Image forming apparatus and method

Abstract: An image forming apparatus includes an image bearing member for carrying a toner image; an image forming means for forming a toner image on the image bearing member;a transfer material carrying member, for carrying a transfer material, wherein the toner image is transferred onto a transfer material carried on the transfer material carrying member or onto the transfer material carrying member; density detecting means for detecting a density of the toner image transferred to the transfer material carrying member; wherein a transfer intensity is smaller when the toner image for density detection is transferred onto the transfer material carrying member than when the toner image is transferred onto the transfer material carried on the transfer material carrying member.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus wherein a toner image is transferred from an image bearing member such as photosensitive drum onto a transfer material carried on a transfer material carrying member such as transfer drum, or transfer belt. Generally, in a color image forming apparatus of electrophotographic type, a positive color tone is not provided if the image density varies due to various conditions such as ambience change, number of prints. Therefore, in order to discriminate the circumstance during image formation, a toner image (patch) for maximum density (Dmax) detection for each color toner is formed on photosensitive drum as a test image, and the density thereof is detected by an optical sensor. The detection result is fed back to the image forming condition such as developing bias to maintain the Dmax for each toner at a predetermined level. This control of the maximum density is known as Dmax control. In order to provide a high quality image, the Dmax for each toner is desirably maintained at a predetermined level, and in addition, the tone gradient reproduction is also desirably correct. In view of this, a plurality of half-tone patches from low density to high density are formed for each toner as test images, and the densities are detected. On the basis of the detection results, a correction (so-called &g& correction) is effected to provide a linear relation between the image signal and the resultant image density (half-tone control). On the other hand, in order to downsize the main assembly of the device, diameter reduction of the photosensitive drum is effective. This is because the circumferential length of the transfer drum has to be at least the length of the transfer material usable with the apparatus. In order to eliminate the necessity of the provision of a sensor around the photosensitive drum, it has been proposed to transfer a patch image formed on the photosensitive drum onto the transfer drum and then to detect the transferred patch image by a sensor provided adjacent the transfer drum. However, there arises a problem that the first sheet after the density control with the patch image on the transfer material drum, involves back side contamination. The cause has been found as being that the patch image formed for the density control is not completely cleaned with the result that the transfer drum is contaminated after the density control. There is a problem that under the low humidity ambience or high humidity ambience, correct image density, or color tone is not provided despite the density control carried out. This is because the correct density control is not carried out because of the deterioration of the transfer action due to the shortage of the transfer charge or the overage of the transfer charge resulting in penetration due to the change of the patch toner polarity. That is, when the image is transferred with low transfer efficiency as a result of transfer defect or penetration (thin image transfer), the density control increases the developing bias despite the fact that the satisfactory development is effected, resulting in the higher density developed image. Thus, positive image density is not provided, and the tone gradient reproducibility becomes poor. From US-A-5 294 959 it is known to transfer test patches to the transfer drum at various transfer currents hereby adjusting image density in dependence of ambient conditions. SUMMARY OF THE INVENTION Accordingly, it is a principal concern of the present invention to provide a control system for an image forming condition of image forming means on the basis of detection of a toner image for density detection. It is another concern of the present invention to provide a transfer system for properly transferring the toner image for the density detection onto the transfer material carrying member. In accordance with a first aspect of the present invention there is provided image forming apparatus as set out in claim 1. A method for forming an image is set out in claim 14. In order that the present invention may be more readily understood, reference will now be made by way of example to the accompanying drawings, in which: Figure 1 is an illustration of an image forming apparatus according to embodiment 1 of the present invention. Figure 2 is a major part illustration of a transfer device of an image forming apparatus according to embodiment 1. Figure 3 is a graph showing a relation btjj a transfer current and Q/M of toner after the transfer. Figure 4 is an illustration of an image forming apparatus according to embodiment 2 of the present invention. Figure 5 is s graph showing a transfer efficiency (for temperature/humidity, respectively) during normal print Figure 6 is a graph showing transfer efficiency (for temperature/humidity, respectively during density detection. Figure 7 is a graph showing transfer efficiency (for respective PWM signal data) during density detection. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 is a sectional view of a full-color image forming apparatus of an electrophotographic type according to an embodiment of the present invention. In the color image forming apparatus, an image bearing member 3 in the form of an electrophotographic photosensitive drum is rotated in a direction indicated by the arrow, and is charged uniformly by charging means 10 during the rotation, and thereafter, it is subjected to a light image projection by a laser exposure device 11 or the like so that the electrostatic latent image is formed on the photosensitive drum 3. The latent image is developed into a visualized image, namely toner image by developing devices 1a, 1b, 1c, 1d containing color developers such as yellow (Y), magenta (M), cyan (C), developers, for example, carried on a rotatable supporting member. In this example, reverse development is used wherein the toner is deposited on the low potential portion provided by the light projection. On the other hand, the transfer material 7 is fixed by a gripper 5 on a transfer device 2, having a drum type transfer material carrying member. More particularly, it is electrostatically attracted on the transfer drum 2 by an attracting device 8. The attracting device 8 comprises, as shown in Figure 2, an aluminum core metal 21, an elastic layer 22, thereon and a dielectric layer 23 for attracting the transfer material on the surface thereof. The toner image on the photosensitive drum 3 is transferred onto a transfer material 7 wound around the transfer device, namely the transfer drum 2 in this example by applying a voltage between the aluminum core metal 21 functioning also as a transfer electrode and the elastic layer 22 from the voltage source 17. More particularly, an electrostatic latent image formed on the photosensitive drum 3 by the exposure based on an image signal for a first color, is visualized by a developing device la accommodating the yellow (Y) developer, and thereafter, it is transferred onto the transfer material 7 carried on the transfer drum 2. Subsequently, the remaining developer on the photosensitive drum 3 is removed by a cleaner 12, and thereafter, an electrostatic latent image for the second color is formed on the photosensitive drum 3 by the exposure based on an image signal for the second color. It is visualized by a developing device lb having a magenta (M) developer, for example. Then, it is overlyingly on transferred on the transfer material 7 on the transfer drum 2 having the yellow visualized image. Subsequently, the same process is repeated, and the cyan (C), and black (Bk) toner images are overlyingly transferred onto the transfer material 7 on the transfer drum 2. Thereafter, the transfer material 7 is discharged by a separation discharger 6, and is separated from the transfer drum 2 by a separation claw 14, and the image is fixed by a fixing device 4 into a permanent image. The transfer drum 2 after the transfer material 7 separation, is cleaned by a transfer member cleaner 13 so that the developer is removed from the surface thereof, and is discharged by a discharger 9 to be electrically initialized. In this embodiment, the density detection is carried out in the following manner. First, a density detection patch image (patch) of the maximum density (Dmax) of yellow (Y) is formed on the photosensitive drum 3. The patch is transferred onto the transfer drum 2, and the density of the patch is detected by a density sensor 15. Subsequently, a patch image for the Dmax detection is formed with magenta (M) color toner on the photosensitive drum 3, and is transferred onto the transfer drum at a position different from that of the Y toner patch. The density of the patch is detected by the density sensor 15. Similarly, the densities of the cyan (C), and black (Bk) toner images are detected to effect the Dmax control. The order of the colors of the patch images for the density detection may be different. On the basis of the output of the density sensor, the image forming condition such as a application voltage, or developing bias of the charger 10 is controlled. In this embodiment, a transfer intensity upon the transfer of the density detection patch image onto the transfer drum 2, is made smaller than the transfer intensity upon the transfer of the toner image onto the transfer material 7 carried on the transfer drum 2. Therefore, the patch image which is not transferred to the transfer material can be easily removed. In this embodiment, in order to reduce the transfer intensity, the transfer bias V pat applied from the voltage source 17 upon the density detection operation is made smaller than the transfer bias V tr applied from the voltage source 17 upon the transfer of the toner image onto the transfer material. Preferably, V pat ≤(4/5)V tr is satisfied. Conventionally, the transfer bias upon density detection is the same as the transfer bias upon the normal print. However, the total electrostatic capacity of the nip is larger during the density detection than during the normal