US3822139A

US3822139A - Electrostatic developer device

Info

Publication number: US3822139A
Application number: US00297580A
Authority: US
Inventors: V Westdale
Original assignee: Multigraphics Inc
Current assignee: AB Dick Co
Priority date: 1972-10-13
Filing date: 1972-10-13
Publication date: 1974-07-02
Anticipated expiration: 1991-07-02

Abstract

PAPER IN PROCESS WILL CONFINE THE MIX. OVER MOST TO THE DEVELOPMENT AREA, BUT SOME WILL ESCAPE THE EDGES. THE NORMAL RISE OF THE MIX IS CONTROLLED BY SHAPING THE BRUSH FORM TO SUPPRESS THE RISE AT THE CYLINDER ENDS. THIS IS PREFERABLY ACCOMPLISHED BY A DOCTOR BLADE HAVING FORWARDLY EXTENDING EXTREME END MEMBERS. IN EFFECT, A "U" SHAPED DOCTOR BLADE.

AN ELECTROSATIC IMAGE DEVELOPING UNIT EMPOLYING A ROTATING MAGNETIC TONER BRUSH. THE BRUSH IS CREATED BY IRON CARRIER PARTICLES IN THE DEVELOPER MIX. THE IRON FOLLOWS A FLUX PATH CREATED BY STATIONARY MAGNETS WITHIN A ROTATING CYLINDER. PARTICLES OF TONER ARE CARRIER BY THE IRON. AT HIGH ROTATION SPEEDS THE MIX TENDS TO EXPAND OUTWARDLY TO AN EXCESSIVE DEGREE, AND CAUSE A DISTURBANCE OF THE ELECTRICAL BIAS NORMALLY USED IN SUCH DEVICES. THE

Description

July 2, 1974 v. w. WESTDALE ELECTROSTATIC DEVELOPER DEVICE 2 Sheets-Sheet 1 Filed Oct. 1.3, 1972 July 2, 1974 v. w. WESTDALE 3,322,139

ELECTROSTATIC DEVELOPER DEVICE Filed Oct. 13, 1972 2 Sheets-Sheet 2 United States Patent Office 3,822,139 Patented July 2, 1974 Filed Oct. 13, 1972, Ser. No. 297,580 Int. Cl. G03g 13/08, /08

US. Cl. 117--17.5 4 Claims ABSTRACT OF THE DISCLOSURE An electrostatic image developing unit employing a rotating magnetic toner brush. The brush is created by iron carrier particles in the developer mix. The iron follows a flux path created by stationary magnets within a rotating cylinder. Particles of toner are carried by the iron. At high rotation speeds the mix tends to expand outwardly to an excessive degree, and cause a disturbance of the electrical bias normally used in such devices. The paper in process will confine the mix, over most of the development area, but some will escape the edges. The normal rise of the mix is controlled by shaping the brush form to suppress the rise at the cylinder ends. This is preferably accomplished by a doctor blade having forwardly extending extreme end members. In effect, a U shaped doctor blade.

BACKGROUND OF THE INVENTION The construction of developer brushes useful in photoelectrostatic copying is well known and is a Well developed art. In general the principles of operation are well defined calling for magnetic means mounted inside a rotating cylinder which is partially immersed in a supply of developer mix. A developer mix usually consists of a mixture of thermoplastic particles and magnetic iron particles. As the cylinder rotates, a quantity of the developer mix is accumulated on its surface in a brush like formation which is then caused to sweep across a photoconductive member carrying a latent electrostatic image thereon thereby developing the image.

Magnetic brush high speed dry electrostatic copiers are required to produce in the range of from fifty to eighty copies, or even more, a minute. The magnetic brush causes the toner mix to stand up in a flux field pattern. Older copiers had an output speed of only fifteen to twenty copies a minute, and therefore a magnetic brush could rotate slowly enough that the toner stayed in the normal flux field pattern. The high speed machines greatly magnify the forces tending to throw the developer mix into an excursion path far beyond that normally developed by magnetic flux effect. Consequently, the conductive developer mix, as it extends from the brush too close to the bias devices used in such copies, decreases the efficiency of the bias and causes edge toning of the copy. Sometimes full shorting of the bias will cause streaks across the page.

