GB2048172A - Electrostatic printer - Google Patents

Abstract

An electrostatic printer is provided with a user replaceable, movable electrostatically receptive belt (10) disposed about spaced rollers (3 and 8). Electrical charge patterns are laid down thereon by a printhead (2), developed by the application of toner (17), and transferred and fixed to ordinary paper (14). <IMAGE>

Description

SPECIFICATION Electrostatic printer The present invention relates to an apparatus for the production of images on paper by means of an electrostatic printing process, a pigment blend, known as toner, being first transferred to an electrical change pattern laid down on a charge bearing member and subsequently transferred as a final image to paper.

The laying down of pigment on to electrical charge patterns is extensively employed not only in document photo-copying equipment but also in electrostatic printers, for example, for use as output devices for computational systems.

The pigment used in such machines, known as toner, generally consists of pigment of a desired colour admixed with materials known as carriers for conferring desirable, physical properties to the toner. The mixture may be a liquid, but in general is a powder composed of particles which are each a combination of pigment and carrier or may be a mixture of different types of particles. By variation of the recipe and inclusion of different ingredients, the toner may be caused to have widely varying physical, electrical and magnetic properties.

The final output of all such machines is visuai copy on paper or similar sheet material. The problems of transferring and fixing the toner to the paper has been solved in numerous ways. In one scheme, an electrostatic charge pattern, laid down on the electrostatically respective surface of a rotating drum, is transferred by contact to the electrostatically receptive surface of specially treated paper, toner then being sprayed onto the paper, adhering to the charged areas and subsequently being fixed by heating. This scheme employs an expensive drum which is difficult to replace if damaged, giving high capital and maintenance costs, and also employs expensive, treated paper, giving high running costs.

Another scheme employs the laying down of electrical charge patterns directly onto treated paper, which subsequently has toner deposited and fixed thereon. The low capital cost of this scheme is offset by high paper cost.

All electrostatically responsive drums have a finite working life. Contamination destroys the charge storage properties of the drum surface.

Physical damage, such as scratching and pitting, eventually degrades the quality of the produced copy. Toner adhesion causes blobs and unwanted marks to appear. Drum cleaning is difficult and costly, requiring skilled labour.

It is thus desirable to provide a printer an electrostatically receptive member which may be replaced by the user. It is also desirable that said member is capable of fabrication at low cost. It is further desirable that such a printer is provided where the final copy appears on materials that do not require special treatment.

The invention consists in an electrostatic printer comprising a movable belt, charge depositing means for imprinting electrical charge patterns onto said belt, toner depositing means for depositing toner onto said patterns, and toner transfer means for transferring said toner from said belt to sheet material.

In a preferred embodiment of the present invention, an electrostatic printer is provided with a user replaceable endless belt of electrostatically receptive material which circulates around two rollers, one of which is driven, to move the belt, and which together hold it taut. The belt material comprises a laminated structure of three layers, viz, a substrate layer conferring properties of strength and elasticity to the beit, bonded to the substrate a semiconducting layer, providing an electrically equipotential surface, and bonded to the semiconducting layer, a dielectric layer, for electrostatic charge storage.

The belt, in circulating about the rollers, firstly, has deposited thereon, an electrical charge pattern by an electrostatic printhead, secondly, magnetic toner is deposited thereon from a dispenser which exploits the magnetic properties of the toner to ensure uniform and even distribution of the toner which adheres to the charge pattern on the belt, thirdly, in passing over one of the rollers, the belt rolls in contact with a moving sheet of untreated paper, the toner being transferred from belt to paper by means of a D.C. corona discharge, fourthly, the belt passes adjacently to a toner removing apparatus, again exploiting magnetic properties of the toner, fifthly, the belt is passed through electrical discharging apparatus, and lastly, the belt is returned to the electrostatic printhead for re-use.The rollers are provided with a release mechanism whereby the belt may readily be removed and replaced.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows, schematically, the system of the preferred embodiment.

Figure 2 shows the structure of the materials used in the electrostatically receptive belt.

Figure 3 shows the apparatus used to deposit magnetic toner on to the electrostatically receptive belt.

Figure 4 shows the apparatus used to remove residual toner from the electrostatically receptive belt.

