WO2021014147A1 - Électrode de charge pour imprimante à jet d'encre continu - Google Patents

Électrode de charge pour imprimante à jet d'encre continu Download PDF

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Publication number
WO2021014147A1
WO2021014147A1 PCT/GB2020/051744 GB2020051744W WO2021014147A1 WO 2021014147 A1 WO2021014147 A1 WO 2021014147A1 GB 2020051744 W GB2020051744 W GB 2020051744W WO 2021014147 A1 WO2021014147 A1 WO 2021014147A1
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WO
WIPO (PCT)
Prior art keywords
ink jet
ink
charge electrode
jet printer
print head
Prior art date
Application number
PCT/GB2020/051744
Other languages
English (en)
Inventor
Matthew LANGHELT
David Andrew Horsnell
Original Assignee
Linx Printing Technologies Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Linx Printing Technologies Ltd filed Critical Linx Printing Technologies Ltd
Publication of WO2021014147A1 publication Critical patent/WO2021014147A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/015—Ink jet characterised by the jet generation process
    • B41J2/02—Ink jet characterised by the jet generation process generating a continuous ink jet
    • B41J2/03—Ink jet characterised by the jet generation process generating a continuous ink jet by pressure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/07—Ink jet characterised by jet control
    • B41J2/075—Ink jet characterised by jet control for many-valued deflection
    • B41J2/08—Ink jet characterised by jet control for many-valued deflection charge-control type
    • B41J2/085—Charge means, e.g. electrodes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/07—Ink jet characterised by jet control
    • B41J2/075—Ink jet characterised by jet control for many-valued deflection
    • B41J2/08—Ink jet characterised by jet control for many-valued deflection charge-control type
    • B41J2/09—Deflection means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/17—Ink jet characterised by ink handling
    • B41J2/175—Ink supply systems ; Circuit parts therefor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/17—Ink jet characterised by ink handling
    • B41J2/18—Ink recirculation systems
    • B41J2/185—Ink-collectors; Ink-catchers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/005—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by bringing liquid or particles selectively into contact with a printing material
    • B41J2/01—Ink jet
    • B41J2/17—Ink jet characterised by ink handling
    • B41J2/18—Ink recirculation systems
    • B41J2/185—Ink-collectors; Ink-catchers
    • B41J2002/1853—Ink-collectors; Ink-catchers ink collectors for continuous Inkjet printers, e.g. gutters, mist suction means

Definitions

  • the present invention relates to an electrostatic deflection continuous ink jet printer, for example an industrial printer suitable for printing onto a succession of objects carried past the printer on a conveyor in an industrial filling, packing or processing line.
  • the objects are products such as manufactured articles or packaged food stuffs and the printer is used to print product and batch information,“use by” dates etc.
  • the present invention also relates to a print head assembly for such a printer and a method of making a charge electrode for such a printer.
  • a continuous jet of ink drops is formed at a print head of the printer.
  • the print head comprises an arrangement of electrodes to trap electric charges on some or all of the ink drops and to create an electrostatic field to deflect the charged drops.
  • the arrangement of electrodes includes a charge electrode, the ink is electrically conductive, and the drops will be charged by the influence of a voltage on the charge electrode.
  • the charged drops will usually be deflected by the electrostatic field generated by a pair of deflection electrodes.
  • the drops are deflected in flight so that only some drops are used for printing. Drops of ink that are not required for printing are caught by a gutter and are normally returned to an ink tank within a printer body of the printer.
  • the print head is connected to the printer body by a flexible conduit (sometimes called an umbilical) which is typically from 1 m to 6 m long.
  • the print head of an electrostatic deflection continuous ink jet printer usually has a cover that protects it from the environment, encloses the electrodes for reasons of electrical safety and for preventing external interference with the ink jet, and contains the atmosphere inside the print head cover to minimise mixing with the surrounding air.
  • An opening (exit hole) in the cover allows drops of ink to exit for printing. Occasionally the printer may malfunction. If a piece of dust or dried ink partially clogs the opening where the jet is formed, the jet may be disrupted and ink may contact the electrodes of other components of the print head.
  • the operator may open the cover in order to clean the print head. This can expose the operator to the risk of an electric shock from the electrodes, especially since the ink is usually electrically conductive when wet. Therefore it is known to provide an arrangement, e.g. using a sensor to detect to presence of the cover or using a mechanical interlock, that automatically disconnects the electric connections to the electrodes when the print head cover is opened.
  • An aspect of the present invention provides an electrostatic deflection continuous ink jet printer, or a print head for an electrostatic deflection continuous ink jet printer, having a charge electrode, wherein all the exposed surfaces of the charge electrode comprise a low conductivity layer at least 0.1 mm thick of material having an electrical volume resistivity of at least 10 4 ohnvmetre and no more than 10 9 ohnvmetre.
  • a layer limits the current that can be experienced by a person (such as the operator of a printer) who touches the charge electrode or who touches conductive ink that is also in contact with the charge electrode, even if the normal voltage used to charge the drops of ink is applied to the charge electrode. Therefore the layer may assist with electrical safety.
  • the layer is not made of an absolute insulator in order to allow stray electric charge, that might otherwise accumulate on the surface of the charge electrode and interfere with the drop-charging operation, to be dissipated through the layer.
  • the low conductivity layer is at least 0.2 mm thick, to provide better electrical protection and to reduce the risk of holes, gaps or defects in the layer. More preferably the layer is at least 0.4 mm thick because this is the minimum permitted thickness for an electrically protective layer according to the safety standards of some countries. It may be preferable to make the low conductivity layer at least 0.8 mm thick or even 1 mm thick, in order to make it easier to apply the layer to an underlying part (e.g. a metal core) of the charge electrode, especially if the low conductivity layer is to be applied by moulding.
