and a fur brush cleaning device 12.

In the image forming apparatus having the above-described structure, the residual developer remaining on the outer peripheral surface of the photosensitive drum 1 is wiped off by the cleaning device 9, and the residual electric charge remaining on the outer peripheral surface of the photosensitive drum 1 is removed by the pre-exposure lamp 10. Thereafter, the outer peripheral surface of the photosensitive drum 1 is uniformly charged by the primary charger 2. After the surface of the photosensitive drum 1 is uniformly charged by the primary charger 2, the image exposure light 3 is projected onto the surface of the photosensitive drum 1, thereby forming an electrostatic latent image on the photosensitive drum 1, which has the original image information. After the electrostatic latent image is formed on the surface of the photosensitive drum 1, the developing device 4 is operated to visualize the electrostatic latent image. By continuing to rotate the photosensitive drum 1 (clockwise in Fig. 19), the visualized image is conveyed to an image transfer station in which the outer surface of the photosensitive drum 1 and the transfer charger 8 are opposed to each other.

On the other hand, by an unillustrated sheet feeding system, the transfer material P is fed in the direction shown by an arrow A in Fig. 19. The transfer material P fed to the transfer material conveyor belt 5 is attracted to the transfer material conveyor belt 5 by applying a high DC voltage or a high AC voltage biased with a DC voltage to the attraction charger 6. The transfer material P attracted to the transfer material conveyor belt 5 is injected with the transfer charge by the opposing roller 7 acting as a counter electrode to the attraction charger 6, and the transfer material P is pressed to the transfer material conveyor belt 5 by the roller 7. The transfer material P thus attracted and pressed to the transfer material conveyor belt 5 is conveyed to the above-described station by the movement of the transfer material conveyor belt 5, and the visible image formed on the surface of the photosensitive drum 1 is transferred to the transfer material P by applying to the transfer charger 8 a high voltage having a polarity opposite to that of the charge of the developer which forms the visible image. The transfer material P, which is applied by the The transfer material 5, which has been transferred to the visible image by the transfer charger 8, is electrically discharged by the discharger 11 supplied with a high AC voltage. Then, the transfer material 5 is released from the transfer material conveyor belt 5 and thereafter conveyed in the direction B of Figure 19 to an image fixing device (not shown) in which the image is fixed. The developer remaining on the surface of the photosensitive drum 1 is removed by the cleaning device 9 and the residual electric charge remaining on the photosensitive drum 1 is removed by the pre-exposure lamp 10 with sufficient illumination, thereby preparing the photosensitive drum 1 for the next image forming process.

In the conventional color image forming apparatus described above, the level of the high voltage applied to the attraction charger 6 is constant regardless of changes in the environmental conditions under which the image forming apparatus is installed, and therefore the attraction of the transfer material P to the transfer material conveying belt 5 is carried out at the constant voltage. However, when the image forming apparatus is operated at high temperature and high humidity, in the case that the transfer material P is made of paper, the volume resistivity of the transfer material P used is lower by about two orders of magnitude than when the image forming apparatus is operated at a normal temperature and a normal humidity (for example, at a temperature of 23°C and a relative humidity of 60%), while with respect to the transfer material conveying belt 5, its surface resistance is lowered by the humidity on the surface.

Therefore, the level of the attraction charger 6 applied constant voltage becomes too low, resulting in the attraction of the transfer material P to the transfer material conveyor belt becoming insufficient. If this occurs, the transfer material P is displaced on the transfer material conveyor belt 5 or may be detached from the transfer material conveyor belt 5. On the other hand, when the image forming apparatus is operated at low temperature and low humidity, the volume resistivity of the transfer material P is approximately two orders of magnitude higher than when the image forming apparatus is operated at normal temperature and normal humidity (23°C and 60%) if the transfer material P is made of paper. With respect to the transfer material conveyor belt 5, however, the amount of moisture absorbed on the surface thereof becomes smaller, resulting in the surface resistance of the transfer material conveyor belt 5 becoming larger. Therefore, the constant voltage level is sufficient for forming a sufficient attraction force between the transfer material P and the transfer material conveyor belt 5.

However, the electric charge applied to the back surface of the transfer material conveyor belt 5 and the front surface of the transfer material P by the attraction charge of the attraction charger 6 is not attenuated before the transfer material reaches the transfer station, so that a good image transfer operation cannot be carried out. Generally, in carrying out the transfer process, a surface potential V1 of the transfer material conveyor belt 5 before carrying out the image transfer process and a surface potential V2 after the transfer operation are such that V1 < V2 when the polarity of the transfer charge is positive. It is known from experience that the difference between the Voltages, namely V2 - V1 is not less than 0.5 kV. When the image forming apparatus is in a low temperature and low humidity state, the voltage applied to the attraction charger 6 is too high, so that the surface potential V1 of the transfer material conveying belt 5 tends to approach a saturation potential Vs of the transfer material conveying belt and therefore the above-mentioned requirement V2 - V1 > 0.5 kV cannot be satisfied, resulting in poor image transfer. Such poor image transfer occurs in a full-color electrophotographic copying machine equipped with a transfer material conveying belt, a transfer drum or the like. In the color copying machine, the visible image formed on the surface of the photosensitive drum 1 is repeatedly transferred by superimposed image transfer to the transfer material P three or four times for the respective colors to form a full-color image.

However, if the transfer material conveying belt 5 acquires a high potential by the attraction charging step, not only the difference V2 - V1 but also a difference (V3 - V'2) between the potential V'2 before the execution of the second transfer process and a potential V3 after the execution of the second image transfer process, a difference (V4 - V'3) between a potential V'3 before the execution of the third transfer process and a potential V4 after the execution of the third transfer process, and a difference (V5 - V'4) between the potential V'4 before the execution of the fourth transfer process and the potential V5 after the execution of the fourth transfer process all become less than 0.5 kV. Therefore, in the multi-color electrophotographic copying machine, the poor image transfer described above occurs.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the invention is to provide an image forming apparatus in which the transfer material can be well attracted by the transfer material carrying device regardless of the change in humidity in the environment in which the image forming apparatus is installed and the image can be properly transferred.

It is another object of the invention to provide an image forming apparatus having good electrostatic attraction means so that multiple images can be transferred onto the same transfer material with good registration.

According to the invention, an image forming device is created with the features listed in claim 1. Advantageous embodiments of the invention are listed in the subclaims.

The invention will become more apparent from the following description of preferred embodiments of the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sectional view of an image forming apparatus according to a first embodiment of the invention.

Figure 2 is a block diagram showing a control system in the image forming apparatus according to the first embodiment illustrated.

