Device for the development of an electrostatic charge image with the aid of magnetic brushes.

Abstract

 Device for the development of an electrostatic charge image with the aid of magnetic brushes comprising:

a trough-shaped reservoir (1) of which two long opposite sloping walls (2, 3) lie in planes which are intersecting at the bottom (4) of the reservoir,

two magnetic brush developing rollers (5, 6) which rotate on their axes in directions away from each other at the side which is directed toward the charge image,

a magnetic brush roller (18) for transporting the developing powder toward the developing rollers and

mixing means consisting of groups of parallelly-erected partitions (23, 24) which have been fixed opposite to each other to the sloping walls of the reservoir.

Description

[0001] The invention relates to a device for the development of an electrostatic charge image on a moving substrate with the aid of magnetic brushes, which device comprises a trough-shaped reservoir of which two long opposite sloping walls lie in planes which are intersecting at the bottom of the reservoir, developing rollers installed in the upper part of the reservoir in parallel to the sloping walls, either of which rollers consists of a rotatable, non-magnetizable cylinder, in which stationary magnets have been installed, transport means installed between the bottom of the reservoir and the developing rollers, for the transport of the developing powder from the reservoir to the developing rollers, and means for mixing the developing powder.

[0002] With the aid of a binary developing powder, consisting of a mixture of a magnetizable powder and a toner powder, or with the aid of a one- component developing powder, in which the magnetizable material has been taken up in the toner particles, a magnetic brush with which charge images can be developed, can be formed on the developing rollers of such a device.

[0003] In a simple embodiment the known developing devices comprise one developing roller, which is partially immersed into a reservoir with developing powder, or which is mounted above the stock of developing powder and is provided with developing powder with the aid of a transport screw or a magnetic transport roller of similar nature as the developing roller. An embodiment with a magnetic transport roller is described in U.S. patent specification 3 654 902.

[0004] In copying apparatus with a low copying speed a single developing roller which at the side of the charge image moves in the same direction as the charge image, satisfies excellently. However, with copying speeds of about 10 metres per minute and more too little developing powder is transferred by such a following roller to the charge image, so that copies with a too low optical density are obtained. This effect is even increased, when in case of copying drawings an image with an edge-effect is desired for improving the contour-sharpness. Such an edge-effect can for instance be achieved with the aid of a binary developing powder by increasing the specific resistance of the magnetizable powder. As a result of this the developing speed and consequently the optical density of the copies obtained decreases. A counterrunning roller, which moves in opposite direction with regard to the charge image, has this disadvantage to a much lesser degree, but gives a bad development of the half tones. Moreover, when applying a binary developing powder, a counterrunning roller not only transfers toner but also a part of the magnetizable material to the charge image.

[0005] A developing device which does not have the disadvantages mentioned above, is described in the British patent specification 1 373 010. The device according to the British patent specification comprises four developing rollers running in the same direction as the charge image. The developing powder is supplied to the first developing roller with the aid of a paddle wheel and is transported in a continuous stream over the four developing rollers which have been installed within each other's sphere of influence. By the repeated contact with the charge image a relatively long time of contact for the manufacture of copies with a high optical density is guaranteed, in spite of the high copying speed. After passing the fourth developing roller that part of the developing powder which has not been used, falls into a mixing device via a scraper. This mixing device comprises a first row of tubular channels which have been installed in parallel with regard to each other, and a second row of channels which have been installed in parallel with regard to each other, which second row is fixed to the first row and crosses this first row. All these channels have been installed in a sloping way with regard to the free-fall direction of the developing powder, so that the developing powder is always split up into partial streams which arrive at different places in the reservoir.

[0006] However, the developing device according to the British patent specification 1 373 010 also has various disadvantages. The four magnetic brushes on the four developing rollers form a great friction surface between the substrate of the charge image and the developing powder, so that the duration of use of the substrate and the developing powder is decreased. Further the device with four developing rollers is constructively less attractive, on the one hand because such-a device occupies much space, and a great surface on the substrate, and on the other hand because the construction of the mixing device is complicated. Moreover, the device is only suitable for the development of charge images on a sloping substrate and the paddle wheel causes extra friction forces in the developing powder, so that especially with binary developing powders the particles of the magnetizable material which is not consumed but is reused again and again, are ageing and consequently have to be refreshed more frequently.