  • an underlying part e.g. a metal core
  • the low conductivity layer is no more than 2 mm thick over the majority of the area of the charge electrode that faces the towards the ink jet in use, in order to prevent the low conductivity layer from having an excessive effect on the performance of the charge electrode.
  • the layer may be much thicker at other places on the charge electrode.
  • the charge electrode may have an underlying part such as a metal core that is thinner than the layer of low conductivity material at a place on the charge electrode that does not face towards the position of the ink jet in use, and the low conductivity layer may make a substantial contribution to the overall strength of the charge electrode.
  • the underlying part may have a portion that is no more than half the thickness, or even no more than one fifth of the thickness of the part of the low conductivity layer that overlies it.
  • the material of the low conductivity layer has an electrical volume resistivity of at least 10 5 Qm, in order to provide a greater electrical resistance and so better electrical safety. Even greater electrical safety may be provided by using a material having a volume electrical resistivity of at least 10 6 Qm.
  • the low conductivity material should have a volume electrical resistivity of no more than 10 9 Qm in order to provide some conductivity through the low conductivity layer so as to dissipate any electrical charge that might otherwise build up on the surface of the charge electrode.
  • the inkjet printer or the print head will have a jet source and a deflection electrode arrangement.
  • the ink jet printer has a deflection voltage generator and either the deflection voltage generator is limited to providing an electric current of no more than 10 mA, preferably no more than 5 mA, more preferably no more than 2 mA or the total electrical resistance for current flow between the deflection voltage generator and the exposed surface of the deflection electrode arrangement is at least 400 kQ, preferably at least 800 kQ, more preferably at least 1.6 MW, yet more preferably at least 2 MW and most preferably a least 4 MW.
  • An electric current through the human body above 10 mA is likely to be painful and may cause a muscular spasm.
  • deflection electrodes may have voltages of about 4 kV, possible with one electrode at +4 kV and another electrode at -4 kV to provide an effective potential difference of 8 kV. Alternatively the same field strength may be provided by an electrode at ground and electrode at 8 kV. A resistance of 400 kQ will limit the current from a voltage of 4 kV to 10 mA.
  • a resistance of 800 kQ will limit the current from a voltage of 8 kV to 10 mA and will limit the current from a voltage of 4 kV to 5 mA.
  • a resistance of 1.6 MW will limit the current from a voltage of 8 kV to 5 mA.
  • a resistance of 2 MW will limit the current from a voltage of 4 kV to 2 mA.
  • a resistance of 4 MW will limit the current from a voltage of 8 kV to 2 mA.
  • the inkjet printer or print head will have a removable cover on a portion of the ink jet printer or print head where the ink jet is present in use.
  • the only dangerous electrical voltages supplied to the components that are exposed when this cover is removed are the voltages supplied to the charge electrode and at least one of the deflection electrodes. If safety features such as the low conductivity layer on the charge electrode and features that limit the current to the deflection electrode(s) are provided so that the electrical risk to an operator, from touching any of the components exposed by removing the cover, is adequately reduced, it may be safe to maintain the electrical signals to these components even when the cover is removed. In this case, it is possible to eliminate safety systems that detect the removal of the cover and automatically remove the electrical signals when the cover is opened.
  • the jet-forming orifice is normally provided in an ink gun.
  • the print head is usually connected to, or is connectable to, a printer body of an ink jet printer by a flexible conduit (umbilical).
  • the flexible conduit will normally carry fluid lines, for example for providing pressurised ink to the ink gun and for applying suction to the gutter and transporting ink from the gutter back to the printer body, and electrical lines, for example to provide a drive signal to a piezoelectric crystal or the like for imposing pressure vibrations on the ink jet, to provide electrical connections for the charge electrode and the deflection electrodes, and to provide drive currents for any valves that may be included in the print head.
  • a further aspect of the present invention provides a print head assembly comprising a print head as discussed above attached to one end of a flexible conduit.
  • This assembly may be attachable to, and detachable from, a printer body of an electrostatic deflection continuous ink jet printer.
  • a further aspect of the present invention comprises making a charge electrode as discussed above by moulding the low conductivity layer over an underlying part at all places where the surface of the charge electrode will be exposed in use.
  • Figure 1 shows an ink jet printer embodying the present invention.
  • Figure 2 is a schematic top view of the main components in the print head of the printer of Figure 1.
  • Figure 3 is a schematic side view of the main components in the print head of the printer of Figure 1.
  • FIG. 4 shows simplified schematic diagram of the fluid system of the printer of Figure 1.
  • Figure 5 shows schematically the main components inside the printer body of the printer of Figure 1.
  • Figure 6 is a side view of a charge electrode in an embodiment of the present invention.
  • Figure 7 is a top view of a charge electrode in an embodiment of the present invention.
  • Figure 8 is an end view of a charge electrode in an embodiment of the present invention.
  • Figure 9 illustrates the electrical connections between the layers of the charge electrode and the jet of ink in an embodiment of the present invention.
  • Figure 10 is an equivalent circuit for the charging of the ink jet using the electrical connections shown in Figure 9
  • Figure 11 is a section, along the line XI - XI in Figures 12 and 13, of a charge electrode and ink gun assembly in an alternative embodiment of the present invention.
  • Figure 12 is a section, along the line XII - XII in Figures 11 and 13, of a charge electrode and ink gun assembly in the embodiment of Figure 11.
  • Figure 13 is an end view of the charge electrode of the assembly of Figures 11 and 12.