Figure 3 shows the contents of a table stored in a memory shown in Figure 2.

Figure 4 shows another table, also stored in the memory shown in Figure 2.

Figure 5 shows data according to which the table shown in Figure 4 is determined.

Figure 6 illustrates a method for measuring the attractive force for generating a force Fc shown in Figure 5.

Figure 7 is a sectional view of an image forming apparatus according to a second embodiment of the invention.

Figure 8 is a block diagram illustrating a control system of the image forming apparatus according to the second embodiment.

Figure 9 shows data on the basis of which the data of a table according to Figure 10 are determined.

Figure 10 shows a table stored in a memory shown in Figure 8.

Figure 11 shows data on the basis of which a table according to Figure 12 is determined and which are different from those shown in Figure 10.

Figure 12 shows a table stored in the memory of Figure 8 with data different from that of Figure 10.

Figure 13 is a sectional view of a color image forming apparatus according to a third embodiment of the invention.

Figure 14 is a block diagram illustrating a control system included in the color image forming apparatus according to the third embodiment.

Figure 15 shows data from which correct attraction current data are obtained, which are shown in a table according to Figure 16.

Figure 16 shows a table contained in a memory shown in Figure 14.

Figure 17 shows data from which the appropriate transmission current data stored in the table of Figure 16 are obtained.

Figure 18 shows data obtained when the color image forming apparatus according to the third embodiment is operated and the charge potential of the transfer sheet on the transfer drum is measured in the course of the copying operation.

Figure 19 shows an example of a conventional image forming device.

Figure 20 is a sectional view of a color image forming apparatus according to another embodiment of the invention.

Figure 21 is a sectional view of a color image forming apparatus according to another embodiment of the invention.

Figure 22 is a sectional view of a conventional image forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments are described in conjunction with the accompanying drawings.

Fig. 1 shows an image forming apparatus according to a first embodiment of the invention. The general structure of the image forming apparatus according to the first embodiment is similar to that of the image forming apparatus shown in Fig. 19. The image forming apparatus is provided with a transfer material conveyor belt as a transfer material carrying device. Around an image bearing member, namely a photosensitive drum 1, there are arranged a cleaning device 9, a pre-exposure lamp 10, a primary charger 2, a developing device 4, a transfer charging device, namely a transfer charger 8 and a transfer material conveyor belt 5 stretched around metal rollers 13, 14 and 15 as main components. The apparatus will now be described in more detail: between the primary charger 2 and the developing device 4, a space is formed through which image exposure light 3 is projected onto the outer peripheral surface of the photosensitive drum 1 by an image exposure device (not shown). The transfer material conveyor belt 5 is stretched in the form of a triangle around the metal rollers 13, 14 and 15. The metal rollers 13, 14 and 15 are electrically grounded. The transfer material conveyor belt 5 is rotated in the direction shown by an arrow in Figure 1, namely in the counterclockwise direction. by a drive motor (not shown) operatively coupled to the metal roller 15. Around the transfer material conveyor belt 5, there are arranged an attraction charging device, namely an attraction charger 6 for attracting the transfer material P (image receiving material) to the transfer material conveyor belt 5, an opposing roller 7, a discharger 11 for removing charge, a fur brush cleaning device 12 and so on.

In this embodiment, the attraction charger 6 has an opening width of 22 mm and is arranged such that the distance between its discharge wire and the transfer material conveyor belt 5 is 11 mm. The transfer material conveyor belt 5 is made of VPdF (polyvinylide fluoride) with a thickness of 150 µm. The opposing roller 7 is made of aluminum and has a diameter of 20 mm. It is electrically grounded and rotates following the transfer material conveyor belt 5. In this embodiment, a temperature and humidity measuring device, namely a temperature and humidity sensor 16, is provided for measuring the temperature and humidity of the environment in the color image forming apparatus. The temperature and humidity sensor 16 is arranged close to the transfer material conveyor belt 5 without disturbing the moving transfer material P. The temperature and humidity sensor 16 generates an output voltage by which the temperature and humidity in the apparatus are detected. The image forming function of the color image forming apparatus is the same as that of the apparatus shown in Fig. 19, and therefore the detailed description thereof is omitted for the sake of simplicity.

Figure 2 shows a control system of the The temperature and humidity sensor 16 shown in Fig. 2 generates a temperature signal, hereinafter referred to as a "T" signal, and a humidity detection signal, hereinafter referred to as an "H" signal. An A/D converter 506 converts the analog T signal into a digital signal and supplies it to an input/output unit 508, while an A/D converter 515 converts the H signal into a digital signal and supplies it to an input/output unit 507. A setting changing means, namely a CPU 510, controls the signals supplied to the input/output units 507 and 508 before controlling the sequence of image forming operations of the image forming apparatus. The CPU calls up a table 1 (Fig. 3) stored in a memory 511 and discriminates which of the ranges (1) to (6) of the table 1 the signals fall into. Based on the decision, the CPU 510 calls up a table 2 (Fig. 4) stored in the memory 511 and reads out attraction current level data corresponding to the T signal and the H signal from the table 2. The CPU then outputs the attraction current level data to a D/A converter 513 through an input/output unit 512. The D/A converter 513 receives the attraction current level data generated by the CPU 510 through the input/output unit 512 and converts it into an analog signal, which in turn is supplied to a high voltage source 514. The high voltage source 514 then supplies an attraction current corresponding to the attraction current level data to the attraction charger 6. The sequence of processing by the CPU 510 is carried out before image formation, namely, before the copying operation.

Further details will be given with reference to the Figures 3, 4, 5 and 6 are described.

Figure 3 shows the contents of Table 1 stored in the memory 511 shown in Figure 2. In Table 1, the areas (1) to (6) are divided and limited by several constant humidity level lines determined by temperature and humidity. It is understandable that the charging properties of the developer, the charging properties of the transfer material P and the moisture absorption and charging properties of the transfer material carrier sheet (the transfer material conveyor belt 5) are substantially the same, that is, the environmental conditions are substantially the same.