Figure 5 shows the electrostatic printhead used to lay down charge patterns on the belt.

Figure 6 shows the elements of the belt tensioning and release mechanism by which means the belt is rendered replaceable.

Attention is first drawn to Figure 1, showing in schematic form the preferred embodiment.

An endless belt of electrostatically responsive material 10 circulates in the direction of the arrows over a driven roller 3 and an idling roller 8, which hold the belt 10 taut, the rollers, 3, 8 rotating in the direction indicated by the arrows.

An electrostatic printhead 2, responsive to signals from the printhead drive 1, which in turn is responsive to external signals, layers down on the belt 10 a pattern of electrical charges, representative of a row of characters it is desired to print.

With the movement of the belt 10, between the two rollers 3, 8, the electrical charge pattern is transported to the toner dispenser 6, which deposits toner 17 uniformly on the belt 10, the toner 17 adhering only to those areas which are charged. Residual toner remains on the toner dispenser 6 which is continuously replenished by its rotation.

With the continued movement of the belt 10 on to the idling roller 8, the charge pattern, now developed by the electrostatic adhesion of the toner 17 to the charged areas of the belt 10, comes into contact with paper 14, which rolls, contiguously with and at the same speed as the belt 10, over the end of the idling roller 8. The paper 14 is held by the paper guide and transport 7 which is shown only in rough schematic outline.

At the point of contact of roller 8, belt 10 and paper 14, a fixing and transfer apparatus 9 causes a D.C. corona discharge through paper 14 and belt 10 onto the idler roller 8. This has the effect of causing the toner 17, to transfer from electrostatic adhesion to the belt 1 0 to electrostatic adhesion to the paper 14. Thereafter, the paper 14 is passed through compression rollers, not shown to fix the toner 1 7 to the paper surface.

After this process, the paper 14, continues to proceed in the direction of the arrows, out of the equipment, as final copy.

After the toner transfer, the belt 10 continues round to the toner remover 5, which removes any toner 1 7 residual on the belt 10.

Residual toner having been removed, the belt 10 continues moving past the charge disperser 4 which disperses the charge pattern, originally laid down by the printhead 2 on the belt 10 doing so by means of an A.C. corona discharge, which impinges on the belt 10.

Having thus been completely cycled, from charge pattern deposition, through printing, to ultimate charge pattern dispersion, the belt 10 is returned round the driven roller 3 to the printhead 12 where the cycle restarts.

Attention is now drawn to Figure 3, showing the details of the toner dispenser 6.

The toner dispenser 6 comprises an inner cylinder 1 5 which contains disposed within it, a plurality of radial magnets 14, and is coaxial with an outer cylinder 21. A toner reservoir 16 is provided, the edge of which 22 almost touches the outer surface of the outer cylinder 21. The reservoir holds a stock of powdered magnetic toner 17.

The outer cylinder 1 5 is an idling cylinder, and rolls on the surface of the belt 10. The inner cylinder 1 5 is driven cylinder, and is driven to rotate in the same direction as the outer cylinder 21 but at many times the rotational velocity of the outer cylinder 21.

The spreading magnetic field of the radial magnets 14 in the inner cylinder 1 5 penetrates the outer cylinder 21. The magnetic field causes toner 17 to adhere to the outer surface of the outer cylinder 21 and so be drawn from the toner reservoir 1 6 past the gap between the edge of the reservoir 22 and the surface of the outer cylinder 21, as the outer cylinder 21 rotates, the gap dimension ensuring that a correct amount of toner 17 is metered onto the surface of the outer cylinder 21.

The rapid rotation of the inner cylinder 1 5 causes the toner 17, which is attracted to the individual poles of the magnets 14, to attempt to follow the individual poles round. This has a twofold effect. Firstly, the toner 17 crawls round the surface of the outer cylinder 21 and therefore smears out any discontinuities of the take-up from the reservoir. Secondly, any variaitons in the pole strengths of the magnets 14, and hence in the amount of toner 1 7 they cause to be taken up on the surface ofthe outer cylinder 21 are averaged out since all magnets 14 contribute to the mean field strength.