  • Figure 14 is an end view of the metal core of the charge electrode of Figures 1 1 to 13.
  • Figure 1 shows an electrostatic deflection type continuous inkjet printer.
  • the printer forms a continuous jet of ink and has an arrangement of electrodes for charging drops of ink and deflecting the drops electrostatically in order to print a desired pattern.
  • the main fluid and electrical components are housed within a printer body 1.
  • An operator communicates with the printer via a touchscreen display 3.
  • the ink jet is formed within a print head 5, which also includes the electrode arrangement for charging and deflecting the ink drops, and the print head 5 is connected to the printer body 1 by a flexible connection 7 known as a conduit or an umbilical.
  • the printer is typically an industrial inkjet printer and is suitable to be used with a conveyor 13 that conveys objects 1 1 past the print head to be printed onto. This is in contrast to a document printer that prints onto flat sheets, and which normally conveys the sheets itself rather than being used with a conveyor 13 that is external to the printer.
  • the object 11 may be a manufactured product item, such as a bottle or can of drink, a jar of jam, a ready meal, or a carton containing multiple individual items.
  • the desired pattern may comprise product information such a batch number or a“use by” date.
  • the printer may print onto the object 11 from the side so that the ink jet travels in a direction generally across the conveyor, or from above so that the ink jet travels in a direction generally towards the conveyor, or from any other angle.
  • bottles are normally printed onto from the side whereas ready meals are normally printed onto from above.
  • the printer is set up to print from the side and partially above.
  • Figure 2 is a schematic top view and Figure 3 is a schematic side view of the main components of the print head 5 in the region of the ink jet.
  • the terms“top view” and“side view” represent conventional directions from which to view the print head on the assumption that the printer will print onto an object 1 1 from the side, and do not necessarily correspond to the orientation of the print head when in use.
  • Pressurised ink, delivered from the printer body 1 through the umbilical 7, is provided via an ink feed line 15 to an ink gun (or nozzle) 17.
  • the pressure of the ink drives it out of the ink gun 17 through a small jet-forming orifice to form an ink jet 19.
  • this type of ink jet printer is known as a continuous ink jet printer, by contrast with a drop-on-demand printer in which a drop of ink is ejected only when a dot is to be printed.
  • the ink jet 19 leaves the ink gun 17 as a continuous unbroken stream of ink, it rapidly breaks into separate drops.
  • the break-up of the ink jet 19 into drops is controlled by applying a pressure vibration, e.g. at 80 kHz, to the ink while it is in the ink gun 17. This stimulates the ink to separate into drops at the frequency of the applied pressure vibration.
  • the path of the ink jet passes through a slot in a charge electrode 21 , which is positioned so that the inkjet 19 separates into drops while it is in the slot through the charge electrode 21.
  • charge electrode 21 Other arrangements and other shapes of charge electrode 21 are possible, so long as the ink jet 19 is subject to the electric field of the charge electrode at the position where it separates into drops.
  • the ink is electrically conductive and the ink gun 17 is held at a constant voltage (typically ground). Accordingly, any voltage applied to the charge electrode 21 induces a charge into the part of the ink jet 19 that is subject to the electric field in the slot of the charge electrode 21. As the inkjet 19 separates into drops, any such charge is trapped on the drops.
  • the amount of charge trapped on each drop can be controlled by the voltage on the charge electrode 21 and different amounts of charge can be trapped on different drops by changing the voltage on the charge electrode 21.
  • the voltage on the charge electrode may vary from 0 V to about 300 V. In order to charge each drop to the correct desired amount, this voltage must be changed stepwise at the same frequency as the drops are formed (e.g. 80 kHz) with voltage rise or fall times that need to be substantially shorter than the time between successive drops in order to ensure that the voltage is stable while each drop separates from the ink jet.
  • the ink jet 19 then passes between two deflection electrodes 23, 25.
  • a large potential difference typically several kilovolts, often 8 to 10 kV
  • the drops of ink are deflected by the electric field and the amount of deflection depends on the amount of charge trapped on each drop. In this way, each ink drop can be steered into a selected path.
  • the ink gun 17, the charge electrode 21 , the deflection electrodes 23, 25 and the gutter 27 are mounted on a baseboard 31.
  • the gutter suction line 29 extends beneath the baseboard 31. It may also be convenient to route the electrical connections for the charge electrode 21 and the deflection electrodes 23, 25 beneath the baseboard 31 , as shown in Figure 3.
  • the deflection electrodes 23, 25 may be mounted so that they each extend perpendicular to the plane of the baseboard 31. Alternatively they may extend parallel to the plane of the baseboard, as shown in Figures 2 and 3, with one deflection electrode 23 lying on the baseboard 31 and the other deflection electrode 25 being spaced above the baseboard 31 and supported by one or more electrode supports 33.
  • the deflection electrode 23 lying on the baseboard will be connected to ground and the deflection electrode 25 spaced above the baseboard 31 will be connected to a high voltage supply to create the deflection field.
  • the electrical connection for the deflection electrode 25 spaced above the baseboard 31 may be carried in one of the electrode supports 33.
  • FIG 4 is a simplified schematic diagram of a fluid system for the ink jet printer of Figure 1.
  • Ink is held in an ink feed tank 35 in the printer body 1.
  • the ink feed tank 35 is the main ink tank of the printer.
  • the interior of the ink feed tank 35 is held at atmospheric pressure by a vent 37.
  • Ink is sucked out of the ink feed tank 35 by a pump 39, via a filter 41 and an ink supply line 43.