SUMMARY OF THE INVENTION The magnetic brush assembly of this invention achieved a high degree of success in such high speed system by utilizing the magnetic fields of force to create specific zones 'within the magnetic flux field generated by the magnetic means through which the surface of the cylinder rotates, and then by shaping the excursion path of the developer mix in the developing station.

The present invention provides an improvement in a magnetic brush developer unit for a photoelectrostatic coupying machine, and a method of shaping the developer mix excursion path, that is particularly suited to a high rate of copy output while protecting the corona bias from undue shorting by the granular developer mix.

A particular feature of the developer unit of the instant invention is its ability to create a magnetic brush that is effective to completely develop a latent image bearing member moving relatively rapidly through the developing station.

IN THE DRAWINGS FIG. 1 is a vertical section through a developer embodying the present invention;

FIG. 2 is an enlarged partial view in a developer station in the absence of a shaping doctor blade of this invention;

FIG. 3 is the same section as FIG. 2 with the shaping doctor blade of the present invention; and

FIG. 4 is a plan view of the shaping doctor blade taken along the line 44 of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, there is shown in FIG. 1 the magnetic brush developer assembly for a brush developer adapted to develop electrostatic images carried on a sheet material, usually coated paper, at high speed. The assembly is equipped with a pair of in-

feed rollers

12 and 14.

Spaced a distance from the in-feed rollers is a set of exit rollers 15 and 16 which provides an exit way for sheet material being developed.

Between the in-feed rollers and exit rollers is an area designated as developing station 17.

Leading from the in-

feed rollers

12 and 14 are a pair of spaced apart guide elements 18 and 20 forming a throat through which the sheet is guided into the developing station. Included in the exit way construction is a.

guide element 28 that receives the lead edge of the workpiece sheet guiding it into the nip of exit rollers 15 and 16.

Superimposed over the developer station is an enclosure 34 which includes two terminals 36 one of which is shown in the FIGS. 1-3. A corona bias wire spans between the terminals. This corona discharge wire extends transverse to the path of movement of the sheet material and is disposed above the back of the copy sheet in substantial alignment with the area of contact of the magnetic brush with the image bearing surface of the sheet material. The function and operation of such corona bias wire is fully described in US. Pat. No. 3,369,917 issued to Daniel Granzow et al. issued Feb. 20, 1968.

The assembly includes, at the general center of the apparatus, a rotatable hollow cylinder 40 which is mounted on a fixed shaft 42. A fixed magnetic flux generating assembly 44 is affixed to shaft 42 and includes an array of several magnets disposed adjacent to the inside surface of the cylinder. The purpose of the magnet array is for attracting and releasably retaining a mass of developer mix on the surface of cylinder 40. The magnet array produces a series of flux zones, the first of which is located adjacent a supply sation 46 for picking up a mass of developer mix. Another zone is located adjacent the developer station 17 for causing the mass of developer mix to become brush-like on the cylinder surface. The generator produces a regular fiux pattern within the mean portion of the cylinder, but produces an irregular fiux pattern at the extremes, thereby permitting a condition of excessive developer excursion under rapid cylinder rotation. The structure thus described is generally well known, including a replenishing hopper 47. See US. 3,369,917.

As the developer mix passes the doctor blade, it is smoothed down into a formation known as a hard brush. The developer mix will stand up in a brush configuration because the magnetic field in the second zone beyond the doctor blade is low compared to other of the zones, and the polarity reversed with respect to its adjacent areas. This soft brush formation is a point of concern toward which this invention is directed.

The improvement of this invention is best shown in FIG. 4. This figure illustrates a doctor blade 60 having a central area of generally conventional construction, and two end flaps 63 which extend from the main body of the doctor blade 60 in the direction of rotation of the cylinder and movement of the workpiece sheet.

The doctor blade may be said to be located upstream of said developing station, because the cylinder turns in a direction away from the doctor blade. It is set a distance from the cylinder surface to iron down excessive high developer mix strands. The doctor blade is best described as having a mean portion extending across the cylinder a distance less than the length of the cylinder, with extreme portions in the form of flaps 63, projecting in the direction of the developing station 17 a distance greater than said means portion and juxtaposed the ends of said cylinder.