The data shown in Fig. 4 are the contents of Table 2 stored in the memory 511. In Table 2, corresponding to the areas (1) to (6), optimum attraction current levels at representative points in the areas according to the temperature and humidity in the environment in which the image forming apparatus is installed are included. The representative points are indicated by "x" in Fig. 3. The proper attraction current levels for the areas (1) to (6) shown in Fig. 4 are determined by the following process: first, a representative point ("x" in Fig. 3) is determined in each of the areas (1) to (6) in Fig. 3. Then, under the environmental conditions indicated by "x", a relationship between the attraction current level and the attraction force between the transfer material P (80g/paper) and the transfer material conveyor belt 5 is measured. The attraction force Fad between the transfer material P and the transfer material conveyor belt 5 is determined in this embodiment in the following manner:

As shown in Fig. 6, to attract the transfer material P to the transfer material conveyor belt 5, the attraction current Iad is supplied to the attraction charger 6, and immediately thereafter, a spring balance is attached to a leading edge of the transfer material with respect to the conveying direction of the transfer material P, by which the transfer material P is pulled along the conveying direction of the transfer material conveyor belt. The critical tensile force F (dyne) at which the transfer material P starts to slide on the transfer material conveyor belt 5 is measured. Then, the attraction force Fad is determined as the critical tensile force F (dyne) divided by a contact area S between the transfer material P and the transfer material conveyor belt 5. (1 dyne = 10-5 N).

Figure 5 shows data obtained by carrying out the above-described measuring method for the respective areas (1) to (6). In Figure 5, "Fc" indicates the minimum attraction force required for conveying the transfer material P with the transfer material conveyor belt 5. In this embodiment, the force is approximately 50 dyne/cm². The optimum attraction current Iad shown in Figure 4 is set to give the attraction force Fad which is slightly larger than the attraction force Fc shown in Figure 5. For the area (1) shown in Figure 4, the optimum attraction current is set to 40 µA which is slightly higher than the specified optimum level, since this area is within an unstable area in which the discharge from the attraction charger is likely to occur with the specified attraction current.

According to the above description of the first embodiment of the invention, due to the temperature detection signal and humidity detection signal output from the temperature and humidity sensor 16, a range is selected from the ranges shown in Fig. 3 or the like, and the attraction current supplied to the attraction charger 6 is controlled to the set value which is equal to the optimum attraction current determined in accordance with the selected range. Therefore, the attraction charger 6 can be supplied with the attraction current which changes in accordance with the change in the volume resistivity of the transfer material P and the change in the surface resistance of the transfer material conveyor belt 5 caused by the change in the moisture absorption of the transfer material P.

Fig. 7 shows an image forming apparatus according to a second embodiment of the invention. In this embodiment, an outside attraction charger 17 (corona charger) is used instead of the opposed roller 7 shown in Fig. 1. The other configurations are the same as those of the image forming apparatus according to the first embodiment, and therefore, their detailed description is omitted for the sake of simplicity. The outside attraction charger 17 has the same structure as the attraction charger 6. The outside attraction charger 17 has an opening width of 22 mm, and the distance between the discharge wire and the transfer material conveying belt 5 is 11 mm.

Figure 8 shows a control system incorporated in the image forming apparatus according to the second embodiment. In this embodiment, the outside attraction charger 17 is used instead of the opposing roller 7 and therefore the control system in this embodiment includes in addition to Among the elements included in the control system of the first embodiment, an input/output unit 516 connected to the outside attraction charger 17, a D/A converter 517, and a high voltage source 518. The input/output unit 516 corresponds to the input/output unit 512, the D/A converter 517 corresponds to the D/A converter 513, and the high voltage source 518 corresponds to the high voltage source 514, and therefore, for the sake of simplicity, the detailed description of these elements is omitted. The sequence of processing operations by the CPU 510 is similar to that in the first embodiment, and therefore will not be described in detail for the sake of simplicity.

In the memory 511, a table 3 is stored instead of the table 2 described above. The data contained in the table 3 relate to the environmental conditions (areas (1) to (6)) to which the image forming apparatus is subjected, for each of the areas, an optimum attraction current (Iadi) to be supplied to the inside attraction charger 6, and an optimum attraction current (Iado) to be supplied to the outside attraction charger 17 (Figs. 10 and 12). The optimum attraction currents for the inside and outside for each of the areas (1) to (6) shown in Fig. 10 are determined by the following process:

First, representative points ("x") are determined with respect to the environmental conditions defined as areas (1) to (6) in Figure 3, and at each of the representative points, the relationship between the inside attraction current Iadi (current for inside attraction), the outside attraction current Iado (current for outside attraction) and the attraction force between the transfer material conveyor belt 5 and the transfer material measured. Various combinations of the inside attraction current Iadi and the outside attraction current Iado can be considered. The inventors have carried out experiments on (1) the relationship between the currents Iadi and Iado and the attraction force between the transfer material P (80 g paper) and the transfer material conveyor belt 5 at Iadi = - Iado and (2) the relationship between the current Iadi and the attraction force between the transfer material p (80 g paper) and the transfer material conveyor belt 5 at Iado = - 100 µA.

As a result of experiment (1), the data shown in Figures 9 and 10 were obtained.

Figure 9 shows the relationship between the inside attraction current Iadi and the outside attraction current Iado in the areas (1) to (6) in the case that the inside attraction current Iadi and the outside attraction current Iado are changed in the same ratio.

Figure 10 shows the optimum inside attraction current and the optimum outside attraction current determined from Figure 9 as described above. The curves determining the regions (1) to (6) in Figure 9 are generally steep and in particular in the regions (1) and (2) the optimum values for stabilizing against the steepness of the curves are set at the shoulder of the respective curve. For this reason, the actual attraction force is considerably higher than the force represented by the point Fc which indicates the critical attraction force in Figure 9. As a result of the test (2), the values shown in Figures 11 and 12 were obtained. Figure 11 shows the relationship between the current Iadi and the attraction force in the case where the current Iado is fixed at - 100 µA. Figure 12 shows the optimum inside attraction current determined in accordance with Figure 11. Under the conditions determined in accordance with Experiments (1) and (2), the image forming operation of the image forming apparatus was carried out and good copy images of high quality were obtained without poor image transfer or slant conveyance of the transfer material.

As described above, in the image forming apparatus according to the second embodiment, based on the temperature detection signal and the humidity detection signal supplied from the temperature and humidity sensors 16, the areas shown in Fig. 3 are defined, and the inside attraction current and the outside attraction current, which are supplied via the inside attraction charger 6 and the outside attraction charger 17, respectively, are controlled to the set values for the optimum inside attraction current and the optimum outside attraction current, respectively, which are determined according to a selected range of the ranges shown in Fig. 3. Therefore, the inner attraction charger 6 and the outer attraction charger 17 can be supplied with attraction currents corresponding to the change in surface resistance of the transfer material conveyor belt 5 and the change in volume resistivity of the transfer material caused by the moisture absorption of the transfer material P.

Figure 13 shows a color image forming apparatus according to a third embodiment of the invention. This color image forming apparatus is provided with a transfer material carrier device in the form of a transfer drum. The general structure of the device is known and will therefore only be described briefly.