  • the ink pressurised by the pump 39, flows through a Venturi 45 and back to the ink feed tank 35 via an ink return line 47.
  • a pressure transducer (pressure sensor) 49 is used to sense the ink pressure on the outlet side of the ink pump 39.
  • the ink feed line 15 is also connected to the outlet side of the ink pump 39 and receives pressurised ink.
  • the ink feed line 15 provides an ink feed path to supply pressurised ink from the ink pump 39 to the ink gun 17.
  • An ink feed valve 51 controls the flow of ink along the ink feed line 15.
  • the pump 39 can drive ink continuously through the Venturi 45 and back to the ink feed tank 35, even when the ink feed valve 51 prevents ink from flowing along the ink feed line 15.
  • the flow of ink through the Venturi 45 generates suction and accordingly the Venturi acts as a suction source.
  • the gutter suction line 29 is connected to a suction inlet of the Venturi 45 to receive suction which sucks ink from the gutter 27 through the umbilical 7 back to the printer body 1.
  • the ink from the gutter suction line 29 is sucked into the Venturi 45 and returns to the ink feed tank 35. Fluid flow in the gutter suction line 29 is controlled by a gutter valve 53.
  • Spare solvent is held in a solvent reservoir 55 which receives suction from the Venturi 45 through a solvent top-up line 57. If solvent needs to be added to the ink in the ink feed tank 35 to dilute the ink and correct its viscosity, a solvent top-up valve 59 in the solvent top-up line 57 is opened briefly. This allows the Venturi 45 to suck a small quantity of solvent from the solvent reservoir 55 into the ink flow through the Venturi 45. The solvent sucked into the Venturi 45 then passes into the ink feed tank 35 to dilute the ink.
  • Spare ink is held in an ink reservoir 61 which receives suction from the Venturi 45 through an ink top-up line 63.
  • an ink top-up valve 65 in the ink top-up line 63 is opened. Ink is sucked out of the ink reservoir 61 by the Venturi 45 and is delivered to the ink feed tank 35 in a similar manner to the operation for topping up with solvent from the solvent reservoir 55.
  • the solvent reservoir 55 and the ink reservoir 61 are supplied from a solvent container 67 and an ink container 69 respectively, and the operator replaces the containers 67, 69 as necessary. In practice, it is not always necessary to provide the solvent reservoir 55 and the ink reservoir 61 , and the respective top-up lines 57, 63 may be connected directly to the containers 67, 69.
  • FIG 5 shows schematically some of the components inside the printer body 1 of the printer.
  • the printer has a printer body ink system 71 , which includes the components in Figure 4 that are shown inside the printer body 1.
  • the printer body ink system 71 and other parts of the printer operate under the control of a control system 73.
  • the control system 73 sends drive currents to the ink pump 39 and to the various valves, 51 , 53, 59, 65 of the printer body ink system 71.
  • the control system 73 receives outputs from the pressure sensor 49 and also from level sensors in the ink feed tank 35, the solvent reservoir 55 and the ink reservoir 61.
  • the control system 73 also provides outputs to, and receives inputs from, the touchscreen display 3.
  • the control system will include a processor such as a microprocessor and other electronic components as is well known in the art.
  • Fluid lines 75 connect the printer body ink system 71 to the print head 5 through the umbilical 7. These fluid lines will include the ink feed line 15, and the gutter suction line 29 shown in Figure 4.
  • Electrical lines 77 connect the control system 73 to the print head 5 via the umbilical 7. These electrical lines include lines for applying the appropriate voltages to the charge electrode 21 and the deflection of electrodes 23, 25, and for applying a drive signal to a piezoelectric crystal inside the ink gun 17 that applies the vibration to the ink in order to control the manner in which the ink jet 19 breaks into drops.
  • the printer receives electric power at a power socket 79, which is converted in a voltage converter 81 to the various voltages required internally within the printer.
  • the printer may be designed to receive 24 volt DC at the power socket 79, since power supplies for generating 24 volts DC from an electric mains supply are widely available.
  • the voltage converter 81 uses the received 24 volt supply to generate the voltages required to power the electronics in the control system 73, which may for example be 5 volts. It also supplies power to a component, either in or controlled by the control system 73, to generate the voltages (e.g. up to about 300 V) applied to the charge electrode 21 , the EHT voltage (e.g. about 8 kV) applied to the upper deflection electrode 23 and to generate the drive signal for the piezoelectric crystal inside the ink gun 17.
  • the voltage converter 81 uses the received 24 volt supply to generate the voltages required to power the electronics in the control system 73, which may for example be 5 volts. It also supplies power to a component,
  • Figures 6, 7 and 8 show an enlarged view of the charge electrode 21 respectively from the side, from above and from one end. All of the exposed surface of the charge electrode 21 is formed by a layer 83 of low conductivity material having a volume (bulk) electric resistivity of 10 4 W metres to 10 9 ohm metres. Volume electrical resistivity may be measured in accordance with IEC 62631-3-1 :2016.
  • the low conductivity material is preferably a mouldable plastic (e.g. one or more thermoplastic polymer materials, which may be inherently dissipative polymers or may be other polymers mixed with inherently dissipative polymers and/or non-polymeric conductive materials).
  • an electrically conductive core 85 typically of metal, shown in broken lines in Figures 6, 7 and 8. In operation of the printer, the charge electrode voltage is applied to the metal core of the charge electrode 21 by a charging signal line 85.
  • the low conductivity layer 83 provides electrical safety. If an operator touches the charge electrode 21 while the charge electrode voltage is applied to the metal core 85, the low conductivity layer 83 prevents the operator from coming into direct contact with the metal core 85 and limits the electrical current to which the operator can be exposed.