The cylinder is longer than the sheet widths intended to be served, because the path of a sheet through the developer station is subject to a considerable variation from side to side, and sometimes paper is not exactly to specification dimensions.

In FIG. 4 the cylinder 40 is shown to be conveying a workpiece sheet 65. The sheet 65 is not as wide as the total length of the cylinder 40. Thus, the sheet 65 is assured a complete support at all times, even though the sheet may enter the developing station off center for some reason.

The magnets in the interior of cylinder 40 are substantially as long as the cylinder, but by the very nature of permanent magnets, they produce a weakened and erratic flux pattern at the cylinder ends. To compound the situation, because the magnets in the developing station 17 are of weaker power in order to cause a better lifting action to produce an upstanding pattern, the end flux zone is exaggerated in its erratic nature. Hence, the magnetic brush formation of developer mix allows a weak irregular fluff of developer at the ends of the cylinder, which is the area that supports the edge of sheet 65.

Refer to FIG. 2. This is an enlarged section of the developing apparatus illustrating principally the in-

feed rollers

12 and 14 into the developing station 17. A conventional doctor blade is designated by the reference character 60A. It is identical in every respect to the doctor blade 60 of FIG. 4 except that it lacks end flaps 63.

In the slow speed development acceptable in prior art devices, such construction would be satisfactory. However, under high speed conditions of fifty to eighty copies per minute, or more, the magnetic brush upstanding formation, due to the weaker magnets, allows the centrifugal force to expand the brush pile in a lifting action considerably beyond the normal pattern it would assume in slow or static condition.

The excursion of the developer mix is particularly extensive in the end zones of the cylinder 40 because of the irregular and weak pattern formerly described. In FIG. 2 the normal excursion of the end areas under high speed condition is suggested by the light flux pattern indicated generally by the reference characters 70 in FIG. 2.

During periods of time when there is no workpiece sheet 65 in the developer zone, the excursion of the developer mix into the region of the bias wire extending between terminals 36 is of no concern. Terminals 36 is one of two such terminals and a corona bias wire extends between the terminals. The corona produces a bias effect which is well known in electrostatic copy art and serves the purpose of intensifying the image desired and causing unwanted toner to drop away from the background areas of the work sheet where it is not desired. If the developer mix containing iron particles is permitted to approach to the point where it approaches closely to the corona wire,

or actually touches the wire, the effect of the corona is seriously reduced or fully lost. So long as there is no workpiece sheet 65 in the developer zone it does not matter that the bias effect of the corona discharge is grounded through the developer mix. However, it is not desired to have any portion of the effect shunted or grounded during the process of workpiece sheet 65.

Experience has shown that in a structure shown and described, as the speed increases, the developer mix at the edge areas where the strength of the magnet influence is weak, and where the cylinder 40 is not covered by the work sheet 65, maintains a wild excursion as suggested by the shadow line 68 in FIG. 2. If the mix actually reaches the corona wire, the bias effect is completely shorted and the usual result is black streak of retained toner across the face of the sheet 65. More usually is a deleterious effect upon the corona which allows an irregular shading along the edge. The gray edge effect is well known in electrostatic copying devices.

According to this invention, the doctor blade 60 is equipped with the end flaps 63 to cover the end area of the cylinder 40 and extend the effect of the doctor blade farther into the developing station 17 in the end areas. Refer now to FIG. 3. FIG. 3 illustrates the effect of the end flap 63 in the correction of the projection of the end area developer mix excursion as compared with FIG. 2. The pattern is much the same, but the commencement point of the excursion is delayed to a position where the excursion maximum is sufficiently far from the corona bias wire as to be substantially of no effect. Hence, the lifting action brush effect is permitted to take place in the strongest area of the development zone, and throughout the entire development zone for good and complete development of the image, but the edge area which is necessary for assurance of full development of sheet 65, is controlled to prevent disruption of the biasing effect of the corona dis charge.