print, corresponding to the absence of the transfer material, and therefore, a larger transfer current flows during density detection if the same bias voltage is applied. In a transfer drum type as in this embodiment, the larger the transfer current (positive) as shown in Figure 3, the larger the charge of the opposite polarity (negative) from the transfer charge is induced in the toner, with the result of higher Q/M (-µC/g) of the toner after the transfer increases. The reason is that by application of the charge (positive) of the same polarity as the transfer onto the rear surface of the dielectric layer 23, the air is ionized in the small clearance downstream of the nip between the transfer drum 2 and the photosensitive drum 3, so that negative charge is applied on the surface of the dielectric layer 23 and the dielectric layer 23. Thus, with increase of the negative charge of the toner and the positive charge on the dielectric layer 23 rear surface, the Coulomb force between the toner and the transfer drum dielectric layer 23 increases, and therefore, the cleaning property become poor. The following Table 1 shows a relation between the transfer bias for the first color density detection and cleaning property : (First Color) Vtr1=1000V Transfer Bias (V) 300 500 800 900 1000 1200 Cleaning Property G G G F NG NG G: good F: fair NG: Not good Here, upon 1000V of transfer bias, the transfer current is 14.1µA, and upon 900V, the current is 10.6µA, and upon 800V, it is 7.2µA. It is understood that with the increase of the transfer current, the Q/M of the toner after the transfer increases with the result of the poor cleaning property. Tables 2-4 show relations between the transfer biases for the density detections for the second to the fourth colors and the cleaning property. (Second color) VTr2 = 1200V Transfer Bias (V) 550 900 1000 1100 1200 1400 Cleaning Property G G F NG NG NG (Third color) VTr3 = 1400V Transfer Bias (V) 600 1100 1200 1300 1400 1600 Cleaning Property G G F NG NG NG (Fourth color) VTr4 = 1400V Transfer Bias (V) 650 900 1200 1400 1600 1800 Cleaning Property G G G F NG NG It has been found that there is an interrelation between the transfer bias and the cleaning property for each color upon the density detection and the transfer bias upon the normal print, more particularly, if the transfer bias during the density detection is not more than 4/5 of the transfer bias during the normal print, the cleaning property is good. In this embodiment, the photosensitive drum is of OPC having a negative charging property. It comprises a charge generating layer and the charge transfer layer having a thickness of 25 microns. The transfer drum comprises a core metal 21 of aluminum as a transfer electrode, an elastic member 22 having a thickness of 5.5 aluminum and a volume resistivity of 10 4 Ohm.cm or smaller, and a dielectric member 23 having a thickness of 75 microns and a volume resistivity of 10 14 -10 16 Ohm.cm. The transfer bias during the normal print was 1000V, 1200V, 1400V, 1600V, for the first to fourth colors, and the transfer bias upon density detection was 500V, 550V, 600V, 650V, by which the cleaning was easy, and the back side contamination of the first sheet after the density control could be prevented. If the transfer bias during the transfer of the density detection patch is too small, the transfer efficiency of the patch image is low, and therefore, the V pat ≥(1/5)V tr is preferable. In this embodiment, the transfer biases are different during the density detection and the normal print, but the DC current to be supplied from the voltage source 17 during the density detection may be made smaller than the normal print. Embodiment 2 Referring to Figure 4, a second embodiment will be described. The same reference numerals as in the first embodiment are assigned to the elements having the corresponding functions, and detailed descriptions thereof are omitted for simplicity. In this embodiment, the temperature/humidity of the ambience is detected by an ambient condition detecting sensor 16, and the transfer bias is changed on the basis of the detection result. In this embodiment, even if the temperature/humidity of the ambience changes, the transfer of the patch image during the density detection is made optimum and the proper density control is assured. If the temperature/humidity of the ambience changes, the resistance, and the electrostatic capacity of the dielectric layer 23 and the like change. For example, under a low temperature and low humidity ambience, the resistance of the dielectric layer 23 is high, and the electrostatic capacity is low. The resistance and electrostatic capacity of the transfer material 7 changes. In this embodiment, the toner is transferred onto the transfer drum 2 by the potential difference between the photosensitive drum 3 and the transfer drum 2. Therefore, when the electrostatic capacity at the transfer position decreases, the potential difference between the photosensitive drum 3 and the transfer drum 2 reduces as compared with the case of the normal temperature normal humidity ambience even if the same bias is applied. So, improper transfer results. On the contrary, under a high temperature and high humidity ambience, the potential difference is large with the result of discharge at the transfer position, and therefore, the improper transfer. In this embodiment, in order to provide a high transfer