As shown in Fig. 13, an image transfer drum 18 having an outer peripheral opening portion covered with a transfer sheet formed by a PVdF sheet having a thickness of 150 µm is arranged substantially at the center of the color image forming apparatus 100. The transfer drum 18 is supported for rotation in the direction shown by an arrow (clockwise direction). In the transfer drum 18, an attraction charger 6, a transfer charger 8, a transfer sheet discharger 17a and a support brush 12b are arranged. On the outside of the transfer drum 18, a counter roller 7 is arranged opposite to the attraction charger 6, and a transfer material discharger 17b is arranged opposite to the transfer sheet discharger 17a. A peeling discharger 11 and a peeling blade 21 are disposed near the transfer material discharger 17b, and a transfer sheet cleaning brush 12a and a temperature and humidity sensor 16 are also disposed. At the position where the transfer charger 6 and the counter roller 7 face each other, there is one end of a transfer material guide mechanism for conveying and guiding the transfer material fed from a sheet supply tray 22 disposed on the right side of the apparatus 100 in Fig. 13. At the portion in the image forming apparatus 100 (the upper right portion in Fig. 13) where the peeling pawl 21 is disposed, there is an image fixing device 19, and a transfer material conveying belt is disposed between the fixing device 19 and the peeling pawl 21. In the upper right area in the image forming device is one end of a Discharge tray 20 at a location corresponding to the fixing device 19. In the upper region of the image forming apparatus 100 there is an original scanning station 3a which forms an optical system 3. In the upper left region of the apparatus 100 according to Figure 13 there is a color separation filter 3b which together with the original scanning station 3a forms the optical system 3. The original scanning station 3a contains an original illumination lamp, various deflection mirrors, a lens system, a color image sensor or the like. Substantially in the center of the image forming apparatus 100 there is an image bearing part in the form of a photosensitive drum 1, the outer circumference of which can be brought into contact with the outer circumference of the transfer drum 18. In the lower region of the apparatus there are four developing devices which are movable in a horizontal plane close to the outer circumference of the photosensitive drum. The horizontally movable developing devices 4 are described in detail below. The photosensitive drum 1 is rotatable in the direction of an arrow in Fig. 13 (counterclockwise). Around the photosensitive drum 1 are arranged various elements which are necessary together with the photosensitive drum 1 for carrying out the sequential image formation. They are the transfer drum 18, the transfer charger 8 and the horizontally movable developing devices described above, as well as a cleaning device 9, a primary charger 2 and the like. The horizontally movable developing devices 4 will now be described. They include a movable member 4a which is movable substantially in a horizontal plane, a yellow developing device 4Y, a magenta developing device 4M, a cyan developing device 4C and a black developing device. 4BK supported by the movable part 4a. The details of the respective elements and the image forming processes are not explained here since they are known.

Fig. 14 shows a control system used in the color image forming apparatus according to the third embodiment of the invention. In this embodiment, the attraction current supplied to the attraction charger 6 is controlled and also the transfer current supplied to the transfer charger 8 is controlled. Therefore, in addition to the elements described in connection with Fig. 2, the control system in this embodiment includes an input/output unit 519 connected to the transfer charger 8, a D/A converter 520 and a high voltage source 521. The input/output unit 519 corresponds to the input/output unit 512, the D/A converter 521 corresponds to the D/A converter 513 and the high voltage source 521 corresponds to the high voltage source 514, and therefore, for the sake of simplicity, the detailed description of these elements is omitted. The sequence of operations of the CPU 510 is similar to that of the first embodiment, and therefore the description thereof is omitted for the sake of simplicity. A Table 4 is stored in the memory 511 in place of the Table 2 described above. The data in the Table 4 include environmental conditions (ranges (1) to (6)) such as temperature and humidity to which the color image forming apparatus shown in Fig. 13 is subjected, appropriate attraction currents (Iad) for the attraction charger 6 determined for the respective environmental conditions, and optimum values for the transfer current supplied to the transfer charger 8 for the respective image transfer operations of the images developed in yellow, magenta, cyan and black (Fig. 16).

The optimum attraction current and the optimum transfer current for each of the areas (1) to (6) are determined in the following process: First, a representative point is selected for each of the areas (1) to (6) in Figure 3 ("x" in Figure 3). Then, at each of the representative points, the relationship between the attraction current Iad and the attraction force between the transfer material P (80g paper) and the transfer sheet is determined. Figures 15 and 16 show the obtained data.

In Figure 15, at a point Fc', an attractive force is shown which is at least necessary for the transfer sheet stretched over the opening of the transfer drum 18 to convey the transfer material P. As can be seen from Figure 15, in this embodiment, the force is approximately 55 dyne/cm². The reason why the attractive force Fc' is slightly larger than the attractive force Fc in the previous embodiments is that in this embodiment, the transfer drum 18 is used and therefore the effect of the curvature of the transfer material support member must be taken into account. Due to the curvature, the transfer material P tends to separate from the transfer drum due to the rigidity of the transfer material P or to slide on the transfer drum.

With respect to the data in Figure 16, an optimum attraction current value Iad is selected such that the attraction force Iad can be achieved which is slightly larger than the attraction force Fc. (In the region (1) in Figures 15 and 16, however, the optimum attraction current Fad giving the attraction force Fc' falls in a region in which the discharge process is not stable, and therefore (In this embodiment, although the optimum attraction current should be as high as possible, e.g., about 70 to 80 µA, the relatively low value of 40 µA is selected. The reason for this will be described below.) As is apparent from Fig. 16, in this embodiment, in addition to the optimum attraction current, an optimum transfer current for transferring the respective visible images in yellow, magenta, cyan and black is determined for each of the environmental conditions defined by the ranges (1) to (6). The optimum transfer current shown in Fig. 16 is determined in the manner shown in Fig. 17. In the graph in Fig. 17, the abscissa represents a transfer current supplied to the transfer charger from the high voltage source 521 and the ordinate represents the transfer efficiency. Here, the transfer efficiency is determined as follows: an area of 50 mm x 50 mm is determined on the outer peripheral surface of the photosensitive drum 1. The latent image forming conditions and the developing conditions are set so that a reflection image density of about 1.5 is obtained and a visible image is formed on the photosensitive drum 1. The transfer efficiency is determined based on the weight of the developer as follows: Transfer efficiency (%) = (weight of the developer to the transfer material) x 100 / [(weight of the developer to the transfer material) + (weight of the developer to the photosensitive drum after image transfer)].

