Corona device

A corona device suitable for use in electrophotographic apparatus to provide a uniform charge on photoconductive material comprises as the ion-generating means a multiplicity of electrode pins, all of the same diameter, which are provided at the same mutual spacing from one another in a body of insulating material from which all the pins project to the same extent, with the pins limited to a diameter between 10 and 100 microns, a mutual spacing of between 0.3 and 2.5 mm, a projecting length of between 0.7 and 3 mm, a length to diameter ratio of between 10 and 300 and a spacing distance to diameter ratio of between 4 and 250. The electrode pins are formed from lengths of a suitable conductive wire which are laid at the required mutual spacing and soldered to and between conductive strips to form an electrically conductive unit that is embedded in a cast body of the insulating material with free ends of the wire lengths projecting to the required extent.

The present invention relates to a corona device suitable for use in an 
electrophotographic apparatus, of a type in which electrode pins serving 
as the ion generating element are provided at the same mutual spacing each 
from another in a body of insulating material and all the pins have the 
same diameter and project to the same extent beyond the surface of the 
body of insulating material. 
Corona devices of that type are generally used in an electrophotographic 
apparatus for charging a photoconductive element or for the creation of an 
ionizing field when required in order to transfer a powder image from the 
photoconductive element to a receptor material. When connected to a high 
voltage, each electrode pin generates an ion cloud which extends from the 
pin toward a counter-electrode. A material that is to be charged up, such 
as a photoconductive element, is located between the pins and the 
counter-electrode. 
Such corona devices exhibit a disadvantage in that the respective ion 
clouds of the electrode pins repel one another, thus giving rise to 
irregular charge patterns on the material to be charged. 
The present invention is based on the discovery that the stated 
disadvantage can be overcome by employing a very definite choice for the 
diameter and the location of the electrode pins. 
A corona device in accordance with the invention is provided with electrode 
pins as described above and, in addition, is characterized by the fact 
that the diameter of the electrode pins is between 10 and 100 microns, the 
distance between the electrode pins is between 0.3 and 2.5 mm, the pins 
project a distance of between 0.7 and 3 mm beyond the body of insulating 
material, the ratio of the length of the pins to their diameter is between 
10 and 300, and the ratio of the distance between the pins to their 
diameter is between 4 and 250. 
A corona device in which the electrode pins conform with these limitations 
provides a uniform charge on the material to be charged and functions over 
a wider range of high voltages than the corona devices conventionally 
employed. 
Preferably the diameter of the pins of the corona device is limited to 
between 20 and 75 .mu.m, with the pins projecting a distance of between 
0.9 and 2 mm beyond the body of insulating material, and the distance 
between the pins is limited to between 0.5 and 1.5 mm. 
The pins of the corona device can be made of materials such as those used 
for wire coronas. Materials especially suitable for the pins are, for 
example, wires of tungsten, of stainless steel, and of tungsten plated 
with a thin layer of gold. As the insulating material of the body holding 
the pins use preferably is made of ozone-resistant insulating plastics, 
such, for example, as polyester resins. 
As a further feature of the invention, an advantageous structure of the 
corona device is provided which can be produced satisfactorily with pins 
formed from cut lengths of a proper wire stock, which lengths are soldered 
at the required precise mutual spacing to and between conductive strips to 
form a conductive unit that is largely embedded in a cast body of the 
insulating material.