efficiency irrespective of the ambient condition change, the temperature and humidity in the device are detected by a sensor 16, and the transfer bias is controlled on the basis of the detection result. For example, as shown in Figure 5, during the normal print, the transfer bias for the first color is 800(V), under 38°C, 80% ambience, and 1000(V), under 23°C, 60% ambience, and 1200(V) under 15°C, 10% ambience. As shown in Table 5, the transfer bias for the density detection is controlled on the basis of the detection result of the sensor 16. This is because there is no transfer material 7 at the transfer position during the density detection, but the electrostatic capacity of the dielectric layer 23 changes depending on the ambience. During the density detection, there is not transfer material 7 in the transfer position, and therefore, the total electrostatic capacity is larger than during the normal print operation. Accordingly, as shown in Table 5, for example, during the density detection, transfer bias, for the first color is 350(V), under 30°C, 80% ambience, and 500(V), under 23°C, 60% ambience, and 700(V) under 15°C, 10% ambience. In this embodiment, transfer bias for the density detection is smaller than the transfer bias for the normal print under the same ambient condition. In this embodiment, the photosensitive drum is of OPC having a negative charging property. It comprises a charge generating layer and the charge transfer layer having a thickness of 25 microns. The transfer drum comprises a core metal 21 of aluminum as a transfer electrode, an elastic member 22 having a thickness of 5.5mm core metal 21 and a volume resistivity of 10 4 Ohm.cm or smaller, and a dielectric member 23 having a thickness of 75 microns and a volume resistivity of 10 14 -10 16 Ohm cm. 15°C10% 23°C60% 30°C80% Bias for first color 700V 500V 350V Bias for second color 770V 550V 380V Bias for third color 840V 600V 410V Bias for fourth color 910V 650V 440V Embodiment 3 The same reference numerals as in the foregoing embodiments are assigned to the elements having the corresponding functions, and detailed descriptions thereof are omitted for simplicity. In this embodiment, density control process includes a first control process for Dmax control, and a second for transfer of a halftone image, V HT is the bias required to transmit a halftone image such that V HT satisfies the inequality: vdMAX>V HT In this embodiment, the transfer is optimized by both of the Dmax control and the half-tone control. More particularly, in the Dmax control, one patch image data corresponding to a certain density F0H of PWM (Pulse Width Modulation) signal, for example, is formed with varied developing bias. In the half-tone control, a plurality of low density patch images corresponding to 10H, 20H, 40H, 80H, are formed. 0H, 10H, 20H, 40H and 80H are standard hexadecimal expressions. At this time, the patch images of different PWM signal data have different latent image potentials, since the exposure amounts are different. In this embodiment, the latent image potential when the PWM signal data is F0H, is -220V, and -580V when it is 10H. In this embodiment, the toner is transferred onto the transfer drum by the potential difference between the photosensitive drum and the transfer drum. Therefore, if the latent image potential is different, the most preferable transfer bias is different. Figure 7 shows a relation between the transfer bias and the transfer efficiency upon the density detection relative to different PWM signal data. With decrease of the PWM signal, the most preferable transfer bias decreases, and with the increase of the PWM signal, the most preferable transfer bias increases. If only the patches for 10H to 80H are looked at, the most preferable transfer is possible with the same bias voltage. Therefore, in this embodiment, the transfer bias during the Dmax control is 500V, and the transfer bias during the half-tone control is 350V, by which the transfer for both can be optimized. The density control is proper, and the the correct image density, and color tone are provided. Most preferable transfer biases may be set for the PWM signals of 10H to 80H, respectively. It is preferable to detect the temperature/humidity of the ambience, and the transfer bias is controlled on the basis of the result of the detection. In this embodiment, the photosensitive drum is of OPC having a negative charging property. It comprises a charge generating layer and the charge transfer layer having a thickness of 25 microns. The transfer drum comprises a core metal 21 of aluminum as a transfer electrode, an elastic member 22 having a thickness of 5.5mm core metal 21 and a volume resistivity of 10 4 Ohm.cm or smaller, and a dielectric member 23 having a thickness of 75 microns and a volume resistivity of 10 14 -10 16 Ohm cm. The description is omitted for the second and subsequent colors, since there are the same tendencies. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: An image forming apparatus comprising: an image bearing member (3) for carrying a toner image; image forming means (1a, 1b, 1c, 1d) for forming a toner image on said image bearing member; a transfer material carrying member (2) capable of carrying a transfer material (7) so that a toner image can be transferred onto a transfer material carried on said transfer material carrying member or directly onto said transfer material carrying member; means (17) for applying a transfer bias to transfer a toner image from said image bearing member; density detecting means (15) for detecting the density of a toner image when a toner image has been transferred to said transfer material carrying member; wherein the applied transfer bias is smaller when a toner image is transferred onto said transfer material carrying member for density detection than when a toner image is transferred onto a transfer material carried on said transfer material carrying member. Apparatus according to claim 1, further comprising transfer means supplied in operation with a voltage to transfer the toner image, wherein the applied transfer bias is the voltage supplied to said transfer means. Apparatus according to claim 2, wherein said transfer means includes an electroconductive member located on the side of the transfer material carrying member opposite to the side for carrying the transfer material, and the voltage is applied in operation to the electroconductive member. Apparatus according to claim 3, wherein said electroconductive member includes a base member and an elastic layer between the base member and said transfer material carrying member. Apparatus according to any one of claims 2, 3, or 4 wherein the voltage V tr applied to said transfer means when the toner image is transferred onto transfer material carried on the transfer material carrying member, and the voltage V pat applied to said transfer means when a toner image for density detection is transferred onto the transfer material carrying member, satisfy (1/5) xV tr ≤Vpat≤(4/5)xV tr . Apparatus according to any one of the preceding claims, further comprising ambient condition detecting means (16) for detecting an ambient condition, wherein the transfer bias is also controlled on the basis of the output of said detector. Apparatus according to claim 6, wherein the transfer bias is smaller when the toner image for the density detection is transferred onto said transfer material carrying member than when the toner image is transferred onto a transfer material carried on said transfer material carrying member, provided that the output of said ambient condition detecting means is the same. Apparatus according to any one of the preceding claims, wherein in operation first and second density detection toner images of different densities are formed on said image bearing member, and the transfer bias when the first detection toner image is transferred onto said transfer material carrying member is different from the transfer bias when the second density detection toner image is transferred onto said transfer material carrying member. Apparatus according to any preceding claim, wherein an image forming condition of said image forming means is controlled on the basis of an output of said density detecting means. Apparatus according to any preceding claim, wherein in operation a plurality of said toner images are sequentially overlaid on said transfer material carrying member. Apparatus according to claim 6, wherein said ambient condition detecting means includes temperature sensing means for measuring the ambient temperature. Apparatus according to claim 6 or claim 11 wherein said ambient condition detecting means includes humidity detecting means for measuring the ambient humidity. Apparatus according to any one of the preceding claims, wherein the image bearing member is a photosensitive drum. A method of forming an image comprising forming a toner image on an image bearing member and transferring the toner image so formed onto a transfer material carried by a transfer material carrying member or as a density detection image onto the transfer material carrying member, and detecting the density of a density detection image when transferred onto the transfer material carrying member, and wherein the applied transfer bias used to transfer the toner image onto transfer material in carrying member is smaller than the applied transfer bias used to transfer the toner image onto a transfer material carried by the transfer material carrying member. A method according to claim 14, wherein the applied transfer bias is a voltage applied by transfer means, and wherein the voltage (V tr ) applied to said transfer means when the toner image is transferred onto transfer material carried on the transfer material carrying member, and the voltage V pat applied to said transfer means when a toner image for density detection is transferred onto the transfer material carrying member, satisfy (1/5) xV tr ≤Vpat≤(4/5)xV tr . A method according to claim 14 or claim 15, including detecting an ambient condition and controlling the applied transfer bias on the basis of the detected ambient condition. A method according to any one of claims 14 to 16 wherein first and second detection toner images are formed on said image bearing member, and the transfer bias is different when the first detection toner image is transferred onto said transfer material carrying member from the transfer bias when the second density detection toner image is transferred onto said transfer material carrying member.

Inventors: KATOH MOTOI, KUME TAKAO, MIYASHIRO TOSHIAKI, OCHIAI TOSHIHIKO, SUZUKI TAKEHIKO, TAKEUCHI AKIHIKO, KATOH, MOTOI, KUME, TAKAO, MIYASHIRO, TOSHIAKI, OCHIAI, TOSHIHIKO, SUZUKI, TAKEHIKO, TAKEUCHI, AKIHIKO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G03G15
IPC codes: G03G15