GB1559641A - Electrographic printing head - Google Patents

Description

(54) ELECTROGRAPHIC PRINTING HEAD (71) We, XEROX CORPORATION, of Xerox Square, Rochester, New York, United States of America, a corporation organised under the laws of the State of New York, United States of America, do hereby decIare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- The invention relates to electrographic printing heads.

In electrographic printing heads it is usual to have a plurality of exposed styli and to move a recording medium past the exposed styli. The record medium is conventionally a conductive material having a dielectrically coated surface which is presented to the styli as the record medium moves past the head. The head is responsive to input signals and arranged to select individual or groups of styli so as to form a surface charge image on the recording medium, which image is later developed elsewhere in the printing system. Considerable electrical energy is often required to ensure that the relative potential between styli and record medium is achieved to enable writing by causing ionisation of the gap between the styli and the medium so that the surface charge image can be formed.

In USA 3859960 for example, selected styli and a backing plate behind the record medium are energised simultaneously with signals of opposite polarity to enable writing. This arrangement has the disadvantage that feeding of the record medium is not easy and it is particularlv difficult to incorporate sheet feeding with'such a printing system. In USA 3611419, sheet feeding problems are overcome because the backin, plates are replaced by"front"plates on either side of the styli array. As in USA 3859960, the front plates and styli are energised by signals of opposite polarity, on the elvli and front plates, and a capacitive coupling is formed with the record medium to enable writing. In both USA 3859960 and USA 3611419, it is normally required in practice, to avoid false writing, to energise groups of styli separated by groups of nonenergised styli. This is achieved as specifically described in USA 3611419 by making sure adjacent front plates are not energised simultaneously.

According to the invention there is provided an electrographic head comprising a plurality of styli and including for each stylus a resistive and capacitive coupling for connection to two different drive circuits respectively including at one or both sides of said styli a conductive plate mounted on said head for contacting the recording medium in use adjacent said styli and plates at or near a datum means for maintaining said plate or potential.

Unlike the prior art the resistive and capacitive coupling are preferably formed as an integral part of the head. In any event, the couplings of the head of this invention are not part of the electrical interaction between the record medium and electro- graphic head.

A similar electrographic printing head is disclosed in USA Patent 4030107 and in copending UK Application 26533/78 (Serial No. 1559643). A method of fabrication of printing heads is disclosed in copending T. TK Application 30820/77 (Serial No. 1559642).

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which : ; Figure 1 shows schematically the electrical configuration of the head ; Figure 2 shows diagrammatically a plan view of the head ; Figure 3 shows a side view of the head taken through TTI-TIT of Figure 2 ; and Figure 4 shows isometrically a block of material from which the head can be formed.

Referring to the drawings, in Figure 1 a part of a stylus array is shown. The styli 10 to 13 are each connected through respective resistors 14 to 17 to different drive pulse supplies in 18 to 21. The styli 10 is capacitively coupled by capacitor 22 to drive pulse supply 23 and the styli 11 to 13 are all capacitively coupled by common capacitor 24 to drive pulse supply 25. Thus, each stylus is connected to receive drive pulses from two different drive pulse supplies via a resistor and a capacitor respectively.

In use, to enable a stylus to write in one configuration it is supplied in sequence with a first drive signal through its resistor and a second drive signal of the same polarity through its capacitor. If the first signal is present, then on application of the second drive signal the voltage at the stylus is raised above the Paschen breakdown voltage so than an electrostatic charge can be applied to a record medium opposite that stylus. One earthing plate 26 is shown in Figure 2 which contacts the record medium 27 (see Figure I) maving past the end of the styli in use to prevent the record medium developing an overall charge poten tial. In practice, in the configuration shown, we apply a negative potential of around 300 volts as the first drive signal. The second drive signal is also at a negative potential of around 300 volts. It will be appreciated that if only either a first or second drive signal is applied alone to any stylus, the stylus voltage will not go below about 300 volts in this case, which is less than the Paschsn breakdown voltage for the one configuration.

The"Paschen breakdown voltage"is the voltage at which the insulation of the air breaks down and an avalanche condition ensues allowing ions to flow from the stylus to the record medium. The breakdown voltage varies with the separation of stvlus and record medium and ambient conditions but in the configuration of Figure 1, it (an be expected to be around 450 volts.

Tn the connguration above, writing is achieved by the application of both drive sisnals. In another configuration writing is prevented by such conditions and writing achieved by the absence of the first drive signal. Tn this other configuration the threshold is set for examole at around-200 volts for the styli and the first drive signal arraneecl tn be at +250 volts, say. Tlius, on the aonlication of the first drive signal the potential of the stylus rises to around +250 volts so that the result of applying the second drive signal is to reduce the potential of the stvlus to near zero volts. However, in the absence of the first drive signal. the second drive signal causes te potential of the stvlus to dron to-250 volts. so that as the Paschen voltage le el is-200 volts. in this case writing occurs. It will be noted that this latter Paschen voltage level can be achieved by noving the record med'"m closer to the stylus than before if possible and ! or maintaining the surface of the record medium at a non-zero datum voltage level, of around say +200 volts in this case.

It will be appreciated that other drive signal combinations can be used to control the write and non-write conditions of the styli and that the datum voltage of the record medium adjusted as required. However. in each case the stvli are driven by the application or non-appliciticn of a first sig- nal applied to the resistive coupling which is then followed by the application of a second drive signal to the capacitor coupling. The sequence, of applic. t'on of first and second drive signals is controlled by conventional logic circuit mean,,.