FIG. 1 shows a portion of a corona device in which several illustrative 
electrode pins 1, 2, 3 are contained in a body 4 of insulating material. 
The electrode pins of the corona device are all of the same diameter and 
all project the same distance or length beyond the body 4. The pins are 
electrically connected with each other and with conductive connecting 
elements 5 and 6, e.g. by means of a layer of solder 7 (FIG. 2). The 
connecting elements 5 and 6 are metal strips which can be connected with a 
high voltage source for the generation of a corona discharge at the free 
ends of the pins. 
FIGS. 3-7 illustrate details of consecutive stages of a method for making a 
corona device in accordance with the invention. In FIG. 3, numeral 11 
denotes a stock roll of wire 12 from which the electrode pins are made, 
the diameter of which corresponds to the desired diameter of the pins and 
amounts, preferably, to between 20 and 75 microns. The wire 12 is led from 
the roll 11 to a mandrel bar 13 having an H-shaped profile, which bar is 
carried by a winding machine (not shown) in such manner that the bar can 
be rotated around its longitudinal axis in the direction of arrow A. The 
opposite end edges of the bar 13 are provided with notches 14, 15, 16 and 
17 recessed in the longitudinal direction. Four strips 18, 19, 20 and 21 
of an electrically conductive material, such as brass, are fastened 
detachably to the opposite longitudinal side edges of the bar 13, with 
each of these strips extending perpendicular to the plane of the bar. 
In an initial stage, as indicated in FIG. 3, the wire 12 is wound around 
the assembly of the bar 13 with a set of the strips 18, 19, 20 and 21. 
This winding is effected so as to space the successive convolutions of the 
wire at a distance apart, or pitch, equal to the desired spacing between 
the electrode pins; for instance, at a spacing of 1 mm. 
When the winding process has been completed, a second stage takes place as 
indicated in FIG. 4. In this stage, the convolutions of the wire 12 are 
fixed in place relative to the strips 18, 19, 20 and 21 by applying a 
layer of solder to these strips and over the lengths of wire laid on them, 
and fastening to them by the solder the strips 22, 23, 24 and 25, 
respectively, of a second set of electrically conductive strips. 
Subsequently, as indicated by the symbols in FIG. 5, the convolutions of 
the wire 12 are cut at locations aligned with the bottoms of the notches 
14, 15, 16 and 17. This cutting gives two, substantially identical 
assemblies of wire lengths soldered and sandwiched between conductive 
strips. Each of these assemblies consists of two pairs of strips soldered 
to each other, such as the pair 19, 23 and the pair 20, 24, with a large 
number of wires properly spaced apart and anchored conductively between 
the strips of each pair. At least one of the strips of each of the 
assemblies 18, 22, 21, 25 and 19, 23, 20, 24, respectively, is also 
provided with a connecting element, such as a pin soldered to the strip, 
for connecting the strips with a voltage source. 
The two assembles of wires and strips are detached from the mandrel bar 13, 
after which each of these assemblies is processed further as indicated in 
FIGS. 6 and 7. One of the strip pairs of the assembly, such as that of 
strips 20 and 24 are shown in FIG. 6, is placed in a channel-shaped 
covering 8 of insulating material. The other pair of the same assembly, as 
formed by the strips 19 and 23, serves as a spacing element by means of 
which the wires joining the two pairs of strips can be kept tensioned so 
that no bends or buckles will occur in the wires between the two pairs of 
strips. Then a mass of insulating material, such as a self-hardening 
liquid polyester resin, is poured into the channel-shaped covering 8 until 
it is filled to the brim and the strips 20 and 24 are completely covered, 
as indicated at 4 in FIG. 6. 
After the insulating material has become solid, the wires protruding from 
the body of this material are held in a fixed position by a mass 26 of a 
low melting point wax applied about them, as indicated in FIG. 7. Then, 
during a final stage shown in FIG. 7, the wires are cut off to the 
required distance of projecting from the insulating body, for instance to 
a length of 1.5 mm from its surface. The cutting can be effected by use of 
a cutting element or knife 27 passed through the mass of wax holding each 
wire upright from the insulating body. The length of wire left to serve as 
an electrode pin is the same for all wires, measured from the surface of 
the insulating material 4. The wax 26 eventually is removed by heating. 
The wires can also be held fixed during the cutting in other ways. For 
example, one or more flat elements the thickness of which corresponds to 
the desired length of the electrode pins can be placed on the surface of 
the insulating material 4 and pressed against the wires, followed by 
movement of a knife over the surface of the flat element to cut off the 
excess length of the wires. It will also be apparent that the order of 
steps in the manufacturing process can be varied from the described above; 
for instance, the wires joining the pairs of sandwiched strips can first 
be cut to length and subsequently the whole of a pair of strips with wires 
protruding from between them can be embedded in poured insulating material 
until the required precise length of the electrode pins remains above the 
surface of the resultant body of insulating material.