  • the metal core 85 is uncoated at the bottom surface of the charge electrode 21 , which sits on the baseboard 31 , so that an electrical connection can be made to the charging signal line 87 through the baseboard 31.
  • the ink is electrically conductive when wet. Therefore the ink can conduct any voltage that it comes into contact with.
  • the bottom surface of the charge electrode 21 is sealed to the baseboard 31 by an ink-resistant sealing material, and the baseboard 31 is itself electrically insulating. Accordingly, even if ink spills onto the charge electrode 21 it cannot come into electrical contact with the metal core 85 of the charge electrode 21 except to the extent that conduction is possible through the low conductivity layer 83.
  • the operator is protected from a direct contact with the charge electrode voltage even in the case of an ink spill, and the low conductivity layer 83 limits the current that can flow if the operator touches the charge electrode 21 or the ink.
  • the actual resistance provided by the low conductivity layer 83 will depend on various factors such as the geometry of the charge electrode 21 , the thickness of the layer 83 and the volume resistivity of the low conductivity material.
  • the total exposed surface area of the charge electrode is likely to be 2 cm 2 or less.
  • the low conductivity material should have a volume resistivity of at least 10 4 Qm.
  • the layer 83 should be at least 0.1 mm thick. If an ink spill contacted the entire 2 cm 2 area of the electrode 21 , and the whole of the low conductivity layer 83 was 0.1 mm thick and made of a material with a volume resistivity of 10 4 Qm, the resistance provided by the layer 83 between the metal core 85 of the charge electrode 21 and the ink would be 5 kQ.
  • the maximum voltage on the charge electrode 21 might typically be 300 V, in which case the maximum current through the low conductivity layer 83 would be 60 mA.
  • the level of current that is dangerous to a human varies depending, for example, on which part of the body the current passes through. Roughly, a current above 1 mA provides a noticeable shock, a current above 10 mA is painful and a current above 100 mA causes heart fibrillation and may be fatal. A current of 60 mA is unlikely to be fatal to a human but could cause breathing difficulty and injury. Thus it can be seen that in this case the low conductivity layer provides a resistance that is useful but would not on its own be sufficient to avoid harm.
  • the protection would be better in the case of a smaller charge electrode 21 since the resistance through the layer 83 is inversely related to the surface area of the electrode 21.
  • the total exposed surface area of the charge electrode can be designed to be in the range of 0.5 to 1 cm 2 , which would result in a smaller maximum current.
  • the layer 83 of low conductivity material is thicker, this provides a greater resistance against current flow through the layer 83 from the metal core 85.
  • the layer 83 is thicker on the part of the surface of the charge electrode 21 that faces the inkjet 19, this may increase the effect that the layer 83 has on the performance of the electrode 21 in charging the drops of ink that separate from the ink jet 19. Consequently it may be preferred to make the layer 83 thinner on most or all of the part of the exposed surface of the charge electrode 21 that faces the inkjet 19 in use than on most or all of the remainder of the exposed surface of the charge electrode 21 , as shown in Figures 7 and 8.
  • the outer face of the charge electrode 21 which the operator is more likely to touch, has a thicker low conductivity layer 83 providing a greater electrical resistance and therefore better electrical safety. If the entire charge electrode is wetted by ink in an ink spill, the area where the low conductivity layer 83 is thinnest, and so provides the lowest resistance, is likely to be less than half of the total exposed surface area of the charge electrode, and so this design also helps to provide better electrical safety in an ink spill. However, because the part of the surface of the charge electrode 21 that faces the ink jet 19 has a thinner layer of low conductivity material, this design preserves the ability of the charge electrode 21 to charge the drops of ink.
  • the low conductivity layer 83 on most or all of the exposed surface of the charge electrode 21 is at least 0.2 mm thick, to give better electrical protection. More preferably it is at least 0.4 mm thick, as this is the minimum thickness for a layer of insulating material according to the current safety standards of some countries. In practice, this part of the low conductivity layer 83 may be at least 0.8 m thick or at least 1 mm thick as these thicknesses are easier to apply reliably by existing coating or moulding processes than thinner layers.
  • the low conductivity layer 83 is no more than 2 mm thick on most or all of the part of the exposed surface of the charge electrode 21 that faces the inkjet 19 in use, because of the adverse effect that a thicker layer may have on the ability of the electrode to charge the ink drops.
  • the low conductivity material of the layer 83 has a volume resistivity of at least 10 5 Qm, in order to provide a greater electrical resistance and so better electrical safety. Even greater electrical safety may be provided by using a material having a volume electrical resistivity of at least 10 6 Qm.
  • the low conductivity material should have a volume electrical resistivity of no more than 10 9 Qm in order to provide some conductivity through the low conductivity layer 83 to the metal core 85.
  • Microdrops (ink drops that are much smaller than the normal drops of ink) may be formed as the ink drops separate from the unbroken part of the ink jet 19 during the operation of the printer. These microdrops will tend to carry electrical charges, and may come into contact with the charge electrode 21. Accordingly, the surface of the charge electrode 21 may receive electric charges during operation of the printer. If these charges accumulate on the surface of the charge electrode 21 they will tend to influence the electric field generated by the charge electrode 21 and therefore they will interfere with the process of charging the drops of ink that separate from the unbroken part of the ink jet 19. The slight conductivity of the layer 83 of the charge electrode 21 allows such charges to leak away to the conductive core 85 of the charge electrode 21 so that accumulation of charge on the surface of the charge electrode 21 is avoided.
  • Figure 9 shows a representation of the parts of the charge electrode 21 as electrical components.