It will be noted that the choices of the values of the respective (El) and capacitive couplings (C) represent t comproraise. On application of the first signal a finite time rise (or fall as the case may be) can be reduced if the time constant of the RC circuit is kept low enabling the second signal to be applied very soon after the application of the first signal. However, if the.timeconstant is low, on application of tlle second siqnal, the voltage pulse at the end of the stylus caused by the second signal will then be very narrow thus causing a very brief voltage excursion through the Paschen breakdown level. By contrast, if the time constant is higller, lhis Paschen voltaOe excursion. s longer but the second drive signa ! must be delased somevillat ; fter the ann1icattrns of the first signal to allow the stylus potential caused thereby to rise or fall to the first signal level. In practice in one arrangement, we use a resistance value of 200k ohms ap proximately and a capacitive value of 15 picofarads approximately.

In Figure 2 and 3, the styli 10 to 14 arc formed of copper plating mounted on a substrate 30. The resistors 14 to 17 are formed of resistance material doped resin mounted on the substrate 30. The capacitors 22 and 24 are provided by a thin epoxy resin sheet fixed on to the stylus hea-1 and discrete copper plates or plating mounted on the resin sheet.

A second row of styli are shown fixed to a second substrate 31 and form in effect a mirror image of the first row of styli. The second row of styli are however staggered with respect to the first row of stvli to increase the lines per inch of writing, as is known in other styli head proposas, and to fill in gaps between styli of the previous rows to improve print quality. End plates 26 are provided at each side of the pair of stylus arrays. In practice these end plates are usually earthed and engage the surface of the record medium during printing.

The method of making each stylus array of the electrographic printing head is as follows :- A substrate of Kapton (trade mark) whicu is clad with 18 micron copper plate is dipped into Kodak (trade mark) printed circuit liquid resist. A negative is placed against the copper plating for exposure and development. Etching liquid is then used to etch away the copper where the light did not fall to produce the configuration shown in Figure 4.

The capacitors are then formed by twice dipping in polyamide/imide eg. RHODEF- TAL 200 (trade mark), drying between the dipning to form a dielectric coating. The coating is cured by heating to around 250 C.

A copper coating is then addec ! to the dielectric by electroless plating and finally separation to form discrete plates (such as 22 and 24 Fig. 2) is achieved by photo etching.

To form the resistors, a 0.001" thick lamination of RISTON (tradz mark) is pressed on to the surface of the stylus array making sure to line up art work on the laminate to form the styli and the supply lines. The arrangement is then exposed and developed to form channels in the laminate extending between respective styli and sup- ply lines. The channels are then filled with colloidal graphite and the surfaces of the channels machined to remove any surplus graphite.

An additional step can be provided to increase the size of the styli tips which uses a further RISTON laminate. This additional step, which could be carried out simul- taneously with the forming of the resistors, involves Iaminating a RISTON sheet over the styli tips, alignir, o thc art worls xvi h styli tips. Channels which are formed ovcr the styli by exposure and development, are then filled with copper. In this way the styli ends can be increased from 18 microns to around 125 microns, say.

Whereas copper plating and styli tips are used the above method can be carried ou) using other conductive materials such as nickel.

It will be appreciated that the styli arrays described herein are required to be joined together and aligned into substantial arrays generally extending the width of a page or document. As such several techniques which may include further coating and etching of the substrats are required to enable groups and selections of groups and individual styli to be energised during printing operation of the electrographic head. The present invention is not concerned with the formulation of the overall stvli arrav connections and respective commoning and logic control of various groups of drive circuit arrangements for electrographic heads.

The method of forming the electrographic head may include encapsulation in epoxy resin of at least part of the head described.

In one arrangement however, the head in the region of the styli tips at least is deliberately made flexible and in use the heat is arranged to press against the record medium. In the arrangement the plane of the head is positioned at an angle to the normal to the medium, say around 45 to the normal, but other angles between 0'and 60 can also be used.

Claims (7)

1. WHAT WE CLAIM IS :- l. An electrographic printing head com- prising a plurality of styli and including for each stylus a resistive and capacitive couplink for connection to two different drive circuits respectively including at ne or both sides of said styli a conductive plate mounted on said head for contacting the recording medium in use adjacent said styli and means for maintaining said plate or plates at or near a datum potential.
2. 2. An electrographic printing head according to Claim 1. in which said styli are arranged in mutually spaced rows of styli, successive styli within each row being spaced from one another and said styli of successive rows being staggered with respect to each other.
3. 3. An electroaraphic printing head according to any of the preceding Claims in which said resistive coupJing comprises resin impreenated with conductive material.
4. 4. An electrographic printing head according to any of the preceding Claims in which said capacitively coupling is provided through an epoxy resin layer on saicl styli.
5. 5. An electrographic printing head according to anv of the preceding Claims wherein said styli comprise copper or nickel mounted on a substrate.
6. 6. Electrographic printing heads having mounted thereon one or more conductive plates substantially as herein described with reference to the accompanying drawings.
7. 7. A printing system including an elec trographic head comprising a plurality of styli and including for each statuts a resistive and capacitive coupling for connection to two different drive circuits respectively. logic circuit means arranged to connect the respective drive circuits such that the resistive coupling is connected in time séquence before the canacitive coupling and including at one or both sides of said styli a conduc- tive plate mounted on said head for con tacting the recording medium in use adjacent said styli and means for maintaining said plate or plates at or near a datum potential.