In Figure 17, a curve (1) shows a relationship between the transfer current and the transfer efficiency in the case where an image visualized with a yellow developer (first developer) is transferred to the transfer material P under the condition that the transfer material P is attracted to the transfer sheet with an attraction current Iad of 40 µA. In the range between 0 and 100 µA, the transfer current is so weak that the transfer is insufficient, whereas in the range between 120 and 320 µA, the transfer current is sufficient for good image transfer. In the range above 340 µA, the transfer current is so strong that the polarity of the charge of the developer initially attracted away from the surface of the photosensitive drum 1 to the transfer material P is reversed by the transfer charge supplied from the transfer charger 8 and therefore a back transfer of the developer from the transfer material P to the surface of the photosensitive drum 1 begins. According to the characteristic curve (1), the optimum transfer current (IY) in the range (1) when transferring the first color developer is set to 140 µA.

A curve (2) in Fig. 17 shows a relationship between the transfer current IM and the transfer efficiency in image transfer for an image with magenta developer (a second color developer) in the case where the transfer current IY during transfer of the first color developer is 140 µA under the condition that the attraction current Iad is 40 µA. The characteristic curve (2) shows the relationship between the transfer current IM and the transfer efficiency as a result of the process in which the attraction current is set to 40 µA and the transfer current for the first color is set to 140 µA during execution of the image forming sequence at the region (1), and then the transfer current IM for the second color is supplied to the transfer charger 8. According to the characteristic curve (2), the optimum transfer current (IM) in the region (1) during transfer for the second color developer set to 240 µA.

A curve (3) in Fig. 17 shows the relationship between the transfer current IC and the transfer efficiency in the image transfer process for an image with a Zian developer (a third developer) when the transfer current IY during image transfer for the first color developer is 140 µA and the transfer current IM during image transfer for the second color developer is 240 µA, both under the condition that the attraction current Iad in the region (1) is 40 µA.

A curve (4) in Figure 17 shows the relationship between a transfer current Ibk and the transfer efficiency during transfer of an image with black developer (a fourth developer) in the case that, under the condition that the attraction current Iad in the region (1) is 40 µA, the transfer current IY during image transfer for the first color developer is 140 µA, the transfer current IM during image transfer for the second color developer is 240 µA, and the transfer current IC during image transfer for the third color developer is 340 µA. When the characteristics (3) and (4) were obtained, the same method as in obtaining the characteristics (1) and (2) was used. According to the characteristic curve (3), the optimal transfer current (IC) during the transfer of the third color developer is set to 340 µA and according to the characteristic curve (4), the optimal transfer current (IBk) during the transfer of the image with the fourth color developer is set to 440 µA. In the areas (2) to (6), the currents are determined in a similar manner.

In Figure 17, a curve (4') shows a relationship between a transfer current IBk with respect to the fourth color developer and the transfer efficiency in the case where the same experiments as those described above are carried out under the condition that the attraction current Iad is 70 µA. As can be seen from the curve (4'), the level of the transfer current IBk has a peak at a position where IBk is about 400 µA, but the transfer efficiency is only 65%. The transferred image obtained is not good and contains blank spots. In general, the transfer efficiency which gives a good image of high quality is not less than 75%. Therefore, it is considered that the insufficient transfer results from an excessive attraction current which causes saturation of the charge potential of the transfer sheet when transferring the visible image formed by the black developer (the fourth developer).

According to the above description, in a so-called superimposed image transfer step, the good image transfer is possible when the rise of the surface potential of the transfer sheet in each of the image transfer steps is not less than 0.5 kV. Actually, with respect to the region (1), the inventors operated the color image forming apparatus with the optimum attraction current and the optimum transfer current determined for the region (1), and measured the surface potential of the transfer sheet.

Figure 18 shows the results. The voltages (V2 - V1), (V3 - V2'), (V4 - V3') and (V5 - V4') were approximately 0.6 kV. When the current Iad was 70 µA, the voltage (V5 - V4') was 0.3 kV.

From the series of test results described above, the data shown in Figure 16, i.e., Table 4 shown in Figure 14 and stored in the memory 511, were obtained.

As described above, in the third embodiment of the invention, good color images of high quality can be obtained in the same way as in the first and second embodiments. In this embodiment, for ease of explanation, the currents to the attraction charger 6 and the transfer charger 8 are controlled to constant values, but constant voltage control is also possible.

As for the attraction charge, the polarity is determined to be the same as that of the transfer charge, but it may be opposite. The number of areas ((1) to (6)) may be increased or decreased as desired. As described, in the foregoing embodiments, the transfer material is always well attracted to the transfer material carrying device regardless of variations in the environmental conditions to which the image forming apparatus is subjected, and further, the image transfer operation can be properly carried out.

In the foregoing embodiments, a single photosensitive drum is used. Therefore, when toner images are transferred to the same transfer material in superposition, the transfer material passes through the same transfer position multiple times. However, superposed image formation on the same transfer material is also possible by using multiple photosensitive drums.

As for the method of attracting the transfer material, there is a method in which charging devices are arranged on the opposite sides of the transfer material conveying belt, and the electrostatic force for attracting the transfer material to the belt is applied from the belt and the transfer material. Such a case will now be described.

In Fig. 22, a color image forming apparatus is shown. The apparatus includes a transfer material conveyor belt 608 (as a conveyor device) for conveying transfer material 606, a fixing station 607, and four image forming stations or image forming units Pa, Pb, Pc and Pd which are arranged side by side in the conveying direction of the transfer material conveying belt 608. The image forming unit Pa, Pb, Pc or Pd each includes a photosensitive drum 601a, 601b, 601c or 601d, a latent image forming station 602a, 602b, 602c or 602d, a developing station 603a, 603b, 603c or 603d, a transfer station 604a, 604b, 604c or 604d and a cleaning device 605a, 605b, 605c or 605d which are arranged around the photosensitive drum 601a, 601b, 601c or 601d.

In the structure described above, a latent image of a yellow component of an original image is formed on the photosensitive drum 601a by the latent image forming station 602a of the first image forming unit Pa by a known electrophotographic process. Thereafter, the latent image is made visible in the developing station 603a with a developer containing yellow toner, and the yellow toner image thus formed is transferred to the transfer material 606 in the transfer station 604a.

While transferring the yellow image to the transfer material 606 in the transfer station 604a, the second image forming unit Pb forms a latent image through the latent image forming station 602b on the photosensitive drum 601b as a latent image for a magenta component of the original image. Then, the developing station 603b develops the latent image into a magenta toner image. The transfer material 602 having received the image from the first image forming unit Pa is fed into the transfer station 604b of the second image forming unit Pb. Then, the magenta toner image is transferred to the predetermined position on the transfer material 606.