  • the part of the low conductivity layer 83 that faces the ink jet 19 can be regarded as a resistor Rp in parallel with a capacitor C1. If the low conductivity layer 83 was not present, the ink jet 19 would be charged by its capacitance C2 to the surface of the metal core 85. However, the presence of the low conductivity layer 83 alters the situation. The ink jet 19 is closer to the surface of the low conductivity payer 83 than to the surface of the metal core 85, and there is a capacitance C3 between the ink jet 19 and the surface of the low conductivity layer 83.
  • the ink jet 19 is capacitively coupled more strongly to the surface of the low conductivity layer 83 than to the metal core 85 (i.e. C3 is larger than C2) and the capacitance C2 between the inkjet 19 and the surface of the metal core 85 can be ignored.
  • Figure 10 shows a simplified equivalent circuit for the process of charging the drops of the ink jet.
  • the ink jet 19 is emitted by the ink gun 17, initially as an unbroken steam of ink.
  • the ink gun 17 is electrically connected to a reference potential (usually earth) in the printer body 1.
  • the charging signal generator in the printer body 1 generates the charge electrode voltage relative to the reference potential, and provides the charge electrode voltage to the metal core 85 of the charge electrode 21 via the charging signal line 87.
  • the control system 73 of printer specifies a particular charging voltage for each individual drop of ink in the ink jet 19 and these voltages are generated in turn by the charging signal generator.
  • the operation of the charging signal generator is frequency and phase locked to the separation of drops of ink from the unbroken part of the ink jet 19 to ensure that each drop is subject to the correct voltage on the charge electrode 21 while that drop is charged and separates from the unbroken part of the ink jet.
  • the part of the ink jet 19 that is in the slot of the charge electrode 21 would be capacitively coupled to the metal core 85 by the capacitance C2 of the air gap between the ink jet 19 and the metal core 85. If the part of the inkjet 19 in the slot of the charge electrode 21 was uncharged and electrically isolated, its voltage would become the same as the voltage of the metal core 85 (i.e. the charge electrode voltage) owing to capacitive coupling through capacitor C2. However, the inkjet 19 is electrically conductive and is connected to the ink gun 17.
  • the charge electrode voltage is held the same for successive ink drops, but is different from the reference potential, there will be a charge on the part of the ink jet 19 in the slot of the charge electrode 21.
  • the charge in the ink that forms the ink drop is trapped on the drop and is carried away from the charge electrode 21.
  • additional ink is always entering the slot of the charge electrode 21 and becoming influenced by the electric field in the slot.
  • the charge on each ink drop leaves capacitor C2 as that ink drop leaves the slot of the charge electrode, and uncharged ink joins the capacitor C2 as the additional ink enters the slot in the charge electrode 21.
  • the same voltage change is coupled through the air gap capacitor C2 to the part of the ink jet 19 that is in the slot of the charge electrode 21 , changing the voltage of this part of the ink jet 19.
  • Any ink drops that have already separated from the unbroken part of the jet are electrically isolated and the charge on them cannot be changed.
  • the unbroken part of the ink jet 19 is connected to the reference potential via the jet resistance Rj, and so a current flows in the unbroken part of the jet to bring its voltage back to the reference potential. This current is in addition to the current required to replace the charge lost as drops of ink separate from the unbroken part of the jet.
  • the current that flows in response to a change in the charge electrode voltage is the current necessary to change the voltage across capacitor C2 in response to the change in the charge electrode voltage.
  • the charging current of a capacitor flows equally on both sides of the capacitor, and so this current must also flow between the charge electrode 21 and the charging signal generator.
  • the current in the ink jet 19 that is required to supply the charge trapped on the ink drops flows along the inkjet but does not result in any net current in the charge electrode 21 (although there may be brief transient currents that are immediately reversed).
  • a change in the charge electrode voltage results in a current in both the ink jet 19 and the charge electrode 21 in order to change the level of charge on the capacitor C2.
  • the presence of the low conductivity layer 83 provides capacitance C1 and resistance Rp across the thickness of the layer. As shown in Figures 9 and 10, C1 and Rp are in parallel with each other. They are in series with the capacitance C3 across the air gap from the ink jet 19 to the surface of the low conductivity layer 83. The capacitor C2 can now be ignored, because its value is small compared with the other capacitances.
  • the instantaneous effect is that the voltage change on the metal core 85 is coupled to the surface of the low conductivity layer 83 by the capacitor C1 and is further coupled to the inkjet 19 by capacitor C3.
  • the voltage of the part of the ink jet in the slot of the charge electrode changes by the same amount as the change in the charge electrode voltage, leading to a potential difference across Rj.
  • the value of Rj is low, since the ink is electrically conductive, and current immediately flows in the inkjet 19 to return the voltage of the unbroken part of the inkjet to the reference potential.
  • This current changes the level of charge on capacitor C3, so that the voltage of the ink jet 19 can change relative to the voltage of the surface of the low conductivity layer 83.
  • the same current must also flow between capacitors C3 and C1 , changing the level of charge on capacitor C1 and causing the voltage at the surface of the low conductivity layer 83 to change relative to the charge electrode voltage on the metal core 85.
  • the surface on each side of the slot in the charge electrode has an area of about 0.24 cm 2 and the low conductivity layer is about 0.4 mm thick and is formed of a material having a volume resistivity of about 10 5 ohm meters.
  • C1 has a capacitance of about 10 -12 F and C3 has a capacitance of about 10 14 F, i.e. the capacitance of C1 is about 100 times the capacitance of C3.