[0007] The invention provides for a developing device which has the favourable properties of the developing device according to British patent specification 1 373 010 without the disadvantages of that device. The developing device according to the invention is a device, as described in the first paragraph, characterized in that a) the device comprises two developing rollers which have been installed out of each other's sphere of influence and the cylinders of these developing rollers have an opposite direction of rotation with which the cylinder surfaces, at the side which is directed towards the substrate, move away from each other, b) the transport means at least consist of a rotatable, non-magnetizable cylinder, installed in parallel to the developing rollers, and magnets, stationary installed in the cylinder, for the supply of developing powder between the developing rollers, and c) the mixing means consist of groups of parallelly erected partitions which have been fixed opposite to each other to the sloping walls of the reservoir, and the partitions of the one sloping wall lie in planes making a sharp angle with the shafts of the developing rollers and the partitions of the other sloping wall, seen in the same direction, lie in planes making an obtuse angle with the shafts of the developing rollers. When applying a developing device according to the invention a charge image is first developed with a counterrunning roller and subsequently with a following roller. In this way on the one hand a high optical density and on the other hand an optimal development of halftones is achieved. This combination of properties is not achieved, when the development is first made with a following roller and subsequently with a counterrunning roller. Moreover, iron is then transferred to the image. This has the consequence, that the photoconductor is quickly damaged, and that when the toner image is transferred electrically, the toner is no more transferred round the iron particles and a copy with white spots in the image parts exists. Although a developing device with a first counterrunning roller and a second following roller is known from German patent specification 15 22 727, that developing device makes use of external magnets so that magnetizable material arrives in the developed image. This magnetizable material is again removed with additional magnets. This leads to an accelerated ageing of the magnetizable material, and moreover the optical density again becomes lower. Further a transport belt is used for the transport of the developing powder which transport belt causes friction with the developing powder in the reservoir and consequently reduces the duration of life of the powder. Moreover, the developing powder is not mixed sufficiently, because a mixing device is absent. The transport and mixing means in the device according to the invention are extremely effective with a minimum of friction.

[0008] The invention is further clarified with the aid of the Figures 1 and 2.

Fig. 1 schematically represents a developing device according to the invention, seen in a section according to the line I-I' of Fig. 2, perpendicularly on the shafts of the developing rollers.

Fig. 2 is an upper view, reduced with regard to Fig. 1, of the bottom of the same developing device, stretched out in a plane.

[0009] The developing device according to the Figures 1 and 2 comprises a trough-shaped reservoir 1 with two sloping walls 2 and 3 which lie in planes which are intersecting at the bottom side 4 of the reservoir. In the upper part of the reservoir a first developing roller 5 and a second developing roller 6 are installed with their shafts in parallel to each other and to the line of intersection of the sloping walls. Either of these developing rollers comprises a non-magnetizable metal cylinder 7, 8 which is rotatable in the direction of the arrow. When the developing device is operating, the cylinders of the developing rollers rotate out of each other at the side which is turned away from the bottom of the reservoir. At that side and near each developing roller two guide rollers 9, 10, 11 and 12 have been installed, which convey a photoconductive belt 13 past the developing rollers. In each non-magnetizable cylinder 7, 8 a stationary magnetic system, consisting of a hexagonal soft-iron core 14, 15 is installed. At a side directed towards the photoconductive belt 13 each core is provided over its full length with a recess 16, 17 in which magnet strips have been installed. On these magnet strips a second layer of magnet strips has been installed. All these magnet strips_have their south pole directed outwards. In the first developing roller 5, seen against the direction of rotation of the enveloping cylinder, the following three sides of the core are provided with magnet strips, directed outwards with the north-, north- and south pole respectively. In the second developing roller 6, also seen against the direction of rotation of the pertaining cylinder, the following three sides of the core are provided with magnet strips, directed outwards with the north-, south- and north pole respectively. In Fig. 1 a magnet with a south pole directed outwards is indicated with an S and a magnet with a north pole directed outwards is indicated with an N.

[0010] Below the developing rollers a transport roller 18 is installed with the shaft 25 in parallel to the shafts of the developing rollers. Just like the two developing rollers this transport roller comprises a rotatable, non-magnetizable cylinder and a stationary magnet system consisting of a soft-iron core with hexagonal section which at four successive sides is provided with magnet strips and is directed towards the developing rollers with the two magnet-free sides. The magnet strips are directed outwards alternately with the north- and south pole. The cylinder of the transport roller rotates in the same direction as the second developing roller 6 and opposite to the first developing roller 5, as indicated by the arrow-direction. The transport roller 18 transports developing powder from the reservoir 1 to the space between the two developing rollers where the developing powder is picked up by the developing rollers. In this space a scraper 19, 20 is installed near each developing roller, which scraper extends in length- direction over the whole cylinder surface of the developing roller and which limits the layer-thickness of the developing powder on the developing roller. Opposite to the scraper 19, 20 a plate-shaped scraper 21, 22 is installed on each developing roller, which scraper extends in length- direction of the pertaining developing roller over the whole surface of the cylinder of this roller. Each scraper extends in the other direction from the pertaining developing roller up to the top side of the nearest sloping wall 2, 3. Developing powder which is transported by the developing rollers in the direction of the sloping walls, is removed by the scrapers from the developing rollers, after it has passed the photoconductive belt, and is transported via these scrapers to the top side of the sloping walls, where the developing powder falls down from the scrapers and falls back over the sloping walls into the reservoir.