Also, the cyan color image and the black color image are similarly formed and transferred to the transfer material 606 to obtain a superimposed toner image of four colors. The transfer material 606 is conveyed to the image fixing station 607 where it is subjected to image fixing, whereby the multi-color or full-color image is fixed to the transfer material 606. After the image transfer operations, the respective photosensitive drums 601a, 601b, 601c and 601d are cleaned by the cleaning devices 605a, 605b, 605c and 605d, respectively, so that the respective remaining toners are removed for subsequent latent image forming operations.

It has been proposed to use a thin sheet of polyethylene terephthalate resin or polyimide resin as an insulating material as the material for the transfer material conveying belt 608. The proposed material has high tensile elasticity and high transmittance for speed control of the transfer material conveying belt 608, and the volume resistivity is generally as high as 10¹⁶ ohm·cm, so that it is therefore preferable for attracting the transfer material 606 to the transfer material conveying belt 608. However, when the belt made of such a material is used as the transfer material conveying belt 608 of the color image forming apparatus, a plurality of image transfer operations are carried out for forming an image, and the transfer material conveying belt 608 is electrically charged each time the image transfer process is carried out. Therefore, uniform image transfer cannot be maintained unless the transfer current is successively increased with the repetition of the transfer process. As a result, it is advantageous to create of an image, the electric charge remaining on the transfer material conveyor belt 608 down to a predetermined low potential level by means of some device such as a discharge brush or an AC discharger. If the discharge brush is used, which is advantageous in terms of cost, the discharge tends to be uneven and those parts of the transfer material conveyor belt 608 which are not sufficiently discharged result in insufficient image transfer in the transfer process in the next image formation. On the other hand, if the AC discharger is used, the attraction charge must be carried out after the discharge, resulting in wasteful power consumption, although the above-mentioned uneven discharge can be avoided.

To solve the problems, a system has been developed in which the belt discharging and the electrostatic attraction are accomplished at once. In the color image forming apparatus of this type, before carrying out the image transfer process, AC discharging operations are simultaneously carried out on the transfer material conveyor belt 608 and the transfer material 606, whereby the conveyor belt 608 and the transfer material 606 are uniformly discharged while simultaneously attracting the transfer material 606 to the transfer material conveyor belt 608. This system reduces the cost of the equipment and allows the space in the apparatus to be used effectively.

However, even when using the system described above, a problem arises: the attraction force between the transfer material conveyor belt and the transfer material 606 changes considerably according to the environmental conditions to which the apparatus is subjected, particularly the humidity of the environment, even to such an extent that it becomes difficult to separate the transfer material 606 from the transfer material conveyor belt 608 after completion of the sub-image transfer.

In view of this, as shown in Fig. 21, an outlet 614 for the transfer material and an image fixing device 607 facing the outlet 614 are provided on the left side of the main body 610 of the image forming apparatus as shown in Fig. 21. On the other hand, a sheet feeding mechanism 613 is arranged on the right side of the main body 610 of the apparatus as shown in Fig. 21. In the area in the main body 610 from the sheet feeding mechanism 613 to the fixing device 607, the transfer material conveying belt 608 extends. The belt 608 has the form of an endless belt which extends between a drive roller device, namely a drive roller 611 arranged close to the sheet feeding mechanism 613, and a follower roller device, namely a deflection roller 612 arranged close to the fixing device 607. The tension on the belt is adjustable by means of an adjusting roller 676. Furthermore, in the area from the drive roller 611 to the guide roller 612 close to the transfer material conveying belt 605, the image forming units Pa, Pb, Pc and Pd are placed next to each other in the above-mentioned sequence from the sheet feeding mechanism 613.

The transfer material conveying belt 608 is driven by the drive roller 611 in the direction of an arrow in Fig. 21 to pick up the transfer material 606 fed from the sheet feeding mechanism 613 and sequentially convey it to the image forming units Pa, Pb, Pc and Pd. In this embodiment, the transfer material conveying belt 608 is made of a material which has a small longitudinal elongation to effectively transmit the rotation control of the drive roller 611 and which has no significant effect on the transfer corona current during the transfer process, such as a polyurethane belt having a thickness of 100 µm, a rubber hardness of 97º D and an elongation elasticity of 16000 kg/cm², which is available from Hokushin Kogyo Kabushiki Kaisha, Japan. The sheet feeding mechanism 613 includes a sheet feeding guide 651 for guiding the transfer material 606 fed from the outside, a pair of registration rollers and a sensor 652 which generates an output signal upon detecting a leading edge of the transfer material 606 moving in the sheet feeding guide 651. The mechanism supplies the transfer material 606 from the drive roller 611 to the transfer material conveyor belt 608. The fixing device 607 takes up the transfer material 606 from the guide roller 612 and fixes the visible image transferred onto the transfer material 606 by the image forming units Pa, Pb, Pc and Pd. The image forming units Pa, Pb, Pc and Pd have substantially the same structure. Each of the image forming units Pa, Pb, Pc and Pd includes a latent image bearing member in the form of an electrophotographic photosensitive drum 601a, 601b, 601c or 601d rotatable in the direction shown by an arrow, a charger 615a, 615b, 615c or 615d, a developing device 603a, 603b, 603c or 603d, a transfer discharger 604a, 604b, 604c or 604d, a cleaning device 605a, 605b, 605c or 605d and a laser beam scanner 616a, 616b, 616c or 616d, which are arranged around the respective photosensitive drum in the above-mentioned sequence in the direction of drum rotation.

The developing devices 603a, 603b, 603c and 603d contain yellow toner, magenta toner, cyan toner and black toner, respectively.

Each of the laser beam scanners 616a, 616b, 616c and 616d includes a semiconductor laser, a polygonal mirror and an f-θ lens. The scanner receives electrical digital dot signals to generate a laser beam modulated in accordance with the signal for scanning the drum surface in the direction of the drum generation line at a location between the charger 615a, 615b, 615c or 615d and the developing device 603a, 603b, 603c or 603d for imagewise exposing the respective drum with the respective laser beam scanner 616a, 616b, 616c and 616d according to picture element signals supplied in accordance with a yellow component image, a magenta component image, a cyan component image and a black component image, respectively. In this embodiment, a first charging device, namely, an attraction charger 659, and a second charging device, namely, an attraction charger 662, are arranged between the image forming unit Pa and the sheet feeding mechanism 613, sandwiching the transfer material conveying belt 608 therebetween. The attraction chargers 659 and 662 perform corona discharge for securely attracting the transfer material 606 fed from the sheet feeding mechanism 613 to the transfer material conveying belt 608. The attraction charger 659 and the attraction charger 662 will be described in detail below. A discharger 661 is arranged substantially right above the guide roller 612 between the image forming unit Pd and the fixing device 607. An alternating voltage is applied to the unloader 661 to release the transfer material 606 from the conveyor belt 608.