  • the amount of charge required to change the voltage across C3, so that the unbroken part of the ink jet returns to the reference potential will change the voltage across C1 by about 1 % of the change in the voltage across C3.
  • the level of charge on C3 and C1 changes rapidly so that the unbroken part of the inkjet 19 in the slot of the charge electrode 21 returns to the reference potential, and the voltage at the surface of the low conductivity layer 83 changes by 99% of the change in the charge electrode voltage applied to the metal core 85. Therefore the level of charge in the ink jet changes by 99% of the desired amount, leaving a small error in the level of charge.
  • Rp is believed to be in the range of 400 kQ to 450 kQ. This value is provided for electrical safety.
  • the charge electrode voltage may vary between 0 V and 300 V. If 300 V flows through 400 kQ, the resulting current is 0.75 mA, which is well below safety limits.
  • the time constant for the discharge of C1 through Rp is about 4 x 10 _1 ° seconds to 4.5 x 10 10 seconds, or about 40 to 45 ns.
  • the drop frequency is assumed to be about 80 kHz, so that the drop period is about 1.25 x 10 -5 seconds or about 12.5 ps. Therefore length of time equal to three times the time constant of the discharge of C1 through Rp, by which time it is effectively fully discharged, is about 120 to 135 ns, which is about 1 % of a drop period.
  • C1 is much larger than C3
  • Rp is small
  • this initial error is corrected quickly.
  • the resistance Rp provides the electrical safety that is the reason for providing the low conductivity layer 83, and so reducing the value of Rp reduces the degree of electrical safety provided.
  • C1 can be expected to be higher.
  • a large value of C1 reduces the initial error in the level of charge on the ink jet 19 following a change in the charge electrode voltage, but increases the time taken for the initial error to be corrected by current flowing through Rp.
  • the design of the charge electrode 21 is a matter of trade-offs between conflicting requirements. It can be seen from the examples given above for the values for C1 , C3 and Rp that there is a range of values that are possible while providing both electrical safety and useful performance of the charge electrode 21. However, it can be understood that if the thickness of the low conductivity layer 83 increases excessively, the value of C1 may become undesirably low (since capacitance is inversely related to the layer thickness) and Rp may become unnecessarily high. Additionally the thickness of the layer 83 can make the design of the charge electrode 21 undesirably bulky.
  • the low conductivity layer 83 is no more than 2 mm thick for most of the part of the surface of the charge electrode 21 that faces the ink jet 19. Additionally, it is preferred that the volume electrical resistivity of the material of the low conductivity layer 83 is no more than 10 7 ohm metres. If an area of 2 cm 2 is coated with a layer 0.1 mm thick of a material having a volume electrical resistivity of 10 7 ohm metres, and contact is made with the entire area, the resistance provided would be 5 MW. This would limit the current to well below the threshold for sensation (about 1 mA) for any likely charge electrode voltage, and so the use of materials having a higher electrical volume resistivity would increase the value of Rp without providing any real improvement in electrical safety. Therefore, although it is anticipated that a functioning charge electrode can be made using a material with a volume electrical resistivity of up to 10 9 ohm meters, this will usually not provide any practical benefits over a volume electrical resistivity of 10 7 ohm meters.
  • microdrops may be formed when the ink jet 19 separates into drops of ink. Electric charge will be trapped on these microdrops in the same way as it is trapped on the normal drops. At least some of these microdrops may travel to the surface of the charge electrode 21. If the low conductivity layer 83 was not present, the charges on the microdrops would immediately be conducted away by the metal core 85 of the charge electrode 21 , resulting in a small current flow along the charging signal line 87. The presence of the low conductivity layer 83 prevents the microdrops from reaching the metal core 85, and therefore the charge on the microdrops has to be conducted to the metal core 85 by the resistance Rp of the low conductivity layer 83.
  • the rate at which microdrops deliver charge to the surface of the low conductivity layer 83 depends on the rate at which microdrops are formed, the degree to which microdrops land on the charge electrode 21 as opposed to other available surfaces, and the charge on each microdrop. These factors are in turn influenced by other factors such as the geometry of the ink gun 17 and the charge electrode 21 , the viscosity and other properties of the ink, the ink pressure and jet speed, and the voltage on the charge electrode. However, the total amount of charge on each microdrop will be very small. Additionally, the printer typically prints a message on a surface 9 of each object 11 as it passes the print head 5, and then waits for the next object to arrive. Therefore it only prints intermittently and all drops of ink generated by the printer between messages are uncharged.
  • Figures 11 to 14 show an alternative embodiment of the charge electrode 21 together with the ink gun 17.
  • the ink gun 17 and the charge electrode 21 are assembled together and this assembly is not mounted on the baseboard 31 , but instead the ink gun 17 is mounted on an endpiece (not shown) of the print head 5 that is connected to the umbilical 7 so that the ink gun 17 extends above and spaced from the baseboard 31.
  • the charge electrode 21 is mounted on the ink gun 17, and also extends above and spaced from the baseboard 31.
  • the metal core 85 of the charge electrode 21 is a thin metal piece, e.g. formed by moulding. It has two flat plates 89 that extend each side of the ink jet 19.
  • the flat plates 89 are joined to one end of a hollow cylindrical body 91 of the charge electrode.
  • This end of the hollow cylindrical body 91 is closed by end plates 93 on either side of the flat plates 89, but the end is open between the flat plates 89 in order to allow the ink jet 19 to pass through the charge electrode 21.
  • the other end of the hollow cylindrical body 91 is open.
  • Figure 1 1 is a section through the ink gun and charge electrode assembly in a plane parallel to the flat plates 89 and Figure 12 is a section through the ink gun and charge electrode assembly in a plane transverse to the flat plates 89, except that the ink gun 17 is not shown in section in Figures 1 1 and 12.