As the outer cylinder 21 rotates, the toner 17, now uniformly spread on its surface, contacts the moving belt 10 and is made evenly available for adhesion to any charged areas. Toner 17 not taken up by charged areas, continues to adhere to the outer surface of the outer cylinder 21, and is returned, as the outer cylinder 21 rotates, back to the reservoir 1 6.

Attention is now drawn to Figure 4, showing the toner remover 5.

The toner remover 5 includes a magnetised cylinder 20 which is an idling cylinder, rotating about its axis 23, driven by its friction against the belt 10 so that it rolls on the surface of the belt 10.

Toner 17, residual on the belt 10 after the transfer to the paper 14 of the bulk of the deposited toner, is removed from the belt 10, by the magnetised cylinder 20, the magnetic forces on the toner 1 7 being greater than the electrostatic forces of attraction of the belt 10 for the toner 17.

The toner 1 7 adheres to the surface of the magnetised cylinder 20 and is carried round by it to the doctor blade 1 8. The doctor blade, being fixed in position, and touching, with a sharp edge, the outer surface of the magnetised cylinder 20, scrapes the toner 17 from the surface of the magnetised cylinder, against its motion.

The toner 17, removed from the cylinder surface, is deposited by gravity into a disposed toner receptacle 1 9.

Attention is next drawn to Figure 2 showing the fabrication of the electrostatically responsive flexible belt 1 0. A substrate 1 3 a few thousandths of an inch thick, composed of a polymeric material such as mylar, has bonded to it a layer of similar thickness of semiconducting material 12. This material 12, may be a conducting plastic or similar flexible substance, of intermediate conductivity between those of metals and insulators. Bonded to the semiconducting layer 12 is a thin dielectric layer 11.

The substrate 1 3 material is selected for type and thickness on the grounds of the desirable mechanical qualities of the belt 10, which the substrate 13 dominates.

The material type of the semiconducting layer 1 2 and its thickness, are selected for their ability to smear out electrical field strength discontinuities encountered in certain styles of electrostatic printhead, where adjacent selector plates, on the substrate side of the belt, with gaps between them, are energised together to select group of electrodes on the active side of the belt 10, but leave, in their mutual gap, an electrical field deficiency which must be compensated for by the inclusion of a partially conducting layer 1 2 in the belt, to smooth out the deficiency by induced charge.The material and thickness of the dielectric layer 11 are selected for their ability to take up and retain electrical charge, a good insulator being required for charge retention and a readily electrically polarised material being required for an ability to take up electrical charge.

Many flexible polymeric materials fulfill this criterion. In addition, the outer surface of the dielectric layer 11, being the active surface of the belt, must be capable of accepting and retaining a smooth surface finish in order to obtain good print quality.

Attention is finally drawn to figure 5 showing details of the construction of the electrostatic printhead 2 and a schematic expansion of the associated drive circuits 1.

The printhead 2 comprises a leading shoe, 100, a trailing shoe 102 and a wire matrix 104. The belt 10 passes beneath all three of these members. The dielectric layer of the belt 10 is in close, parallel planar proximity to them all.

The leading and trailing shoes 100 Er 102 comprise flat plates, each comprising a plurality of rectangular, electrically separate, conducting areas 106 with edges lying along the line of travel of the belt 10. Each of the areas 106 is identical to every other. The width of each of the conducting areas 106 is equal to the width of the character it is desired to print. The conducting areas 106 are aligned between the leading shoe 100 and the trailing shoe 102 such that the part of the width of the belt covered by an area 106 on the leading shoe 100 corresponds precisely to the area of the belt 10 covered by the corresponding area 106 on the trailing shoe 102. The aligned pairs of corresponding conducting areas 106 are electrically connected in pairs.

The wire matrix 104 comprises an insulating block 110 in which are embedded and through which pass a plurality of wires 108 in a regularly spaced, rectangular array, down to the surface of the belt 10, against which they are disposed in close adjacence. The rectangular, regularly spaced array of wires 108 is divided into sub arrays, each rectangular and occupying its own area across the width of the belt 10, being nine wires wide by seven wires deep. The matrix thus comprises a plurality of separate, 7 x 9 wire arrays, spread out evenly and contiguously across the width of the belt 10. The two outer columns of each sub array are never energised, representing the space between characters. The wires 108 in these positions may be omitted, if so desired.The wires 108 in the remaining 7 x 7 square may be selectively energised in various combinations to form dot patterns recognisable as characters it is desired to reproduce, for example, the Roman aiphabet and the Arabic numeral set, including the zero. Chinese, Japanese, Indian and other character sets are equally possible.