A sensor 660a, 660b, 660c and 660d is arranged upstream of each of the image forming units Pa, Pb, Pc and Pd. Each of the sensors 660a, 660b, 660c and 660d detects a leading edge of the transfer material 606 conveyed by the transfer material conveyor belt 608 to supply an electronic control circuit device, i.e., a control unit (not shown), with a signal for starting the image forming process in the respective image forming units Pa, Pb, Pc and Pd.

When the transfer material 606 in the form of a cut sheet is fed to the sheet feed guide 651 of the sheet feed mechanism 613, the leading edge of the transfer material 606 is detected by the sensor 652, and a start signal for starting the rotation of the photosensitive drums 601a, 601b, 601c and 601d of the image forming units Pa, Pb, Pc and Pd is generated by the sensor 652. At the same time, the drive roller 611 is driven so that the transfer material conveying belt 608 starts to rotate in the direction shown by an arrow.

When the transfer material 606 is guided along the sheet feed guide 651 and placed on the transfer material conveying belt 608, it is subjected to corona discharge by the attraction charger 659 and is securely attracted to the transfer material conveying belt 608. When the transfer material conveying belt 608 moves in the direction shown by an arrow in Fig. 21, the leading edge of the transfer material 606 is detected by the sensors 660a, 660b, 660c and 660d, respectively, whereupon the respective image forming operations are sequentially initiated on the photosensitive drum 601a, 601b, 601c and 601d. In detail, the first image forming unit The first image forming unit Pa forms a yellow image on the photosensitive drum 601a, the second image forming unit Pb forms a magenta image, the third image forming unit Pc forms a cyan image, and the fourth image forming unit Pd forms a black image. The image forming process in each of the image forming units Pa, Pb, Pc, and Pd is the well-known Carlson process, so a detailed description is omitted for the sake of simplicity.

By the movement of the transfer material conveying belt 608, the transfer material 606 is conveyed to the fixing device 607 via the portions below the photosensitive drum 601a to 601d of the first, second, third and fourth image forming units Pa to Pd, while the transfer chargers 604a, 604b, 604c and 604d sequentially transfer the respective color images to the same transfer material 606 to form a composite color image. After the transfer material has passed through the fourth image forming unit Pd, it is electrically discharged by the discharger 661 supplied with an AC voltage and released from the transfer material conveying belt 608. The transfer material 606 released from the transfer material conveyor belt 608 is introduced into the fixing device 607 where it is subjected to image fixing. Thereafter, it is discharged from the apparatus 610 via the outlet 614. In this way, one printing cycle ends.

In this embodiment, the polarity of the high voltage applied to the attraction charger 662 is the same as that of the high voltage applied to the transfer chargers 604a, 604b, 604c and 604d. The polarity of the high voltage applied to the attraction charger 662 is opposite to that applied to the charger 659.

In this embodiment, the distance between the wire of the respective attraction chargers 659 and 662 for attraction discharge and the transfer material conveyor belt 608 is 15 mm, while the distance between the attraction discharge wire and the backed electrode plate of the respective attraction charger is 8.5 mm. The total amount of current supplied to the attraction charger 659 is 500 µA, and that to the attraction charger 662 is 300 µA. As shown in Fig. 20, the attraction charger 659 is connected only to an AC constant voltage source 680, so that only an AC voltage is supplied thereto. On the other hand, the attraction charger 662 is connected to an AC high voltage source 681 connected in series with a DC voltage source 682 so that an AC voltage with DC bias is supplied thereto. A known humidity sensor (not shown) is arranged at a suitable location in the device 610. The humidity sensor will be described below. The power supply system will be described in more detail. The AC constant high voltage source 680 and the AC constant high voltage source 681 have the same rating. The DC voltage source 682 serves to add a DC voltage of positive polarity to the AC voltage from the constant voltage source 681 and the composite voltage is supplied to the attraction charger 662.

In the image forming apparatus described above, normally used copy paper (80 g paper) is used as the transfer material 606, and the force required to separate the transfer material 606 from the transfer material conveyor belt 608 is determined by measuring with a force gauge (a spring balance) the force required to displace the transfer material 606 electrostatically attracted to the transfer material conveyor belt 608 in the horizontal direction as shown in Fig. 20 is measured. The following are data at normal temperature and normal humidity (25ºC, 60% RH), high temperature and high humidity (30ºC, 90% RH), and low temperature and low humidity (10ºC, 10% RH). Table 1 Temperature and humidity Invention State of the art Other embodiment of the invention Normal High Low

Increasing the force for attracting the transfer material 606 to the transfer material conveyor belt 608 at low humidity as shown by the data in Table 1 could reduce difficulties in releasing the transfer material 606 from the transfer material conveyor belt 608 after the overlay image transfer. on the transfer material 606. The separation becomes difficult particularly when the transfer material 606 used is thin, such as 60 g paper. The difficulty of separating the transfer material 606 from the transfer material conveyor belt 608 varies depending on various conditions during the separation, such as the camber of the guide roller 612 (see Fig. 21) or the moving speed of the transfer material conveyor belt 608. In experiments by the inventors, unsatisfactory separation occurred at an attractive force of not less than 200 g when the rollers 611 and 612 had a diameter of 40 mm, the moving speed of the conveyor belt 608 was 85 mm/s, the unloader 661 was not turned on, the relative humidity was 10%, and the transfer material 606 was a copy paper with a nominal weight of 60 g.

On the other hand, when the transfer material 606 used is thicker, namely, not less than 120 g in nominal weight, the reduction in the force of attraction of the transfer material 606 to the transfer material conveyor belt is considerable under high humidity. In this case, the attraction force is insufficient, resulting in the alignment between the images formed by the image forming units Pa to Pd being disturbed.

To solve the problem, the color image forming apparatus according to this embodiment is provided with a humidity sensor (of a known type) in the main body of the apparatus 610. Based on the detection of the relative humidity by the humidity sensor, the attraction force between the transfer material 606 and the transfer material conveyor belt 608 is controlled. In Specifically, in this embodiment, the humidity state is divided into three regions, namely, the region below 30%, from 30 to 70%, and over 70%, and according to the regions, the attraction of the transfer material 606 to the transfer material conveyor belt 608 is changed. For example, when the relative humidity is not more than 30%, the DC voltage applied to the attraction charger 662 is lowered from +2.32 kV, which is the voltage at the normal state (the relative humidity of 30 to 70%), to about +1.0 kV. On the other hand, when the relative humidity is at least 70%, the DC voltage is increased to about +4.0 kV. The attraction force between the transfer material 606 and the transfer material conveyor belt 608 controlled in this way is shown in the left column of Table 1.