  • Figure 13 is an end view of the charge electrode 21 looking in the direction from the gutter 27 and
  • Figure 14 is an end view of the metal core 85 of the charge electrode 21 looking in the direction from the gutter.
  • Broken line XI - XI in Figures 12 and 13 shows the line of the section shown in Figure 11
  • broken line XII - XII in Figures 1 1 and 13 shows the line of the section shown in Figure 12.
  • the metal core 85 of the charge electrode 21 is encased in the layer 83 of low conductivity material except for a short distance of the hollow cylindrical body 91 at its end remote from the flat plates 89.
  • the layer 83 of low conductivity material is thinnest between the flat plates 89. Elsewhere it is thicker in order to provide strength for the charge electrode 21 , and the low conductivity layer 83 is thickest on the outer sides of the flat plates 89 to protect the flat plates.
  • the ink gun 17 is mounted in a plastic insulating body 95.
  • the outer edge of a metal contact ring 97 is embedded in the plastic insulating body 95.
  • the inner edge of the metal contact ring 97 is exposed.
  • the charge electrode 21 fits onto the end of the ink gun 17 and the plastic insulating body 95, and the exposed end of the hollow cylindrical body 91 of the charge electrode 21 presses against the metal contact ring 97.
  • the metal contact ring 97 is electrically connected to the charging signal line 87 by a conductor (not shown) through the plastic insulating body 95. In this way the charging signal from the charging signal generator is connected to the metal core 85 of the charge electrode 21.
  • a fitting ring 99 preferably made of insulating plastic or low conductivity plastic, fits around the charge electrode 21 and captures a ledge on the outside of the charge electrode 21 as can be seen in Figures 11 and 12.
  • the fitting ring 99 screws onto the plastic insulating body 95 around the ink gun 17, to hold the charge electrode 21 in place and to force the exposed end to the hollow cylindrical body 91 of metal core 85 of the charge electrode 21 against the metal contact ring 97 to ensure a good electrical contact between them.
  • the charge electrode 21 is easily assembled to the insulating plastic body 95 that houses the ink gun 17, and the charge electrode 21 is held in the correct position relative to the ink gun 17.
  • Sealing rings 101 protect the metal contact ring 97 and the exposed end of the metal core 85 of the charge electrode 21 from any contact with spilt ink, in order to ensure that the charge electrode voltage is not shorted to the reference potential of the ink gun 17.
  • Embodiments of the present invention provide an operator with electrical protection from the charge electrode voltage, by providing the low conductivity layer 83 over all exposed parts of the metal core 85 of the charge electrode 21. If the thickness of the low conductivity layer 83 and its volume resistivity are sufficient, it may be possible to avoid the need to turn off the charge electrode voltage when the cover of the print head 5 is opened.

Landscapes

  • Particle Formation And Scattering Control In Inkjet Printers (AREA)
  • Ink Jet (AREA)

Abstract

Selon l'invention, une couche à faible conductivité (83) est disposée sur un noyau électroconducteur (85) de l'électrode de charge (21) d'une imprimante à jet d'encre continu à déviation électrostatique au niveau de la totalité de la partie exposée de l'électrode de charge (21). La couche à faible conductivité (83) a une épaisseur d'au moins 0,1 mm et présente une résistivité électrique volumique de 104 à 109 ohm mètres. La couche à faible conductivité (83) fournit une sécurité électrique au moins partielle dans le cas où un opérateur touche la charge (électrode 21) tandis que la tension d'électrode de charge est appliquée, mais est également susceptible de dissiper toute charge électrostatique qui pourrait par ailleurs s'accumuler sur la surface de l'électrode de charge (21).
PCT/GB2020/051744 2019-07-24 2020-07-22 Électrode de charge pour imprimante à jet d'encre continu WO2021014147A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB1910600.4 2019-07-24
GB1910600.4A GB2585928B (en) 2019-07-24 2019-07-24 Charge electrode for a continuous Ink jet printer

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WO2021014147A1 true WO2021014147A1 (fr) 2021-01-28

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4587527A (en) * 1985-05-15 1986-05-06 Eastman Kodak Company Charging electrodes bearing a doped semiconductor coating
GB2316364A (en) * 1996-08-15 1998-02-25 Linx Printing Tech An ink jet printer and a cleaning arrangement thereof
WO2015187983A2 (fr) * 2014-06-05 2015-12-10 Videojet Technologies Inc. Tête d'impression à jet d'encre continu avec réglage du zéro pour électrode de charge intégrée

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3984843A (en) * 1974-07-01 1976-10-05 International Business Machines Corporation Recording apparatus having a semiconductor charge electrode
GB1453571A (en) * 1974-07-01 1976-10-27 Ibm Liquid droplet recording apparatus
JPS55166259A (en) * 1979-06-11 1980-12-25 Ricoh Co Ltd Ink jet recording device
GB2551321B (en) * 2016-06-07 2021-06-09 Linx Printing Tech Ink jet printer

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4587527A (en) * 1985-05-15 1986-05-06 Eastman Kodak Company Charging electrodes bearing a doped semiconductor coating
GB2316364A (en) * 1996-08-15 1998-02-25 Linx Printing Tech An ink jet printer and a cleaning arrangement thereof
WO2015187983A2 (fr) * 2014-06-05 2015-12-10 Videojet Technologies Inc. Tête d'impression à jet d'encre continu avec réglage du zéro pour électrode de charge intégrée

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GB201910600D0 (en) 2019-09-04
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