An individual wire 108 in any particular sub array is electrically connected in parallel with the.

wires 108 of corresponding position in every other sub array. The entire matrix 104 is thus energised by a total of forty nine drive wires, each drive wire energising every one of the wires in the sub arrays which occupies one of the forty nine positions in the 7 x 7 wires 108 remaining at the centre of the character space.

The associated drive circuit 1 comprises a matrix driver 120 and a shoe driver 122. The matrix driver 120 provides a selectable zero or + 400 volts energisation signal to each of its forty nine output electrical connections to the matrix 104. In this way, any selectable combination of wires 108 out of the forty nine available within each sub array, may be energised to produce a character shape. That character shape is repeated in every sub array across the belt 10.

The particular position across the width of the belt 10 where the electrostatic charge pattern, corresponding to the desired character shape, is actually electrostatically imprinted onto the surface of the belt, is determined by the selective activation ozone of the corresponding pairs of conducting areas 106 on the leading and trailing shoes 100 and 102. The shoe driver 1 22 provides one output connection to each of the pairs of conducting areas 1 06. These outputs may be energised, individually or in simultaneous combinations. The output of the shoe driver 122 comprises, selectably,either a zero potential or a - 400 volts energisation signal.There are as many independent outputs from the shoe driver 122 as there are pairs of conductors 106 across the width of the belt 1 0. The imposing of negative potential onto a pair of conducting areas 106 causes the electrostatic field strength, beneath any activated wire 108, to exceed the breakdown fieldstrength of the dielectric surface of the belt 1 0. Charge migration ensures imprinting a charge pattern onto the surface of the belt 10.

The shoe driver acts in either a time serial mode or a line simultaneous mode. In the time serial mode, each character is presented to the matrix 104 in its order of position on the line and individual conducting area pairs 106 energised, one by one, across the width of the belt, changing for each character presentation on the matrix until the line of characters is full. In the line simultaneous mode, a character is presented to the matrix and all conductor pairs 106 on the shoes 100 and 102, where that character is to be printed on the line, are simultaneously energised.

Thus, all of the A's will be simultaneously printed, then all of the B's thereafter all of the C's, and so on.

Attention is finally drawn to Figure 6, showing the elements of the belt tensioning and release mechanism, by which the belt is rendered user replaceable.

The shaft 138 of the driver roller 3 passes through a fixed bearing 140, attached immovably to the body of the printer. The shaft 1 38 thereafter engages with a drive motor 142, which causes the roller 3 to rotate, and thereby move the belt 10.

The idling roller 8 has its shaft 143 passing into a movable bearing 144. The movable bearing 144 is coupled to the body of the printer via a one dimensional slide 146, which allows the distance between the centres of the rollers 3 and 8 to be changed.

Tension is maintained in the belt 10 by means of a bearing return spring 148 which pulls the idling roller 8 along its slide 146 away from the driver roller 3.

A user operable lever 1 50 with a handle 1 52 is provided, the position of the pivot 1 54 being such that mechanical advantage accrues to the user.

The free end of the lever 1 50 impinges onto the movable bearing 144. The lever 150, when turned counter clockwise (as seen in Figure 6), causes the movable bearing 144 to move along its slide 146 in the direction of the driver roller 3. The return spring 148 is thereby extended and the belt 10 rendered loose on its rollers 3 and 8. The lever 150, when in its extreme counter clockwise position, may be locked in place.

With the belt 10, loose on the rollers 3 and 8, it may be slipped from the free ends of the rollers 3 and 8 and replaced with another. The belt 10, having been replaced, the lever 1 50 is unlocked, and returns automatically, tension being reestablished in the belt 10.

It is to be appreciated that the matrix 104 may be partitioned otherwise than heretofore described, and provided with more than one driver 12, as can the shoes 100 and 102 further providing further and different utility in printing modes.