The inventors' repeated investigations and experiments have shown that the same effects can be obtained by phase shifting the AC voltages applied to the attraction charger 659 and the attraction charger 662. Specifically, in the structure shown in Fig. 20, the phase of the AC voltage applied to the attraction charger 659 and the phase of the AC voltage applied to the attraction charger 662 were shifted by 180° (antiphase) from each other, and the force required to peel off the transfer material was measured. The data are shown in the right column of Table 1. The data in the right column of Table 1 are, as in the previous description, those at a nominal weight of 80 g of the transfer material 606 (copy sheet) and at normal temperature and normal humidity (25º C and 60%), at high temperature and high humidity (30º C, 90%) and at low temperature and low humidity (10º C, 10%). According to the above, The level of the DC voltage applied to the attraction charger 662 is controlled at high humidity and at low humidity, respectively.

When comparing the data in the left column of Table 1 with the data in the right column, the attraction force in this control system is generally stronger than in the control system described above. The attraction state in this control system can be used satisfactorily when the separation between the transfer material 606 and the transfer material conveyor belt 608 is facilitated by, for example, increasing the camber of the guide roller 612. Alternatively, to obtain the attraction force corresponding to the data in the left column, the level of the DC voltage applied to the attraction charger 662 may be generally lowered. It has been confirmed by a surface potential meter, image data or the like that the transfer material conveyor belt 608 is electrically discharged uniformly by the AC voltage applied to the attraction chargers 659 and 662 so that it has a uniform surface potential.

As described in the foregoing, according to the embodiments, an image forming apparatus can be provided in which, without increasing the cost and without requiring additional space, the transfer material conveying device can be uniformly discharged regardless of the ambient humidity, the transfer material can be electrostatically attracted to the transfer material conveying device, and after the overlay transfer process is completed, the transfer material can be easily separated from the transfer material conveying device.

The invention is not limited to color image formation, but is also effective in a black transfer device or a single color transfer device. The attraction device has been described as a corona discharger, but may also have another form if it applies a bias voltage to generate the electrostatic attraction force.

The invention is not limited to an image forming apparatus in which the image transfer step is required, but is also applicable to an image forming apparatus in which an image is formed directly on an image receiving material.

While the invention has been described with reference to the embodiments disclosed herein, it is not limited to the details given and this application is intended to cover such modifications or changes as come within the scope of the following claims.

Claims (15)

1. Image forming apparatus for forming an image on an image receiving material (P, 606) in which the image receiving material (P, 606) is electrostatically attracted to a carrier device (5, 18, 608) by the action of an attraction device (6, 7, 17, 659, 662), characterized in that a sensor device (16, 510) for detecting the humidity and/or temperature of the image receiving material (P, 606) and a change device (510, 511) for controlling the function of the attraction device (6, 7, 17, 659, 662) in accordance with output signals of the sensor device (16, 510) are provided. 2. An image forming apparatus according to claim 1, further comprising an image bearing member (1, 601), an image forming device (2, 3, 4, 615, 616, 603) for forming an image on the image bearing member (1, 601), and a transfer device (8, 604) for electrostatically transferring the image from the image bearing member (1, 601) to the image receiving material (P, 606) conveyed on the carrier device (5, 18, 608), wherein the attracting device (6, 7, 17, 659, 662) is designed to electrostatically attract the image receiving material (P, 606) to the carrier device (5, 18, 608) before the image transfer process, and the changing device (510, 511) is designed to change the electrostatic output power of the transfer device (8, 604) and the output power of the attraction device (6, 7, 17, 659, 662) with respect to the same image receiving material (P, 608) according to the output signals is formed from the sensor device (16, 510). 3. Device according to claim 1 or 2, characterized in that the carrier device (5, 18, 608) for conveying the image receiving material (5, 608) contains a dielectric part which is movable along an endless path. 4. Apparatus according to claim 2 or 3, characterized in that means are provided for repeating the image transfer by the transfer device (8, 604) to the same image receiving material (P, 608) and that the electrostatic output power of the transfer device (8, 604) is increased with the repetition. 5. Device according to one of claims 2 to 4, characterized in that the sensor device (16) serves to detect the temperature and the humidity in the device, the changing device (510, 511) contains several environmental areas delimited by several constant moisture quantity curves determined by the temperature and the humidity, wherein a area is selected according to the temperature and humidity detected by the sensor device and the changing device causes the change of the electrostatic output powers of the transmission device and the attraction device according to the selected area. 6. Device according to one of claims 2 to 5, characterized in that the attraction device (6, 7, 17, 659, 662) contains a corona discharge device facing the carrier device (5, 18, 608) and the changing device changes an output power of the corona discharge device. 7. Device according to claim 6, characterized in that the attraction device contains a grounded rotatable part (7) which is in contact with that side of the carrier device which faces away from the corona discharge device (6). 8. Device according to one of claims 2 to 7, characterized in that the image carrier element (1, 601) is a photosensitive material. 9. Device according to one of claims 2 to 8, characterized in that the changing device (510, 511) changes the electric current supplied to the attraction device (6). 10. Device according to one of claims 2 to 9, characterized in that the transfer device (8) transfers several images to the same image receiving material (P, 606) conveyed on the carrier device (5, 18, 608). 11. Device according to claim 10, characterized in that the device is suitable for producing a full-color image. 12. Apparatus according to any one of claims 2 to 10, characterized in that it comprises a device (604a - d) for sequentially transferring a plurality of developed images to an image receiving material (P, 606) and in that the changing means (510, 511) increases the output of the transfer device (8, 604) in the initial image transfer of the sequence together with an increase in the output of the attraction device (6). 13. Device according to one of claims 2 to 12, characterized in that the attraction device comprises a Carrier device (5, 18, 608) includes an inside attraction device (6, 662) having a charge polarity which is the same as that of the transfer device (8, 604), and that an output power of the transfer device is increased together with an increase in an output power of the inside attraction device (6, 662). 14. Device according to one of claims 1 to 13, characterized in that the attraction device (6, 662) is supplied with a periodic alternating voltage which is biased with a direct voltage. 15. Device according to claim 14, characterized in that the changing device changes the direct voltage.