Although the solution to this vexing problem is simple in its physical construction, the results have been outstanding and the solution to the problem has been complete.

In the FIG. 4, it will be noted that the end flaps 63 are provided with elongated slots 75 in order that the distance of projection of the end flaps 63 from the face of the central mean portion of the doctor blade may be established correctly with respect to the speed of the cylinder 40 and the operating dynamic characteristic of the development function. Although structural and magnetic design are capable of relatively close engineering certainty, a degree of variation between particular machines is possible. Hence, the extension of the fiaps 63 is calculated with repect to the dynamics of a particular machine. The degree of excursion can be determined theoretically, and can be established in practical testing, and the extent of the flap 63 is adjustable to produce just sufficient delay of the excursion in the end zones to achieve the maximum results without unduly restricting the soft brush nature of the developer mix in the end zone.

What is claimed is:

1. In a magnetic brush assembly for a brush developer adapted to develop electrostatic images carried on a sheet material at high speed, said assembly including a rotatable hollow cylinder having a fixed magnetic flux generating assembly disposed adjacent to the inside surface of the cylinder for attracting and releasably retaining a mass of developer mix on the surface thereof;

said flux generating assembly producing an irregular flux pattern at the extremes, thereby permitting a condition of excessive developer excursion under rapid cylinder rotation;

a supply station for supplying developer mix juxtaposed a portion of the outer surface of the cylinder;

a developing station having ingress and egress means for said sheet material and for feeding said sheet material immediately adjacent the cylinder;

a corona discharge wire extending transverse to the path of movement of the sheet material and disposed above the back of the copy sheet in substantial alignment with the area of contact of the magnetic brush with the image bearing surface of the sheet material,

the improvement comprising:

a doctor blade located upstream of said developing station, said doctor blade set a distance from the cylinder surface to iron down excessive high developer mix strands, said doctor blade having a mean portion extending across said cylinder a distance less than the length of said cylinder, and extreme portions projecting in the direction of said developing station a distance greater than said mean portion and juxtaposed the ends of said cylinder;

said extreme portions extending the commencement point of free excursion from the cylinder end areas to a location within said development station wherein the free excursion path will be spaced from said corona biasing means a distance sufficient to avoid shorting effect of the corona discharge.

2. The improved doctor blade as defined in Claim 1, wherein said extreme portions projecting a distance dynamically related to the cylinder rotation speed, being adjustable to project farther into said developer station in harmony with the existing cylinder speed, as determined by the induced excursion path of said developer mix at the cylinder end edges for a given cylinder speed.

3. The improved doctor blade as defined in Claim 1, wherein the extreme portions extend beyond the ends of said cylinder.

4. The method of providing a magnetic brush application of toner to a sheet material, comprising the steps of:

providing a rotatable hollow cylinder having a magnetic flux generating assembly disposed adjacent the inside surface of the cylinder, including a series of magnets of such strength and location as to produce a first zone to pick up a mass of developer mix and a second zone of a power and magnet location causing the developer mass to lift oif and become upstanding in brush-like formation on the cylinder surface;

providing a corona discharge bias device to produce a corona discharge to the upper surface of the sheet material in the area in which the downwardly facing surface is engaged by the magnetic brush to produce a direct current flow through the sheet material successively contacted by the magnetic brush;

rotating said cylinder at a speed which would cause the developer mix to develop an excursion path at the cylinder ends greater than at static conditions to the extent that the mix effectively shorts out the corona discharge device, and positioning a doctor blade near the surface of said cylinder at the beginning of said second zone, said doctor blade extending in the direction of cylinder rotation and partially overlaying the cylinder edge and beyond to shape said developer mix excursion at the cylinder ends to prevent developer from reaching to the corona bias device to cause a corona short.

References Cited UNITED STATES PATENTS 3,098,765 7/1963 Keller et a1 117--17.5 3,145,122 8/1964 Streich 1l8---637 3,369,917 2/1968 Granzow et al 117-17.5 3,387,586 6/1968 Shelffo et a1 118637 MICHAEL SOFOCLEOUS, Primary Examiner US. Cl. X.R.

ll711l R; 118-637