[0011] Both sloping walls are provided with groups of parallel partitions 23, 24 which are installed perpendicularly to the sloping wall and extend from the top side of the sloping wall to the lower part of the reservoir. All partitions on the one sloping wall 2 make a sharp angle with the shaft 25 of the transport roller, which is not represented in Fig. 2, and with the shafts of the developing rollers. Seen in the same direction, the partitions on the other sloping wall 3 make an obtuse angle with the shaft 25 of the transport roller and with the shafts of the developing rollers installed in parallel to these. In this way it is achieved, that the developing powder which falls downwards from the scrapers, is deflected from the direction of falling into the direction of the ends of the trough-shaped reservoir, in such a way that the deflection on the one sloping wall is directed to the one end and the deflection on the other sloping wall is directed to the other end. When all partitions are installed in parallel to each other, the developing powder may sometimes accumulate at one end of the reservoir, for instance when the reservoir is not installed horizontally. Therefore in the preferred embodiment the partitions on each sloping wall are divided in two groups, in such a way that seen in the same direction each first group has a smaller sharp respectively smaller obtuse angle with the shafts of the transport roller and the developing rollers than the second group.In the situation according to Fig..2, with which the sloping walls are stretched out in a plane, the first group of partitions of the one sloping wall makes an angle of 66° with the shaft of the transport roller and the second group on the same wall makes an angle of 73° with the shaft of the transport roller. On the other sloping wall, seen in the same direction, the angles which the first and second group make with the shaft of the transport roller are 107° and 114⁰ respectively. With this installation a light accumulation of developing powder occurs in the centre of the reservoir but this does not cause any trouble. The angles indicated result into an optimal mixing, but they are not critical. When the developing unit has a length of 21 cm, concentration-differences can be eliminated within 30 seconds. For this test 2% of developing powder, calculated on the weight of the iron powder, was added to the iron powder at an end of the reservoir which only contained iron powder. A good mixing is also obtained, when the sharp angles are adjusted on values between 50° and 80° and the obtuse angles are adjusted on values between 100° and 130°.

[0012] In the developing device described above the first developing roller 5 is provided with two north poles beside each other. In an alternate embodiment also the order south-north-south-north for the position of the magnet poles directed outwards can be applied in the corresponding developing roller, but in that case a layer of toner can be transferred electrostatically to the cylinder of the developing roller, when developing with binary toner and simultaneous application of a high bias voltage between the photoconductive belt and the developing roller is applied. In this way the developing current and consequently the optical density of the developed image, may start decreasing. When two like poles have been installed beside each other, the layer formed is again whiped away from the developing roller by the developing powder which is supplied between the developing rollers. Two like poles beside each other are superfluous in the second developing roller 6, possibly because any layer of toner formed is automatically whiped away from the developing roller, because the surface of the second developing roller moves oppositely to the surface of the transport roller in the nip with the transport roller. Of course the problem of the formation of a toner layer on the developing rollers can also be eliminated by applying, according to another alternate embodiment of the developing device according to the invention, instead of one transport roller, two smaller transport rollers, which with regard to the pertaining developing rollers are rotatable in the proper direction.

Claims

1. Device for the development of an electrostatic charge image on a moving substrate with the aid of magnetic brushes, comprising a trough-shaped reservoir (1) of which two long opposite sloping walls (2,3) lie in planes which are intersecting at the bottom (4) of the reservoir, developing rollers (5,6) installed in the upper part of the reservoir in parallel to the sloping walls, which rollers each consist of a rotatable non-magnetizable cylinder (7,8), in which stationary magnets (14,15) have been installed, transport means (18) installed between the bottom of the reservoir and the developing rollers, for the transport of developing powder from the reservoir to the developing rollers and means for mixing the developing powder, characterized in that

a) the device comprises two developing rollers (5,6) which have been installed out of each other's sphere of influence, and the cylinders (7,8) of these developing rollers have an opposite direction of rotation, with which the cylinder surfaces at the side which is directed towards the substrate (13) move away from each other,

b) the transport means (18) at least consist of a rotatable non-magnetizable cylinder, installed in parallel to the developing rollers, and magnets stationary installed in the cylinder, for the supply of developing powder between the developing rollers, and

c) the mixing means consist of groups of parallelly erected partitions (23,24) which have been fixed opposite to each other to the sloping walls (2,3) of the reservoir (1) and the partitions of the one sloping wall lie in planes which make a sharp angle with the shafts of the developing rollers (5,6), and the partitions of the other sloping wall, seen in the same direction, lie in planes which make an obtuse angle with the shafts of the developing rollers.

2. Device according to claim 1, characterized in that the two sloping walls (2,3) are each provided with two groups of partitions (23,24) of which, seen in one and the same direction, on the one wall the first group makes a smaller sharp angle with the shafts of the developing rollers (5,6) than the second group and on the other wall the first group makes a smaller obtuse angle with the shafts of the developing rollers than the second group.

Drawing