It will also be apparent to those skilled in the art that the electrostatic printhead 2 may be replaced by an optical imaging system which may cause the belt to take up charge by the photon-induced expulsion of electrons. In this way, the apparatus may be used as an automatically printing camera or a document photocopier.

It will also be apparent to those skilled in the art that with small, obvious modifications, the apparatus may be made to use liquid rather than powdered toner.

It will also be apparent to those skilled in the art that more than two spaced rollers may be used with replaceable belt.

Finally, it will be apparent to those skilled in the art, that the rollers may be driven either continuously or in a stop-start manner so that either a continuous copy may be obtained from the machine, or sheet by sheet copies.

Claims (14)

1. An electrostatic printer comprising; a movable belt, charge depositing means for the imprinting, onto said belt, of electrical charge patterns, toner depositing means for depositing toner onto said patterns, and toner transfer means for transferring said toner from said belt to sheet material.

2. An electrostatic printer, according to claim 1, wherein said belt is endless, extending around two or more spaced rollers.

3. A printer, according to claim 2 comprising; belt release means, operable to release said belt frorn said rollers, said belt being removable from and replaceable over said rollers, and said release means being reoperable to re-engage said belt with said rollers.

4. An electrostatic printer, according to any of the preceding claims, comprising; belt discharge means, for discharging said charge patterns, from said belt, subsequently to the operation of said transfer means.

5. An electrostatic printer, according to any of the preceding claims comprising; belt cleaning means, for removing said toner from said belt subsequently to the operation of said transfer means.

6. An electrostatic printer, according to any of the preceding claims, wherein said belt comprises three bonded layers, a substrate layer for providing the overall mechanical properties of said belt, a partially conducting layer for providing an equipotential surface within said belt, and a dielectric layer for receiving and storing said charge patterns.

7. An electrostatic printer, according to any of the preceding claims, wherein said toner transfer means comprises, electrical charging means for charging said sheet material oppositely to said toner, and contact means, for bringing said sheet material into adjacence with said belt, whereby said toner is attracted from said charge patterns onto the surface of said sheet material.

8. An electrostatic printer, according to claim 7, wherein said electrical charging means comprises; D.C. corona discharge means, disposed adjacently to said sheet material.

9. An electrostatic printer, according to any of the preceding claims wherein said toner is magnetic.

1 0. An electrostatic printer, according to any of the preceding claims, wherein said toner depositing means comprises, a toner reservoir, a magnetically penetrable cylinder for rolling against said belt and for removing said toner from said reservoir, and one or more magnets, disposed within said cylinder to rotate about the axis of said cylinder.

11. An electrostatic printer, according to claim 10, wherein said magnet or magnets rotates or rotate relatively to said cylinder.

1 2. An electrostatic printer, according to any of claims, 9, 10 or 11 wherein said toner removing means comprises; a magnetised cylinder, disposed to roll against and remove said toner from said belt, cylinder cleaning means for removing said toner from said cylinder, and a toner receptacle, for receiving said toner from said cylinder cleaning means.

1 3. An electrostatic printer, according to any of the preceding claims, wherein said charge depositing means comprises; a plurality of charge depositing electrodes and a plurality of selection electrodes, said charge depositing electrodes being disposed adjacently to said belt and being selectably energisable, said selection electrodes being disposed adjacently to said belt and being selectably energisable, each of said selection electrodes being associated with a predetermined group from among said plurality of charge depositing electrodes, such that; any one out of said plurality of charge depositing electrodes if energised, lays down electrical charge, onto said belt, and only if, the selection electrode associated with the group of which said energised charge depositing electrode is a member, is simultaneously energised.

14. An electrostatic printer, according to any of claims 4 to 1 3 wherein said belt discharging means comprises A.C. corona discharge means, disposed adjacently to said belt.

1 5. An electrostatic printer, according to any of the preceding claims, which also comprises; fixing means, operable for making fast said toner on said sheet material subsequently to the operation of said transfer means.

1 6. n electrostatic printer, according to claim 13, wherein said fixing means comprises pressure rolling means.

1 7. An electrostatic printer, according to any of the preceding claims, wherein said sheet material is paper.

1 8. An electrostatic printer, substantially as herein described with reference to the accompanying drawings.